Aluminum dust concentration effect on combustion in hydrocarbon
Bunsen flames
Michael Soo
Master of Science
Department of Mechanical Engineering
McGill University, Montreal
December 2012
A thesis submitted to McGill University in partial fulfillment of the requirements of the
degree of Masters of Science in Mechanical Engineering
Copyright 2012 All rights reserved.
DEDICATION
For D, S, H, and J.
ii
ACKNOWLEDGEMENTS
I owe much more than a page of this thesis to thank the numerous people involved
in this project. I thank my supervisors, Jeff Bergthorson, David Frost, and Sam Goroshin
for their trust and willingness to allow me almost full control over a lab and a project.
Moreover, I am grateful for the unhesitant support in pursuing outlandish ideas beyond
the scope of the project, and the support to bring them to fruition. The experience and
opportunities allowed to me during the course of this degree were beyond expectation,
and for that I am indebted.
My colleagues in the lab, I thank you for your time and effort in lab construction
and experiments. Thanks to Nick Glumac for providing the means to do AlO temperature
measurements. My final acknowledgement goes to Gary Savard, for his invaluable
guidance in the construction of the experiment.
iii
TABLE OF CONTENTS
FIGURES .................................................................................................................v
LIST OF SYMBOLS ............................................................................................. vi
ABSTRACT .......................................................................................................... vii
1 INTRODUCTION...............................................................................................1
1.1 Overview ........................................................................................................1
1.2 Laminar Flames in Gas Mixtures...................................................................3
1.3 Laminar Flames in Heterogeneous Mixtures .................................................6
1.4 Consecutive and Parallel Reactions .............................................................13
1.4.1 Consecutive reactions ...........................................................................13
1.4.2 Parallel Reactions ..................................................................................17
1.5 Hybrid flame studies ....................................................................................18
2 HYBRID FLAME EXPERIMENT AND DIAGNOSTICS ..........................20
2.1 Dust Burner ..................................................................................................20
2.2 Aluminum Powder .......................................................................................21
2.3 Diagnostics Overview ..................................................................................22
2.3.1 Dust concentration ................................................................................23
2.3.2 Flame Speed ..........................................................................................26
2.3.3 Spectroscopy .........................................................................................27
2.3.4 Condensed phase Temperature .............................................................28
2.3.5 AlO molecular temperature...................................................................29
3 EXPERIMENTAL RESULTS.........................................................................31
3.1 Low concentrations of aluminum ................................................................32
3.2 Flame front formation ..................................................................................35
3.3 High concentrations of aluminum................................................................41
3.4 The presence of excess oxygen ....................................................................45
3.5 AlO molecular temperature measurement ...................................................47
4 DISCUSSION ....................................................................................................48
4.1 Explanation for the aluminum flame front formation ..................................48
4.2 Kinetic and diffusive regimes of combustion ..............................................50
4.3 The choice of emissivity and the AlO temperature .....................................51
5 CONCLUSIONS ...............................................................................................53
5.1 Structure of hybrid flames ...........................................................................53
5.2 Future work ..................................................................................................53
REFERENCES .....................................................................................................55
iv
FIGURES
Figure 1-1: Gas phase flame structure. (I) unburned gas zone, where temperature and mass species are
constant. (II) Preheat zone, heat and mass diffusion govern this zone up to the point of thermal runaway.
(III) Thermal runaway where reaction peaks and decreases due to species consumption (IV) post flame
zone, where flame temperature has been reached and species consumed ...................................................... 5
Figure 1-2: the flame structure of a particle fuel in gaseous oxidizer undergoing diffusive combustion. (I)
the preheat zone, (II) the reaction/flame front zone, (III) the post flame zone. .............................................. 9
Figure 1-3: CH4/NO2/O2 in a flat flame burner demonstrating the separation of flame fronts [28]. .............14
Figure 1-4: Preliminary results for the iron-methane-air flame demonstrating the separation of flame fronts.
.......................................................................................................................................................................15
Figure 1-5: Schematic of the methodology: progressively increase the concentration of dust in the methane
and determine the effects. ..............................................................................................................................18
Figure 2-1: the complete dust burner cross section. Gas is dispersed into the main flow line via and air knife
which disperses a column of powder. The powder-gas flow expands in the dispersion cone, and is
laminarized in the long tube leading up to the nozzle where it is ignited. The ejector allows the ejection of
flow after the powder is dispersed to control the flow at the exit of the nozzle. ...........................................21
Figure 2-2: SEM images and particle size distribution of Ampal 637. ..........................................................22
Figure 2-3: Diagnostic configuration around the dust burner ........................................................................23
Figure 2-4: Dust concentration calibration for Ampal 637 ............................................................................25
Figure 2-6: Planck curve approximation fitting to sample spectra. ...............................................................29
Figure 3-1: concentrations of aluminum between 0-100 g/m3 seeded into the stoichiometric methane flame
.......................................................................................................................................................................32
Figure 3-2: Flame temperature at concentrations of aluminum between 0-100 g/m3. ...................................33
Figure 3-3: A typical spectrum of a flame with concentrations of aluminum from 0-100 g/m3 ....................34
Figure 3-4: Measured flame speed in concentrations of aluminum of 0-100 g/m3 ........................................35
Figure 3-5: formation and stabilization of the aluminum flame front. (A) low concentrations of aluminum.
(B) aluminum front begins to form at the tip of the cone. (C) aluminum front moves down over the methane
cone. (D) aluminum front stabilizes. .............................................................................................................36
Figure 3-6: Temperature increase after the formation of the aluminum front above 100 g/m 3. ....................37
Figure 3-7: Aluminum oxide band sequence formation after flame front formation. Indication of relatively
fast aluminum combustion. ...........................................................................................................................38
Figure 3-8: Flame speed dependence on aluminum concentration. After flame front formation, the flame
speed remains stable, as oppose to inert loadings of SiC which eventually quench the flame. Standard
deviations for concentration are plotted for a few points of aluminum to illustrate the error propagation at
larger concentrations. ....................................................................................................................................39
Figure 3-9: An open tip flame in high loading of SiC particles. ....................................................................40
Figure 3-10: (left) appearance of the secondary diffusion flame at higher concentrations. (right) neutral
density filter increased to see Bunsen cone features. .....................................................................................41
Figure 3-11: evolution of the AlO spectra at high concentrations of aluminum. ..........................................42
Figure 3-12: Temperature plateau at higher concentrations of aluminum. ....................................................43
Figure 3-13: measured temperature plotted with adiabatic flame temperature prediction and aluminum
species concentrations. ..................................................................................................................................44
Figure 3-14: High resolution spectra showing the presence of the atomic transition of aluminum gas in
concentrations of aluminum greater than 180 g/m3. ......................................................................................45
Figure 3-15: Comparison of flame speeds in stoichiometric methane and in oxygen excess. .......................46
Figure 3-16: Comparison of temperature in stoichiometric methane and in oxygen excess. ........................47
Figure 4-1: A) Low concentrations of aluminum where thermal runaway of aluminum happens far
downstream of the methane flame. B) A mid-range of concentration where the thermal runaway of
aluminum happens in the vicinity of the methane flame front but where concentration fluctuations cause the
front to be unstable. C) Sufficiently high concentration of aluminum to couple the methane front to the
aluminum front. .............................................................................................................................................48
v
LIST OF SYMBOLS






