IFRF Combustion Journal
Article Number 200510, December 2005
ISSN 1562-479X
Design, evaluation measurements and modelling of a small
swirl stabilised laboratory burner
N. G. Orfanoudakis1, A. Hatziapostolou2, K. Krallis3, E. Mastorakos4, K. Sardi5,
D. G. Pavlou6 & N. Vlachakis6
1
2
3
4
5
6
TEI Chalkis, Mechanical Engineering Department, Laboratory for Steam Boilers, Turbines & Thermal
Plants, Greece.
TEI Athens, Energy Technology Dept., Athens, Greece.
Heron Consultants Engineers, Greece
University of Cambridge, UK.
Imperial College, London
TEI Chalkis, Mechanical Engineering Department, Greece.
Corresponding Author:
N. G. Orfanoudakis
TEI Chalkis
Mechanical Engineering Department
Laboratory for Steam Boilers
Turbines & Thermal Plants, Greece
Email: [email protected]
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ABSTRACT
The effect of swirl on the motion of coal particles in the near-burner region of a multi-fuel
swirl-stabilised laboratory burner of total thermal input of 100kW was investigated experimentally and also by the use of CFD. The burner was designed as a scale model of a 10MW coal
burner operating in a cement rotary kiln or boiler and was able to burn a combination of gaseous, liquid and pulverised solid fuels. Temperature and Laser Doppler measurements of all
three velocity components were obtained in reacting single and multiphase flows as a function
of swirl number in the range of 0.6 to 0.9 and confirmed the ability of the burner to produce
close-to-industrial conditions making it a powerful tool in the field of combustion research.
In detail, the velocity measurements showed that the flow field was axisymmetric and an internal recirculation zone (IRZ) in the shape of a toroidal vortex was formed around the centreline
for swirl numbers of at least 0.65. A 60% increase in the swirl number, from 0.65 to 0.9, resulted in a 30% widening of the IRZ. Solid particle measurements revealed that the width of
the zone where coal particles recirculate is by 20% larger than that formed in the single phase
case and that most of the coal particles are centrifuged away from the IRZ, particularly at high
swirl numbers.
A numerical investigation on the effect of the swirl number on the fluid and particle motion
downstream the burner in isothermal flows has also been performed. The flow field was modelled as 2D axisymmetric and results were obtained with both the renormalisation group
(RNG) and k-ε turbulence models and compared to measurements obtained in non combusting
flows. As expected, the RNG model provided a more realistic description of the flow field than
the k-ε turbulence model, with numerical results being in good agreement with the measurements even at the high swirl number. Lagrangian tracking of coal particles in the range of 1 to
150 µm has also been performed as a function of swirl number. The calculations revealed that
particles of diameter larger than about 20 µm are centrifuged away from the IRZ in accordance
to the measurements, while particles larger than 100 µm, due to their high inertia remain on the
IRZ boundary and are neither centrifuged, nor entrained inside the recirculation zone. In accordance with the measurements, the calculations also showed that the effect of centrifuging is
decreased when the swirl number is reduced.
An attempt to scale-up the results, through the application of Stokes number similarity and of
the standard “constant velocity scaling” concept, has also been pursued and comparisons of the
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present findings to the flow field downstream a 120 kW and a 12 MW industrial burner were
made.
1 INTRODUCTION
An important way of understanding flame processes is to scale-up experimental results obtained in small burners to larger ones [1]. Ideally, the result of a burner and flame scaling
would be the complete similarity of all the combustion processes (turbulent transport and mixing, heat generation, heat transfer) in the scale down domain. In reality however, this is not
possible, as all physical and chemical processes do not scale down in the same way. Extensive
work on volatile evolution and ignition and their scale up to larger burners has been reported in
[2] where two non-dimensional numbers, the En and the Fl characterising respectively coal
particle ignition and devolatilisation, were introduced as a means to interpret and scale up experimental data. Other criteria, based on scaling the large macro-scale turbulent mixing processes, the so called constant velocity (CV) or constant residence or mixing time (CRT) are also
employed in practice, in order to scale experimental data. In constant velocity scaling both air
and fuel particle velocities are maintained constant with scale reduction. For the constant residence time criterion, details can be found in [1] and [2], however application is generally limited due to problems in optimising the burner air box.
Scaling based on the constant velocity criterion has its own shortcomings, particularly at laboratory size burners, so that small scale burner design can be a challenging task. In a laboratory
burner flame, the surface to volume ratio i.e. the specific flame area, is larger than in an industrial flame leading to cooling due to radiation loses [2]. Also residence time of coal particles in
a laboratory flame is less than in a large scale industrial flame, so that less volatiles, i.e. less
combustion gases, are released resulting to an unstable flame. It is generally not possible to
maintain a steady flame in a laboratory scale burner burning only pulverised coal or atomised
liquid fuel and laboratory flames burning non gaseous fuels are commonly supported by cofiring a gaseous fuel. The latter essentially substitutes the quantity of volatile gases, which
would have been ideally produced from a coal flame [2].
