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Experimental Thermal and Fluid Science 56 (2014) 33–44
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Experimental Thermal and Fluid Science
journal homepage: www.elsevier .com/locate /et fs
Effects of dense concentrations of aluminum nanoparticleson the evaporation behavior of kerosene droplet at elevatedtemperatures: The phenomenon of microexplosion
0894-1777/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.expthermflusci.2013.11.006
⇑ 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.
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 a b s t r a c t
Article history:Available online 19 November 2013
Keywords:Kerosene vaporizationHigh-energy–density fuelsNanofluid fuelsMicroexplosionNanoparticlesElevated temperature
The evaporation behavior of kerosene droplets containing dense concentrations (2.5%, 5.0%, and 7.0% byweight) of aluminum (Al) nanoparticles (NPs) suspended on silicon carbide fiber was studied experimen-tally over a range of ambient temperatures (400–800 �C) under normal gravity. The evaporation charac-teristics of the pure and stabilized kerosene droplets were also examined to provide a comparison. Theresults show that at all of the tested temperatures, the evaporation behavior of suspended kerosene drop-lets containing dense concentrations of Al NPs was different from that of pure kerosene droplets andexhibited three stages of evaporation; an initial heating up stage, d2 -law evaporation and then the mic-roexplosion stage. The phenomenon of microexplosion was not observed during the evaporation of pureor stabilized kerosene droplets at the same temperatures. The microexplosions occurred early in thedroplet’s lifetime and with a much greater intensity, for either an increase in the ambient temperatureor 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 tem-peratures (700–800 �C). The intense microexplosions occurring at these temperatures led to a substantialenhancement in the evaporation rate. However, at lower temperatures (400–500 �C), the delayed onsetwith 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.
� 2013 Elsevier Inc. All rights reserved.
1. Introduction
The specifically tailored high-energy–density fuels are a com-pelling need of the next generation combustion and propulsionsystems. The energy–density of available conventional fuels is amajor limiting factor in the current liquid propulsion systems;therefore, there is an immense necessity of enhancing the energycontent of both traditional and future synthetic fuels. One possibleway to accomplish this is to load a liquid hydrocarbon fuel withhigh-energy–density solid NPs in form of a stable suspension, slur-ry or gel. A stable suspension of solid NPs in a liquid fuel is callednanofluid fuel which is a new class of nanofluids and is fundamen-tally different from slurry fuels. These are suspensions with lowconcentrations (�10 wt.%) of NPs compared to slurry fuels which
contain relatively high solid loadings (40–80 wt.%) of micron sizedparticles and were studied decades ago [1]. Low concentrations ofNPs will not significantly change the physical properties such asthe density, viscosity or boiling point of the base fuel so that cur-rent fuel delivery systems can still be used, with only slight mod-ifications [2].
High loadings of NPs (�10 wt.%) would enable the production ofhigh-energy–density fuels but they can also induce some potentialside effects, as found in slurry fuels, which have limited their prac-tical applications. The major problems associated with the use ofslurry fuels are the stability of the micron-sized particles in the li-quid fuels and particle agglomeration during evaporation and com-bustion which resulted in slow burning rates and incompletecombustion of the agglomerations and consequently reduced thecombustion efficiencies [1]. However, reducing the particle sizefrom micro to nano scale can provide a solution to such problems,for as metallic particles size approaches the nano scale, their ther-mophysical properties often change significantly. Several studieshave described lower melting temperatures [3,4] and a reducedheat of fusion for nano-sized metal particles [5,6]. Also, nano scale
34 I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
energetic materials have a much higher surface to volume ratiocompared to micron-sized particles, which allows more fuel tobe in contact with the oxidizer [7] and consequently results in ahigher combustion efficiency. Furthermore, it is expected thatdue to the addition of nano scale particles, the size of the agglom-erates should be reduced. The agglomerates may also be shatteredby the microexplosions which occur due to the addition of NPs andare previously observed in kerosene based nanofluid fuels at ele-vated temperatures [8].
