Experimental Thermal and Fluid Science 56 (2014) 33–44
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Experimental Thermal and Fluid Science
journal homepage: www.elsevier.com/locate/etfs
Effects of dense concentrations of aluminum nanoparticles
on the evaporation behavior of kerosene droplet at elevated
temperatures: The phenomenon of microexplosion
Irfan Javed ⇑,1, Seung Wook Baek ⇑, Khalid Waheed 1
Division of Aerospace Engineering, School of Mechanical, Aerospace and Systems Engineering, Korea Advanced Institute of Science and Technology (KAIST),
291 Daehak-ro, Yuseong-Gu, Daejeon 305-701, Republic of Korea
a r t i c l e
i n f o
Article history:
Available online 19 November 2013
Keywords:
Kerosene vaporization
High-energy–density fuels
Nanofluid fuels
Microexplosion
Nanoparticles
Elevated temperature
a b s t r a c t
The evaporation behavior of kerosene droplets containing dense concentrations (2.5%, 5.0%, and 7.0% by
weight) of aluminum (Al) nanoparticles (NPs) suspended on silicon carbide fiber was studied experimentally over a range of ambient temperatures (400–800 °C) under normal gravity. The evaporation characteristics of the pure and stabilized kerosene droplets were also examined to provide a comparison. The
results show that at all of the tested temperatures, the evaporation behavior of suspended kerosene droplets containing dense concentrations of Al NPs was different from that of pure kerosene droplets and
exhibited three stages of evaporation; an initial heating up stage, d2 -law evaporation and then the microexplosion stage. The phenomenon of microexplosion was not observed during the evaporation of pure
or stabilized kerosene droplets at the same temperatures. The microexplosions occurred early in the
droplet’s lifetime and with a much greater intensity, for either an increase in the ambient temperature
or an increase in the NP loading rate. For all of the Al NP suspensions, regardless of the concentration,
the evaporation rate remained higher than that of either pure or stabilized kerosene droplets at high temperatures (700–800 °C). The intense microexplosions occurring at these temperatures led to a substantial
enhancement in the evaporation rate. However, at lower temperatures (400–500 °C), the delayed onset
with lower intensity of the microexplosions was not able to increase the evaporation rate significantly,
and it remained similar to that of pure fuel droplets. The maximum increase in the evaporation rate
(48.7%) was observed for the 2.5% Al NP suspension droplet at 800 °C.
Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction
The specifically tailored high-energy–density fuels are a compelling need of the next generation combustion and propulsion
systems. The energy–density of available conventional fuels is a
major limiting factor in the current liquid propulsion systems;
therefore, there is an immense necessity of enhancing the energy
content of both traditional and future synthetic fuels. One possible
way to accomplish this is to load a liquid hydrocarbon fuel with
high-energy–density solid NPs in form of a stable suspension, slurry or gel. A stable suspension of solid NPs in a liquid fuel is called
nanofluid fuel which is a new class of nanofluids and is fundamentally different from slurry fuels. These are suspensions with low
concentrations (10 wt.%) of NPs compared to slurry fuels which
⇑ Corresponding authors. Tel.: +82 42 350 3754; fax: +82 42 350 3710 (I. Javed).
Tel.: +82 42 350 3714; fax: +82 42 350 3710 (S.W. Baek).
E-mail addresses: [email protected] (I. Javed), [email protected] (S.W. Baek).
1
Permanent address: Pakistan Institute of Engineering and Applied Sciences
(PIEAS), Nilore 45650, Islamabad, Pakistan.
0894-1777/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.expthermflusci.2013.11.006
contain relatively high solid loadings (40–80 wt.%) of micron sized
particles and were studied decades ago [1]. Low concentrations of
NPs will not significantly change the physical properties such as
the density, viscosity or boiling point of the base fuel so that current fuel delivery systems can still be used, with only slight modifications [2].
High loadings of NPs (10 wt.%) would enable the production of
high-energy–density fuels but they can also induce some potential
side effects, as found in slurry fuels, which have limited their practical applications. The major problems associated with the use of
slurry fuels are the stability of the micron-sized particles in the liquid fuels and particle agglomeration during evaporation and combustion which resulted in slow burning rates and incomplete
combustion of the agglomerations and consequently reduced the
combustion efficiencies [1]. However, reducing the particle size
from micro to nano scale can provide a solution to such problems,
for as metallic particles size approaches the nano scale, their thermophysical properties often change significantly. Several studies
have described lower melting temperatures [3,4] and a reduced
heat of fusion for nano-sized metal particles [5,6]. Also, nano scale
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I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
energetic materials have a much higher surface to volume ratio
compared to micron-sized particles, which allows more fuel to
be in contact with the oxidizer [7] and consequently results in a
higher combustion efficiency. Furthermore, it is expected that
due to the addition of nano scale particles, the size of the agglomerates should be reduced. The agglomerates may also be shattered
by the microexplosions which occur due to the addition of NPs and
are previously observed in kerosene based nanofluid fuels at elevated temperatures [8].
Tyagi et al. [9] found experimentally that with the addition of
0.1% and 0.5% (by volume) of Al and Al2O3 NPs, the hot plate ignition probability of nanoparticle laden diesel fuel was significantly
higher than that of pure diesel fuel. Allen et al. [10] performed
experiments using an aerosol shock tube and observed that the
addition of 2% (by weight) of Al NPs to ethanol and JP-8 reduced
their ignition delays by 32% and 50%, respectively. Jones et al.
[11] determined experimentally the heat of combustion (HoC) of
ethanol, containing various volume fractions of nano-aluminum
(n-Al) and nano-aluminum oxide (n-Al2O3) particles. Their results
indicated that 1% and 3% n-Al contents did not show enhancement,
but that 5%, 7% and 10% n-Al contents increased the volumetric
HoC by 5.82%, 8.65% and 15.31% respectively. Van Devener and
Anderson [12] observed that the ignition temperature of JP-10
was significantly reduced when using soluble CeO2 NPs as a catalyst. Later, their group produced oxide free, fuel soluble and air stable boron NPs that were also coated with the combustion catalyst
ceria through a unique process [13,14]. Gan and Qiao [15] experimentally investigated the effects of nano and micro sized Al particles on the burning behavior of ethanol and n-decane fuel droplets.