A
B
cp
C
D
Ea
I
k
m
q
Q
R
Tad
Ti
To
uf
w
Y
V
thermal diffusivity
mass transfer coefficient
flame thickness
emissivity
thermal conductivity, wavelength
density (subscript is indicative of ‘s’ for solid or ‘g’ for gas)
characteristic reaction time
Arrhenius pre-exponential term
dust concentration
specific heat capacity
concentration of oxidizer
diffusion coefficient
activation energy
intensity
Arrhenius term
slope
heat flux
heat release per unit mass
universal gas constant
adiabatic flame temperature
ignition temperature
initial temperature
flame speed
reaction rate, species reaction rate
concentration of reactant
voltage
vi
ABSTRACT
A premixed methane–air bunsen-type flame is seeded with micron-sized (d32 = 5.6 μm)
atomized aluminum powder over a wide range of solid fuel concentrations. It is found
that an increase in aluminum dust concentration changes the aluminum combustion
regime from slow, low-temperature oxidation to full-fledged aluminum flame front
propagation. The critical concentration for this transition occurs in a range of 100–
180 g/m3. The transition is manifested by a sharp increase in temperature up to 2600 K,
following closely with thermodynamic predictions at these higher concentrations, and the
appearance of AlO sub-oxide bands. In concentrations that precede the formation of the
aluminum flame front, the burning velocity decreases linearly, similar to inert SiC
loadings. After aluminum flame front formation, the flame speed in aluminum-methane
mixtures remains constant even after increasing concentrations in contrast to the rapid
decrease in flame speed, followed by quenching that is observed in flames seeded with
inert SiC particles. It is likely that thermal coupling is required between the aluminum
and methane flame fronts for stabilized flame propagation and, moreover, given the effect
of concentration on the front formation and temperatures in the flame, there is evidence
of aluminum combustion in the kinetically-limited regime.
vii
ABRÉGÉ
Une flamme du type de bec de Bunsen, composée de méthane et d'air pré-mélangés, est
ensemencée en particules d'aluminium pulvérisé de l'ordre du micromètre (d32 = 5.6 μm)
pour une large gamme de concentrations. Il a été constaté que l'augmentation de la
concentration en aluminium modifie le régime de combustion qui passe d'une oxydation
lente et à basse température à une propagation à part entière de la flamme frontale. La
concentration critique pour cette transition se produit dans un éventail de 100–180 g/m3.
La transition se manifeste par une forte hausse de température jusqu'à environ 2600 K,
suivant étroitement les prédictions thermodynamiques pour ces hautes concentrations et
l'apparition de bandes sous-oxydes de AlO. Pour les concentrations qui précèdent la
formation du front de la flamme d'aluminium, la vitesse de la flamme décroît de façon
linéaire - un comportement similaire aux tests avec une charge de SiC. Suite à la
formation du front de la flamme, la vitesse de la flamme des mélanges d'aluminium et de
méthane demeure constante pour des concentrations croissantes, contrairement à la
diminution rapide de la vitesse de flamme, suivie d'extinction, observée pour les flammes
à particules de SiC. Il est probable que le couplage thermique entre les front d'aluminium
et de méthane est indispensable pour la stabilisation du système de propagation de la
flamme. De plus, compte tenu de l'effet de la concentration sur la formation du front et de
la température des flammes, il est probable que l'aluminium brûle dans un
régime cinétiquement limité. Ceci contraste avec l'hypothèse du régime limité par
diffusion,
présumé
par
les
études
viii
de
systèmes
à
particule
unique.
1 INTRODUCTION
1.1 Overview
In combustion systems, mixed fuels are often used to tailor the fuel to the specific
needs of the engine or application. In the aerospace industry, this might be encountered in
fuel blends such as Jet A. In military and rocket applications, fuel blends involving
multiple phases are encountered. Many explosive and propellant systems utilize the high
energy of metal combustion in combination with hydrocarbon fuels and explosives to
enhance performance. Theoretical prediction of the behavior of fuel blends has received
relatively little attention, and actual behavior predictions have remained largely
empirical. In multiphase fuel systems, the theoretical treatment is complicated by liquid,
gel, or gas components with dispersed solid phase materials, typically in particulate form.
The effect of the solid phase is commonly quantified by observation, but there is no
comprehensive theory able to predict the phase dynamics when presented with
multiphase reacting systems. Even the simplest case of a theoretical fuel blend with a one
step reaction is not well understood. With the exception of Margolis, Matkowsky and
Khakin and their colleagues [1-3], little research has been conducted to theoretically
analyze the nature of fuel mixtures, especially multiphase mixtures which have the
possibility of different modes of combustion, that ultimately change the details of the
kinetics.
Hybrid Systems
Multiphase fuel blends have not received much attention despite their practical
importance in several areas of combustion engineering, as well as in the fundamental
understanding of the physics between the different phase reactions in the combustion
wave. Distinguished from “heterogeneous” combustion (solid fuel-gaseous oxidizer),
multiphase fuels with a gaseous oxidizer are defined to be a “hybrid” systems. Examples
of such systems are few but of great engineering importance.
In coal combustion, coal particles are known to burn in two phases:
heterogeneously with the solid phase carbon fuel and homogeneously with the gas phase
fuel released from the volatilization of the particle. Numerous studies on the combustion
of coal have been conducted, concluding that the kinetics involve condensed phase
1
reactions between the solid and gas resulting in our classification of coal combustion as a
“hybrid” flame [4-6].
The reaction of particulate suspensions of light metals, aluminum particles in
particular, with the products of hydrocarbon flames is one of the critical stages in the
combustion of solid and metalized gelled propellants, pyrotechnics, and metalized
explosives. Other hybrid mixtures are also found in process industries and in the mining
industry, where dust explosions are often enhanced by the presence of hydrocarbon fuels
[7]. The majority of experimental work in this area is focused on the combustion of large
(tens of microns) individual particles and agglomerates [8-11] or on the combustion of
specifically formulated solid propellant compositions with very low metal particle
loading [12]. It is often implicitly assumed that the results can be extrapolated to smaller
particles and dense suspensions which are found in applications like explosions in dust
clouds and in propellant compositions. However, these studies ignore flame front
propagation in the bulk suspension and the effect of concentration on the reaction of the
metal.
These motivations initiate the study of the structure and properties of hybrid
deflagrations in metal-hydrocarbon fuel mixtures. Beginning with laminar premixed
flames, the most elementary flame configuration, studies can progress into more
advanced turbulence associated with practical systems. The work in this thesis is meant to
provide a basic understanding of the properties of multiphase metal-hydrocarbon in
laminar flames and to provide insight into the mechanism of flame propagation for future
model development. Preliminary work in this area was completed by Bryant et al. and
Brzutowski et al. [13, 14], but little attention was paid to the particular flame structure in
the mixed fuels. The fuels used in this study will be gaseous methane and aluminum
powder. The experimental and theoretical precedent for laminar flames in both these
fuels individually makes them ideal candidates for study. This thesis does not attempt to
formulate a theory for hybrid flame combustion, but outlines the important factors in
determining such a theory as well as qualitative experimental description of the
combustion properties of premixed aluminum-methane mixtures.
2
It is first important to understand the combustion of the individual fuel
constituents. While most studies of metal combustion focus on the particles, here the
focus in on the reaction of a suspension. The “gas-like” properties of a particle
suspension allow for the possibility of flame propagation. Thus, background in premixed
flame propagation and flame structure of heterogeneous systems and homogenous
systems is required for full understanding of the hybrid problem. The basic theoretical
principles of fuel blends are also covered in the introduction to give context to the
observations in the results of the experiment.
1.2 Laminar Flames in Gas Mixtures
The normal burning velocity of a given flame is perhaps the most important
calculation of flame theory. It is related directly to the chemical kinetics and diffusive
transport properties in the fuel-oxidizer system and depends on the rate of these chemical
reactions in converting the fuel mixture into combustion products [15] . Mathematical
treatment of homogenous, gas phase combustion has been studied in great detail evolving
from the basic principles of Mallard and Le Chatelier to the more widely accepted
Zeldovich and Frank-Kamenetskii theory of laminar flames. These formulations serve as
the basis for combustion physics.
Without any insight into the structure of the flame front, Frank-Kamenetskii first
used the thermal flux in the reaction to estimate the flame speed in one dimension [16].
He approximated the temperature gradient between the cold gas and the reaction zone as
a discontinuity. Assuming that the energy generated in the reaction zone is used to heat
the upstream cold gas mixture, the relation can be shown in (2.1).
q
T T
dT
  ad o  c p u f (Tad  To )
dx

(2.1)
The thermal flux can be approximated by knowing the flame thickness  and that
the heat released in the flame thickness goes to heating the unburned gas. Because the
unburned gases enter normal to the wave, by definition, a one-dimension problem
assumes the flame speed must be equal to the incoming gas speed.
3
Assuming that the flame front thickness is proportional to the speed of the front,
the characteristic reaction time  is determined.
  u f
(2.2)
Thus the flame speed expression is given by substitution of (2.2) into (2.1).
uf   /
(2.3)
where  is the thermal diffusivity given by  /  c p . This expression is also obtained by
dimensional analysis of the important parameters of the flame propagation as shown in
Yarin et al. [17]. It expresses the dependence of the laminar speed on the transport
properties and the reaction rate. In a slow reaction, the characteristic reaction time in the
flame zone will be longer resulting in a lower flame speed.
A more rigorous mathematical treatment of the flames requires the relevant
transport and energy conservation equations of the problem with reaction kinetics.
In
order to formulate the equations it is important to have an understanding of the flame
structure.
For a single fuel in a homogenous mixture of oxidizer, the thermal and species
structure of the combustion wave is shown in Figure 1-1. The structure is separated into
four distinct zones that describe the flame. In (I), the mixture is unburned and unaffected
by heat and species diffusion processes. In (II), the preheat zone, the mixture is governed
by heat and mass diffusion processes. In (III), the reaction zone, the reaction rate peaks
and quickly drops off with loss of reactive mixture. In (IV), the post reaction zone, the
hot products and unreacted mixture flows out of the system unaffected by heat transfer
upstream.
4
Figure 1-1: Gas phase flame structure. (I) unburned gas zone, where temperature and mass species are constant. (II)
Preheat zone, heat and mass diffusion govern this zone up to the point of thermal runaway. (III) Thermal runaway
where reaction peaks and decreases due to species consumption (IV) post flame zone, where flame temperature has
been reached and species consumed
The formulation of the problem utilizes the conservation of mass, momentum and
energy equations. The treatment is limited to a one-dimensional system in the frame of
reference of the flame front, moving with constant speed u f . The equations are shown
in (2.4) and (2.5) are the conservation of energy and the conservation of the limiting
reactant species respectively.
dT  d 2T
u

 qw(Y , T )
dx c p dx 2
dY
d 2Y
u
  D 2  w(Y , T )
dx
dx
(2.4)
(2.5)
The complete step-by-step formulation of the governing equations can be found in
Law [18]. Assume that the reaction rate w stems from one step Arrhenius kinetics. The
equations are non-dimensionalized as explained in Law [18]. The dependence on
concentration can be decoupled to yield a non-dimensional temperature energy equation
with boundary conditions determined by the initial temperature and reacting species and
5
assumption that the flame temperature tends towards the adiabatic flame temperature
when all species are consumed.
The boundary conditions over-determine the system of equations causing
invariance in the origin of the position coordinate. The extra boundary condition imposes
that the system of equations will only have a solution at a certain value for flame speed.
Thus, the flame speed is an eigenvalue.
The Zeldovich, Frank- Kamenetskii, Semenov (ZFS) treatment of the flame speed
assumes that the majority of the reaction happens in a temperature very close to the
adiabatic flame temperature [15]. The treatment of these equations for an analytical
solution is beyond the scope of this thesis, but utilizing the ZFS model, the calculation for
flame speed is shown in (2.6) for Lewis numbers of unity.