Tyagi et al. [9] found experimentally that with the addition of0.1% and 0.5% (by volume) of Al and Al2O3 NPs, the hot plate igni-tion probability of nanoparticle laden diesel fuel was significantlyhigher than that of pure diesel fuel. Allen et al. [10] performedexperiments using an aerosol shock tube and observed that theaddition of 2% (by weight) of Al NPs to ethanol and JP-8 reducedtheir ignition delays by 32% and 50%, respectively. Jones et al.[11] determined experimentally the heat of combustion (HoC) ofethanol, containing various volume fractions of nano-aluminum(n-Al) and nano-aluminum oxide (n-Al2O3) particles. Their resultsindicated that 1% and 3% n-Al contents did not show enhancement,but that 5%, 7% and 10% n-Al contents increased the volumetricHoC by 5.82%, 8.65% and 15.31% respectively. Van Devener andAnderson [12] observed that the ignition temperature of JP-10was significantly reduced when using soluble CeO2 NPs as a cata-lyst. Later, their group produced oxide free, fuel soluble and air sta-ble boron NPs that were also coated with the combustion catalystceria through a unique process [13,14]. Gan and Qiao [15] experi-mentally investigated the effects of nano and micro sized Al parti-cles on the burning behavior of ethanol and n-decane fuel droplets.Their results indicated that for the same surfactant and particleconcentrations, the disruption behavior of the micro sized suspen-sions occurred later and with a much greater intensity. Later, Ganet al. [16] investigated the combustion behavior of dilute anddense suspensions of boron and iron NPs in n-decane and ethanol.In dilute suspensions, simultaneous combustion of both the drop-let and the NPs was observed, whereas in dense suspensions, mostparticles were combusted as large agglomerations after the con-sumption of the liquid fuel.
Isolated, single droplet combustion is considered a classicalsubject in combustion, which provides the opportunity to investi-gate the interactions of the physical and chemical processes, in asimplified geometrical configuration. The importance of the drop-let vaporization/combustion phenomena in practical liquid fuelcombustion systems has motivated a large number of researchersin the past. Droplet vaporization and combustion are similar inmany aspects and the knowledge obtained from studying one ofthem provides insights to the other. Enhancement of vaporizationleads to complete combustion with higher combustion efficiency.Nanofluid fuel droplet vaporization is a highly complex phenome-non compared to liquid fuel droplet vaporization due to itsmulti-component, multi-phase and multi-scale nature. A simplecomparative analysis shows it involves at least six interacting pro-cesses amongst them, whereas three are present in pure fuel drop-let vaporization; liquid phase transport, gas phase transport andphase change at the liquid–gas interface [17]. The additional threeprocesses are the solid phase transport, phase change at the solid-liquid interface (heterogeneous nucleation sites) and the dynamicsof the NPs (particle diffusions, collisions, aggregation, agglomera-tion, etc.). The interaction of these processes with each othermakes fundamental understanding even more difficult. Moreover,the thermophysical properties of nanofluid fuels such as the ther-mal 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. Theseproperties can also vary with time due to the continuous changesin the concentration of NPs within a droplet. Therefore, experimen-
tal studies, which are very rare, are required to provide the basicunderstanding of the evaporation behavior of a nanofluid fueldroplet.
