Combustion and Flame 160 (2013) 170–183
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Combustion and Flame
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e
Evaporation characteristics of heptane droplets with the addition
of aluminum nanoparticles at elevated temperatures
Irfan Javed ⇑, Seung Wook Baek, Khalid Waheed
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:
Received 11 June 2012
Received in revised form 31 August 2012
Accepted 7 September 2012
Available online 15 October 2012
Keywords:
Droplet vaporization
High-energy-density fuels
Nanofluid fuels
Nanoparticles
Aluminum
Heptane
a b s t r a c t
The evaporation characteristics of n-heptane droplets with varying concentrations of aluminum (Al)
nanoparticles (NPs) hanging at a silicon carbide fiber were studied experimentally at different environmental temperatures (100–600 °C) under normal gravity. The evaporation of pure and stabilized heptane
droplets has been also examined for comparison. The characteristics of the shell formation due to evaporation of the NPs suspensions and its effects on evaporation rate were also investigated. The results
show that the evaporation of suspended heptane droplets containing Al NPs follows the classical
d2-law at all temperatures. The phenomenon of bubble formation in stabilized heptane droplets is
reduced with the addition of Al NPs. For all Al NPs suspensions; regardless of their concentrations, the
evaporation rate obtained was lower than pure heptane droplets from 100 to 300 °C, but it monotonically
increased and became higher than the evaporation rate of pure heptane droplets above 400 °C. However
for 2.5% (by weight) Al NPs suspension, the increasing trend in evaporation rate is exponential above
400 °C. At relatively low temperatures the formation of large agglomerates results in a compact shell
development which suppresses the evaporation. On the other hand, at high temperatures a highly porous
shell was formed by small agglomerates so that Al NPs lead to evaporation enhancement. Maximum
reduction of 15.5% in the evaporation rate at 200 °C with 5% Al NPs and maximum increase of 50%
in the evaporation rate at 600 °C with 2.5% Al NPs suspension was observed.
Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction
The design of high-energy-density fuels is an area of significant
interest for high speed propulsion systems. There is a tremendous
need to augment the energy content of conventional and future
synthetic fuels. One possible approach to accomplish this is the
addition of highly exothermic and energetic NPs to liquid fuels.
Energetic NPs are usually metallic with a passivated oxide layer,
offering high reactivity, fast ignition, and high rate of energy
release [1]. The metallic NPs have potential to enhance the volumetric energy density of the liquid fuels, which is an utmost
requirement of high-speed propulsion systems.
Nanofluids are stable suspensions of solid NPs (10–100 nm) in a
base fluid. They used to show different thermo-physical properties
from their base fluids such as thermal conductivity [2], mass
diffusivity [3], surface tension [4], radiative property [5] and nonNewtonian viscosity [6]. The alternative fuel blends containing
energetic NPs are a new class of nanofluids and such nanofluid
fuels have been rarely studied.
⇑ Corresponding author. Fax: +82 42 350 3710.
E-mail address: [email protected] (I. Javed).
Tyagi et al. [7] conducted hot-plate experiments and observed
that with addition of small amounts of Al and Al2O3 NPs, the ignition probability for nanoparticle-laden diesel fuel was significantly
higher than that of pure diesel fuel. Jackson et al. [8] measured
ignition delay time in a shock tube and observed that an addition
of Al NPs could substantially decrease the ignition delay time of
n-dodecane above 1175 K. Using an aerosol shock tube, Allen
et al. [9] found that an addition of 2% (by weight) Al NPs in ethanol
and JP-8 can reduce their ignition delays by 32% and 50%,
respectively. Gan and Qiao [10] studied the effect of nano and micron-sized Al particles on burning characteristics of n-decane and
ethanol fuel droplets. Their results show that for the same particles
and surfactant concentrations, the disruption and microexplosion
behavior of the micron suspension occurred later with much stronger intensity. Gan et al. [11] recently compared the burning behavior of dilute and dense suspensions of boron and iron NPs in
ethanol and n-decane. A simultaneous burning of both the droplet
and the particles was observed for dilute suspensions and in dense
suspensions; it was found that most particles were burned as large
agglomerate after the consumption of liquid fuel.
A basic mechanism in spray combustion is vaporization of a
liquid fuel droplet at high temperature environments, which must
be taken into account in design and optimization of various practical
0010-2180/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.combustflame.2012.09.005
I. Javed et al. / Combustion and Flame 160 (2013) 170–183
combustion systems such as liquid propellant rocket engines, diesel
engines, gas turbines and oil fired furnaces. The liquid fuels are
injected into combustor as spray of droplets; the vaporization and
oxidation characteristics of these droplets demonstrate the combustion performance. In fact, droplet evaporation is an important
process in the combustion of liquid fuels and knowledge of the laws
governing the rate of droplet vaporization, induction times and
combustion is an utmost importance for the design of efficient
combustion systems. Droplet vaporization is also important in
highly complex phenomenon of combustion of nanofluid fuels
which is a multiphase, multicomponent and multiscale process
[11]. However studies on droplet evaporation behavior of nanofluid
fuels are rare.
Chon et al. [12] studied the effect of NPs size on evaporation and
dry-out characteristics of a strongly pinned water droplet on a
heated substrate and identified three periods: liquid dominant
evaporation, dryout progress, and NPs strain. The pattern of formation of NPs strain strongly depends upon the NPs size. Sefiane and
Bennacer [13] investigated the influence of Al NPs on evaporation
kinetics and wetting dynamics of ethanol sessile droplets on rough
heated substrates. A reduction of evaporation rate compared to the
base fuel was found during the pinning phase. Chen et al. [14] studied the effect of three different types of NPs (laponite, Fe2O3 and
Ag) on the evaporation rate of deionized water under natural convection at room temperature. The results show that these
nanofluid droplets evaporate at different rates from the base fluid.
The evaporation rate of various Ag and Fe2O3 nanofluids goes
through a transition from one constant value to another during
the evaporation process. The authors explained the effect of various NPs on evaporation from the perspective of apparent heat of
evaporation. Gan and Qiao [15] recently studied the effect of Al
NPs on evaporation characteristics of ethanol and n-decane fuel
droplets under natural and weak forced convections at temperatures up to 380 K. They observed a deviation from classical
d2-law under these convections at 300 K and 320 K. This is the only
study reported in literature so far regarding the vaporization of
nanofluid fuels, in-spite of their practical importance. To the best
of our knowledge, no report has been available about the evaporation behavior of nanofluid fuels under the elevated temperatures
which is the real environment in the practical combustors. Therefore, an experimental investigation about the evaporation characteristics of nanofluid fuel droplets at elevated temperatures was
highly desirable from practical point of view.