Their results indicated that for the same surfactant and particle
concentrations, the disruption behavior of the micro sized suspensions occurred later and with a much greater intensity. Later, Gan
et al. [16] investigated the combustion behavior of dilute and
dense suspensions of boron and iron NPs in n-decane and ethanol.
In dilute suspensions, simultaneous combustion of both the droplet and the NPs was observed, whereas in dense suspensions, most
particles were combusted as large agglomerations after the consumption of the liquid fuel.
Isolated, single droplet combustion is considered a classical
subject in combustion, which provides the opportunity to investigate the interactions of the physical and chemical processes, in a
simplified geometrical configuration. The importance of the droplet vaporization/combustion phenomena in practical liquid fuel
combustion systems has motivated a large number of researchers
in the past. Droplet vaporization and combustion are similar in
many aspects and the knowledge obtained from studying one of
them provides insights to the other. Enhancement of vaporization
leads to complete combustion with higher combustion efficiency.
Nanofluid fuel droplet vaporization is a highly complex phenomenon compared to liquid fuel droplet vaporization due to its
multi-component, multi-phase and multi-scale nature. A simple
comparative analysis shows it involves at least six interacting processes amongst them, whereas three are present in pure fuel droplet vaporization; liquid phase transport, gas phase transport and
phase change at the liquid–gas interface [17]. The additional three
processes are the solid phase transport, phase change at the solidliquid interface (heterogeneous nucleation sites) and the dynamics
of the NPs (particle diffusions, collisions, aggregation, agglomeration, etc.). The interaction of these processes with each other
makes fundamental understanding even more difficult. Moreover,
the thermophysical properties of nanofluid fuels such as the thermal conductivity [18], mass diffusivity [19], surface tension [20],
radiative properties [21,22] and the non-Newtonian viscosity
[23] are significantly different from those of their base fuels. These
properties can also vary with time due to the continuous changes
in the concentration of NPs within a droplet. Therefore, experimen-
tal studies, which are very rare, are required to provide the basic
understanding of the evaporation behavior of a nanofluid fuel
droplet.
Chen et al. [24] observed that with the addition of laponite,
Fe2O3, and Ag NPs in deionized water, the nanofluid droplets evaporate at different rates from the base fluid and that the evaporation
rate shifts from one constant value to another during the evaporation process. The authors explained these effects based on a change
in the apparent heat of evaporation. With the addition of Al NPs to
ethanol and n-decane fuel droplets, Gan and Qiao [25] detected a
deviation from the classical d2-law under natural and weak forced
convections at 300 and 320 K during their evaporation process. In
another study they observed that with an addition of multi-walled
carbon nanotubes (MWCNTs) or carbon nanoparticles (CNPs) to
ethanol, the evaporation rates of nanofluids are increased compared to the evaporation rate of pure ethanol [22]. They also found
that the addition of a small amount of Al NPs to ethanol increases
the radiation absorption, which increases the nanofluid droplet
temperature and enhances the droplet evaporation rate [26].
Recently, it has been demonstrated [27] that the addition of Al
NPs to heptane enhances its evaporation rate at high temperatures
(above 400 °C). At lower temperatures (below 400 °C), the formation of large agglomerates results in a compact shell and suppresses
the evaporation rate of heptane. More recently, experimental investigations on the effect of dilute concentrations (0.1%, 0.5% and 1.0%)
of Al NPs on the evaporation characteristics of kerosene droplets at
elevated temperatures (400–800 °C) have been conducted [8].
These nano-Al and kerosene (n-Al/kerosene) suspension droplets
evaporate in the same way as pure kerosene droplets at temperatures in the range 400–600 °C and the added Al NPs enhanced the
evaporation rate at all tested temperatures. Microexplosions were
observed at high temperatures (700–800 °C) due to the addition
of the Al NPs to kerosene droplets. The current study investigated
the effects of high loadings of ligand-protected Al NPs on the evaporation behavior of kerosene droplets at elevated temperatures.
Such an experimental study was necessary to verify the occurrence
of microexplosions in the case of dense NP/kerosene suspension
droplets and to develop an understanding of this phenomenon.
The problem associated with this study is the stability of high loadings of metal NPs in liquid hydrocarbon fuels.
In the previous work [8], surface modified and ligand protected
Al NPs were suspended in kerosene. Oleic acid (OA) was used for
the surface coating of the Al NPs. The same surfactant, surface
modification technique and procedure were adopted in the present
study to prepare stable suspensions of dense concentrations of NPs
in kerosene. The evaporation behavior of kerosene based nanofluid
droplets at elevated temperatures with high loadings of Al NPs will
provide valuable insights into the occurrence of microexplosions
due to NPs and their effects on the evaporation rate of such multi-component hydrocarbon fuels. Therefore, in this study the effects of high loadings of NPs on the evaporation behavior of
multi-component liquid hydrocarbon fuel (kerosene) droplets at
elevated temperatures were experimentally investigated.
2. Experimental methods
The materials and instruments used in this study were the same
as described in other recent research work [8]. All the materials,
except for the Al NPs, were used in their original form as received
without further treatment.
Preparation of uniformly dispersed and stable NP suspensions
in fuel is the key step before performing any experimental study
with nanofluid fuels. The post synthesis surface modification concept was applied to the Al NPs purchased. The NPs were ground in
a planetary ball mill (PM100) along with the OA and their surfaces
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I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
3. Results and discussion
3.1. Pure kerosene droplet evaporation
The evaporation rate of pure kerosene droplets was measured,
compared with previous data and discussed in detail in recent research work [8]. The general evaporation behavior and measurements of the evaporation rates of pure kerosene droplets at
elevated temperatures (400–800 °C) are essential for understanding the effects of the addition of high concentrations of OA and
Al NPs on the droplet evaporation behavior. In pure kerosene droplets after a finite heating up period, the temporal variation in the
droplet diameter squared becomes approximately linear while following the d2-law in the last stage of evaporation. The evaporation
rate increases consistently as the ambient temperature increases.