Chen et al. [24] observed that with the addition of laponite,Fe2O3, and Ag NPs in deionized water, the nanofluid droplets evap-orate at different rates from the base fluid and that the evaporationrate shifts from one constant value to another during the evapora-tion process. The authors explained these effects based on a changein the apparent heat of evaporation. With the addition of Al NPs toethanol and n-decane fuel droplets, Gan and Qiao [25] detected adeviation from the classical d2-law under natural and weak forcedconvections at 300 and 320 K during their evaporation process. Inanother study they observed that with an addition of multi-walledcarbon nanotubes (MWCNTs) or carbon nanoparticles (CNPs) toethanol, the evaporation rates of nanofluids are increased com-pared to the evaporation rate of pure ethanol [22]. They also foundthat the addition of a small amount of Al NPs to ethanol increasesthe radiation absorption, which increases the nanofluid droplettemperature and enhances the droplet evaporation rate [26].Recently, it has been demonstrated [27] that the addition of AlNPs to heptane enhances its evaporation rate at high temperatures(above 400 �C). At lower temperatures (below 400 �C), the forma-tion of large agglomerates results in a compact shell and suppressesthe evaporation rate of heptane. More recently, experimental inves-tigations on the effect of dilute concentrations (0.1%, 0.5% and 1.0%)of Al NPs on the evaporation characteristics of kerosene droplets atelevated temperatures (400–800 �C) have been conducted [8].These nano-Al and kerosene (n-Al/kerosene) suspension dropletsevaporate in the same way as pure kerosene droplets at tempera-tures in the range 400–600 �C and the added Al NPs enhanced theevaporation rate at all tested temperatures. Microexplosions wereobserved at high temperatures (700–800 �C) due to the additionof the Al NPs to kerosene droplets. The current study investigatedthe effects of high loadings of ligand-protected Al NPs on the evap-oration behavior of kerosene droplets at elevated temperatures.Such an experimental study was necessary to verify the occurrenceof microexplosions in the case of dense NP/kerosene suspensiondroplets and to develop an understanding of this phenomenon.The problem associated with this study is the stability of high load-ings of metal NPs in liquid hydrocarbon fuels.
In the previous work [8], surface modified and ligand protectedAl NPs were suspended in kerosene. Oleic acid (OA) was used forthe surface coating of the Al NPs. The same surfactant, surfacemodification technique and procedure were adopted in the presentstudy to prepare stable suspensions of dense concentrations of NPsin kerosene. The evaporation behavior of kerosene based nanofluiddroplets at elevated temperatures with high loadings of Al NPs willprovide valuable insights into the occurrence of microexplosionsdue to NPs and their effects on the evaporation rate of such mul-ti-component hydrocarbon fuels. Therefore, in this study the ef-fects of high loadings of NPs on the evaporation behavior ofmulti-component liquid hydrocarbon fuel (kerosene) droplets atelevated temperatures were experimentally investigated.
2. Experimental methods
The materials and instruments used in this study were the sameas described in other recent research work [8]. All the materials,except for the Al NPs, were used in their original form as receivedwithout further treatment.
Preparation of uniformly dispersed and stable NP suspensionsin fuel is the key step before performing any experimental studywith nanofluid fuels. The post synthesis surface modification con-cept was applied to the Al NPs purchased. The NPs were ground ina planetary ball mill (PM100) along with the OA and their surfaces
P= 0.1 MPa500 C 400 C
0 1 2 3 4 5 6 7 8
-1 0 1 2 3 4 5 6 7
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KeroseneKerosene+1.25% OAKerosene+2.50% OA
P= 0.1 MPa
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d2 /d2 0
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(a)
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44 35
were coated with the ligand which improves their dispersion sta-bility in liquid hydrocarbons. The amount of OA and the time ofball milling were optimized. The ligand protected Al NPs were dis-persed in the kerosene fuel and 2.5%, 5.0% and 7.0% NP/kerosenesuspensions were prepared. In this case and for the remainder ofthis paper mass percentage values are used. The surface modifica-tion technique used in the current study has been described else-where [14] and the procedure and amendments adopted herehave been also explained in another recent study [8].