The main purpose of this work is to experimentally determine
the evaporation behavior of a hydrocarbon-based nanofluid fuel
with varying concentrations of Al NPs at elevated temperatures.
The n-heptane is selected as a suitable fuel due to its high purity,
high volatility and abundant availability of experimental data
regarding its evaporation rate at widest ranges of temperature
and pressure. Experiments were performed with an isolated suspended droplet at a fine silicon carbide fiber. The initial diameter
of the droplets was 1.0 ± 0.11 mm. The ambient temperature varied from 100 to 600 °C, higher than the boiling point of heptane
and below the melting point of Al NPs and ambient pressure was
kept constant at 0.1 MPa. High temperature environment has been
provided by a falling electric furnace. The evaporation process was
recorded by a high-speed charge-coupled device (CCD) camera.
Droplet evaporation histories were obtained from the measured
temporal variations of droplet diameter to calculate evaporation
rates. A brief description of materials and characterization methods is presented in the next section, after which the nanofluid fuel
preparation and experimental apparatus are discussed. Data reduction/analysis methods and a discussion about sources of error are
presented in the following sub-section. In the section of results
and discussions, the general evaporation behavior of pure fuel
droplets, the effects of surfactant at elevated temperature on
171
evaporation process, and a comprehensive discussion about the effects of NPs addition on evaporation at high temperature environments are presented.
2. Experimental methods
2.1. Materials
Al NPs (99.9%, metal basis, 70 nm) were purchased from US Research Nanomaterials (Houston, Texas). Heptane (99% pure) was
obtained from Junsei Chemical Co., (Japan) and Sorbitan Trioleate
(C60H108O8, better known as Span 85) was purchased from Sigma–Aldrich. Silicon carbide (SiC) fiber (100 lm diameter) was obtained from Goodfellow (England). These materials were used in
their as-received form without further treatment.
2.2. Characterization methods
The morphology of Al NPs was studied using high resolution
transmission electron microscope (HRTEM, F30, FEI Company
Eindhoven, Netherlands). The residues were examined by a field
emission scanning electron microscope (FESEM, Nova 230, FEI
Company Eindhoven, Netherlands). Quantitative analysis was carried out with energy-dispersive X-ray spectroscopy (EDX) attached
with the FESEM. Ultrasonic disrupter (Fisher Scientific sonic dismembrator, model 505, Pittsburg, PA) was used to disperse Al
NPs in the base fuel homogeneously.
Figure 1a is a TEM image of Al NPs which clearly shows that NPs
are spherical in shape with smooth surface. Most of the NPs are well
separated from each other. This TEM image has been analyzed using
ImageJ software and the obtained plot is also shown in Fig. 1a. The
plot shows that NPs vary in diameter from 30 to 250 nm with
average diameter of most probable particles of 70 nm. High
resolution TEM image of a single NP is shown in Fig. 1b. An oxide
layer of thicknesses 2–3 nm can be clearly seen in the image. The
active Al contents in the 70 nm sample were estimated to be
65–76%.
2.3. Nanofluid fuel preparation
A liquid hydrocarbon fuel, n-heptane, was considered as the
base fuel with normal boiling temperature of 98 °C, critical temperature of 276 °C and critical pressure 2.74 MPa. Al NPs were used
as energetic material additives. Agglomeration and clusters formation of NPs, due to their high surface energy, in a base fuel is the
major drawback which limits the application of nanofluids. However, homogenous dispersion of NPs in the base fuel can be
obtained by using ultrasonic dispersion and addition of some
appropriate surfactants [16].
An ultrasonic disrupter generates alternating high and low
pressure cycles, applies mechanical stress to the attracting forces
between the individual NPs and thus breaks down the agglomerates and suppresses forming clusters of NPs. Addition of surfactant
can improve the stability of NPs in suspensions by changing the
hydrophobic surfaces of NPs to hydrophilic and vice versa which
helps to overcome the van der Waals force of attraction between
the NPs and thus reduces the agglomeration [17]. In addition the
surfactants can also significantly affect the evaporation behavior
of the fuel suspension, which will be discussed later.
Al NPs were stirred and mixed with base fuels by hand and an
ultrasonic disruptor was used to disperse NPs homogeneously.
The ultrasonication was performed in an ice bath to quickly dissipate heat from the system to avoid agglomeration of NPs. The
ultrasonic disruptor was turned on for 5 min which generates
4 s-long pulses 4 s apart and ultrasonic vibrations was set at 40%
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I. Javed et al. / Combustion and Flame 160 (2013) 170–183
2.4. Experimental apparatus
The experimental apparatus used in this study has already been
used for the vaporization, ignition and combustion studies of single, binary and multicomponent fuels at elevated pressures and
temperatures without NPs (0%) [20–22]. A schematic view of the
experimental apparatus is presented in Fig. 2. In this section,
experimental procedures and facilities are described and further
detailed descriptions of the experimental setup can be found in
Ghassemi et al. [20,21]. First, the nanofluid fuel is filled in plunger
micro-pump (17) as shown in Fig. 2. Then electric furnace (4) is
lifted up away from the droplet suspension system and its inner
temperature is maintained to an intended value using temperature
controller (7) and thermocouple (6). At the same time, the pressure
vessel (1) (height = 0.8 m, inner diameter = 0.15 m) is purged by
pressurized dry nitrogen (9) at room temperature, which refreshes
the ambience of droplet. Once the control of droplet ambient temperature is completed, then single droplet of nanofluid fuel was
suspended on SiC fiber (12) using a droplet maker (16) which consists of a hypodermic needle, connecting tube, and a lever. The
lever can carry the needle very close to the suspended fiber by vertical displacement and then, the nanofluid fuel is transferred to the
fiber by turning the lever, thereby generating a droplet (13). For
continuous monitoring of droplet ambient temperature, a K-type
thermocouple (6) was placed about 30 mm beside suspended
droplet. Next electric furnace (4) brings to the position as shown
in dotted line in Fig. 2. Then, this leads the suspended nanofluid
fuel droplet to be exposed to high temperature, which eventually
Fig. 1. TEM micrographs of (a) Al NPs and associated particle size histogram and (b)
the passivated oxide layer of Al2O3 of thickness about 2–3 nm.