3.2. Evaporation of kerosene droplets with the addition of oleic acid
Before investigating the evaporation behavior of n-Al/kerosene
droplets at elevated temperatures, the evaporation behavior of kerosene/OA droplets with high concentrations of OA (surfactant)
were studied at the same temperatures. The evaporation behavior
of the stabilized kerosene (kerosene/OA) droplets helps us distinguish the effect of NP addition on the droplet evaporation behavior.
Fig. 1 shows the normalized temporal histories of the diameter
squared of kerosene droplets with 1.25% and 2.5% of OA over a
range of ambient temperatures (400–800 °C). The droplets were
selected from the experimental data within a narrow range of initial diameters from 0.978 to 1.079 mm to reduce the effect of the
initial droplet diameter on the evaporation rate. The general evaporation behavior of the kerosene/OA droplets is similar to that for
pure kerosene droplets, i.e., after a finite heating up period, the
reduction in the droplet diameter squared became linear and followed the d2-law. At low temperatures (400–600 °C), the total life-
1.2
500 C
P= 0.1 MPa
Kerosene
Kerosene+1.25% OA
Kerosene+2.50% OA
400 C
1
2
d /d0
2
0.8
0.6
0.4
0.2
(a)
0
0
1
-1
2
0
3
1
4
5
2
3
2
0
6
800 C
700 C
600 C
7
4
5
8
6
7
2
(s/mm )
1.2
P= 0.1 MPa
Kerosene
Kerosene+1.25% OA
Kerosene+2.50% OA
1
2
0.8
2
d /d0
were coated with the ligand which improves their dispersion stability in liquid hydrocarbons. The amount of OA and the time of
ball milling were optimized. The ligand protected Al NPs were dispersed in the kerosene fuel and 2.5%, 5.0% and 7.0% NP/kerosene
suspensions were prepared. In this case and for the remainder of
this paper mass percentage values are used. The surface modification technique used in the current study has been described elsewhere [14] and the procedure and amendments adopted here
have been also explained in another recent study [8].
The design, fabrication and installation of the experimental
apparatus is discussed in detail in previous studies [28–30]. The
experimental procedure for the evaporation of the nanofluid fuel
droplet along with the data reduction and analysis are described
in the previous work [27]. An isolated n-Al/kerosene droplet was
suspended by a fine SiC fiber (100 lm diameter). The initial average diameter of a droplet was 1.0 ± 0.10 mm. Nitrogen was used
to provide an inert environment for evaporation. The ambient temperature was varied in the range 400–800 °C and the ambient pressure was kept constant at 0.1 MPa. High ambient temperatures
were provided by an electric furnace. The evaporation process
was recorded using a high speed charge coupled device (CCD) camera. The captured images were analyzed using a flexible image processing code to extract the droplet diameter. The temporal
variation of the droplet diameter was obtained during the whole
evaporation process. If the droplet evaporation follows the d2law then the evaporation rate can be expressed as the time derivative of the droplet diameter squared, Cv = d(d2)/dt [31,32]. In
that case, the droplet evaporation rate was obtained from the temporal history of the droplet diameter squared, by measuring the
slope of its linear regression.
0.6
0.4
0.2
(b)
0
0
0.5
1
1.5
2
2.5
3
3.5
4
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
-1
-0.5
0
0.5
1
2
0
1.5
2
2.5
4.5
4
3
2
(s/mm )
Fig. 1. Droplet diameter squared histories of evaporating stabilized kerosene
droplets containing 1.25% and 2.50% OA at various ambient temperatures; (a) at 400
and 500 °C, (b) at 600, 700 and 800 °C.
time of the stabilized kerosene droplets was slightly longer than
that for pure kerosene droplets as shown in the figure. OA is a
low volatility component compared to kerosene while its addition
to kerosene produces a monolayer at the surface of the droplet
which inhibits the diffusion of kerosene so that slightly reduces
the evaporation rate [33]. However, at high temperatures (700–
800 °C), the total lifetime of the stabilized kerosene droplets was
comparable to that of pure kerosene droplets and the effects of
the addition of OA were diminished as shown in Fig. 1b.
Fig. 2 shows that the evaporation behavior of kerosene/OA
droplets with 5.0% of OA is similar to that of binary component fuel
droplets. Likewise with binary component droplets [28], the stabilized kerosene droplets show a three stage evaporation process; a
finite heating up period, d2-law evaporation of the highly volatile
component (kerosene) and d2-law evaporation of the low volatility
component (OA) as depicted in Fig. 2. The evaporation history at
the above mentioned temperatures can be divided into two distinct stages, with an obvious difference in their diminishing rates,
as shown in the figure. During the first stage of the evaporation
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I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
1.2
Fuel: Kerosene+5.0% Oleic Acid
P= 0.1 MPa
T=400
T=500
T=600
T=700
T=800
1
d2 /d20
0.8
o
C
C
C
o
C
o
C
o
o
Cv for Kerosene component
0.6
Cv for Oleic Acid
component
0.4
0.2
0
0
1
2
3
4
2
0
5
6
7
2
(s/mm )
Fig. 2. Normalized temporal variations of the diameter squared of evaporating
stabilized kerosene droplets containing 5.0% OA with each least-squares fit to the
linear kerosene evaporation part (first stage) and to the linear OA evaporation part
(second stage) at various ambient temperatures (400–800 °C) and constant
pressure of 0.1 MPa.
period, the kerosene component from the stabilized kerosene
droplet evaporates. In the second stage the droplet becomes rich
with OA and the OA near the droplet surface starts to evaporate.