The design, fabrication and installation of the experimentalapparatus is discussed in detail in previous studies [28–30]. Theexperimental procedure for the evaporation of the nanofluid fueldroplet along with the data reduction and analysis are describedin the previous work [27]. An isolated n-Al/kerosene droplet wassuspended by a fine SiC fiber (100 lm diameter). The initial aver-age diameter of a droplet was 1.0 ± 0.10 mm. Nitrogen was usedto provide an inert environment for evaporation. The ambient tem-perature was varied in the range 400–800 �C and the ambient pres-sure was kept constant at 0.1 MPa. High ambient temperatureswere provided by an electric furnace. The evaporation processwas recorded using a high speed charge coupled device (CCD) cam-era. The captured images were analyzed using a flexible image pro-cessing code to extract the droplet diameter. The temporalvariation of the droplet diameter was obtained during the wholeevaporation process. If the droplet evaporation follows the d2-law then the evaporation rate can be expressed as the time deriv-ative of the droplet diameter squared, Cv = �d(d2)/dt [31,32]. Inthat case, the droplet evaporation rate was obtained from the tem-poral history of the droplet diameter squared, by measuring theslope of its linear regression.
20 (s/mm2)
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4
-1 -0.5 0 0.5 1 1.5 2 2.5 3
0
0.2
0.4
(b)
Fig. 1. Droplet diameter squared histories of evaporating stabilized kerosenedroplets containing 1.25% and 2.50% OA at various ambient temperatures; (a) at 400and 500 �C, (b) at 600, 700 and 800 �C.
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 re-search work [8]. The general evaporation behavior and measure-ments of the evaporation rates of pure kerosene droplets atelevated temperatures (400–800 �C) are essential for understand-ing the effects of the addition of high concentrations of OA andAl NPs on the droplet evaporation behavior. In pure kerosene drop-lets after a finite heating up period, the temporal variation in thedroplet diameter squared becomes approximately linear while fol-lowing the d2-law in the last stage of evaporation. The evaporationrate 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/kerosenedroplets at elevated temperatures, the evaporation behavior of ker-osene/OA droplets with high concentrations of OA (surfactant)were studied at the same temperatures. The evaporation behaviorof the stabilized kerosene (kerosene/OA) droplets helps us distin-guish the effect of NP addition on the droplet evaporation behavior.
Fig. 1 shows the normalized temporal histories of the diametersquared of kerosene droplets with 1.25% and 2.5% of OA over arange of ambient temperatures (400–800 �C). The droplets wereselected from the experimental data within a narrow range of ini-tial diameters from 0.978 to 1.079 mm to reduce the effect of theinitial droplet diameter on the evaporation rate. The general evap-oration behavior of the kerosene/OA droplets is similar to that forpure kerosene droplets, i.e., after a finite heating up period, thereduction in the droplet diameter squared became linear and fol-lowed the d2-law. At low temperatures (400–600 �C), the total life-
time of the stabilized kerosene droplets was slightly longer thanthat for pure kerosene droplets as shown in the figure. OA is alow volatility component compared to kerosene while its additionto kerosene produces a monolayer at the surface of the dropletwhich inhibits the diffusion of kerosene so that slightly reducesthe evaporation rate [33]. However, at high temperatures (700–800 �C), the total lifetime of the stabilized kerosene droplets wascomparable to that of pure kerosene droplets and the effects ofthe addition of OA were diminished as shown in Fig. 1b.
Fig. 2 shows that the evaporation behavior of kerosene/OAdroplets with 5.0% of OA is similar to that of binary component fueldroplets. Likewise with binary component droplets [28], the stabi-lized kerosene droplets show a three stage evaporation process; afinite heating up period, d2-law evaporation of the highly volatilecomponent (kerosene) and d2-law evaporation of the low volatilitycomponent (OA) as depicted in Fig. 2. The evaporation history atthe above mentioned temperatures can be divided into two dis-tinct stages, with an obvious difference in their diminishing rates,as shown in the figure. During the first stage of the evaporation
Fuel: Kerosene+5.0% Oleic AcidP= 0.1 MPa
Cv for Oleic Acidcomponent
Cv for Kerosene component
20 (s/mm2)
d2 /d2 0
0 1 2 3 4 5 6 70
0.2
0.4
0.6
0.8
1
1.2
T=400 oCT=500 oCT=600 oCT=700 oCT=800 oC
Fig. 2. Normalized temporal variations of the diameter squared of evaporatingstabilized kerosene droplets containing 5.0% OA with each least-squares fit to thelinear kerosene evaporation part (first stage) and to the linear OA evaporation part(second stage) at various ambient temperatures (400–800 �C) and constantpressure of 0.1 MPa.