w
RTad2
u f  2 
 E T  T   Y
 a ad o 
(2.6)
where  is the heat diffusivity of the gas, R is the universal gas constant, Tad is the
adiabatic flame temperature, To is the initial temperature, w is the reaction rate, and Y
is the fuel concentration.
The dependence of the system on the square root of the diffusivity multiplied by
the reaction rate is seen again in the more detailed model.
1.3 Laminar Flames in Heterogeneous Mixtures
The fundamental work in laminar dust combustion was completed by Cassel in
his seminal work for the US bureau of mines. Cassel designed and created a dust burner
for metal dusts premixed with oxidizer [19]. His approach to study metal combustion was
fundamentally different from other research in metal combustion in that he studied
suspensions and clouds of dust as if they were the same as a gaseous mixture, as opposed
to the combustion of single particles in research such as Freidman and Maček [8].
In gaseous oxidizer-particulate fuel mixtures, the size of the particles determines the
treatment of the problem. In very fine particulate suspensions, heat and mass transfer
behave similar to a gaseous species and hence, the temperature of the particulate phase
6
closely follows the gas phase indicating that the kinetics of the reaction are the major
factor of the combustion, known as kinetically-controlled combustion.
For larger
particles, the rate of reaction may depend on the temperature but will have greater
dependence on oxidizer diffusion to the particle surface due to the lower specific surface
area, also known as a diffusion-controlled regime of combustion. It has been shown in
[20, 21] that decreasing the particle size and changing the oxidizer environment produces
evidence of this transition from diffusion-controlled to kinetically-controlled regimes.
The effect can be described by the value of the Damköhler number, which is the ratio of
the surface reaction rate to the oxygen diffusion rate. This effect becomes important in
the modeling and understanding of hybrid flames as the Damköhler number is likely to
change across the profile of the flame, possibly changing between regimes of kineticallylimited and diffusion-limited combustion or some combination of both [20].
An order of magnitude estimate of the flame speed in particle-fuel/gaseous
oxidizer mixtures is made by Yarin et al. [17]. The very rudimentary treatment assumes
that the particles heat up to an ignition temperature before entering the reaction zone. The
ignition temperature Ti
is defined as the temperature where heat flux from the reaction
is greater than heat loss to the environment (This temperature is dependent on the initial
temperature To , so it is not an intrinsic property of a material). In purely conductive heat
transfer, the heat flux from the reaction zone is all used to heat the incoming mixture
from its unburned temperature (assuming a single temperature of the mixture) to the
ignition temperature shown in (2.7). In dust flames of this scale, the radiation mean free
path is at least an order of magnitude larger than the preheat zone, meaning that radiative
heat transfer is negligible [22]. Larger clouds of dust may have a stronger dependence on
radiative heat transfer.
 c
g g
  s cs  u f Ti  To   
dTg
dx

Tad  To

(2.7)
If the temperature distribution in the reaction zone is assumed linear as was done
in the order-of-magnitude estimation for the homogenous flames, the flame speed of the
flame front in(2.8) is determined.
7
uf 
 c
g g
Tad  To  1

  s cs  (Ti  To ) 
(2.8)
The flame front thickness  is proportional to the burning time of the particle.
The burning time is proportional to of the particle diameter d in the kinetically-limited
regime and to d 2 in the diffusion-limited regime [17]. Beckstead indicates that the most
accepted mode of burn time for particles (droplets) is the d 2 law, though admits that in
the combustion of aluminum, the rate is described by d n where n can vary depending
on the size and oxidizing medium of the particle [23].
In past work by Goroshin, the solid fuel is assumed to undergo only diffusive
combustion at the surface of the particle [24]. The combustion of each particle is assumed
to be initiated at some temperature and rise quickly to the reaction temperature and
maintain constant temperature for the duration of the combustion time of the particle. The
corresponding flame structure for a particle suspension undergoing diffusion-controlled
combustion is shown in Figure 1-2.
8
Figure 1-2: the flame structure of a particle fuel in gaseous oxidizer undergoing diffusive combustion. (I) the preheat
zone, (II) the reaction/flame front zone, (III) the post flame zone.
The thermal structure is separated into three groups. In (I), the preheat zone, the
heat of the reaction is small and the particles are heated by the downstream gas phase. At
some point when the heat of the reaction of the particle surpasses the heat flux to the gas,
the particle ignites and quickly moves into the stable particle combustion controlled by
oxygen diffusion in region (II). In this region, the particle burning time in the flame front
 is close to the burning time of a single particle because there is little effect from the
temperature of the ambient gas on diffusion limited combustion. In the reaction zone, it is
assumed that the volumetric heat release in a given time is determined by the mass
concentration of particles and the theoretical heat release of the reaction with the
oxidizer.
It is important to note that in the diffusion-limited regime of combustion that the
gas temperature and particle temperature will separate. Experimentally, determination of
the regime of combustion can be accomplished by measuring and comparing the gas
phase temperature and the particle temperature in a heterogeneous mixture.
9
Goroshin presents an analytical solution to this treatment of solid particle fuel
flames by assuming the lean fuel case [24]. A ‘g’ subscript is assumed to be a parameter
related to the gas temperature, an ‘s’ indicates the solid. As there is no dependence of the
reaction rate on the local concentration explicitly, there is only need to solve one
differential equation in the three different zones described above.
I 
 g u f c pg
dTg
 II 
gu f cp
dTg
 III 
dx
dx
 g u f c pg

 g
dTg
dx
d 2Tg
  x  0
dx 2
d 2Tg
dx
 g
2

d 2Tg
dx 2
BQ

0  x  u f
u f  x  
With boundary conditions in the region (I):
Tg  x     To
(2.9)
Tg  x  0   Ti
(2.10)
In region (II) the boundary conditions are:
Tg  x  0   Ti
dTg
dx

x 0
dTg
dx
(2.11)
(2.12)
x 0
The third region boundary conditions are unnecessary because it provides a
redundant final form. In zone (I), the preheat zone, the particle temperature profile is
given by the rate that heat is transferred to the gas up to the ignition temperature assumed
in quasi-stationary heating of the particle. The heat transfer coefficient is assumed to be
the ratio of the characteristic particle heat exchange time, and the combustion time of the
particle.
(I )
s u f c ps
dTs 3

(Tg  Ts )   x  0
dx r 2 2
(2.13)
In this zone, the following boundary conditions apply:
Ts  x     Tso  Tgo
10
(2.14)
There is only a need to solve the bounded equations of the gas phase in either
zone (I) and (II) or (II) and (III) because of the number of boundary conditions defined,
and the particle temperature equation can be solved independently to find a relationship
that allows the particle temperature to cancel out from the final expression for flame
speed. An analytical solution to this set of equations is given in Goroshin [25, 26] by first
non-dimensionalizing them and solving in each individual zone. The burning velocity can
be determined by the transcendental equation:
   (1   )[1  exp   ]
(2.15)
where   u f 2 /  ,   [r 2c ps s / 3 g c pg  g ] /  and   BQ / c ps  (Tsi  Tgo ) .
The limitation of analytical models for heterogeneous reactions and reactive metal
particles is the expectation that the reaction is either entirely in the diffusive limited
regime, where the particle, burning rate is dependent only on the rate of oxygen diffusion
to the surface of the particle or in the kinetically limited regime of combustion [24]. A
more general case of the single step reaction would combine the rates determined by the
kinetics that take place at the surface and partly by the transport rate of the reactants to
the surface of the particle by diffusion. This was briefly examined conceptually and
numerically by Goroshin et al. [27].
The basis for this treatment was proposed by Frank-Kamenetskii [16] where it is
assumed that a reactive particle will be subject to both diffusion kinetics and surface
reaction kinetics. It is assumed that oxidizer mass transfer rate to the particle is given by
the equation for interfacial mass transfer shown in (2.16) .
w  C   (C  Cs )
(2.16)
where  is the mass transfer coefficient and C is the concentration of the bulk of
oxidizer, and Cs is the concentration at the surface of the particle. The solution to the
spherical diffusion equation for a single particle combined with Fourier’s law yields the
value for  in(2.17).