As the temperature increased from 400 to 800 °C, the difference
between the evaporation of the high volatility and low volatility
components in the third stage was decreased. Due to the reduced
difference between the boiling points of kerosene and OA and
the multi-component nature of the kerosene, the phenomena of
bubble formation and microexplosion were not seen to take place
even at high temperatures (700–800 °C). The evaporation rates of
the stabilized kerosene droplets were calculated using the least
squares regression method with the data corresponding to the linear part of the evaporation history except for the initial heat up
period.
1.2
P=0.1MPa
Kerosene
Kerosene+1.25%
Kerosene+2.50%
Kerosene+5.00%
Kerosene+5.00%
1.0
OA
OA
OA (Kerosene component)
OA (OA component)
Cv (mm2/s)
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
Temperature (oC)
Fig. 3. A comparison of the evaporation rates of the stabilized kerosene droplets
containing various concentrations of OA with pure kerosene droplets at different
ambient temperatures and constant pressure of 0.1 MPa.
3.2.1. Effect of temperature on the evaporation rate of stabilized
kerosene droplets
Fig. 3 shows a comparison of the evaporation rates of kerosene
droplets containing 1.25%, 2.5% and 5.0% of OA and pure kerosene
droplets for a range of ambient temperatures (400–800 °C). The
evaporation rate of the kerosene/OA droplets is lower than or equal
to that of the pure kerosene at all of the above mentioned temperatures as shown in the figure. The evaporation rate of the kerosene/
OA droplets with relatively low concentrations of OA (1.25% and
2.5%) remained slightly lower than the evaporation rate of the pure
kerosene droplets at all temperatures tested. The kerosene/OA
droplets with 5.0% of OA have two evaporation rate curves; one
for the kerosene component and the other for the OA component
present in the stabilized kerosene droplets. The evaporation rate
of the kerosene component in the 5.0% stabilized kerosene droplets
is comparable to that of the pure kerosene droplets and remains
the same in the range 400–700 °C and is slightly higher at
800 °C. However, the evaporation rate of the OA component in
the 5.0% stabilized kerosene droplets is considerably lower than
the evaporation rate of the pure kerosene droplets at all temperatures tested. These two curves are similar in that the evaporation
rate of each component (kerosene and OA) of the stabilized kerosene droplets was increased by an increase in the temperature.
The effect of the increase in the temperature has different impacts
on the evaporation rate of the kerosene component and the OA
component in stabilized kerosene droplets. A comparison of these
two curves also shows that the evaporation rate of the kerosene
component truly represents the evaporation rates of the 5.0% stabilized kerosene droplets.
These results indicate that similar to the addition of dilute concentrations of OA to kerosene [8], the addition of dense concentrations of OA to kerosene also has no considerable effect on the
evaporation rate of kerosene droplets. The evaporation rate of
the stabilized kerosene droplets, containing 1.25%, 2.5% and 5.0%
of OA, is also nearly equal to the evaporation rate of pure kerosene
droplets at all temperatures tested.
3.3. Evaporation of kerosene based nanofluid fuel droplets
In this section, the evaporation characteristics of n-Al/kerosene
droplets are discussed.
Fig. 4 displays normalized temporal histories of the kerosene
droplets’ diameter squared with the addition of 2.5%, 5.0% and
7.0% of Al NPs whose surfaces were coated with 1.25%, 2.50% and
3.50% of OA, respectively, at ambient temperatures ranging from
400 to 800 °C. To minimize the effect of the initial droplet size
on the evaporation rate, droplets having similar initial diameters
ranging from 0.977 to 1.074 mm were selected from the experimental data.
Unlike the dilute n-Al/kerosene suspension droplets [8], the
general evaporation behavior of the dense n-Al/kerosene suspension droplets was not similar to that of pure kerosene droplets.
The droplets with high loadings of Al NPs in kerosene showed three
stages of evaporation; a finite heating up period, d2-law evaporation followed by microexplosions. Fig. 4 also clearly shows that
the addition of dense concentrations of Al NPs to kerosene caused
bubble formation and microexplosions of different strengths at different temperatures.
Fig. 4a shows normalized evaporation histories for kerosene
based nanofluid droplets with 2.5%, 5.0% and 7.0% of Al NPs at
400 and 500 °C. The kerosene droplets with 2.5% of Al NPs suspension show no bubble formation or microexplosions at the above
mentioned temperatures. However, at these temperatures, the
droplets containing 5.0% and 7.0% of Al NPs suspension exhibit
bubble formation and they rupture at the end of the droplet’s lifetime. Due to the high loadings of NPs (5.0% and 7.0%), enough NPs
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
1.2
500 C
P= 0.1 MPa
Kerosene
Kerosene+1.25% OA+2.5% Al NPs
Kerosene+2.50% OA+5.0% Al NPs
Kerosene+3.50% OA+7.0% Al NPs
400 C
1
d2/d20
0.8
0.6
0.4
0.2
(a)
0
0
1
-1
2
0
3
1
4
2
5
3
2
0
700 C
7
5
8
6
9
7
10
8
(s/mm2)
P= 0.1 MPa
Kerosene
Kerosene+1.25% OA+2.5% Al NPs
Kerosene+2.50% OA+5.0% Al NPs
Kerosene+3.50% OA+7.0% Al NPs
1.2
800 C
6
4
600 C
1
d2/d20
0.8
0.6
0.4
0.2
(b)
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
-1.5
-1
-0.5
0
0.5
2
0
1
1.5
2
2.5
3
(s/mm2)
Fig. 4. Temporal variation of the diameter squared of kerosene droplets with the
dense concentrations (2.5%, 5.0% and 7.0%) of Al NPs (coated with OA) at different
environmental temperatures (a) at 400 and 500 °C, (b) at 600, 700 and 800 °C.
were accumulated at the droplet’s surface during evaporation that
they produce a shell there in the last phase of the evaporation period. The shell temperature can exceed the boiling point of the liquid fuel causing nucleation sites to form inside the droplet
which results in the gasification of the surrounding liquid, pressure
build up and ultimately fragmentation of the droplet. As the temperature increased from 400 to 500 °C or the NP concentration increased from 5.0% to 7.0%, the microexplosions started earlier in
the droplet’s lifetime.