of amplitude. After ultrasonication, the NPs suspension quality was
evaluated by observing their sedimentation and counting their settling time, i.e., time taken by NPs to completely settle down at the
bottom of a test tube. We performed a series of experiments for
selecting an appropriate surfactant and optimizing its concentration to obtain stable nanofluid fuel. Sorbitan Trioleate (C60H108O8,
Span 85) was found to be the suitable surfactant. It has been used
in heptane and metallic particles slurries [18,19]. The concentration of Span 85 was varied from 0.5% to 5% for 5% Al NPs. Here
and below, mass percent values are used. The surfactant was
mixed with the liquid fuel first, and then NPs were added,
subsequently the samples were sonicated for 5 min each.
Maximum 3 h stability of the nanofluid fuel was observed with
5% Span 85 for 5% Al NPs. With this appropriate concentration of
surfactant, nanofluid fuels are prepared by suspending different
concentrations (0.5%, 2.5% and 5%) of Al NPs in heptane. For low
NPs concentrations, the surfactant concentration is proportionately lower.
Fig. 2. A sketch of experimental setup. (1) Pressure vessel, (2) guide bar, (3) furnace
entrance, (4) electric furnace, (5) quartz glass window on furnace, (6) thermocouple, (7) temperature controller, (8) lever, (9) nitrogen vessel, (10) quartz glass
window on pressure vessel, (11) LED backlight, (12) silicon carbide fiber, (13)
droplet, (14) shock absorber, (15) heating wire, (16) droplet maker, (17) plunger
micro pump, (18) CCD camera.
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I. Javed et al. / Combustion and Flame 160 (2013) 170–183
Temporal variations of droplet diameter during evaporation
process are recorded on a computer. Comparison of evaporation
rates obtained on several recording speeds did not show a significant difference for 60, 100, and 200 frames per second. In the cases
with low evaporation rate 100 frames per second is selected for
image recording speed whereas in the cases of high evaporation
rate 200 frames per second has been used. For each test, about
3000–5000 images were recorded to achieve a satisfactory time
resolution. In all tests, microlens and the camera were placed at
a fixed distance (15 cm) from the droplet, and the magnification
was kept constant. For each experimental condition, tests were
conducted at least three times to ensure reproducibility and consistency and the most frequent values are chosen to display.
To extract droplet diameter from the captured image, a flexible
image-processing code was developed using Matlab. A needle with
a known diameter is used as reference scale. In this code, a threshold value for pixel gray level was carefully set to count the pixels in
the droplet zone. First, the number of pixels corresponding to the
diameter of reference needle is calculated. Then the droplet area
was calculated from the number of pixels. Finally, the area of a
concentric circle having the same number of pixels is calculated,
which in turns gives droplet diameter from the law of proportion
with reference needle. This code is executed iteratively for each
image file, which yields a temporal variation of droplet diameter
during evaporation. The well-known d2-law says that d2 decreases
2
2
linearly with time as, d ¼ d0 C v t where d0 is the initial droplet
diameter and Cv is the evaporation rate (evaporation coefficient)
and is dependent on the thermo-physical properties of the liquid
itself and the surroundings [24,25]. The evaporation rate can be
3. Results and discussions
3.1. Pure fuel droplets evaporation
The evaporation rate of pure heptane fuel droplets was measured first as a baseline to understand the effect of addition of various concentrations of surfactants and NPs on droplet evaporation
behavior. Figure 3 shows variations of normalized squared diame2
2
2
ter (d =do ) with normalized time (t=do ) for different environment
temperatures and at a constant pressure of 0.1 MPa for heptane
droplets. The droplet diameter d and time t was normalized by
the original droplet diameter do, as suggested by the classical theory of droplet evaporation [24]. Each evaporation history follows
the same general behavior. After a finite heating up period, the
variation of square of droplet diameter becomes approximately linear with time while keeping d2-law at the last stage of evaporation.
A little deviation from d2-law in heptane evaporation has been
detected at low temperature (100 °C) at the beginning but later it
follows the d2-law. This is due to the fact that temperature within
the droplet doesn’t remain uniform when the ambient temperature
is not too high compared to the fuel boiling temperature [19]. As
depicted in Fig. 3, evaporation at different temperatures shows
different droplet lifetime. Increase in the ambient temperature decreases the total lifetime of the heptane droplets.
The evaporation rates extracted from the linear part of the heptane evaporation history from Fig. 3 is plotted in Fig. 4. The effects
of ambient temperature on the evaporation rates of heptane have
been investigated at constant environmental pressure of 0.1 MPa.
It is clear from the graph that as the ambient temperature increases; the evaporation rate also increases monotonically as
1.2
Fuel: n-Heptane
P= 0.1MPa
T=100
T=200
T=300
T=400
T=500
T=600
1
0.8
2
2.5. Data reduction/analysis
expressed as the time derivative of droplet squared diameter, Cv =
d(d2)/dt. This evaporation coefficient can be extracted from the
linear part of the evaporation history curve. Thus the droplet
evaporation rate is obtained from the temporal history of squared
droplet diameter by measuring the slope of its linear regression
part. Note that the shape of suspended droplet is not an exactly
concentric circle due to both gravitation and droplet–fiber interaction. This eccentricity increases as initial droplet size decreases,
which may lead to some measurement errors. The uncertainties
of the evaporation rate measurements were estimated within ±3%.