Nu D
d
11
(2.17)
Where d is the particle diameter, D is the diffusivity, and Nu is the Nusselt number.
With this in mind, the mass consumption rate is set equal to a general Arrhenius, one step
reaction rate dependent on the concentration of oxidizer at the surface as shown in (2.18).
w  kCs
(2.18)
where k is the Arrhenius dependence given by k  Ae Ea / RT . It is assumed that no mass
accumulates on the surface of the particle.
By equating (2.16) to (2.18) (mass
consumption to mass transfer rate) and finding Cs , the concentration at the surface, the
mass consumption rate (2.18) can be rewritten by substituting the value of Cs to yield
(2.19).
w
k
C
k 
(2.19)
This is equivalent to an addition of parallel resistances if the expression is written
in the form:
w  k *C
(2.20)
1 1 1
 
k* k 
(2.21)
Where
Frank-Kamenetskii’s method reveals a reaction rate involving the rates of both the
kinetics of the reaction and the diffusion of oxygen to the surface. The model does not
limit itself to the scope of a single particle because the surface oxidizer concentration can
be cancelled out and thus the model can describe the bulk properties of single sized
particle suspensions. The model indicates strong dependence on the size of the particle to
determine the regime (kinetic or diffusive) of combustion as demonstrated by the work in
[20, 21]. During combustion, a particle of fuel will likely undergo a change in size as the
fuel either volatilizes or is consumed. This allows for the possibility of a combustion
regime change based on the rate of particle size decrease. Goroshin et al. proposed a set
of unsteady one dimensional flame equations with a gas temperature, a particle
temperature, oxidizer species consumption equation and an expression to track the
12
particle radius through the flame [27]. The resulting numerical simulation reveals the
dependence of flame speed on particle size which agrees similarly to the experimental
results in dust suspensions of different initial particle sizes [20].
The Frank-Kamenetskii model serves as a basis for the treatment of hybrid flames
depending on the actual flame structure. For instance, if the gaseous fuel ignites before
the solid fuel in the flame (a likely scenario given the relatively low activation energies
associated with gaseous fuel compared to solid fuels) the resulting flame structure will
have the particles of solid fuel passing through the flame front of the gaseous fuel. This
will change the effective “resistance” of the reaction rate in the bulk mixture of gaseous
and solid fuels, either by introducing higher initial temperatures increasing diffusive rates
or by changing the oxidizer constituents. It is not entirely certain whether the solid fuel
will need to be treated as separate phenomena or as a simultaneous reaction with the
gaseous fuel. Goroshin examines this phenomenon for the case of a binary mixture of
particle sizes, or two different metal fuels, in [26]. The equations are similar to those
found in the treatment of a mono-size suspension reviewed in this section. Two possible
configurations of heat release result in Goroshin’s treatment of the problem. One stable
solution predicts a separated zone of heat release, while the other predicts the merging of
the zones of heat release from the two different fuel constituents. The possibility of
multiple fronts or interacting fronts requires review of the terminology developed to
describe chain reactions and parallel reactions.
1.4 Consecutive and Parallel Reactions
1.4.1 Consecutive reactions
Sets of parallel and consecutive reaction steps dictate the flame structure of a
given fuel mixture. In homogenous mixtures of fuels, for instance in the different fuel
constituents of jet fuel, activation energies, intermediate species and their kinetic rates are
largely the same, meaning that the mixture acts more or less as a single fuel producing a
single flame front, but in mixtures with vastly different activation barriers and reaction
rates, the parallel and consecutive reaction steps may manifest themselves physically as
separated zones of heat release. For instance, the fuel system of CH4/O2/NO2 utilizes two
13
oxidizers which forms two flame fronts when burned in a counterflow burner [28].
Between the flame fronts is a distinguishable dark zone indicating a non-uniform heat
release across the flame region in Figure 1-3.
Branch et al. [28] showed that in each distinct flame zone was a peak of CH
radical concentration, which is highly unusual but seemingly correlates to the two
luminous zones seen in the flame. Khaikin considers this fuel system to fall under the
category of consecutive reactions [3].
Figure 1-3: CH4/NO2/O2 in a flat flame burner demonstrating the separation of flame fronts [28].
While Branch et al. tend to dismiss the notion of consecutive or parallel reactions
when describing the CH4/O2/NO2 fuel system to favor a more detailed description of the
reaction pathways, the description developed by Khaikin et al. is useful to describe bulk
reaction schemes in which individual reaction schemes are not necessarily known, i.e.
with certain fuel systems that exhibit multiple flame fronts as in the aforementioned fuel
system and, potentially, in hybrid flames. In Figure 1-4, a preliminary study of a hybrid,
methane-iron-air flame is shown to exhibit similar visual characteristics to the Branch
study with a dark zone in-between the flame fronts. A similar separation of flame fronts
has been shown to occur in the flame propagation of monogermane fuels in tubes.
Aivazyan et al. [29] attributes the unknown combustion mechanism to Khaikin’s
terminology, describing the flame fronts formed first by the decomposition of the
tetrahydride and the subsequent oxidation reaction.
14
Figure 1-4: Preliminary results for the iron-methane-air flame demonstrating the separation of flame fronts.
It is assumed that the bulk reaction can be broken down into a consecutive
reaction where the final product, C , relies on the formation of B in order to proceed.
A B C
In this case, there are two reaction rates given by
dA
 k1 A
dt
(2.22)
dC
 k2 B
dt
(2.23)
w1  
w2 
The simplest formulation of the problem is assumed. The letters, A, B, and C
indicate the concentrations of each individual species. The binary diffusion coefficient is
close to the thermal diffusivity and the mixture components have similar molecular
weight and specific heat. A one dimensional temperature profile and species profile is
15
calculated numerically in [3]. The result reveals three different types of regimes defined
as separation, merging, and control discussed next.
Separation Regime
If the activation energy of the reaction of A is much less than that of B , the
majority of the heat release from the second reaction happens after a delay time from the
first. The result is two distinct zones of heat release corresponding to the double flame
front. The first step affects the second step only by delivery of fuel downstream. The
second step has no reciprocal effect on the first flame front. The first step is the dominant
step and determines the overall flame speed and temperature.
Control Regime
If the activation energies of both steps are comparable, but the reaction energy of
the first step is significantly smaller than the second step, then the first step can only
propagate with heat from the second step. Hence, the first step is “controlled” by the
second step. The second step will be the dominant reaction, determining the combustion
temperature and flame speed.
Merging Regime
If the activation energy of the second step is less than that of the first step, B is
consumed in reaction immediately after being formed and releases the heat in the same
region as the first step. The concentration of B is stationary.
dB
 k1 A  k2 B  0
dt
w2 
dC
 k1 A
dt
(2.24)
(2.25)
Thus the flame propagation speed is controlled by the first step and the
temperature is controlled by the heat release from the second step. Khaikin showed that
the burning velocity calculated is close to the values in each of the regimes approximated
by the Zeldovich formula. Based on a general observation, the nature of the velocities
16
solved by Zeldovich formulas predict that a transition from “separation” to “merging” is
impossible without passing the “control” phase because the velocity curves predicted do
not intersect [3].
1.4.2 Parallel Reactions
The notion of parallel reactions is useful to describe the simultaneous reactions of
multifuel systems with a single oxizider, though in itself, can describe any reaction that
has the possibility of two products.
The simplified, general case of parallel reactions:
A  A1or A  A2
Yields the two separate reactions
w1 
dA1
 k1 A
dt
w2 
dA2
 k2 A
dt
(2.26)
(2.27)
The reaction rates and various chemical and thermodynamic properties can be
different for different mixtures allowing for the possibility that the combustion products
can differ from thermodynamic equilibrium, and thus various combustion temperatures
can be realized. Flame speed can be determined by comparing the burning velocity of
each individual reactant. If the reaction coefficients, reaction energies, and activation
energies are relatively similar but the transport properties vary, the flame velocity should
be closer to the greater of the velocities of the independent steps [15]. Similar thermal
and kinetic properties will produce the greatest interaction and effect on the structure of
the flame front. For vastly different chemical and transport parameters of the two