36 I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
period, the kerosene component from the stabilized kerosenedroplet evaporates. In the second stage the droplet becomes richwith OA and the OA near the droplet surface starts to evaporate.As the temperature increased from 400 to 800 �C, the differencebetween the evaporation of the high volatility and low volatilitycomponents in the third stage was decreased. Due to the reduceddifference between the boiling points of kerosene and OA andthe multi-component nature of the kerosene, the phenomena ofbubble formation and microexplosion were not seen to take placeeven at high temperatures (700–800 �C). The evaporation rates ofthe stabilized kerosene droplets were calculated using the leastsquares regression method with the data corresponding to the lin-ear part of the evaporation history except for the initial heat upperiod.
P=0.1MPa
Temperature (oC)
Cv
(mm
2 /s)
400 500 600 700 8000.0
0.2
0.4
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1.0
1.2
KeroseneKerosene+1.25% OAKerosene+2.50% OAKerosene+5.00% OA (Kerosene component)Kerosene+5.00% OA (OA component)
Fig. 3. A comparison of the evaporation rates of the stabilized kerosene dropletscontaining various concentrations of OA with pure kerosene droplets at differentambient temperatures and constant pressure of 0.1 MPa.
3.2.1. Effect of temperature on the evaporation rate of stabilizedkerosene droplets
Fig. 3 shows a comparison of the evaporation rates of kerosenedroplets containing 1.25%, 2.5% and 5.0% of OA and pure kerosenedroplets for a range of ambient temperatures (400–800 �C). Theevaporation rate of the kerosene/OA droplets is lower than or equalto that of the pure kerosene at all of the above mentioned temper-atures as shown in the figure. The evaporation rate of the kerosene/OA droplets with relatively low concentrations of OA (1.25% and2.5%) remained slightly lower than the evaporation rate of the purekerosene droplets at all temperatures tested. The kerosene/OAdroplets with 5.0% of OA have two evaporation rate curves; onefor the kerosene component and the other for the OA componentpresent in the stabilized kerosene droplets. The evaporation rateof the kerosene component in the 5.0% stabilized kerosene dropletsis comparable to that of the pure kerosene droplets and remainsthe same in the range 400–700 �C and is slightly higher at800 �C. However, the evaporation rate of the OA component inthe 5.0% stabilized kerosene droplets is considerably lower thanthe evaporation rate of the pure kerosene droplets at all tempera-tures tested. These two curves are similar in that the evaporationrate of each component (kerosene and OA) of the stabilized kero-sene droplets was increased by an increase in the temperature.The effect of the increase in the temperature has different impactson the evaporation rate of the kerosene component and the OAcomponent in stabilized kerosene droplets. A comparison of thesetwo curves also shows that the evaporation rate of the kerosenecomponent truly represents the evaporation rates of the 5.0% sta-bilized kerosene droplets.
These results indicate that similar to the addition of dilute con-centrations of OA to kerosene [8], the addition of dense concentra-tions of OA to kerosene also has no considerable effect on theevaporation rate of kerosene droplets. The evaporation rate ofthe stabilized kerosene droplets, containing 1.25%, 2.5% and 5.0%of OA, is also nearly equal to the evaporation rate of pure kerosenedroplets at all temperatures tested.
3.3. Evaporation of kerosene based nanofluid fuel droplets
In this section, the evaporation characteristics of n-Al/kerosenedroplets are discussed.
Fig. 4 displays normalized temporal histories of the kerosenedroplets’ diameter squared with the addition of 2.5%, 5.0% and7.0% of Al NPs whose surfaces were coated with 1.25%, 2.50% and3.50% of OA, respectively, at ambient temperatures ranging from400 to 800 �C. To minimize the effect of the initial droplet sizeon the evaporation rate, droplets having similar initial diametersranging from 0.977 to 1.074 mm were selected from the experi-mental data.