d /d02
yields its evaporation. The residue remains on the SiC fiber (12)
after evaporation of nanofluid fuel droplet. It was removed by
using a heating wire (15) which is made of nickel and chromium
alloy with a resistance of about 4.5 X. Quartz glass window (5) enables us to observe the evaporation process of droplet using a highspeed CCD camera (18) and its pictured images are recorded on a
computer. A LED light source (11) was placed on the opposite side
of the camera to backlight the droplet. Post-processing procedures
to get quantitative data from these sequential pictures are explained in the next section. It should be noted that there are several
error sources in measuring the evaporation rate of nanofluid fuel
droplets under a constant temperature condition. Regardless of
these errors, a certain degree of measurement accuracy may be
conserved in this study. When the electric furnace is brought
down, the temperature rises from the room temperature to the
98% of the set value in less than 50 ms for the low ambient
pressure (less than 1.0 MPa) tests [22]. By virtue of self-controlling
temperature system as well as rapid increase in temperature using
electrical heating element, it is possible to maintain the ambient
temperature of droplet to a nearly constant value without significant variation. In addition, a thin (100 lm) SiC fiber is used for
droplet suspension in order to minimize the heat transfer between
droplet and fiber. The heat loss from the fiber can be neglected during most of the droplet life time for fiber diameters less than
100 lm [23]. Radiation heat transfer from heating element coils
placed at furnace inner walls to the droplet is also minimized using
radiative shields equipped around the coils. However, an amount
of fresh nitrogen at room temperature inevitably flows into hightemperature furnace during its fall. As a result, furnace inner temperature fluctuates a little; while nanofluid fuel droplet is evaporated, especially in early evaporation period. Note that this
fluctuation bandwidth becomes wider as the initial temperature
of furnace increases due to a larger temperature difference between furnace inside and fresh nitrogen.
o
C
C
C
o
C
o
C
o
C
o
o
0.6
0.4
0.2
0
0
5
10
15
20
25
t/d 02 ,s/mm 2
Fig. 3. Variations of normalized diameter squared with the normalized time for
different ambient temperatures at 0.1 MPa for pure heptane droplets.
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I. Javed et al. / Combustion and Flame 160 (2013) 170–183
reported in our previous work [20]. A simple comparison of heptane evaporation rates between the previous results of our group
[20] and current study at 0.1 MPa pressure has been made
(Fig. 4) to show the consistency of the experimental apparatus
and minimum effect of human error. The evaporation rates match
well each other except for high temperature region in which some
difference was observed.
(a)
3.2. Evaporation of fuel droplets with the addition of surfactant
To understand the evaporation behavior of nanofluid fuel
droplets at elevated temperatures, it is necessary to measure the
evaporation rates of fuel droplets with the addition of various concentrations of surfactant at the same temperatures. These evaporation rates provide another baseline to distinguish the effect of NPs
addition on droplet evaporation behavior.
Figure 5 displays normalized temporal histories of the diameter
squared of heptane droplets with 1%, 2.5% and 5% of surfactant at
various ambient temperatures (100–600 °C). It is important to note
that to minimize the initial droplet size effect on its evaporation,
the droplets having similar initial diameters (from 0.989 to
1.108 mm) were selected from a number of experimental data.
The general evaporation behavior of the stabilized heptane droplets is similar to that for a binary component droplet. Like binary
component droplets [20], the stabilized heptane droplets show
three-staged evaporation; a finite heating up period, d2-law evaporation of high volatile component (heptane) and in last thermal
decomposition of low volatile component (surfactant) as depicted
in Fig. 5. In the third stage at high temperatures, bubble formation
within the droplet is seen to take place due to entrapment of high
volatile component, which leads to droplet distortion and fragmentation. As is clearly shown in the figure, the evaporation behaviors of
stabilized heptane droplets are evidently dependent on their ambient temperatures. Based on this observation, the evaporation characteristics of stabilized heptane droplets can be classified into the
following two different cases according to ambient temperatures.
(b)
3.2.1. For ambient temperatures of 100, 200 and 300 °C
Figure 5a presents the droplet lifetime history of stabilized
heptane droplets evaporated at 100, 200 and 300 °C. Here, each
Fig. 5. Droplet diameter squared histories of stabilized heptane for evaporation at
different temperatures and surfactant concentrations: (a) at 100, 200 and 300 °C,
(b) at 400, 500 and 600 °C.
1.2
1.0
0.8
Fuel: n-Heptane
P=0.1MPa
Ghassemi et al. [20]
Present Study
0.6
Cv (mm2/s)
0.4
0.2
100
200
300
400
500
600
Temperature (oC)
Fig. 4. Comparison of the evaporation rate of n-heptane between present study and
previous study by Ghassemi et al. [20] at different ambient temperatures and
constant pressure of 0.1 MPa.
evaporation history of normalized droplet diameter squared is displayed up to the point, after which almost no more reduction in
overall droplet size is observed.
The boiling temperature of heptane is known to be about 98 °C
and by using thermogravimetric analysis (TGA) the temperature of
492 K (219 °C) corresponding to a 2% weight loss of surfactant (Sorbitan Trioleate, Span 85) is selected as the reference temperature
for surfactant pyrolysis [19]. As shown in the figure, the evaporation history at the above mentioned temperatures can be divided
into two distinguished stages with an obvious difference in their
diminishing rates. During the first stage of the evaporation period,
the heptane component from stabilized heptane droplet evaporates first. In the second stage the droplet becomes rich with surfactant and the surfactant near the droplet surface starts to
thermally decompose and evaporate. It can be also seen from the
figure that as the concentration of surfactant increases the size of
2
2
droplet (d =do ) obtained at the end of first stage also increases at
all temperatures. This fact illustrates that the second stage signifies
the thermal decomposition and vaporization of surfactant. The
evaporation rates of stabilized heptane droplets are calculated
using the least-squares regression method with the data
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I. Javed et al. / Combustion and Flame 160 (2013) 170–183
t = 0.00 s
t = 2.50 s
t = 2.505 s
t = 2.51 s
t = 2.59 s
t = 2.74 s
t = 3.38 s
(a)
t = 0.00 s
t = 1.815 s
t = 1.835 s
t = 1.82 s
t = 1.825 s
t = 1.855 s
t = 1.83 s
t = 1.86 s
t = 5.495 s
(b)
t = 0.00 s
t = 1.235 s
t = 1.335 s
t = 1.425 s
t = 1.44 s
t = 1.465 s
t = 1.47 s
t = 3.19 s
(c)
t = 0.00 s
t = 1.815 s
t = 1.84 s
t = 1.845 s
t = 1.85 s
t = 1.855 s t = 1.86 s t = 4.725 s
(d)
Fig. 6. Sequential photographs of stabilized heptane droplet vaporization under elevated temperatures. (a) 2.5% Span 85 at 500 °C (b) 5% Span 85 at 500 °C, (c) 1% Span 85 at
600 °C, (d) 2.5% Span 85 at 600 °C, (e) 5% Span 85 at 600 °C. Camera speed: 200 fps.
corresponding to the linear part of evaporation history except for
both initial heat-up and final solidification periods.