Another method to increase residence time and to enhance flame stabilisation is by introducing
a swirling motion in the air flow [3]. Swirling flows are characterised by a non-dimensional
number, the swirl number S=2Gθ/GZR. where Gθ and Gz are the axial fluxes of the angular
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momentum and the axial thrust respectively and R the exit radius of the burner nozzle. Experiments have shown that the swirl number is a significant similarity criterion of swirling jets
produced by geometrically similar swirl generators and that swirl can be successfully used to
control flames in combustion chambers. In this direction, swirling air flows have been used in
various burner types in order to achieve the desired ignition and burnout characteristics for a
given fuel [4].
This paper presents a purpose-built laboratory burner with a swirling air flow firing simultaneously pulverised coal and methane. The burner was designed as a scale model of an industrial
coal burner operating in a cement rotary kiln or in a power plant boiler and the design criteria
are described in detail in order to justify the technical solutions employed in both the burner
and its support equipment. Detailed velocity and temperature measurements were obtained by
means of a Laser Doppler Velocimetry system (LDV) and a thermocouple as a function of the
swirl number. The measurements aim to provide additional understanding on the swirl and
combustion process interaction in industrial burners and can lead to useful conclusions about
the behaviour of coal particles or droplets within the flame and the production of nitrogen oxides. The latter is closely related to the different trajectories followed by large and small particles [9] -dependent on near burner aerodynamics - and being subject to different local temperatures and oxygen levels. The measurements also contribute towards forming a database for the
validation of a computer code simulating the two phase combustion processes and, to this effect, preliminary CFD calculations on the effect of the swirl number on the fluid and particle
motion, have also been performed and are reported herein.
The present work is part of a larger program undertaken by the current group for the study of
processes occurring in kilns of the cement industry and in boilers of power plants. The main
objective of this program is the development of the necessary know-how for the clean combustion of organic wastes in cement kilns, as well as in lignite fired power plants.
2 EXPERIMENTAL SET-UP
2.1. Laboratory Burner
The laboratory burner used in this study is shown in Figure 1. It allows for mixtures of gaseous, liquid and pulverised solid fuel and flows of different degrees of swirl. The similarity to
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the industrial scale burner was accomplished by employing the constant velocity (CV) scaling
criterion as in [1].
Due to the small size of the laboratory flame, in comparison to the respective flame in the industrial burner, the residence time of the coal particles in the former is reduced and the use of
gaseous fuel is necessary to ensure flame stability. Note that the residence time of the coal particles in the laboratory flame, is of the order of 20ms and hence a very small portion of the
volatiles (which contributes a lot to the flame stabilisation) is expected to be released, so that
the main fuel, in terms of thermal input, is the gaseous fuel (methane).
The burner consists of a cylindrical body for the secondary air flow and a central fuel pipe
(also referred to as the “fuel gun”). The exit of the burner (throat) of diameter, De, of 64.5 mm
is located at the top. Provisions have been made to allow for a quarl and an enclosure to be
fitted downstream the burner exit. However, for the measurements reported in this work, air
and fuel flow were released as free jets. The surrounding air flow in the laboratory was stagnant and its influence on the flame is considered to be minimal. Adding a quarl or an enclosure is expected to enhance flame stability as reported by [2] and [17], the former because it
prevents lift off and the latter because it leads to higher temperatures of the surrounding air.
The central fuel pipe consists of three individually sealed coaxial tubes. The inner tube is for
the liquid fuel, the annular area between the inner and the middle tube is for the pulverised coal
and the annular area between the outer and the middle tube is for the gas fuel (Fig.1.b). The gas
fuel is injected radially through two rows of 20 holes of diameter of 1mm around the outer
tube.
The burner is fixed to a three axis traverse-table and can be moved at various positions with
respect to the measurement system with the accuracy achieved by the milling machine-type
table (effectively ±0.5mm). This method for the measurement campaign is usually preferred,
in order to avoid disturbance on the optical arrangement and thus, of the optical alignment.
The amount of swirl in the flow can be adjusted by varying the ratio of axial and tangential air,
while maintaining the same total air flow rate. The air for the coal particles is directed to an air
tight metallic box which encloses the coal feeder. It uses a vibrating tube with variable amplitude to disperse the coal from a hopper into the air flow. Coal is pneumatically transported
through a copper tube to the burner.
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2.2. Instrumentation
Profiles of all three velocity components were measured at the near burner region with a dualbeam LDV system employed in the off-axis (30ο) forward-scatter mode as described in Orfanoudakis et al. (2002) [9]. Isothermal and reacting single phase flows (air and/or methane) as
well as reacting multiphase flows (with coal present) were seeded as described in [9].
Mean temperature profiles were obtained by a 250 µm diameter Pt/Pt10%Rh thermocouple.
The connecting wires were of 5.5 mm in diameter, 300 mm long and protected within a ceramic tube. The thermocouple signal was fed to a commercially available temperature indicator
calibrated for the particular thermocouple type. All reported temperature measurements have
been corrected for radiation loses according to [7].
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3 EXPERIMENTAL RESULTS AND DISCUSSION
Two cases with gaseous and pulverised coal flames have been considered. In both cases, the
total air flow rate was kept constant. However, the tangential to axial air flow rate ratio was
varied so as to create a relatively weak swirling flow of swirl number, S, of 0.65 in Case 1 and
a strongly swirling flow of S equal to 0.9.