reactions, the structure may be different. A fast reaction that releases little heat needs a
mechanism to help to propagate and equilibrate a second reaction with high heat release
but slower reaction time. There are few theoretical studies with parallel reactions, mostly
dealing with analytical asymptotic matching [2, 30], but very little in the way of
fundamental studies of this phenomenon.
17
1.5 Hybrid flame studies
Utilizing the terminology and theory developed in the introduction, the results of
the experiments can be put into context from flame propagation theory of particulate and
gaseous fuels and in terms of the possibility of multiple flame fronts. The ability of the
solid phase flame to propagate a flame in the suspension allows the study of metal
combustion in a fundamentally different way than simply using the hydrocarbon flame as
a heat/oxidizer source which is the typical application in single particle studies. Instead,
the hydrocarbon flame front has the possibility of interacting with flame propagation in
the aluminum resulting in the flame structure of metal-hydrocarbon fuel blends.
The description of the heterogeneous flame front allows for the separation of gas
and particle temperatures in diffusion controlled combustion, but the combustion of
hydrocarbon fuel greatly increases the temperature and the oxidizing environment. Thus,
the regime of combustion and flame structure of the blend may depend on the
concentration of particulate fuel in the suspension.
Figure 1-5: Schematic of the methodology: progressively increase the concentration of dust in the methane and
determine the effects.
It is necessary to begin the study of hybrid mixtures fundamentally as was first
done in the pursuit of a large body of research for gaseous phase flames. Laminar flames
are the first stage in the understanding of the complex nature of multiphase fuel systems.
This thesis examines the combustion temperature and flame speed of laminar flows of
18
oxidizer and methane gas dispersed with aluminum of progressively increasing the dust
concentration (see Figure 1-5). The goal is to understand the effect of solid fuel
concentration in the combustion of the hybrid mixture. Utilizing high resolution imaging
and spectroscopy to determine flame speed and temperature respectively, a qualitative
description of the laminar flames in hybrid mixtures is developed to aid in describing the
characteristics of the combustion as well as for possible pathways for future research and
model development.
19
2 HYBRID FLAME EXPERIMENT AND DIAGNOSTICS
2.1 Dust Burner
A schematic of the newly constructed hybrid fuel burner is shown in Figure 2-1.
The design implements a number of technical solutions developed previously for the
study of dust flames [24, 31]. In particular, it uses a similar dust dispersion system
comprised of a piston dust feeder and a “flow knife” disperser [24]. The thin, highvelocity jet (“flow knife”) is formed by flowing a premixed methane-air mixture through
a 40-micron-wide circular slot concentric with the top surface of the dust column. The jet
removes and disperses the dust pushed by the advancing piston layer-by-layer in a
continuous fashion. The piston is pushed by a linear actuator. Before going through the
nozzle, the initially turbulent flow is laminarized in a narrow angle conical diffuser
followed by a smooth 60-cm-long supply tube. The dust concentration in the flow is
regulated by changing the speed of the dust feeder piston. The flow rate through the
disperser remains constant to maintain a stable dispersion regime. To regulate the flow
rate through the nozzle, a small volume of the main flow is ejected from the main
dispersion line into the ejection line, where it is exhausted without burning. The ejection
tube is connected to the main dispersion flow tube by a small 1/8” pipe. The flow of
nitrogen in the ejection line causes a pressure differential which pulls the dusty flow from
the main dispersion line. The flow rate of the ejected mixture is determined by using pure
nitrogen as the ejector driver gas and measuring the oxygen percentage of the gas (.01%
resolution at  0.2%) the exit of the ejection line with an oxygen analyzer. Knowing the
concentration of oxygen in the main dispersion line and the flow of nitrogen in the ejector
line, the flow rate removed from the main dispersion line is determined. Alternatively, the
ejector can be calibrated using the surface area of a stabilized methane flame as an
indicator of the flow rate through the nozzle. For a given mixture of fuel and oxidizer, the
flame speed should remain constant, and thus the ratio of flame surface area to volume
flow should remain constant so that flow rate in the main dispersion line can be
determined for different levels of ejection flow.
20
Figure 2-1: the complete dust burner cross section. Gas is dispersed into the main flow line via and air knife which
disperses a column of powder. The powder-gas flow expands in the dispersion cone, and is laminarized in the long tube
leading up to the nozzle where it is ignited. The ejector allows the ejection of flow after the powder is dispersed to
control the flow at the exit of the nozzle.
2.2 Aluminum Powder
In order to compare parameters of the hybrid flames with benchmark data of pure
aluminum dust flames, the same atomized aluminum powder (Ampal 637, Ampal NJ) is
used as in previous experiments with stabilized dust flames and aluminum flames in
tubes [24, 31]. As shown in Figure 2-2, the aluminum particles are nodular in shape. The
Sauter mean diameter, d32, is derived from the particle distribution obtained from SEM
images of the powder and is shown to be about 5.6 m, compared with a value of about
6.9 m from the distribution obtained by the light scattering technique with a Mastersizer
2000 Malvern particle sizing instrument.
21
Figure 2-2: SEM images and particle size distribution of Ampal 637.
2.3 Diagnostics Overview
The diagnostics in place are shown in the schematic of Figure 2-3. Dust
concentration is correlated to individual spectra of the flame front and to spatially
calibrated photographs of the flame. The spectra allow the correlation of concentration to
temperature measurements from the Planck fitting of the background radiation and the
characteristics of the aluminum combustion through detection of aluminum oxide species
molecular bands and aluminum atomic lines indicating the gas-phase species combustion.
The photographs allow for the correlation of concentration with flame speed, as well as
providing general physical characteristics of the flame.
22
Figure 2-3: Diagnostic configuration around the dust burner
2.3.1 Dust concentration
The dust concentration in the flow is monitored with a laser light attenuation
probe consisting of a diode laser ( = 632 nm), cylindrical lenses forming a rectangular
62 mm beam, and a diode sensor with a narrow bandpass filter and focusing lenses. The
laser beam is transmitted across the dusty flow through a rectangular slot cut in the walls
of the conical nozzle and protected by a transparent, heat-resistant Teflon film. The
schematic in Figure 2-3 shows the laser and detector in the setup. The dust concentration
probe is calibrated by the complete aspiration of dust from the flow through a set of fine
multilayered filters for a determined period of time. The average dust concentration is
then determined by dividing the total mass of the aspirated dust by the volume of gas that
flowed through the dispersing system during that time interval.
The Beer-Lambert law, shown in (3.1), is then used to determine the linear
correlation of the ratio of measured intensity to the initial intensity as shown in
Figure 2-4.
23
V 
ln  o   m * B
V 
(3.1)
Vo is the unattenuated voltage measured by the photo diode, V is the attenuated
voltage, B is the concentration of dust, and m is a slope constant dependent on the
Where
properties of the dust and tube diameter as determined by Mie theory.
The output of the photo diode sensor is continuously recorded by a data
acquisition system. The length of time that powder is collected is measured using a
switch that produces a signal when the vacuum is over the nozzle collecting powder. The
average intensity over the period of collection is taken to be the measured intensity. The
calibration curve for Ampal 637 is shown in Figure 2-4. The calibration curve for inert
SiC powder is shown in Figure 2-5. The data in both correlations are fit to y-intercept of
zero.
The major source of error in these experiments comes from the calibration of the
powder concentration and the scatter of the voltage attenuation. This scatter is inherent to
the experimental apparatus and measurement method. The scatter in the voltage
measurement is relatively small, so the standard deviation in the slope permits the
calculation of the standard error in concentration measurement. The nature of this error
means that higher concentrations will demonstrate greater uncertainty. The calibration
slope for Ampal 637 aluminum has a standard deviation of 1.747x10-4. The standard
deviation for the slope of SiC is 5.16x10-4.
24
Figure 2-4: Dust concentration calibration for Ampal 637
25
Figure 2-5: Dust concentration for SiC.
2.3.2 Flame Speed
In a premixed Bunsen flame, the velocity profile of a flow exiting a nozzle makes
it difficult to predict local burning velocity. An average burning velocity is determined by
assuming that the flame speed is constant over the entire surface of the flame. The mass
flow across the flame front is constant and so an average flame speed can be found [18].
uf 
m V