Fig. 4b shows normalized evaporation histories for stabilized
kerosene droplets with dense concentrations of Al NPs at 600,
700 and 800 °C. The most prominent characteristic of the evaporating n-Al/kerosene droplets at these ambient temperatures was the
onset of bubble formation and microexplosions irrespective of the
Al NP concentration. Between 600 and 700 °C, with 2.5% of Al NPs,
the droplets were suddenly shrunk to smaller droplets by the release of accumulated vapors without any prior expansion. At 600
and 800 °C with relatively high concentrations (5.0% and 7.0%) of
Al NPs, the evaporating droplets first expanded to a larger size
37
and then ruptured. This expansion and contraction process was repeated several times. For 5.0% and 7.0% n-Al/kerosene droplets at
700 °C, the phenomenon of droplet fragmentation and microexplosion was observed in two stages; primary fragmentation and secondary microexplosion (see Supplementary video). The primary
fragmentation includes the droplet’s diameter reduction without
any prior expansion whereas the secondary microexplosion comprises a droplet diameter expansion and then reduction several
times. These two microexplosion stages are separated by a small
linear evaporation period. The 2.5% n-Al/kerosene droplets also
exhibited the same two microexplosion stages at 800 °C. It is noteworthy to mention that microexplosions would be most effective if
nucleation occurs early in the droplet lifetime and deep within the
droplet’s inner core [17]. Fortunately, the heterogeneous nucleation sites (HNSs) produced at 800 °C, with the addition of 5.0%
and 7.0% of Al NPs to kerosene, satisfied these two requirements
simultaneously. These HNSs were produced early in the droplet
lifetime (after 20–25% of the time span) and deep inside the droplet, consequently multiple intense microexplosions occurred in the
remaining evaporating time. At a high temperature (800 °C), due to
the strong particle dynamics (collisions, diffusion and Brownian
motion) inside the droplet or due to the enhanced thermal diffusivity, the NPs present inside the droplet acquired a temperature
higher than the liquid’s local boiling point and converted to HNSs.
Whereas at 700 °C, the early HNSs were produced at or near the
droplet surface after 50% of the time span. Later, HNSs were created inside the droplet, however by that time much of the liquid
droplet had already been gasified. At low temperatures (400–
600 °C), the onset of microexplosion was further delayed (starting
after 70–80% of the time span) so that most of the liquid fuel had
already evaporated and the intensity was reduced. It is important
to note that the microexplosions started a little earlier with an increase in the concentration of NPs at the same temperature. However, an increase in the temperature significantly affects the onset
of microexplosions and they started quite early in the droplet’s
lifetime with a much greater intensity.
3.3.1. Phenomenon of microexplosion
The phenomenon of microexplosion is very important to the
evaporation and combustion of nanofluid fuels because it has the
potential to enhance their evaporation/combustion performance
by shattering the droplets into smaller particles, which yields faster and complete burning of the added Al NPs. For a better understanding of the nature and types of microexplosions, sequential
photographs of evaporating n-Al/kerosene droplets are presented
in the next figure.
Fig. 5a–c presents some sequential images of the evaporation
behavior of stabilized kerosene droplets with 2.5%, 5.0%, and 7.0%
of Al NPs at 800 °C, respectively. The phenomena of bubble formation and microexplosion are clearly observed here. In Fig. 5a–c, the
second snapshot shows the end of the linear evaporation period
(second stage). The third snapshot of Fig. 5a (at t = 0.695 s) shows
the vapor formation at the droplet surface, due to this a slight
increase in the droplet diameter was observed for the 2.5% n-Al/
kerosene. Due to the escape of these vapors, the droplet diameter
is suddenly reduced in the subsequent pictures (at t = 0.700 and
0.705 s). After this the droplet vaporized linearly up to 1.065 s
and then the droplet starts to expand due to internal bubble formation which is completed by 1.210 s, the droplet then ruptures at
1.215 s.
For the 5.0% n-Al/kerosene droplet (Fig. 5b), after linear evaporation up to 0.850 s, internal bubble formation started and this appears at 0.920 s and the droplet ruptures at 0.925 s as shown in the
captured frames. The second, third, fourth, fifth and sixth times
bubbles appeared at 0.980, 1.120, 1.210, 1.240 and 1.270 s and
the droplets rupture in the subsequent photos at 0.985, 1.125,
38
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
t = 0.000 s
t = 0.690 s
t = 1.065 s
t = 1.210 s
t = 0.695 s
t = 1.215 s
t = 0.700 s
t = 1.220 s
t = 0.705 s
t = 1.335 s
(a)
t = 0.000 s
t = 0.850 s
t = 0.920 s
t = 0.925 s
t = 0.930 s
t = 0.980 s
t = 0.985 s
t = 0.990 s
t = 1.120 s
t = 1.125 s
t = 1.130 s
t = 1.210 s
t = 1.215 s
t = 1.220 s
t = 1.240 s
t = 1.245 s
t = 1.250 s
t = 1.270 s
t = 1.275 s
t = 1.280 s
t = 1.290 s
t = 1.295 s
t = 1.300 s
t = 1.305 s
t = 1.320 s
t = 1.325 s
t = 1.370 s
t = 1.375 s
t = 1.435 s
t = 2.265 s
(b)
Fig. 5. Sequential photographs of vaporization of kerosene droplets containing various dense concentrations of Al NPs at same elevated temperature (a) 2.5% Al NPs (coated
with 1.25% OA) at 800 °C, (b) 5.0% Al NPs (coated with 2.5% OA) at 800 °C, and (c) 7.0% Al NPs (coated with 3.5% OA) at 800 °C; camera speed: 200 fps.