Table 1 summarises the gas flow conditions for
both cases.
Profiles of mean and rms velocity components at four different stations downstream the burner
exit and temperature profiles of gaseous flames are reported in figures 2 to 5. In all figures,
velocity and radial distance are normalised by the bulk exit velocity VB and the burner throat
diameter, De, respectively. The radial velocity component is presented only at two stations at
the vicinity of the burner exit (Figures 2b and 3b), since values tend to zero further downstream. To ensure that the flow was symmetric, profiles of all three velocity components were
measured up to one diameter to the left and to the right of the burner axis for gaseous flames.
As shown in Figures 2 and 3 the flowfield is symmetrical. Thus, mean temperature profiles and
all three velocity components of pulverised coal were measured only to the right of the symmetry axis. For completeness, photographs of single and two phase flames are also shown in figure 6 confirming the symmetry of the flow field.
Case description
Tangential air [m3/h]
Axial air [m3/h]
Gas flow rate [m3/h]
Bulk exit velocity [m/s]1
Swirl number, S
Case 1
Medium swirl
96
84
7
25
0.65
Case 2
Strong swirl
175
0
7
24
0.9
Table 1: Flow characteristics
3.1. Case 1
The mean axial velocity profiles at the vicinity of the burner exit (fig.2a at z/De=0.78 and 1.55)
depict a central recirculation zone (IRZ). Towards larger radii, the axial velocity increases and
two positive peaks with values of the order of the exit bulk velocity VB are identified at
1
Bulk exit velocity, Uo, is the mean velocity at the burner exit determined by the volumetric air flow rate and the annular area at the exit
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r/De≅±0.5. Further downstream (fig.2a at z/De=2 and 3.1), the central negative values disappear, although the influence of the IRZ is still evident. Towards larger radii, velocity peaks are
smoothened out due to molecular and turbulent diffusion.
Mean swirl velocity profiles (fig. 2a) display a forced-vortex core around the central axis of the
burner with maximum values at around r/De=±0.35. Mean values decrease towards larger radii
and distance from the burner exit.
Within the central zone (r/De≅±0.3De) at the very vicinity of the burner (fig. 2a, z/De=0.78),
mean swirl velocity gradients are smoother than at the other three locations downstream. It
maybe argued that this region is at the very vicinity of the combustion initiation zone and that
the sudden temperature increase and the gaseous elements volume expansion lead to disorderly
motions and angular momentum losses. Swirl velocities increase abruptly between r/De≅-0.3
and r/De≅-0.5 and also between r/De≅+0.3 and r/De≅+0.5. This region is shown to coincide
with the interface between the inner combustion zone and the outer non-combusting region as
identified by the mean temperature measurements. Figure 2c at z/De=0.78 reveals a steep decrease in the mean temperature from T=1600 oC at r=0.27De to just above 300 oC at r=0.4D
and to less than 50o C at r=0.47D.
Further downstream, mean temperature profiles show the existence of a wider combustion zone
and a larger interface between a high temperature region of T=1600 oC and ambient air. Mean
swirl profiles are shown to decrease smoothly with radii, probably due to the larger distance
from the combustion initiation zone, so that local pressure differences are less intense.
Mean radial velocities (Fig.2b) at z/De=0.78 are close to zero within the intense combustion
activity region (r/De≅±0.4). At larger radii, radial velocities increase to up to a quarter of the
exit bulk velocity value, revealing a clear tendency for the fluid to move towards the radial direction. The radial velocity profile at z/De=1.55 is the sole example where symmetry around
the burner axis was not preserved possibly due to minor asymmetries in the burner construction. Asymmetry in the radial profile is considered to be of minor importance, since rms levels
were larger than mean values and, consequently, statistical errors were significant.
Rms values of all three velocity components (Figures 2a and b) were independent of the radial
and downstream distances and approximately equal to each other indicating that turbulence
was almost homogeneous and isotropic. Larger rms values can be observed at certain areas in
the flow field (most pronounced at the rms axial component values at z/De=0.78, Fig. 2a), due
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to the presence of steep mean velocity gradients. These were partly attributed to high shear
stresses and partly to the so-called velocity gradient broadening effect [8].
Half radial axial and swirl velocity profiles of coal particles, in the case of a pulverised coal
flame, are shown in Figure 4. It is clear that the basic characteristic of the flow field identified
for the case of gaseous flames, i.e. the internal recirculation zone is preserved. In detail, a recirculation zone of width ranging from 0.25De at z/De=0.31 to almost zero at z/De=1.55 can be
identified in figures 4a, b and c. Negative axial velocities are as high as 0.4 the exit bulk velocity (figure 4b). Positive axial velocities appear at a radial distance between r/De=0.4 to 0.55
and are of the order of VB . The corresponding rms axial velocity profiles reveal turbulence
homogeneity with the exception of the region close to r/De=0.25, see figures 4a and b where
local peaks are identified due to the presence of large shear stresses.