S S
(3.2)
where m is the mass flow rate, V is the volume flow rate and S is the surface area of
the flame front.
The total flow rate is determined by subtraction of the flow ejected from the main
dispersion line from the initial flow as explained in the Dust Burner section. A Canon
EOS D40 DSLR camera was used with a variable neutral density filter to image the
flame. Before each experimental run, an image was taken with a ruler placed in the center
of the nozzle to establish the pixel to length calibration of each subsequent image from
26
the experiment. During the experiment, the ND filter is adjusted to find optimum image
brightness so that the flame cone is clearly visible.
In post processing, the image is first calibrated for pixel to length scales, then the
flame cone is traced from the base along the inner edge of the thickness of the flame
front. The trace is then fit to a 9th order polynomial to ensure goodness of fit, and the
surface curve is rotated around the center axis to find the surface area. Because dust
flames do not exhibit the same sharp cone structure of gas flames, it is necessary to hand
trace the flames to get the full effect of the curvature. This is done on a tablet monitor
utilizing MATLAB’s image processing tools. Correlation between concentration and
image is done by outputting the signal from the camera’s flash to the data acquisition.
Error in the flame speed measurement will come primarily from the trace of the flame
cone. A statistical average of flame speeds calculated for a given concentration for
several traces would give an idea of the uncertainty in this measurement. In the data sets,
it can be seen naturally from the scatter in the data.
2.3.3 Spectroscopy
A flame spectral scanning system similar to one described in previous work [32]
is used to acquire spatially resolved flame spectra across the flame cone. Two new
spectrometers are used: Ocean Optics HR 4000 CG-UV-NIR and USB 4000. The first
spectrometer, equipped with a 5-micron-wide entrance slit and coupled to a 0.6 mm fiber,
is used to acquire spectra in the 250-1000 nm spectral range with a resolution of about
0.75 nm. The second USB 4000 spectrometer uses a 0.1 mm fiber without an entrance slit
and records spectra in the 350-900 nm range with a spectral resolution of about 2.5 nm.
With the second spectrometer, the system has a spatial resolution of about 0.1 mm with
scanning steps of 0.2 or 0.4 mm whereas the larger fiber of the first spectrometer limits
the spatial resolution to about 0.6 mm. The spectrometers were wavelength calibrated
with an Ocean Optics HG-1 Mercury Argon lamp and intensity calibrated with an Ocean
Optics LS-1 Tungsten Halogen Light Source. The first spectrometer is used to acquire
higher quality spectra with resolved signature AlO molecular bands and Al atomic lines
whereas the second, more light-sensitive, low-resolution spectrometer is primarily used to
27
acquire continuous spectra for flame temperature measurements using the continuum.
The signal that triggers the step motor and the acquisition on the spectrometer is split to
produce a signal to determine the correlation for a given spectra to a concentration on the
data acquisition.
2.3.4 Condensed phase Temperature
For a given emitter, Planck’s law gives the intensity of the emitter as a function of
the wavelength of the emitted light and the apparent temperature of the emitter as shown
in (3.3). In the range of wavelength between 100-1000 nm, and for the temperature
ranges investigated in hybrid flames (the adiabatic temperature of pure methane is 2200K
and the adiabatic temperature of pure Al is 3200K), Plank’s law can be reduced to the
form in (3.4) by assuming that the exponent is the dominant term in the denominator
(Wein’s law). This form can be further arranged so the temperature can be determined
from the slope of the data arranged in (3.5).
I
C
I