39
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
t = 0.000 s
t = 0.855 s
t = 0.715 s
t = 0.860 s
t = 0.750 s
t = 0.845 s
t = 0.850 s
t = 0.880 s
t = 0.985 s
t = 0.990 s
t = 1.090 s
t = 0.950 s
t = 1.000 s
t = 1.080 s
t = 1.085 s
t = 1.105 s
t = 1.110 s
t = 1.115 s
t = 1.130s
t = 1.215 s
t = 1.220 s
t = 1.225 s
t = 1.245 s
t = 1.250 s
t = 1.255 s
t = 1.260 s
t = 1.265 s
t = 1.270 s
t = 1.380 s
t = 1.400 s
(c)
Fig. 5 (continued)
1.215, 1.245 and 1.275 s, respectively. It is important to note that
after the generation of the first bubble, the time taken to produce
every subsequent bubble was lower than the time previously spent
in bubble generation. Initially, it took a long time to produce HNSs
inside the droplet, but once the NPs had a temperature higher than
the liquid’s local boiling point they continuously gasified the surrounding liquid. Due to the accumulation of NPs, the heat transfer
inside the droplet was increased so that more inner NPs were providing HNSs. It is also noteworthy that this irregular behavior was
not observed in the evaporation process of pure or stabilized kerosene droplets.
In the case of the 7.0% n-Al/kerosene droplet (Fig. 5c), the internal bubble formation started after the second stage (t = 0.715 s) but
the bubble appeared at 0.750 s and continuously grows up to
0.845 s. In the subsequent two pictures the droplet fragmented
intensively. After that the remaining droplet expanded several
times (at 0.990, 1.080, 1.105, 1.215, 1.245 and 1.260 s) due to bubble formation inside the droplet and then shrank as a result of its
subsequent ruptures, which took place at 0.995, 1.085, 1.110,
1.220, 1.250 and 1.265 s as shown in the pictures. A comparison
of the re-expansion of the droplets and their rupturing behavior
between the 5.0% and 7.0% Al NP suspension droplets reveals that
more intense microexplosions occurred with high Al NP concentrations at high ambient temperatures. A fast internal pressure build
up caused by more HNSs provided by a high Al NP concentration at
high temperatures, results in more aggressive microexplosion
behavior such as a large expansion, severe bubble formation and
more disruptive fragmentation. The microexplosion phenomenon
started earlier with an increase in the concentration of the Al
NPs at the same temperature.
There are two types of microexplosions which were observed
here. One is droplet fragmentation without any prior droplet
expansion. The other is the repeated expansion and subsequent
rupture of the droplet. The first kind of droplet reduction was
due to the sudden escape of vapors from the droplet surface. These
gaseous liquids were produced at or near to the droplet surface by
40
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
Fig. 6. SEM micrographs of the evaporation residue obtained at 600 °C for 2.5% Al
NPs with 1.25% OA (a) overall view and (b) magnified view of surface,
magnification = 30,000.
the HNSs provided by the NPs. The second kind of microexplosion
which was observed with high concentration of Al NPs during the
latter part of the droplet lifetime at 400–700 °C was due to the
accumulation of NPs at the droplet surface. These NPs may gather
to form a shell at the droplet surface and simultaneously raise the
droplet surface temperature to close to the liquid local boiling
point. The NPs present inside the droplet may also be heated beyond the liquid local boiling point, providing multiple HNSs and
converting the surrounding liquid into vapor. This vapor expanded
the droplet while increasing its diameter. By receiving further heat
from the NPs these vapors became superheated, an intense pressure buildup was caused, which resulted in the catastrophic fragmentation of the droplet. An increase in the NPs concentration or
an increase in the temperature resulted in an early accumulation
or shell formation of the NPs at the droplet surface, which resulted
in early onset of microexplosions. This is why microexplosions
were observed during the major portion of the droplet’s lifetime
with a greater intensity at 800 °C in droplets containing a relatively
high concentration of Al NPs. In the case of the 2.5% Al NPs, the
added Al NPs were not enough to form a shell at the droplet surface
which could raise its surface temperature. They only provided
HNSs at or near to the droplet surface.
The residues obtained at the end of evaporation were collected
to investigate the effects of microexplosions on shell formation of
nanofluid fuel droplets as well as to confirm the physical state of
the Al NPs present at the droplets’ surface. Residues of the 2.5%
and 5.0% Al NP suspension droplets were obtained at two different
temperatures (600 and 800 °C) and studied using field-emission
scanning electron microscope (FESEM) and energy-dispersive Xray spectroscopy (EDX). The selected temperature values were below and above the melting temperature of the Al NPs (660 °C),
respectively. The residues were collected very carefully using the
collection procedure as described in a recent study [8].
Figs. 6 and 7 show the FESEM images of the residues after evaporation at 600 and 800 °C for a 2.5% Al NP suspension. An overall
view of the residue (Fig. 6a) shows that it was soft, powdery and
Fig. 7. SEM micrographs of the evaporation residue obtained at 800 °C for 2.5% Al NPs with 1.25% OA (a) overall view, (b) close view of lump, (c) close view of surface and (d)
magnified view of surface, magnification = 30,000.
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
Fig. 8. SEM micrographs of the evaporation residue obtained at 600 °C for 5.0% Al
NPs with 2.5% OA (a) overall view and (b) magnified view of surface,
magnification = 30,000.
41
had been broken during the SEM sample preparation. The magnified view (Fig. 6b) of the residue shows that most of the NPs preserved their original spherical shapes at 600 °C. When the
temperature increased to 800 °C, a large and compact residue
was formed as shown in Fig. 7a. A closer view (Fig. 7b) shows that
the residue was soft, crumbly and porous in nature. A further detailed view of the residue’s surface (Fig. 7c) revealed that it has
been punctured by the NPs coming out of the surface through microexplosion. Fig. 7d shows a magnified view of the residue, which
clearly shows that most of the NPs loosen their shapes, while being
fused and stuck each other to form larger lumps.
Figs. 8 and 9 show the FESEM characterization micrographs of
the 5.0% Al NP suspension residue collected at 600 and 800 °C.