3.2.2. For ambient temperatures of 400, 500 and 600 °C
Figure 5b shows normalized evaporation histories of stabilized
heptane droplets at ambient temperatures of 400, 500, and
600 °C. The most prominent characteristic of evaporating stabilized heptane droplets at these ambient temperatures is the onset
of bubble formation within the droplet at the beginning of second
stage. The onset of micro-explosion is also observed as the concentration of surfactant increases. In this figure, after the linear
variation of droplet diameter squared during heptane evaporation
period, re-expansion of the droplet has been observed with a fluctuating behavior at surfactant concentrations of 2.5% and 5%. The
reason for this fluctuation is that, as more volatile component
trapped inside stabilized heptane droplet due to homogeneous
nucleation and diffusional resistance is heated up, the droplet
internal pressure builds up. Here, the rates of both nucleation
and diffusion compete against each other so that they influence
droplet expansion characteristics. A comparison of these evaporation histories reveals that more intense fluctuations occur at high
ambient temperature and high surfactant concentration. A fast
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I. Javed et al. / Combustion and Flame 160 (2013) 170–183
t = 0.00 s
t = 1.24 s
t = 1.245 s
t = 1.255 s
t = 1.28 s
t = 1.26 s
t = 1.285 s
t = 1.295 s
t = 1.25 s
t = 1.265 s
t = 4.57 s
t = 1.27 s
t = 4.995 s
(e)
Fig. 6. (continued)
internal pressure build-up caused by rapid nucleation at high temperature results in more aggressive evaporation behavior such as
big expansion, severe bubble formation, and more disruptive fragmentation. To obtain the evaporation rate for the first stage, the
least-squares method was also applied to the linear part of experimental data. However, for the second evaporation stage, the use of
the least-squares method is not justified due to droplet expansion
and fluctuation so that its diminishment rate is simply estimated
using the slope determined by two end points.
Figure 6 shows some sequential photographs of evaporation
behavior of stabilized heptane droplets with 1.0%, 2.5% and 5% surfactant at 500 and 600 °C. Figure 6a and b shows the snapshots of
the stabilized heptane droplets with surfactant concentration of
2.5% and 5% at 500 °C respectively. In these figures, the second
snapshots show that after the d2-law evaporation of heptane, i.e.,
at the end of first stage, the droplet mainly comprises surfactant.
In next (third) snapshot expansion of the droplet and bubble
formation is observed due to entrapment of heptane. In Fig. 6a only
droplet distortion is observed, whereas in Fig. 6b due to increase in
surfactant concentration the droplet distortion as well as fragmentation is detected. The droplet rupture is taken place at 1.83 and
1.86 s. Figure 6c–e shows the sequential photographs of the stabilized heptane droplets at 600 °C with 1.0%, 2.5% and 5% surfactants
respectively. Figure 6c shows that the droplet becomes enriched
with surfactant at the second snapshot as the heptane evaporates
and the droplets diameter is reduced. Afterwards the droplet is distorted and moved around the SiC fiber below t = 1.465 s. However,
at time t = 1.465 s, the droplet expands and in subsequent snapshots it then pops up.
In Fig. 6d and e, the microexplosion phenomenon is clearly observed besides the droplet distortion and fragmentation. These
kind of irregular behaviors are typically observed in the evaporation process of the multicomponent droplets [26]. The bubble formation, distortion and partial rupture of droplet take place with
I. Javed et al. / Combustion and Flame 160 (2013) 170–183
different strengths at different ambient temperatures and surfactant concentrations. When the temperature increases, the strength
also increases. In Fig. 6d the droplet is fragmented at t = 1.85 s.
Figure 6e shows that at high temperature and with high surfactant
concentration, the droplet distortion is so strong that bubble formation and its following rupture take place several times. Before
each rupture the droplet diameter increases due to bubble expansion. The partial ruptures of droplet have taken place at 1.25, 1.26
and 1.285 s. The microexplosion phenomenon started earlier with
an increase in temperature for same surfactant concentration.
3.2.3. Effect of temperature on evaporation rate of stabilized heptane
droplet
Figure 7 shows the evaporation rates of heptane droplets with
1.0%, 2.5% and 5% surfactant at various ambient temperatures.
Figure 7a shows the evaporation rate of heptane component, while
Fig. 7b represents the evaporation rate of surfactant component in
the stabilized heptane droplets. The evaporation rate of heptane
component in stabilized heptane droplets is lower than the pure
(a) 1.2
heptane at relatively low temperatures (100–300 °C) as shown in
Fig. 7a. This is due to the fact that surfactant in stabilized heptane
droplets usually retards the evaporation rate of heptane fuel alone
[27]. The surfactant concentration significantly affects the evaporation rate of heptane component in stabilized heptane droplets at
above 300 °C. The evaporation rate of heptane component in stabilized heptane droplets with 1.0% and 2.5% surfactant still remain
low or equal to the evaporation rate of pure heptane droplets.
The evaporation rate of heptane containing 5% surfactant is significantly higher than the pure heptane droplets.
Figure 7b shows the evaporation rate of thermal decomposition
of surfactant component at various ambient temperatures. At relatively low temperatures (100–300 °C), the evaporation rate is very
low (in 103 mm2/s) regardless of surfactant concentration. It is
attributed to its surfactant pyrolysis temperature of 219 °C which
corresponds to 2% weight loss of surfactant during TGA. At relatively high temperatures (400–600 °C), due to more thermal
decomposition of surfactant and several micro-explosions, the
evaporation rate of surfactant part significantly increases as the
surfactant concentration increases.
(a)
P = 0.1 MPa
Heptane
Heptane+1.0% Span 85
Heptane+2.5% Span 85
Heptane+5.0% Span 85
1.0
177
Cv (mm2/s)
0.8
0.6
0.4
0.2
0.0
100
200
300
400
500
600
Temperature (oC)
(b) 1.2
(b)
P = 0.1 MPa
Heptane
Heptane+1.0% Span 85
Heptane+2.5% Span 85
Heptane+5.0% Span 85
1.0
Cv (mm2/s)
0.8
0.6
0.4
0.2
0.0
100
200
300
400
500
600
Temperature (oC)
Fig. 7. Evaporation rate of the stabilized heptane droplets at different environmental temperatures (a) evaporation rate of heptane evaporation part, (b)
evaporation rate of surfactant thermal decomposition and microexplosion part.