e
C

5
ln  I *
 C

1
5
5
C2
T
1
 e CT
(3.3)
2

  C2 / T
 
(3.4)
(3.5)
Ignoring the molecular bands from AlO emission and the atomic lines produced from Na,
Li, and K impurities in the aluminum, the intensity data from spectra in the range of 600800 nm is plotted according to [32]. The data is then fit linearly and the inverse of the
slope yields the condensed phase temperature. The fit of the Planck curve to the spectra
can be seen in Figure 2-6. In the case of hybrid flames with aluminum, the solid and
liquid phase aluminum oxides are the majority of the emitters in the continuous spectra.
Thus, the temperature found by this method is considered to be the condensed phase
28
temperature. The emissivity of the condensed phase products is assumed to behave as
1/  2 [32].
About 25 spectra were acquired during each flame scan from the center of the
flame to the periphery. Temperatures were derived only from the spectra taken within the
flame front, i.e., from spectra with the maximum intensity for a given scan. The
acquisition time for each spatial point was about 2 seconds for low luminosity flames,
about 0.5-0.7 s for flames in the transition regime, and only 30-50 ms for high aluminum
concentration flames.
Figure 2-6: Planck curve approximation fitting to sample spectra.
2.3.5 AlO molecular temperature
The AlO B-X state transition manifests itself as a set of vibrational bands in the
region of 460 – 540 nm that can be seen in Figure 3-13. It is assumed that the fraction of
AlO emitters in the B-state is an exponential function of temperature and therefore,
provided sufficient signal, can be fit to a spectroscopic model as the one used by Glumac
et al. in [33]. This temperature is representative of the gas phase AlO intermediate
29
species of aluminum oxidation and provides information about the temperature field
around the particles.
30
3 EXPERIMENTAL RESULTS
Aluminum-methane-air hybrid flames are studied at two different cases of oxygen
concentration with equivalence ratios of  = 1 (stoichiometric, no excess oxygen) and  =
0.8 (excess oxygen). The flow rates of methane and the oxidizer gas are kept constant. To
change the equivalence ratio, the original 21% O2 (air) oxidizer is replaced by with a gas
mixture with 26% O2 at the same flow rate (an effective equivalence ratio of 0.8). This
was done in order to maintain similar dust dispersion characteristics and hydrocarbon
combustion product concentrations while being able to have an oxygen excess for
comparison to the stoichiometric case.
First, a Bunsen-type conical flame is established at the exit of the nozzle without
activating the piston in the dust dispersion system. Even without moving the piston, the
mass concentration of aluminum in the mixture never falls below ~10 g/m3 due to the
surface erosion of the compacted powder column by the dispersing gas. Thus, the dust
dispersion system must be completely emptied of powder in order to verify flame speeds
in pure methane-air mixtures. Upon activation of the dispersion system, the aluminum
concentration in the flow is slowly increased until reaching a steady plateau in
concentration. Depending on the piston speed, the total dispersion time varies from 3.5 to
6 minutes and the corresponding maximum aluminum concentration at the plateau varies
in the range 300-450 g/m3.
31
3.1 Low concentrations of aluminum
Figure 3-1: concentrations of aluminum between 0-100 g/m3 seeded into the stoichiometric methane flame
From concentrations between 0-100 g/m3 the flame is characterized by a
yellow/orange color as shown in Figure 3-1 and the measured flame temperature is
around 2000 K for the entire range of concentration from 0-100 g/m3, which is around the
methane flame temperature taking into account heat loss to the surroundings. The
temperatures in this range are shown in Figure 3-2. In the photographs, the color is not
indicative of relatively fast aluminum combustion (typically an intense white color).
From the spectra shown in Figure 3-3, there are no characteristic AlO band sequences
which are typically associated with combustion of aluminum, but the aluminum content
analysis of the combustion product yields that 75% percent of the product has oxidized. If
the particles are assumed to be spherical, this is a 40% decrease in the radius of the
particle. It should be noted that the products are collected downstream of the flame front,
and it is likely that they have had significant time to oxidize in the hot flow. This leads to
32
the assumption that the particles slowly oxidize in the hot environment in this
concentration range.
Figure 3-2: Flame temperature at concentrations of aluminum between 0-100 g/m3.
33
Figure 3-3: A typical spectrum of a flame with concentrations of aluminum from 0-100 g/m3
Examination of the flame speed in this concentration range, shown in Figure 3-4,
shows that although there is oxidation of aluminum and heat release from this process, it
is slow and the particles passing through the flame front are effectively heat sinks which
decrease the flame speed with increasing concentration. The aluminum oxidation reaction
does not contribute to the heat release in the flame in this concentration range.
Substitution of the aluminum for inert SiC yields a decreasing flame speed with
increasing concentration, similar to the aluminum particle loading. SiC was chosen
because the specific heat capacity is similar around this temperature (~1.0 J/g-K for SiC
at 2000-3000 K versus 1.15 J/g-K for Al from 2000-2800 K [34]). The size of the readily
available 1000 mesh (ANSI standard) SiC particles was reported to be similarly sized to
the aluminum, but particle sizing analysis after the experiments revealed the mean
diameter to be around 50 μm, almost a whole order of magnitude of particle size
34
difference. Despite this difference, the aluminum behaves similarly to the inert SiC in
this concentration range1.
Figure 3-4: Measured flame speed in concentrations of aluminum of 0-100 g/m3
3.2 Flame front formation
At concentrations above about 100 g/m3, a white front starts to form at the tip of
the Bunsen cone, the hottest part of the flame assuming a Lewis number greater than
unity, and moves down towards the base of the methane flame anchored on the nozzle, as
depicted in Figure 3-5C. The flame in Figure 3-5D shows the fully stabilized white flame
front coupled to the methane flame front. This white front is characteristic of the
combustion of aluminum reaction propagation. The transition to a fully stabilized
aluminum flame front happens in the range of 100 g/m3 to 180 g/m3. This transition is
1
It should be noted that the results for flame speed are shown for the case of excess oxygen. The changes in flame
speed in the stoichiometric case are on the order of the scatter of the data making it difficult to clearly see the data
trend, as will be shown in Section 3.4. The excess oxygen and stoichiometric case both exhibit the same patterns with
increasing concentration. Showing the results for excess oxygen here is meant to clearly demonstrate the trend.
35
marked by a rapid increase in temperature as shown in Figure 3-6 in the range of
concentration from of 100 g/m3 to 180 g/m3. After the flame front forms over the initial
methane, Bunsen cone and stabilizes, diagnostics can be implemented. The secondary
flame front is noticeably whiter compared to the flame at low concentrations, and the
spectra begin to show the characteristic AlO green/blue bands, indicating the relatively
fast combustion of aluminum shown in Figure 3-7. This is again reiterated in the results
of the product analysis. The aluminum content found in the combustion product in this
concentration range was found to be 0.9%, indicating the nearly complete and fast
combustion compared to the low concentration case.
Figure 3-5: formation and stabilization of the aluminum flame front. (A) low concentrations of aluminum. (B)
aluminum front begins to form at the tip of the cone. (C) aluminum front moves down over the methane cone. (D)
aluminum front stabilizes.
36
Figure 3-6: Temperature increase after the formation of the aluminum front above 100 g/m3.
37
Figure 3-7: Aluminum oxide band sequence formation after flame front formation. Indication of relatively fast
aluminum combustion.
The effect of the flame front formation on the flame propagation can be seen in
what happens to the flame speed measurement. Figure 3-8 shows the flame speed taken
from the flame cone in lean methane-26% O2 mixtures vs. the aluminum concentration,
and a same gaseous fuel-oxidizer with silicon carbide loading. The results also show what
happens when inert SiC concentration is increased. The flame speed continues to
decrease until the point where the flame is at an open tip and a flame speed can no longer
be measured with the eventual quenching of the flame after a certain increase in
concentration as shown in Figure 3-9.
38
Figure 3-8: Flame speed dependence on aluminum concentration. After flame front formation, the flame speed remains
stable, as oppose to inert loadings of SiC which eventually quench the flame. Standard deviations for concentration are
plotted for a few points of aluminum to illustrate the error propagation at larger concentrations.
39
Figure 3-9: An open tip flame in high loading of SiC particles.
The flame speed in aluminum loadings decreases initially at low concentration
similarly to the SiC loadings. After flame front formation, the flame speed stabilizes and
continues to change very little with increasing concentration of aluminum. The burning
velocity is constant above 100-150 g/m3 aluminum in methane-26% O2, a behaviour that
was also observed in fuel rich pure aluminum flames [31]. The decrease in flame speed is
less pronounced in the case of methane-air as it is difficult to distinguish the flattening of
the curve from the scatter in the data, but the same trend applies (see Section 3.4). The
flame speed in the stoichiometric case does not quench for the entire concentration range
of aluminum but with quenches at the same SiC concentrations as the 26% O2. At SiC
concentrations higher than 200 g/m3, the tip of the flame opens and a part of the mixture
escapes without reacting. At concentrations above 300 g/m3, the flame covers only the
surface area adjacent to the nozzle walls and the majority of the fuel mixture escapes
unburned. Higher concentrations will cause the flame to quench completely.
40
3.3 High concentrations of aluminum
Figure 3-10: (left) appearance of the secondary diffusion flame at higher concentrations. (right) neutral density filter
increased to see Bunsen cone features.
After the flame fronts are fully coupled and stabilized after about 180 g/m 3, a
second diffusion flame is formed around the Bunsen flame when the aluminum
concentration is sufficiently high due to the liberation of hydrogen enriched products [35]
and excess of aluminum gas. The thin inner flame cone has a well defined inner and outer
boundary. It is enveloped by a much larger flame with a well defined outer boundary that
has the appearance of a gaseous diffusion flame as shown in Figure 3-10. The base of the
outer diffusion flame is usually lifted from the inner cone base by about 2-3 mm. The
spectra again shows the aluminum oxide band sequences indication of the fast
combustion of aluminum oxide where AlO bands are stronger in these the high
concentration case as shown in Figure 3-11. The temperature appears to sharply increase
at concentrations of around 180 g/m3. The temperatures plateau at around 200 g/m3 with
a value around 2600 K for 21% O2 shown in Figure 3-12.
41
Figure 3-11: evolution of the AlO spectra at high concentrations of aluminum.
42
Figure 3-12: Temperature plateau at higher concentrations of aluminum.
It is interesting to note that the product analysis in the high concentration case
beyond 180 g/m3 yields excess of aluminum (about 10%), implying that above the
previous concentration range, the mixture is fuel rich.
Looking at the equilibrium calculations for this mix of fuels gives insight into
what happens in the rich concentration. Figure 3-13 illustrates the equilibrium conditions
for various concentrations of aluminum. It is seen that around 200 g/m3, the presence of
excess aluminum is predicted indicating that there is insufficient oxidizer to complete the
combustion of aluminum, thus allowing the collection of the excess aluminum in the
combustion products and theoretically the presence of gas phase aluminum in the spectra
given the sufficient temperature to vaporize the aluminum. The presence of the gas phase
aluminum is shown in Figure 3-14 for concentrations above 180 g/m3.
43
Figure 3-13: measured temperature plotted with adiabatic flame temperature prediction and aluminum species
concentrations.
44
Figure 3-14: High resolution spectra showing the presence of the atomic transition of aluminum gas in concentrations
of aluminum greater than 180 g/m3.
A thermodynamic equilibrium analysis of constant-pressure aluminum-methaneair flames at different aluminum concentrations is presented in Figure 3-13 and provides
the boundary for such influence in the absence of any limitations by kinetic rates. The
flame temperature as a function of the aluminum concentration is shown. At zero
aluminum concentration, the temperature should asymptote to the combustion
temperature of the methane-21% O2 and is measured to be around 1900 K. After the
formation of the aluminum flame front and the sharp transition in flame temperatures, the
temperature trend closely follows the thermodynamic calculations, with the adiabatic
flame temperature peaking at the predicted aluminum concentrations. Before the
transition, the predicted thermodynamic temperature increasingly deviates from the
measured temperature.
3.4 The presence of excess oxygen
The presence of excess oxygen changes the initial flame speed in the absence of
aluminum loading. A stoichiometric methane flame approaches around 38 cm/s [18]
45
while a lean   0.8 flame in 26% O2 produces a flame speed of around 60 cm/s
calculated in a Cantera flat flame burner simulation. The scatter in the measurement of
flame speed in the experiment causes difficultly in the perception of the flame speed
plateau after flame front formation in the stoichiometric case as shown in Figure 3-15.
Both oxygen concentrations demonstrate formation of the aluminum flame front at a
certain aluminum concentration. It is possible that the formation of the flame front
happens at lower concentrations of aluminum in oxygen-rich compositions. However, the
differences were not examined extensively in this thesis. In Figure 3-16, the temperature
in 26% O2 begins to increase at concentrations of aluminum below the stoichiometric