The profile of the mean swirl velocity component resembles solid body rotation, i.e. almost
linear function of velocity with respect to the
radius at the first station downstream
(z/De=0.31, fig 4a). At z/De=0.78, the mean swirl velocity profile exhibits the same nonmonotonic behaviour identified in gaseous flames. In detail, mean swirl velocity increases
linearly up to r/De=0.2, decreases abruptly up to r/De=0.35, where a local minimum occurs and
then is shown to increase again up to the outmost measured point. As argued in the discussion
concerning Fig. 2, the region at the vicinity of to r/De=0.35 coincides with the interface between the inner combustion zone and the outer non-combusting region. The data are in support
to the argument that two separate vortices rotating at different swirl velocity and separated by
the combustion boundary exist. At z/De=1.55, swirl velocity increases linearly up to r/De=0.25
and then decreases monotonically in agreement to the findings in gaseous flames (fig 2a) suggesting that the second vortex depicted at z/De=0.78 no longer exists. The profile is shown to
be flattening out with downstream distance as commonly observed in developed flows with a
maximum value of 0.4 VB. Rms values of the swirl velocity component are approximately
equal to the axial rms values.
The good agreement between gaseous and coal velocity profiles suggest that due to their relatively small size [2], coal particles generally follow the fluid flow and entrain inside the recirculation zone. However two main differences between gaseous and pulverised flames exist.
The mean negative axial velocities of coal particles were smaller than of the gaseous phase.
Further the LDV data rate within the IRZ and off the burner centreline was reduced for the coal
particle measurements. The reduced data rate can be attributed to the fact that less coal parti9
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cles are entrained in the IRZ as compared to the seeding particles. In the area around the centreline of the burner, particles were shown to penetrate the IRZ and the LDV signal was of
relatively larger amplitude. These trajectories are typical of larger particles [2], which do not
follow the fluid flow, due to their increased inertia.
3.2 Case 2
The mean and rms radial profiles of all three velocity components and the mean temperature
distribution in a gaseous flame of a swirl number of 0.9 are shown in Figure 3. The internal
recirculation zone created by the strong swirling motion is shown to be wider by approximately
30% than in Case 1 of the swirl number of 0.65. Another major difference from Case 1 is that
here maximum negative axial velocities do not occur on the burner symmetry axis but off centreline at r≅±0.25De. As expected, increased swirl enhances mixing so that mean temperature
gradients in the interface between the combusting flow and the outer isothermal stagnant air
are rapidly smoothened out. Increased swirl also leads to lower mean flame temperatures, possibly due to the increased strain rates, and to shorter flame lengths. The latter is also confirmed
from the photographs of the gaseous and pulverised coal flames as a function of swirl number
presented in figure 6.
As in case 1, the mean radial velocity component is close to zero around the centreline and increases towards larger radii.
Half radial, axial and swirl velocity profiles of coal particles are shown in Figure 5 for the case
of a pulverised coal flame. In agreement to the gaseous flame measurements, figure 5a b and c
confirms that an increase in the swirl number leads to an almost proportional increase in the
width of the IRZ and reveals that coal particles continue to follow the fluid flow despite the
increase in the swirl number. Maximum values of the mean negative axial velocity components are shown to be independent of the swirl number and equal to about 0.35 VB as in Case
1. Due to the increased swirl and thus, to the increased number of particles being centrifuged
towards larger radii, it has been possible to obtain measurements at larger radial distances than
in Case 1 at the vicinity of the burner (z/De=0.78).
Figures 5 d, e and f, show the profiles of swirl velocities for coal particles The profile at the
first station downstream has the form of solid body rotation, with 17%-increased value for
swirl velocity as compared to lower swirl case (see figures 4d & 5d). At z/De=0.62 the mean
profile of the axial velocity component exhibits the same non-monotonic behaviour identified
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in Case 1. In particular, at r/De=0.4, there is a local minimum for mean swirl velocity that lies
further in the outer region as compared to the lower swirl case 1. The mean swirl velocity further increases to about 0.53 VB at r/De=0.6. The profile is shown to be flattening out with
downstream distance. At z/De=1.09, there is ring formed in the region from r/De=0.2 to 0.6
where mean swirl velocity remains almost constant as in Case 1.
4 NUMERICAL INVESTIGATION
To enhance understanding on the effect of the swirl number on the fluid and particle motion,
the non-combusting single and two-phase flow fields downstream the burner were further investigated by the use of CFD. Results are reported as a function of swirl number and compared to experimental data also obtained in the same burner in the absence of combustion.
The turbulent flow field downstream the burner was calculated from the solution of the twodimensional, iso-thermal, axisymmetric, steady-state, Reynolds averaged Navier-Stokes equations by use of in house developed software based on the SIMPLE algorithm of Patankar [10]
that is an extension of the original TEACH code [11]. The Reynolds stresses which appear as
unknowns in the Reynolds averaged forms of the Navier-Stokes equations for the velocity
components were modelled by use of both, the standard high Reynolds k-ε model and the RNG
turbulence model as proposed by [12]. The latter is known to have significant advantages over
the former particularly in high turbulence non-isotropic flows (see for example [13] and references therein).
Initial conditions were imposed from the experimentally measured velocity profiles of all three
components at z/De = 0.045 and a total of three cases corresponding to low (Sw = 0.45), medium (Sw = 0. 65) and high (Sw = 0.9) swirl were considered.