Unlike the dilute n-Al/kerosene suspension droplets [8], thegeneral evaporation behavior of the dense n-Al/kerosene suspen-sion droplets was not similar to that of pure kerosene droplets.The droplets with high loadings of Al NPs in kerosene showed threestages of evaporation; a finite heating up period, d2-law evapora-tion followed by microexplosions. Fig. 4 also clearly shows thatthe addition of dense concentrations of Al NPs to kerosene causedbubble formation and microexplosions of different strengths at dif-ferent temperatures.
Fig. 4a shows normalized evaporation histories for kerosenebased nanofluid droplets with 2.5%, 5.0% and 7.0% of Al NPs at400 and 500 �C. The kerosene droplets with 2.5% of Al NPs suspen-sion show no bubble formation or microexplosions at the abovementioned temperatures. However, at these temperatures, thedroplets containing 5.0% and 7.0% of Al NPs suspension exhibitbubble formation and they rupture at the end of the droplet’s life-time. Due to the high loadings of NPs (5.0% and 7.0%), enough NPs
P= 0.1 MPa
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20 (s/mm2)
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1.2 KeroseneKerosene+1.25% OA+2.5% Al NPsKerosene+2.50% OA+5.0% Al NPsKerosene+3.50% OA+7.0% Al NPs
(a)
(b)
Fig. 4. Temporal variation of the diameter squared of kerosene droplets with thedense concentrations (2.5%, 5.0% and 7.0%) of Al NPs (coated with OA) at differentenvironmental temperatures (a) at 400 and 500 �C, (b) at 600, 700 and 800 �C.
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44 37
were accumulated at the droplet’s surface during evaporation thatthey produce a shell there in the last phase of the evaporation per-iod. The shell temperature can exceed the boiling point of the li-quid fuel causing nucleation sites to form inside the dropletwhich results in the gasification of the surrounding liquid, pressurebuild up and ultimately fragmentation of the droplet. As the tem-perature increased from 400 to 500 �C or the NP concentration in-creased from 5.0% to 7.0%, the microexplosions started earlier inthe droplet’s lifetime.
Fig. 4b shows normalized evaporation histories for stabilizedkerosene droplets with dense concentrations of Al NPs at 600,700 and 800 �C. The most prominent characteristic of the evaporat-ing n-Al/kerosene droplets at these ambient temperatures was theonset of bubble formation and microexplosions irrespective of theAl NP concentration. Between 600 and 700 �C, with 2.5% of Al NPs,the droplets were suddenly shrunk to smaller droplets by the re-lease of accumulated vapors without any prior expansion. At 600and 800 �C with relatively high concentrations (5.0% and 7.0%) ofAl NPs, the evaporating droplets first expanded to a larger size
and then ruptured. This expansion and contraction process was re-peated several times. For 5.0% and 7.0% n-Al/kerosene droplets at700 �C, the phenomenon of droplet fragmentation and microexplo-sion was observed in two stages; primary fragmentation and sec-ondary microexplosion (see Supplementary video). The primaryfragmentation includes the droplet’s diameter reduction withoutany prior expansion whereas the secondary microexplosion com-prises a droplet diameter expansion and then reduction severaltimes. These two microexplosion stages are separated by a smalllinear evaporation period. The 2.5% n-Al/kerosene droplets alsoexhibited the same two microexplosion stages at 800 �C. It is note-worthy to mention that microexplosions would be most effective ifnucleation occurs early in the droplet lifetime and deep within thedroplet’s inner core [17]. Fortunately, the heterogeneous nucle-ation sites (HNSs) produced at 800 �C, with the addition of 5.0%and 7.0% of Al NPs to kerosene, satisfied these two requirementssimultaneously. These HNSs were produced early in the dropletlifetime (after 20–25% of the time span) and deep inside the drop-let, consequently multiple intense microexplosions occurred in theremaining evaporating time. At a high temperature (800 �C), due tothe strong particle dynamics (collisions, diffusion and Brownianmotion) inside the droplet or due to the enhanced thermal diffusiv-ity, the NPs present inside the droplet acquired a temperaturehigher than the liquid’s local boiling point and converted to HNSs.Whereas at 700 �C, the early HNSs were produced at or near thedroplet surface after 50% of the time span. Later, HNSs were cre-ated inside the droplet, however by that time much of the liquiddroplet had already been gasified. At low temperatures (400–600 �C), the onset of microexplosion was further delayed (startingafter 70–80% of the time span) so that most of the liquid fuel hadalready evaporated and the intensity was reduced. It is importantto note that the microexplosions started a little earlier with an in-crease in the concentration of NPs at the same temperature. How-ever, an increase in the temperature significantly affects the onsetof microexplosions and they started quite early in the droplet’slifetime with a much greater intensity.