Fig. 8. Temporal variation of diameter squared of heptane droplets with 0.5% Al NPs
at different environmental temperatures (a) at 100, 200 and 300 °C, (b) at 400, 500
and 600 °C.
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I. Javed et al. / Combustion and Flame 160 (2013) 170–183
Figure 7a and b are similar to each other in a way that evaporation rate of each component (heptane and surfactant) of stabilized
heptane droplets has been increased by increasing temperature.
But the effect of increase in concentration of surfactant is different
on evaporation rate of heptane component (Fig. 7a) and surfactant
component (Fig. 7b) in stabilized heptane droplets. A comparison
of Fig. 7a and b also shows that Fig. 7a truly represents the evaporation rates of stabilized heptane droplets (for further details, see
Supplementary material).
3.3. Evaporation of heptane based nanofluid fuel droplets
3.3.1. General evaporation behavior
In this section, the evaporation characteristics of heptane-based
droplets containing Al NPs are to be discussed. Figures 8–10
display normalized temporal histories of the heptane droplet
diameter squared with the addition of 1%, 2.5% and 5% of surfactant
along with 0.5%, 2.5% and 5% of Al NPs respectively at ambient
temperatures ranging from 100 to 600 °C. It is important to note
that, in order to minimize the initial droplet size effect on its
evaporation, the droplets having similar initial diameters (from
0.900 to 1.108 mm) are selected from a number of experimental
data.
In comparison with stabilized heptane droplets, the general
evaporation behavior of the nanofluid fuel droplets containing
0.5% Al NPs is similar to the pure heptane (single component)
droplets irrespective of temperature as shown in Fig. 8. The heptane based nanofluid droplets show two-staged evaporation; a finite heating up period followed by d2-law evaporation. As shown
in Figs. 8a, 9a and 10a, a little deviation from d2-law observed at
low temperature (100 °C) in pure heptane droplet evaporation
has been diminished early by adding Al NPs. It derives from the fact
that due to addition of Al NPs in heptane droplet, the temperature
distribution inside the droplet becomes uniform sooner than pure
heptane droplet. Consequently, addition of Al NPs in heptane droplet results in a slower evaporation. Figure 8b clearly shows that at
high temperature (600 °C), the phenomenon of bubble formation
within the stabilized heptane droplet, has been vanished by adding
Al NPs. Figures 9 and 10 represent similar results as discussed
above except the following discrepancies. In Fig. 9b, at ambient
(a)
(a)
(b)
(b)
Fig. 9. Temporal variation of diameter squared of heptane droplet with 2.5% Al NPs
at different environmental temperatures (a) at 100, 200 and 300 °C, (b) at 400, 500
and 600 °C.
Fig. 10. Temporal variation of diameter squared of heptane droplet with 5% Al NPs
at different environmental temperatures (a) at 100, 200 and 300 °C, (b) at 400, 500
and 600 °C.
179
I. Javed et al. / Combustion and Flame 160 (2013) 170–183
and 5% Al NPs respectively are plotted on the same figure for comparison. It can be seen that the trend of evaporation rate curves of
nanofluid fuel droplets is similar to that for pure heptane, i.e., that
evaporation rate increases monotonically as temperature increases
from 100 °C to 600 °C. These figures also clearly show that addition
of Al NPs in heptane exhibits opposite effects on evaporation
behavior at relatively low and high temperatures. Based on this
observation, the discussion about the evaporation rate of nanofluid
fuel droplets can be classified into following two different cases
according to the variation of ambient temperature.
temperature of 600 °C the heating up period of heptane droplet
with 2.5% Al NPs is significantly higher than that of the pure and
stabilized heptane droplets, since Al NPs are supposed to absorb
heat faster. Figure 10b shows that with high concentration of surfactant (5%), the bubble formation, droplet distortion and fragmentation at relatively high temperatures (500 and 600 °C) do not
vanish even by adding high concentration of Al NPs (5%).
It is observed from Figs. 8–10 that the lifetime of heptane droplets containing Al NPs is found to become longer at relatively low
temperatures (100–300 °C) irrespective of Al NPs concentrations.
This clearly shows that addition of Al NPs reduces the evaporation
rate as compared to pure heptane droplets. At relatively high temperatures (400–600 °C) the droplet lifetime is found to be equal or
shorter than the pure or stabilized heptane droplets, which indicates an increase in evaporation rate with addition of Al NPs as
compared to pure or stabilized heptane droplets.
3.3.2.1. For ambient temperatures of 100, 200 and 300 °C. The evaporation rates of heptane based nanofluid droplets are considerably
lower than the pure and stabilized heptane droplets at relatively
low temperatures (100–300 °C) irrespective of Al NPs concentrations as shown in Fig. 11a–c. The phenomenon of evaporation suppression is well-known and studied in detail by our group [19] and
others [18,27]. Surfactants usually create a thin layer around the
droplets surfaces and the diffusion of fuel component is retarded
by it. The evaporation suppression phenomenon becomes more
severe with the addition of micron size particles in the stabilized
3.3.2. Effect of temperature on evaporation rate of heptane-based
nanofluid fuel droplet
Figure 11a–c shows that the evaporation rates of pure heptane,
stabilized heptane and stabilized heptane droplets with 0.5%, 2.5%
(a) 2.5
2.0
(b) 2.5
2.0
P = 0.1 MPa
Heptane
Heptane+1.0% Span 85
Heptane+1.0% Span 85+0.5% Al NPs
1.5
1.0
P = 0.1 MPa
Heptane
Heptane+2.5% Span 85
Heptane+2.5% Span 85+2.5% Al NPs
1.5
1.0
0.5
Cv (mm2/s)
Cv (mm2/s)
0.5
100
200
300
400
500
600
100
200
Temperature (oC)
300
400
500
600
Temperature (oC)
(c) 2.5
2.0
P = 0.1 MPa
Heptane
Heptane+5.0% Span 85
Heptane+5.0% Span 85+5.0% Al NPs
1.5
1.0
Cv (mm2/s)
0.5
100
200
300
400
500
600
Temperature (oC)
Fig. 11. Comparison of the evaporation rates of heptane-based nanofluid fuel droplets with pure and stabilized heptane droplets under different temperatures; (a) 0.5% Al
NPs, (b) 2.5% Al NPs, (c) 5% Al NPs.