Figure 7 presents profiles of the mean axial velocity component and the turbulence intensity
along the burner centreline as a function of the swirl number. Results obtained with the RNG
model are in good agreement with the measurements even at the high swirl number, whereas,
as expected, the k-ε model tends to underestimate the magnitude of the axial velocity component and the size of the internal recirculation zone (IRZ). At Sw = 0.45 and in contrast to the
experimental measurements and the RNG predictions, the k-ε model fails to depict the existence of the recirculation zone, figure 7(i). However, the k-ε model reproduces successfully
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the increase in the velocity fluctuations within the first diameter downstream the burner exit,
figure 7(ii).
The radial variations of all three velocity components at selected axial locations for all three
swirl numbers are shown in Figures 8-10 for both turbulence models. Good agreement between predictions and experiments for the axial and swirling velocity components is obtained
with the RNG model. However, the model severely underestimates the magnitude of the radial
velocity component at high swirl numbers, figure 10(ii).
Lagrangian tracking of coal particles in the range of 1 to 150 µm was performed by the use of a
second in house software described elsewhere [14]. The particles were tracked in the precalculated isothermal single phase flow field and one way coupling between the fluid and the
particulate flow was assumed. The latter is well justified as the particle flow rate in the burner
during the experiments did not exceed a maximum of 2 gr/min so that the flow could be considered as being adequately dilute [14]. The effect of turbulence on the particle motion was
estimated from the Gosman and Ioannides eddy interaction model [15].
Typical trajectories as a function of particle size are shown in figures 11 and 12 for the highest
and lowest swirl number cases respectively. Particles of diameter of the order of 10 µm follow
the mean and turbulent flowfields and get entrained inside the IRZ. In contrast, particles of
diameter from 20 – 100 µm are centrifuged in accordance to the measurements of the previous
section and to the results of [16], [17] and [18]. The effect of centrifuging diminishes with increasing particle size. As shown in the figures, particles of diameter of the order of 100 µm
follow the IRZ boundary due to their increased inertia and are neither centrifuged nor entrained
inside the IRZ. A decrease in the swirl number, from 0.9 to 0.45, results in a minor decrease in
the centrifuging effect as shown in Figure 12 suggesting that even at low swirl numbers, the
annular injection of solid particles inevitably leads to reduced residence times within the flame.
5 DISCUSSION – SCALE UP OF RESULTS
The problem of scaling up has been investigated through application of Stokes number similarity and of the standard “constant velocity scaling” concept for case IV of the laboratory scale
burner used in [9], for the pilot-scale burner used here which is similar (in thermal output) to
that used in [18] and for an industrial-scale burner (12MW).
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For this purpose the relaxation times and the mean, Stm, turbulent, Stt, and transit, Sttransit, Stokes
numbers defined as
Stm = tm / τrelax
(1)
Stt = Tf / τrelax
(2)
Sttransit = Ttransit / τrelax
(3)
Where: tm is the characteristic mean timescale, τrelax is the relaxation time, Tf is the Langrangian integral timescale of the air turbulence, Ttransit is the transit time of coal particle in the IRZ.
The above numbers were calculated for a selection of 12, 36, 642, 100, 150 and 200µm coal
particles as stated in Orfanoudakis 1994 [2]. The results are summarised in table 2.
Particle
size
τrelax
(µm)
12
36
64
100
150
200
(ms)
0.15
1.4
4.5
10.8
24.5
43.6
Case IV in [9]
Stm
8.75
0.94
0.29
0.12
0.05
0.03
Stt
5.83
0.62
0.19
0.08
0.03
0.02
Sttransit
103
11
1.16
0.48
0.21
0.12
Current Burner
Stm
34
3.64
1.12
0.46
0.19
0.12
Stt
23
2.40
0.74
0.31
0.12
0.08
Sttransit
399
43
4.49
1.84
0.82
0.46
12MW Burner
Stm
339
36
11
4.65
1.94
1.16
Stt
226
24
7
3.10
1.16
0.77
Sttransit
3989
426
45
29
19
14
Table 2: Mean, turbulent and transit Stokes numbers at three burner scales3.
The values of the mean Stokes numbers suggest that the behaviour of the “large” (64 µm) particles in the burner described in this work would correspond to coal particles sizes and well in
excess of 200 µm particles behaviour in a 12MW burner. The latter are present in only in very
low concentrations in conventional pulverised fuel, so that the mean flow characteristics of the
present ‘large’ particles correspond to those of the largest and rarest particles in a power station
burner.
Particle dispersion due to gas phase turbulence, is also an important parameter and depends on
the turbulent Stokes number (as defined in [2]), Stt, and on the time available for dispersion.
This can be estimated from the transit time across the IRZ, and the related transit Stokes number has already been introduced. The magnitude of Ttransit, as defined in [2], is proportional to
the length of the internal recirculation zone (LIRZ), which is, in turn, proportional to the burner
throat diameter, Dth. The turbulent Stokes numbers suggest that, for the 120 kW and 12 MW
2
The preceding three sizes are the arithmetic mean diameters of the “small”, “medium” and “large” particles of
the 8 kW burner presented in [9].