case. The case with enriched oxygen has a temperature that is consistently above the
temperature measured in normal air but exhibits the same transitional behaviour between
low temperature oxidation and flame front formation.
Figure 3-15: Comparison of flame speeds in stoichiometric methane and in oxygen excess.
46
Figure 3-16: Comparison of temperature in stoichiometric methane and in oxygen excess.
3.5 AlO molecular temperature measurement
The AlO temperature derivations for several values of concentration are shown in
Figure 3-17. They indicate a value of temperature about 200 K higher than the condensed
phase temperature measurement.
Figure 3-17: AlO temperatures for high concentrations of aluminum in the excess oxygen case.
47
4 DISCUSSION
4.1 Explanation for the aluminum flame front formation
The formation and stabilization of the secondary flame front with increasing
concentration is an indication that there is contribution of the heat feedback between the
methane flame front and the combustion of aluminum which allows for flame
stabilization. Figure 4-1 illustrates this concept in a one dimensional perspective by
looking at the thermal runaway of the particles in an infinite adiabatic tube. It is assumed
that the cold flow of particles is instantaneously heated by the methane flame front and
that the particles undergo an Arrhenius reaction. The particles will ignite (undergo
thermal runaway) after a period of delay downstream of the flame. It is assumed that heat
diffusion of the reaction of the particles is the dominant mechanism of heat propagation
to the upstream flow and that the methane flame front is stationary to the observer (i.e.,
the effect of the aluminum loading, in reality, will cause a flame speed decrease in the
methane front, but this is not taken into account into the explanation).
Figure 4-1: A) Low concentrations of aluminum where thermal runaway of aluminum happens far downstream of the
methane flame. B) A mid-range of concentration where the thermal runaway of aluminum happens in the vicinity of the
methane flame front but where concentration fluctuations cause the front to be unstable. C) Sufficiently high
concentration of aluminum to couple the methane front to the aluminum front.
In low concentrations, the particles undergo slow oxidation after passing through
the methane flame front and being brought to methane flame temperature and continue in
the hot methane product flow. The particles will reach the point of thermal runaway far
48
down stream of the methane flame. This distance is too far from the methane flame front
to have any noticeable effect from heat diffusion from the aluminum flame front. In the
realistic scenario, heat losses and the fact that the flame is not adiabatic ensure that the
particles will never reach thermal runaway in the low concentration case as was shown in
the experiment.
Increasing the concentration increases the pre-exponential factor in the Arrhenius
expression by increasing the surface area for the reaction to take place and hence, the
delay time to thermal runaway decreases. There is also a dependence of the reaction rate
on temperature. The rising temperature from increase in concentration also affects the
reaction rate and therefore the location of thermal runaway. The consequence that arises
from far distances between formation of flame fronts is that a stabilized combustion
regime is inherently unstable as fluctuations in concentration will move the flame front
up or down stream as shown in Figure 4-1B. After increasing the concentration
sufficiently, the aluminum front will move in the vicinity of the methane flame front and
will begin to couple via the heat diffusion from the heat from aluminum flame front and
the heat from the methane flame. This feedback forms a thermally stable regime of
combustion which is which can have separated fronts (as in the case with the ironmethane flame in Figure 1-4) or overlapping fronts described by Khaikin as “merging” or
“control” regimes. The fronts in the aluminum-methane flame appear to be merging, but
it is difficult to determine due to the relative luminosity of the aluminum front.
The experimental evidence presented in this work clearly demonstrates that the
mechanism of aluminum combustion in a hot, oxidizing flow strongly depends on the
mass concentration of the solid fuel in suspension. The observed transition from lowintensity oxidation to fast, fully-established combustion reflects the ability of the reacting
aluminum suspension to modify the combustion properties, i.e., increase the flame
temperature, which in turn affects the aluminum combustion mode.
The apparent explanation of this qualitative difference in flame behaviour from
low- to high-seeding densities is the relatively rapid coupling of the aluminum
combustion front to the hydrocarbon flame that is observed over some critical
concentration range, and the absence of any coupling for lower concentrations. This
49
relatively rapid transition from uncoupled to coupled aluminum-hydrocarbon flame
combustion gives an appearance of a sharp, "ignition-like" transition. The very possibility
of such a transition indicates that aluminum combustion in hybrid mixtures needs to be
treated in future modeling efforts as a flame front propagation phenomenon with the
corresponding ability to transfer heat and active species upstream of the combustion
front. The coupled, frontal nature of aluminum combustion in hybrid flames is also
evident from the fact that the flame speed of the hybrid mixture is practically unchanged
with an increase of the solid fuel concentration.
In contrast, the addition of inert silicon carbide solid particles leads to a sharp
decrease in burning velocity followed by flame quenching. It is likely that the same
quenching behaviour would occur for any hybrid fuel whose combustion cannot couple to
the hydrocarbon flame front. Analysis by Khaikin and Goroshin to determine the heat
release profiles in elementary consecutive reactions and in binary mixtures of powders
determined a solution of a separated front. Based on the observations in this experiment,
it is likely that a separated front solution is unstable unless there is significant heat
feedback between the two fronts, as is in the ‘merging’ and ‘control’ fronts presented by
Khaikin. In a separated regime, fluctuation in the concentration can shift the aluminum
front forward and away from the methane flame front. The flames are likely coupled
according to a “merging” or “separated” regime as named by Khaikin.
4.2 Kinetic and diffusive regimes of combustion
The observed transition from slow to fast aluminum burning should not be
confused with the ignition phenomenon associated with a large single aluminum particle.
A thermal runaway reaction leading to a sharp separation of particle and surrounding gas
temperatures, as well as the subsequent transition to a particle combustion regime limited
by the diffusion of oxidizer was not observed in the present experiments. After front
formation, the particle temperature increases with aluminum concentration. If the
particles were burning in the diffusion controlled regime of combustion, even the lower
concentrations of aluminum would have found particle temperature around the predicted
adiabatic temperature. The very observation that the concentration increase causes a
50
formation of a flame front necessitates the need for an effective reaction rate limited by
the kinetics, as the effect of concentration increase would cause flame front formation
only in a kinetically limited regime. The possibility that small-particle combustion in the
products of hydrocarbon flames would be kinetically-limited was previously indicated by
Glumac et al. [36] and is not surprising considering the small particle sizes and the high
diffusivity
associated
with
the
high-temperature,
hydrogen-enriched
products
encountered in the present experiments.
This does not discount the possibility of the diffusive regime of combustion. The
results present evidence for a case of aluminum combustion in the kinetically limited
regime, but a true test of regime would be to determine the difference between gas phase
temperature and particle temperature as discussed in Section 1.3. The temperature
measurements in this experiment only show the condensed phase temperature. If the
particle and gas temperature are separated, it is evidence of the diffusion limited regime
of combustion, and if the temperatures are relatively close, this is evidence of a
kinetically limited regime of combustion.
There may be several factors (e.g. the aluminum concentration, the oxidizer, the
initial temperature) which change the effective resistance of either the diffusive or kinetic
rates which will determine the overall rate as discussed in Section 1.3 in accordance with
Frank-Kamenetskii’s theory. It may be required to accept that a pure regime of
multiphase combustion may not be realized, but that different factors will influence the
overall regime of combustion throughout the flame.
4.3 The choice of emissivity and the AlO temperature
There is some debate on the validity of the choice for emissivity in this
experiment as the emissivity of aluminum oxide particles has been shown to depend on
temperature [37]. Thus, a single value for emissivity throughout the entire concentration
range may not be valid. A common assumption for emissivity is that the particles are
grey bodies. It is shown in [37] that for temperatures from 3000-3300 in the 550-900 nm
range that the emissivity of the oxides can be approximated as a grey body. Under that
n
temperature, from 2500-3000, the emissivity depends on 1/  where n increases from -
51
1.4 to 0.5 as temperature increases. The grey body assumption is often made for
pyrometric temperature measurements, but the assumption is often wrong in context of
the optical medium. Due to the effects of Rayleigh scattering of small particles, the
prediction of 1/  2 emissivity dependence is a reasonable assumption [38]. Indeed, a grey
body approximation in this experiment would yield a flame temperature of nearly 3500 K
in the high concentration cases, indicating the temperature exceeds the thermodynamic
equilibrium temperature. This indicates, at the very least, that the value chosen for
emissivity is wavelength dependent, though not necessarily by the inverse square as
chosen in this work.
The temperatures derived from the molecular spectra of AlO also provide insight
into the choice of emissivity. The AlO temperature reflects the temperature of the gas
intermediate in aluminum combustion.
The interpretation of this result is complicated by the fact that in vapor-phase
diffusion combustion, there may be a separation of measured condensed phase
temperature and AlO temperature due to the actual physical separation of the combustion
zone on an individual particle being lifted from the aluminum droplet as shown in [21]. If
the particle burns in a kinetically limited (the aluminum vapor diffusion outward from the
particle is slow compared to the oxygen diffusion to the surface), the particle temperature
and AlO temperature will be relatively similar as the combustion zone and the particle
surface coincide. Since there is a question of the validity of our emissivity assumption, it
is difficult to say whether or not this is diffusion-limited or kinetically-limited
combustion. This again reiterates the need for the ability to do a true gas phase
temperature measurement in the oxidizing media and in the species of condensed phase
aluminum.
52
5 CONCLUSIONS
5.1 Structure of hybrid flames
The observation of the formation of the aluminum flame front requires the
treatment of the aluminum fuel burning in a kinetic regime. However, there is still
uncertainty if the particles are burning in a diffusion-limited or kinetically-limited regime
depending on concentration. The results of this experiment indicate the possibility of the
kinetically controlled regime due to the formation of the flame front on the fuel
concentration. However, the separated AlO and particle temperatures in the range of high
concentration indicate the possibility of a diffusive regime depending on the value chosen
for emissivity. It is possible that there is a shift from kinetic to diffusion limited
throughout the flame based on the local diffusive or kinetic resistance. A comprehensive
model will need to determine this regime of combustion as it affects the overall flame
propagation.
The model will also be complicated by the flame structure in the hybrid mixture.
Since the combustion properties of the fuel constituents are vastly different, the resulting
behavior can be seen to follow the bulk schemes employed by Khaikin et al. to describe
the different types of flame, in this case, the transition from a ‘separated’ regime, to a
coupled ‘merging’ or ‘control’ regime [3]. As seen in this experiment, the formation of
the flame front shifts with increasing concentration. A well defined hybrid flame will
require modeling of the flame structure, the coupling effect of the fuels, and the
determination of combustion regime of the particles.
5.2 Future work
The results indicate the need for a true gas phase temperature measurement and a
better idea of the emissivity of the particle suspension to determine the combustion
regime of the solid phase (either kinetically or diffusion controlled). In multiphase
combustion, the only reasonable way to measure temperature is through optical
temperature measurement since any thermocouple measurement is skewed by the
deposition of the solid phase onto the thermocouple. In general, a redundancy of optical
temperature measurements allows for greater certainty of the temperature. Utilizing the
53
molecular temperature of a gas intermediate species unrelated to aluminum combustion
would help determine the regime of combustion.
When modeling hybrid flames, the blended fuel should be treated as two separate
flame fronts which have the ability to couple. It is likely that the ‘separated’ solution
predicted by Khaikin et al. and Goroshin et al. [3, 26] is unstable given that a fluctuation
in concentration of aluminum can move the flame downstream and upstream as shown in
this thesis. The solutions to time dependent flame equations should be solved to verify
the validity of the separated solution. The aluminum combustion also has the ability to be
kinetically limited fundamentally changing its behavior throughout the flame. Although
Goroshin et al. examined this for a purely heterogeneous case, a similar treatment should
also apply to hybrid flames [27].
54
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