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I. Javed et al. / Combustion and Flame 160 (2013) 170–183
fuel. This is due to the formation of porous shell established near
the droplet surface by accumulation of particles bonded with adsorbed surfactant [19]. Wong et al. [18,27] provides quantitative
measurements about the effect of surfactant on the evaporation
of Al/JP-10 and boron/JP-10 slurry droplets at 425–610 K. They reported that the addition of surfactant and micron size particles into
liquid fuel droplets usually retards its evaporation rate compared
to the pure liquid fuel. Furthermore, they also observed that the
evaporation suppression is enhanced when the particles loading
is increased in the Al/JP-10.
In order to confirm the presence of surfactant monolayer and
shell formation in nanofluid fuel droplets and to investigate its effect on droplet evaporation, the residues obtained at the end of
evaporation were collected. Residues with two different concentrations of Al NPs (2.5% and 5%) were selected at two different temperatures (200 and 600 °C) and studied using FESEM attached
with EDX. The selected values of temperature lie below and above
the pyrolysis temperature of the surfactant (219 °C).
Figures 12 and 13 show the FESEM images of residues after
evaporation at 200 °C for 2.5% and 5% of NPs suspensions respectively. For 2.5% of NPs suspension, the droplet shrank monotonically and elliptical-shaped agglomerate with a visible appearance
of smooth surface was formed on SiC fiber as shown in Fig. 12a.
The magnified view (Fig. 12b) of residue shows that most of the
Al NPs were remained stick to the dried surfactant; however some
pores are also present. The NPs are bonded in cross-linking structure and form large agglomerates of different sizes (in microns)
and shapes. When the concentration of NPs and surfactant increased, large and compact residue was formed as shown in
Fig. 13a. The magnified view of residue for 5% of surfactant and
NPs suspension (Fig. 13b) indicates that the NPs are densely
packed in dried surfactant, leaving few pores in the crust. High
evaporation rates were observed with 2.5% of NPs suspensions as
compared to 5% of NPs suspension because of high porosity of residue. However in either case the overall evaporation rate is lower
than pure heptane evaporation rate at 200 °C.
The composition of the residues obtained after evaporation at
200 °C was analyzed by conducting EDX at several positions of
the agglomerates. The results show that the evaporative residues
are mostly composed of Al and C, with small amounts of O. The
O contents are due to the oxidation of Al NPs present at the surface
of the residues, after vaporization and during preparation of samples for FESEM. The C contents remained are 15.3% and 20.5% in
residues of 2.5% and 5% of NPs suspensions, respectively as shown
in Figs. S2 and S3 (supplementary material). The measured element weight ratio of Al/C is 5.25 for 2.5% and 3.55 for 5% of dried
residue, which proves more carbon weight percentage remained in
agglomerate containing 5% of surfactant and NPs suspension. EDX
results show that the cross-linking structure bonding Al NPs
mainly consists of carbon which is believed to be the dried-out
deposit of the surfactant.
These results imply that similar to micron sized slurry fuel
droplets a shell was formed during the evaporation of nanofluid
fuel droplets as a result of particle aggregation. The presence of
Fig. 12. SEM micrographs of the evaporation residue (an agglomerate) obtained at
200 °C for 2.5% Al NPs with 2.5% surfactant; (a) overall view and (b) Magnified view
of surface, magnification = 10,000.
Fig. 13. SEM micrographs of the evaporation residue (an agglomerate) obtained at
200 °C for 5% Al NPs with 5% surfactant; (a) overall view and (b) magnified view of
surface, magnification = 10,000.
I. Javed et al. / Combustion and Flame 160 (2013) 170–183
surfactant in nanofluid fuels promoted the linking of NPs aggregation during evaporation process to form a more compact and porous shell. The characteristics of the shell formed in nanofluid fuel
droplets evaporation might be different from the shell formed in
slurry fuel droplets evaporation. But the existence of this compact
shell inhibited the diffusion process of the base fuel and thus reduced the evaporation rate at low temperatures. As the concentration of Al NPs increases, more Al NPs are agglomerated to form
bigger, densely packed aggregates. These large aggregates in combination with more adsorbing surfactant molecules form a larger,
stronger and more compact barrier for evaporation while intensifying the evaporation suppression.
3.3.2.2. For ambient temperatures of 400, 500 and 600 °C. It has been
observed that the evaporation rate of stabilized heptane droplets
with Al NPs is found to be equal and higher than that for both pure
heptane and stabilized heptane droplets over 400 °C as shown in
Fig. 11a–c. The evaporation enhancement of Al NPs suspensions
observed at high temperatures has not been detected previously
in micron sized particles slurry fuel droplets evaporation and
combustion.
Figures 14 and 15 represent the FESEM micrographs of residues
obtained after evaporation at 600 °C for 2.5% and 5% of NPs suspensions respectively. Figure 14a shows small, thick but fragile residue
with a size of about 460 lm. A close-up view is shown in Fig. 14b.
It shows that most of the surfactant, which bonds NPs in cross linking structure at low temperature, is decomposed at relatively high
temperatures. Thermogravimetric analysis (TGA) of surfactant
(Span 85) studied by Byun et al. [19] also shows that 50% weight
loss at 400 °C and 95% weight loss at 500 °C and above has taken
Fig. 14. SEM micrographs of the evaporation residue (an agglomerate) obtained at
600 °C for 2.5% Al NPs with 2.5% surfactant; (a) overall view and (b) magnified view
of surface, magnification = 10,000.