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burners respectively, the behaviour of the “medium” (36 µm) particles4 in the burner presented
in this work would correspond to approximately 64 and 200µm particles behaviour in a 12MW
burner. The size of particles that would result in ‘fan-spreading’ and be unresponsive to turbulence, corresponding to the 64 m particles, is estimated to be 150 and well in excess of 200 µm
for the two larger burners, respectively. The magnitudes of Sttransit for all particle size ranges
are, for the 12 MW burners, more than sufficient for the particles to respond to the gas phase
turbulence: the only exception is the large size range (64µm particles) for the laboratory scale
burner measured here.
It is not profitable to pursue the scale up arguments further because the present experiments are
imperfect models of full scale for at least two reasons, other than the methane support. First,
the “chemical” timescales (of devolatilisation and char burn-out) will not scale with the aerodynamic ones and the corresponding weight loss during the transit time will be a more important factor than in the small scale burner. Secondly, at full scale, the momentum ratio of the
primary pulverised coal stream will be an order of magnitude higher than the highest value
considered here: not because the velocity scales are inappropriate, but because the mass loading of the primary stream is too small. The effects of higher mass loading will be important
not only because mean momentum transport will be a fully two way process: there may also be
large modification of turbulence in the gaseous phase as well. In practice, some of the trends
produced by realistically large momentum ratios would be similar to the effects of staging, the
physical insertion of the coal gun simulating the existence of a central jet of high momentum
with relatively little radial dispersion. Research into the two – way coupling between particulate and gas phase mean momentum and turbulence fields remains an active area of basic investigation. Experimentally, it is unlikely that the sizing anemometer presented in [1] can
measure at higher mass loadings than those considered, at least while retaining laboratory scale
models: the shadow Doppler anemometer [17] promises only a modest increase in measurable
mass loadings. It is more likely that planar imaging combined with particle tracking of the dispersed phase, possibly along the lines pursued recently by [21], is the more fruitful avenue to
pursue the investigation of the effects of high mass loadings of the dispersed phase than the
“single point” measurement techniques considered here.
3
“Constant velocity” scaling was performed assuming constant temperature and composition.
The size which showed some departure from what we have taken to be an acceptable representation of the gas
phase turbulence
4
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6 CONCLUSIONS
A versatile laboratory scale burner that allowed for the firing of gaseous, solid and liquid fuel
has been designed and built. Mean temperature and Laser Doppler measurements of all three
velocity components in reacting single and multiphase flows were obtained and presented as a
function of two swirl numbers in the range of 0.65 to 0.9. The measurements confirmed that
the flow field was axisymmetric and revealed the formation of an internal recirculation zone
(IRZ) in the shape of a toroidal vortex around the centreline for swirl of at least 0.65. A 60%
increase in the swirl number, from 0.65 to 0.9, resulted in a 30% widening of the IRZ. Velocity measurements of coal particles in the same configuration revealed that the width of the
zone, where coal particles recirculate, is by 20% higher than the IRZ in single phase flows.
Most of the coal particles were shown to be centrifuged away from the IRZ, particularly for the
high swirl number case.
Further work can be performed with the use of this burner to examine other parameters such as
burner quarl geometry, behaviour of the different size of coal particles and staged combustion.
The laboratory burner can also be used as test bed for other laser techniques such as spectroscopic (CARS, LIV) or particle sizing techniques.
The non-combusting single and two-phase flow fields downstream the burner were further investigated by the use of CFD, as a function of the swirl number. The flow field was modelled
as 2D axisymmetric and additional experimental data obtained in isothermals cases were used
to validate an in-house developed software including both k-ε and RNG turbulence models.
Results obtained with the RNG model were shown to be in good agreement with the measurements even at the high swirl number case, whereas, as expected, the k-ε model tended to underestimate both the magnitude of the axial velocity component and the size of the internal recirculation zone (IRZ).
Lagrangian tracking of coal particles in the range of 1 to 150 µm was performed as a function
of swirl number by means of another in-house software developed as part of previous work.
The particles were tracked in the pre-calculated single phase flow field and a one way coupling
between the fluid and the particulate flow was assumed. In accordance to the measurements,
the calculations revealed that particles of diameter less than 20µm are entrained inside the IRZ,
particles with diameters larger than about 20 µm are centrifuged away from the IRZ and that
particles with diameter larger than 100 µm slide along the IRZ boundary and are neither centri-
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fuged nor entrained inside the recirculation zone. Finally, and also in accordance with the
measurements, the calculations showed that the effect of centrifuging is decreased when the
swirl number is reduced. An attempt to scale up the results through application of Stokes number similarity and of the standard “constant velocity scaling” concept has also been pursued
and comparisons of the present findings to the flow field downstream a 120 kW and 12 MW
industrial burner were made.
REFERENCES
1.
Smart J. P.& D. J. Morgan The comparison between constant velocity and constant residence time scaling of the aerodynamically air staged burner. Netherlands: International
Flame Research Foundation Doc No F37/y/28, 1992.
2.
Orfanoudakis N. Measurements of size and velocity of burning coal, PhD Thesis, Imperial
College of Science, Technology and Medicine, Department of Mechanical Engineering,
1994.
3.
Hagiwara A. & Bortz S. Studies of the near field aerodynamics of swirl burners. Netherland, International Flame Research Foundation, 1984.