The droplet shrank and formed an elliptical shaped agglomerate
on the SiC fiber as shown in Fig. 8a. This agglomeration seemed
compact and porous because during the preparation of the SEM
sample the whole lump was detached from the SiC fiber and was
not broken into pieces as happened to the 2.5% suspension residue
at the same temperature (Fig. 6a). Fig. 8b shows a magnified view
of the residue, which disclosed that most of the NPs preserved their
original shapes, whereas a few NPs fused, stuck together and loosened their original shapes. The agglomeration obtained at 800 °C
(Fig. 9a) was squeezed, sticky and remained on the SiC fiber with
holes at its surface. The effects of microexplosion on the evaporation residue were clearly noticeable here. A closer view of the residue (Fig. 9b) shows that its surface contained many small pores. A
close up of the inside of a hole (Fig. 9c) shows the presence of well
separated NPs. These NPs provided HNSs and produced enough vapors which on rupturing resulted in holes in the droplet surface.
Magnified views (Fig. 9d) of the residue show that the Al NPs were
densely packed; some of them fused a little and loosened their original spherical shapes but they did not form larger agglomerations.
Due to maintaining their smaller size, pores were also present.
With high loadings of NPs to kerosene, large and compact residue
was formed with more perforations at the droplet’s surface due to
microexplosions. However, evaporation rates for the 2.5% NP sus-
Fig. 9. SEM micrographs of the evaporation residue obtained at 800 °C for 5.0% Al NPs with 2.5% OA (a) overall view, (b) close view of lump, (c) close view of a hole at surface
and (d) magnified view of surface, magnification = 30,000.
42
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
pension droplets were higher when compared to those of the 5.0%
NP suspension droplets due to melting of the Al NPs present at the
droplet’s surface. In both cases the overall evaporation rate was
substantially higher than the pure kerosene droplets’ evaporation
rate at 800 °C.
EDX was conducted to analyze the composition of these residues. The evaporative residues were mostly composed of Al and
O, with small amounts of C. The O content was due to the oxidation
of the Al NPs exposed to air after the vaporization and during the
preparation of the samples for FESEM. The O contents were
5.38% and 20.7% (by weight) in the residues of the 2.5% NP suspension, evaporated at 600 and 800 °C, respectively as shown in
Fig. 10. Similarly, Fig. 11 shows the O contents obtained from the
5.0% NP suspension were 4.51% and 16.6% (by weight) in the residues evaporated at 600 and 800 °C, respectively. This indicates
high oxidation of the Al NPs to Al2O3. All of the residues were
cooled down to room temperature in the presence of nitrogen before being taken out into air; therefore, the high oxygen content
was due to the presence of elemental Al at their surfaces, which
was oxidized. The EDX results imply that the residue obtained at
T=600oC
o
T=800 C
Al
O
C
100
200
800 °C contained high amounts of elemental Al at its surface,
which was thought to be the melted Al NPs or the Al NPs exposed
through perforations in the droplet’s surface.
3.3.2. Evaporation rate of n-Al/kerosene droplets
The evaporation rates of the n-Al/kerosene droplets containing
dense concentrations of Al NPs were calculated from their evaporation history curves through a least squares fitting in the linear
evaporation stage and by a two point fit in the microexplosion part.
For the microexplosion stage, the use of the least squares method
is not justified due to the droplet expansion and contraction.
Therefore the rate of diminishment was simply estimated by measuring the slope between the two end points [34,35]. Fig. 12 shows
a sample least squares fit and a two point estimation for n-Al/kerosene droplets containing 5.0% Al NPs.
The evaporation rates of these two stages were obtained using
the above methodology and are plotted in Fig. 13a and b for comparison with the evaporation rates of pure kerosene droplets.
Fig. 13a shows that at low temperatures (400–600 °C), the linear
evaporation stage is dominant in the droplet evaporation lifetime,
therefore their evaporation rates are close to the evaporation rates
of pure kerosene droplets at these temperatures. Whereas at high
temperatures, this stage comprises a small portion of the droplet
lifetime and shows lower evaporation rates than the evaporation
rates of pure kerosene droplets.
In contrast to the second stage, the third stage (the microexplosion stage) comprises less of the evaporating droplets’ lifetime at
low temperatures due to the low intensity and tendency for microexplosions at these temperatures. Even for the 2.5% Al NP suspension, no microexplosion was observed at these temperatures.
However at high temperatures, most of the droplet evaporation
lifetime consists of microexplosions with a greater intensity so that
the diminishment rate of this stage at these temperatures was considerably higher than with the evaporation rates of pure kerosene
droplets as shown in Fig. 13b.
As discussed above, at low temperatures (400–500 °C) the second stage evaporation rates represented the evaporation rates of nAl/kerosene droplets whereas at high temperatures (600–800 °C) it
was the third stage evaporation rates. Therefore, these were plot-
Energy (eV)
Fig. 10. EDX spectrum of the evaporation residues (agglomerates) obtained at 600
and 800 °C for 2.5% Al NPs in kerosene coated with 1.25% OA.
Fuel: Kerosene+2.5% OA+5.0% Al NPs
P= 0.1 MPa
T=400 oC
T=500 oC
2nd stage
o
T=600
C
Linear evaporation
o
Least Square Fit
T=700 C
o
T=800 C
1
o
T=600 C
o
T=800 C
d2/d02
Al
3rd stage
Microexplosions
Two Point Fit
0.5
O
0
C
0
1
2
3
4
2
0
100
200
Energy (eV)
Fig. 11. EDX spectrum of the evaporation residues (agglomerates) obtained at 600
and 800 °C for 5.0% Al NPs in kerosene coated with 2.5% OA.
5
6
7
2
(s/mm )
Fig. 12. Normalized temporal variations of the diameter squared of evaporating
stabilized kerosene droplets containing 5.0% Al NPs with each least-squares fit to
the linear evaporation part (second stage) and two-point estimation to the
microexplosion part (third stage) at various ambient temperatures (400–800 °C)
and constant pressure of 0.1 MPa.