181
place due to pyrolysis of surfactant. Due to the decomposition of
surfactant at high temperatures, the agglomerates formed by NPs
are not big in size and also may not be bonded in a compact way
to make a compact shell which used to inhibit the diffusion of base
fuel. As discussed in Fig. 10b, for 5% of NPs suspension at 600 °C, at
the end of heptane evaporation, after a short shrinking the significant droplet swelling occurs which is then followed by a few consecutive strong eruptions leaving a ruptured crust on SiC fiber as
shown in Fig. 15a. The agglomerate is so fragile and ruptured that
it breaks into many pieces during FESEM analysis. A magnified
view is presented in Fig. 15b which reveals that by increasing the
concentration of Al NPs in base fuel, the agglomerates formed by
NPs are more compact and densely packed than the residue obtained with 2.5% of NPs suspension. Formation of these agglomerates inhibits the internal diffusion of liquid fuel from inside toward
droplet surface. This shows that increasing the NPs loading to a
critical value (2.5%) decreases the evaporation rate at high temperature. EDX results (Figs. S4 and S5) indicate that the carbon contents are 2.8% and 3.4% in residues obtained with 2.5% and 5% of
NPs suspensions respectively at 600 °C. A considerable reduction
in carbon contents proves that most of the surfactant is decomposed and evaporated at high temperature. The large spherical
particles (shown in Fig. 14b) are composed of Al2O3 which was
Fig. 15. SEM micrographs of the evaporation residue (an agglomerate) obtained at
600 °C for 5% Al NPs with 5% surfactant; (a) overall view and (b) magnified view of
surface, magnification = 10,000.
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I. Javed et al. / Combustion and Flame 160 (2013) 170–183
formed due to oxidation of Al NPs present at the surface of residue
after vaporization and before conducting FESEM analysis.
These results indicate that the evaporation suppression does
not take place at high temperatures because the shell formed is
highly porous and containing smaller agglomerates (size in hundreds of nanometers) due to the decomposition of surfactant. The
phenomenon of evaporation enhancement at high temperatures
is only due to presence of NPs in base fuel. Possibly at high temperatures due to unavailability of surfactant, the NPs present in the
droplets results in increasing effective thermal diffusivity so that
the heat transfer to liquid fuel would be enhanced in their surroundings. At high temperatures the heated NPs near the droplet
surface may provide multiple nucleation sites, thereby, enhancing
evaporation rate. As the evaporation process continues, more NPs
are concentrated near the droplet surface. The NPs could also enhance the diffusive heat transfer through changing the thermo
physical properties of nanofluid fuel such as enhancing the thermal
conductivity and reducing the surface tension. Further studies
focusing on nanofluid fuel evaporation at high temperatures with
and without surfactant will provide valuable insight into the
mechanism by which NPs affect the evaporation rate.
3.3.3. Effect of nanoparticles concentration
Figure 16 reveals that the evaporation rate of heptane-based
nanofluid droplets is also affected by the concentration of NPs.
As the concentration of NPs increases from 0.5% to 5%, the evaporation rates at relatively low ambient temperatures (100, 200 and
300 °C) are observed to decrease. The maximum decreasing effect
in evaporation rate is obtained with 5% Al NPs in low ambient temperatures. It results from the fact that the evaporation suppression
observed previously at low ambient temperatures is intensified as
the concentration of NPs and surfactant increased. The evaporation
suppression for 2.5% NPs suspension at low temperatures is lower
than that observed with 5% NPs suspension due to higher porosity
of shell formed with lower NPs loading. The maximum reduction of
15.5% in evaporation rate of heptane-based nanofluid droplet is obtained at 200 °C with 5% Al NPs (from 0.11189 to 0.09445 mm2/s).
At relatively high ambient temperatures (500 and 600 °C), with
increase in NPs concentration the evaporation rate is found to be
increased up to 2.5% and then decreased, but higher than the evaporation rate of pure heptane. The maximum increasing effect is
2.5
2.0
1.5
P = 0.1 MPa
Heptane
Heptane+1.0% Span 85+0.5% Al NPs
Heptane+2.5% Span 85+2.5% Al NPs
Heptane+5.0% Span 85+5.0% Al NPs
1.0
obtained with 2.5% Al NPs in high ambient temperatures, where
evaporation rate increases exponentially above 400 °C. This is because when the concentration of NPs and surfactant is increased
beyond a critical value (2.5% in this case); the shell formed is densely packed, thereby, reducing the evaporation enhancement. The
maximum increase in evaporation rate of heptane-based nanofluid
droplets with 2.5% Al NPs is about 50% compared to pure heptane
droplet evaporation rate (with increment from 0.6107 to
0.9184 mm2/s) at 600 °C.
4. Conclusions
The evaporation behavior of heptane-based nanofluid droplets
with varying concentrations of Al NPs (0.5%, 2.5% and 5%) were
studied at different elevated temperatures (100–600 °C) under
normal gravity. The characteristics of shell formation and its effects
on the evaporation rate were evaluated by collecting the Al NPs
suspensions (2.5% and 5%) droplets residues at temperature below
and above the surfactant pyrolysis temperature. The main conclusions of our studies are summarized as follows:
(1) Heptane based nanofluid fuel droplets vaporize in the same
way as liquid fuel droplets and follow classical d2-law at elevated temperatures (100–600 °C).
(2) The phenomenon of bubble formation in stabilized heptane
droplets is reduced with the addition of Al NPs.
(3) The ambient temperature significantly affects the evaporation rates of heptane-based nanofluid fuel droplets. The
added Al NPs exhibit opposite effects at relatively low and
high temperatures. At relatively low temperatures, the Al
NPs in the droplets are bonded by surfactant to form large
agglomerates, thereby, making a compact shell which suppresses the evaporation and thus evaporation rate decreases
below 400 °C. However, at relatively high temperatures due
to decomposition of most of surfactant, the NPs are bonded
to produce small agglomerates while the shell formed
becomes highly porous. These effects are shown to lead to
the evaporation enhancement above 400 °C.
(4) The nanofluid fuel droplets evaporation rate is also affected
by the concentration of NPs. The evaporation suppression at
low temperatures is intensified with an increase in concentration while the evaporation enhancement at high temperatures increases with increase in NPs concentration up to
2.5%, after which it is reduced. The maximum effect is
obtained at 2.5% NPs concentration, in which evaporation
rate is observed to increase exponentially over 400 °C.
Acknowledgments
0.5
Cv (mm2/s)
This work was supported by the Midcareer Researcher Program
through the NRF grant funded by MEST (2010-0000353). Special
Thanks to Professor Sung Oh Cho and Mr. Ghafar Ali for their endless support in sample preparation and analysis of solid residues
remained on SiC fiber.
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.combustflame.
2012.09.005.
100
200
300
400
500
600
Temperature (oC)
Fig. 16. Comparison between the evaporation rates of heptane-based nanofluid fuel
droplets with 0.5%, 2.5% and 5% Al NPs under different ambient temperatures.
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