4.
Beer, J. M. & Chigier N. A. Combustion aerodynamics, Applied Science Publishers Ltd,
1984.
5.
Durst F., Melling A., Whitelaw J. H. Principles and practice of Laser-Doppler anemometry, London: Academic Press, 1990.
6.
Yanta W. J., Turbulence measurement with a laser Doppler velocimeter. Maryland: Naval
Ordnance Laboratory, 1973.
7.
Heitor, M. V. and Moreira, A.L.N. Thermocouples and sample probes for combustion
studies. Prog. Energy Combust. Sci. 19, pp. 259-278,1993.
8.
Hatziapostolou A. Swirling flows in direct-injection diesel engines, PhD Thesis, Imperial
College of Science, Technology and Medicine, De-partment of Mechanical Engineering,
1991.
9.
Orfanoudakis N., Taylor A.M.K.P., and Whitelaw J.H. Measurements of particle size, velocity and flux in the near burner zone of a model pulver-ised coal swirl burner; submitted,
Comb. Flame 2002.
10. Patankar, S.V. Numerical Heat Transfer and Fluid Flow, Mc Graw Hill, 1980.
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December 2005
11. Gosman, A.D. and Ideriah, F.J.K. “TEACH-T”, Imperial College Mechanical Engineering
Dept. Report, 1976.
12. Yakhot, V. and Smith, L.M. The renormalisation group the -expantion and derivation of
turbulence models, J. Sci. Comp. 7:35-61, 1992.
13. Abbas, T, Charoenduk, J., Costen, P. and Lockwood, F.C. The perform-ance of pulverised-coal flames in a simulated combined cycle unit. Comb. Flame, 11, pp. 111-123,
1997.
14. Inage S.I., Taylor A.M.K.P., Sardi, K., Marquis, A.J. Hamada, I. Hu-kuda, Y. Ichinose, N.
& Kobayashi, N. Development of an erosion evaluation technique using a new model on a
pipe surface. Trans. Japan Soc. Mech. Eng. 64(623 B) pp. 157-164, 1998.
15. Sommerfeld, M. Theoretical and experimental modelling of particulate flows, Lecture Series 2000-06, von Karman Institute for Fluid Dynamics, April 3-7, 2000.
16. Gosman, A.D. & Ioannides, E. Aspects of computer simulation of liquid-fueled combustors. Journal of Energy 7 pp. 482-490.
17. Prassas, I. Combustion of pulverised coal in swirl burners, PhD Thesis, Imperial College
of Science, Technology and Medicine, Department of Mechanical Engineering, 1998.
18. Abbas, T., Costen, P., Kandamby, N.H., Lockwood, F.C. & Ou J.J. The influence of
burner injection mode on pulverised coal and biomass co-fired flames. Comb. Flame 99
pp. 617-625.
19. Smart, J.P., Knill, K.J., Visser, B.M. & Weber, R. Reduction of NOx emissions in swirled
coal flameby particle injection into the recirculation zone, 22nd Symposium (Int’l) on
Combustion, The Combustion Institute pp. 1117-1125.
20. Orfanoudakis N., Hatziapostolou A.& Mastorakos Ε. Design, evaluation measurements of
a small swirl stabilised laboratory burner; International Conference on Clean Coal Technologies for our future. Sardinia Italy 21-23 October 2002.
21. D A Khalitov, E K Longmire, Exp Fluids 32, (2002), 252 – 268, “Simultaneous two-phase
PIV by two parameter phase discrimination”
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(a)
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Orfanoudakis et al.
December 2005
(b)
Liquid
fuel
Pulverised
solid fuel
Gaseous
fuel
(c)
a)
b)
(d)
Figure 1: Laboratory burner (a) with enlarged view of the fuel gun (b) and horizontal cross-sections of the
burner (c) showing i) tangential and ii) axial inlets (d) coordinate system and velocity components.
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Orfanoudakis et al.
December 2005
T (o C)
-19 -
v
v′
,
VB
VB
u
u′ w
w′
,
,
,
VB
V B VB
VB
IFRF Combustion Journal
Article No 200510
Figure 2: Combusting flow for case 1(gaseous fuel only): (a); axial and swirl mean and rms velocity components, (b); radial mean and rms velocity components, (c); mean temperature profiles.
19
Orfanoudakis et al.
December 2005
T (o C)
-20 -
v
v′
,
VB
VB
u
u′ w
w′
,
,
,
VB
V B VB
VB
IFRF Combustion Journal
Article No 200510
Figure 3: Combusting flow for case 2(gaseous fuel only): (a); axial and swirl mean and rms velocity components, (b); radial mean and rms velocity components, (c); mean temperature profiles.
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(a)
1.0
(b)
1.2
1.0
u/VB
0.8
Orfanoudakis et al.