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
1.4
P=0.1MPa
Kerosene
Kerosene+1.25% OA+2.5% Al NPs
Kerosene+2.50% OA+5.0% Al NPs
Kerosene+3.50% OA+7.0% Al NPs
1.2
(a)
Cv (mm2/s)
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
Temperature (oC)
1.4
P=0.1MPa
Kerosene
Kerosene+1.25% OA+2.5% Al NPs
Kerosene+2.50% OA+5.0% Al NPs
Kerosene+3.50% OA+7.0% Al NPs
1.2
(b)
Cv (mm2/s)
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
Temperature (oC)
1.4
P=0.1MPa
Kerosene
Kerosene+1.25% OA+2.5% Al NPs
Kerosene+2.50% OA+5.0% Al NPs
Kerosene+3.50% OA+7.0% Al NPs
1.2
(c)
Cv (mm2/s)
1.0
0.8
0.6
0.4
Linear evaporation
stage
0.2
400
500
ted together in Fig. 13c. This figure shows that the trend of the
evaporation rate curves for the n-Al/kerosene droplets containing
dense concentrations of Al NPs is similar to those of pure kerosene,
i.e., the evaporation rate increased monotonically as the temperature increased from 400 to 800 °C. However, the trend for the
increasing evaporation rate of pure kerosene was represented by
a power-law fit, whereas the evaporation rate of the kerosene
based nanofluid fuels increased exponentially with the temperature. This figure also clearly shows that the addition of dense concentrations of Al NPs to the kerosene enhanced the evaporation
rates substantially at high temperatures (600–800 °C) through intense microexplosions in the nanofluid fuel droplets. However at
low temperatures (400–500 °C), evaporation rates were slightly
enhanced with 2.5%, remained the same with 5.0% and slightly reduced with 7.0% Al NP suspension droplets. The reduction in the
evaporation rates of the n-Al/kerosene droplets with 7.0% Al NPs
from 400 to 500 °C was due to the delayed onset and lower intensity of the microexplosions at these temperatures.
3.3.3. Effect of Al NP concentration on the evaporation rate
Fig. 13c indicates that the evaporation rates of the n-Al/kerosene droplets containing dense concentrations of Al NPs were also
affected by the concentration of NPs. As the NP concentration increased from 2.5% to 7.0%, the evaporation rates obtained at low
ambient temperatures (400–500 °C) went from being slightly higher to being slightly lower than the evaporation rates of pure kerosene droplets. This is because an increase in the concentration of
NPs in a droplet also increased their agglomeration in the final portion of its lifetime.
At relatively high ambient temperatures (700 and 800 °C), the
evaporation rates were substantially increased with the NP concentration. The maximum increase was obtained with the 2.5% Al
NP suspension. A further increase in the NP concentration decreased the enhancement in the evaporation rates but kept them
higher than the evaporation rates of pure kerosene droplets at
these temperatures. The enhancement in the evaporation rates
was due to microexplosions observed in the n-Al/kerosene droplets
at these temperatures and it is believed that these microexplosions
occurred due to the presence of NPs in the droplets. In relatively
higher NP suspension density (5.0% and 7.0%), the NPs accumulated at the droplet surface and formed a shell which controls
the droplet’s surface temperature. Due to this shell, microexplosions started earlier in the droplet lifetime and occurred several
times with an expansion and subsequent contraction each time.
In addition, due to the presence of this shell, the droplet takes a
longer time to shatter into smaller droplets and particles. Consequently, a low enhancement in the droplet diminishment rates
was obtained for high NP suspension droplets. Whereas, for low
(2.5%) NP loading rates, such a shell was not formed and the droplet easily ruptured into smaller sized droplets which resulted in
higher evaporation rates.
4. Conclusions
Microexplosion stage
0.0
43
600
700
800
Temperature (oC)
Fig. 13. A comparison of the evaporation rates of kerosene-based nanofluid fuel
droplets containing dense concentrations (2.5%, 5.0% and 7.0%) of Al NPs with pure
kerosene droplets under different ambient temperatures; (a) evaporation rate
constants of linear evaporation part (second stage), (b) evaporation rate constants
of microexplosion part (third stage) and (c) true representative evaporation rate
constants.
The evaporation characteristics of kerosene based nanofluid
droplets with dense concentrations of Al NPs (2.5%, 5.0%, and
7.0%) were studied at various elevated temperatures (400–
800 °C) under normal gravity. The main conclusions of this experimental study are summarized as follows.
1. With high loadings of Al NPs, the kerosene based nanofluid
droplet vaporized in a different way as compared to the pure
kerosene droplet and exhibited an additional stage known as
‘microexplosion’. The classical d2-law was not valid during this
stage for kerosene based nanofluid fuel droplets.
44
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
2. The phenomenon of microexplosion was observed at all tested
temperatures (400–800 °C) in kerosene based nanofluid fuel
droplets containing dense concentrations of Al NPs. The absence
of microexplosions in the pure and stabilized kerosene droplets
indicates that this phenomenon was based solely on the presence of the Al NPs.
3. The microexplosions occurred earlier in the droplet’s lifetime
and with a much greater intensity with either an increase in
the ambient temperature or an increase in the NP loading rate
at the same temperature. However, the effect of an increase in
the ambient temperature on the intensity and onset of microexplosions was substantially higher than the NP loading rate.
4. The ambient temperature significantly affected the evaporation
rates of the kerosene based nanofluid fuel droplets. Regardless
of the Al NPs concentration, the intense microexplosions at
700 and 800 °C led to a substantial enhancement in the evaporation rate of kerosene droplets. However, at low temperatures
(400–500 °C), due to the delayed onset and lower intensity of
the microexplosions, the evaporation rate of the nanofluid
droplets was not significantly affected and remained similar
to that of pure kerosene droplets.
5. The evaporation rate of nanofluid fuel droplets was also affected
by the concentration of the NPs. The evaporation rate enhancement increased with the NP concentration up to 2.5%, after
which it decreased. The maximum increase was obtained at
800 °C with the 2.5% NP suspension, where the evaporation rate
was increased by 48.7%.
Acknowledgment
This work was supported by the Midcareer Researcher Program
through the NRF grant funded by MEST (2010-0000353).
Appendix A. Supplementary material
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.expthermflusci.
2013.11.006.
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