December 2005
Mean
rms
0.8
0.6
0.6
0.4
0.4
0.8
0.6
0.4
0.2
0.2
0.0
0.0
(c)
1.0
0.2
-0.2
-0.2
0.0
-0.4
0.0
0.1
0.2
0.3
0.4
0.5
(d)
0.5
0.0
0.1
0.2
0.3
0.4
0.5
0.6
(e)
0.5
0.4
0.4
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
(f)
0.5
0.4
0.3
0.3
w/VB
0.3
0.2
0.2
0.2
0.1
0.1
0.1
0.0
0.0
0.0
0.1
0.2
x/De
0.3
0.4
-0.1
0.5
-0.1
0.0
0.0
0.1
0.2
x/De
0.3
0.4
0.5
0.6
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
x/De
Figure 4: Combusting two phase flow- Case 1: Mean and rms axial velocity component of coal particles (a)
z/De=0.31, (b) z/De=0.78, (c) z/De=1.55. Mean and rms swirl velocity component (d) z/De=0.31, (e) z/De=0.78,
(f) z/De=1.55.
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(b)
(a)
1.0
1.0
0.8
u/VB
Mean
rms
0.8
0.6
0.8
0.6
0.6
0.4
0.4
0.4
0.2
0.2
0.2
0.0
0.0
0.0
-0.2
-0.2
-0.2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
(c)
1.2
1.2
1.0
-0.4
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
-0.4
0.8 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
(e)
(d)
0.5
0.4
0.8
0.9
(f)
0.5
0.5
0.4
0.4
w/VB
0.3
0.3
0.3
0.2
0.2
0.2
0.1
0.1
0.0
0.0
0.1
0.0
0.1
0.2
x/De
0.3
0.4
0.5
0.0
-0.1
-0.1
0.0
0.1
0.2
x/De
0.3
0.4
0.5
0.6
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
x/De
Figure 5: Combusting two phase flow- Case 2: Mean and rms swirl velocity component of coal particles (a)
z/De=0.31, (b) z/De=0.78, (c) z/De=1.55. Mean and rms swirl velocity component (d) z/De=0.31, (e) z/De=0.78,
(f) z/De=1.55.
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(a) –Case 1
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(b)-Case 2
(c) – Case 1
Orfanoudakis et al.
December 2005
(d)- Case 2
Figure 6: Effect of swirl on gaseous (a & b) and coal flames (c) and (d).
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Orfanoudakis et al.
December 2005
(i)
0.8
0.6
U/Vb
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
0
1
2
3
4
5
z/De
0.6
(ii)
S = 0.45:
S = 0.65:
S = 0.90:
measured;
measured;
measured;
RNG;
RNG;
RNG;
k-ε
k-ε
k-ε
u'/Vb
0.4
0.2
0.0
0
1
2
3
4
z/De
Figure 7: Mean and rms of the axial velocity components normalized by the burner exit bulk velocity, VB,
for all swirl numbers. Symbols are the same in all graphs.
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Orfanoudakis et al.
December 2005
(i)
1.2
1.0
0.8
U/Vb
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0.0
1.4
0.4
0.6
0.8
1.0
1.2
(ii)
1.2
1.0
0.8
0.6
V/VB
0.2
measured;
measured;
measured;
measured;
z/De=0.78:
z/De=1.32:
z/De=2.00:
z/De=3.10:
RNG;
RNG;
RNG;
RNG;
k-ε
k-ε
k-ε
k-ε
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0.0
(iii)
0.2
0.4
0.6
0.8
1.0
1.2
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.2
1.0
0.8
W/VB
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0.0
x/De
Figure 8: Radial profiles of the axial (i), radial (ii) and swirl (iii) velocity components at selected axial locations for S = 0.45. Symbols are the same in all graphs.
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Orfanoudakis et al.
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(i)
1.2
1.0
0.8
U/VB
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0.0
1.4
0.4
0.6
0.8
1.0
1.2
(ii)
1.2
1.0
0.8
0.6
V/VB
0.2
measured;
measured;
measured;
measured;
z/De=0.78:
z/De=1.32:
z/De=2.00:
z/De=3.10:
RNG;
RNG;
RNG;
RNG;
k-ε
k-ε
k-ε
k-ε
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0.0
(iii)
0.2
0.4
0.6
0.8
1.0
1.2
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.2
1.0
0.8
W/VB
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0.0
x/De
Figure 9: Radial profiles of the axial (i), radial (ii) and swirl (iii) velocity components at selected axial locations for S = 0.65. Symbols are the same in all graphs.
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Orfanoudakis et al.
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(i)
1.2
1.0
0.8
U/VB
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0.0
1.4
(ii)
0.2
0.4
0.6
0.8
1.0
1.2
1.2
1.0
0.8
V/Vb
0.6
measured;
measured;
measured;
measured;
z/De=0.78:
z/De=1.32:
z/De=2.00:
z/De=3.10:
RNG;
RNG;
RNG;
RNG;
k-ε
k-ε
k-ε
k-ε
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0.0
(iii)
0.2
0.4
0.6
0.8
1.0
1.2
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.2
1.0
0.8
W/VB
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0.0
x/De
Figure 10: Radial profiles of the axial (i), radial (ii) and swirl (iii) velocity components at selected axial locations for S = 0.9. Symbols are the same in all graphs
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Figure 11: Particle trajectories as a function of size for the high swirl case (Sw = 0.9). The thick gray line
marks the IRZ boundary.
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December 2005
Figure 12: Trajectories as a function of particle size for the low swirl case (Sw = 0.45). The thick gray line
marks the IRZ boundary.
29