3.3.1. Phenomenon of microexplosionThe phenomenon of microexplosion is very important to the
evaporation and combustion of nanofluid fuels because it has thepotential to enhance their evaporation/combustion performanceby shattering the droplets into smaller particles, which yields fas-ter and complete burning of the added Al NPs. For a better under-standing of the nature and types of microexplosions, sequentialphotographs of evaporating n-Al/kerosene droplets are presentedin the next figure.
Fig. 5a–c presents some sequential images of the evaporationbehavior of stabilized kerosene droplets with 2.5%, 5.0%, and 7.0%of Al NPs at 800 �C, respectively. The phenomena of bubble forma-tion and microexplosion are clearly observed here. In Fig. 5a–c, thesecond snapshot shows the end of the linear evaporation period(second stage). The third snapshot of Fig. 5a (at t = 0.695 s) showsthe vapor formation at the droplet surface, due to this a slightincrease in the droplet diameter was observed for the 2.5% n-Al/kerosene. Due to the escape of these vapors, the droplet diameteris suddenly reduced in the subsequent pictures (at t = 0.700 and0.705 s). After this the droplet vaporized linearly up to 1.065 sand then the droplet starts to expand due to internal bubble forma-tion which is completed by 1.210 s, the droplet then ruptures at1.215 s.
For the 5.0% n-Al/kerosene droplet (Fig. 5b), after linear evapo-ration up to 0.850 s, internal bubble formation started and this ap-pears at 0.920 s and the droplet ruptures at 0.925 s as shown in thecaptured frames. The second, third, fourth, fifth and sixth timesbubbles appeared at 0.980, 1.120, 1.210, 1.240 and 1.270 s andthe droplets rupture in the subsequent photos at 0.985, 1.125,
t = 0.000 s t = 0.690 s t = 0.695 s t = 0.700 s t = 0.705 s
t = 1.065 s t = 1.210 s t = 1.215 s t = 1.220 s t = 1.335 s
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
(a)
(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 (coatedwith 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.
38 I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
t = 0.000 s t = 0.715 s t = 0.750 s t = 0.845 s t = 0.850 s
t = 0.855 s t = 0.860 s t = 0.880 s t = 0.985 s t = 0.990 s
t = 0.950 s t = 1.000 s t = 1.080 s t = 1.085 s t = 1.090 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)
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44 39
1.215, 1.245 and 1.275 s, respectively. It is important to note thatafter the generation of the first bubble, the time taken to produceevery subsequent bubble was lower than the time previously spentin bubble generation. Initially, it took a long time to produce HNSsinside the droplet, but once the NPs had a temperature higher thanthe liquid’s local boiling point they continuously gasified the sur-rounding liquid. Due to the accumulation of NPs, the heat transferinside the droplet was increased so that more inner NPs were pro-viding HNSs. It is also noteworthy that this irregular behavior wasnot observed in the evaporation process of pure or stabilized kero-sene droplets.
In the case of the 7.0% n-Al/kerosene droplet (Fig. 5c), the inter-nal bubble formation started after the second stage (t = 0.715 s) butthe bubble appeared at 0.750 s and continuously grows up to0.845 s. In the subsequent two pictures the droplet fragmentedintensively. After that the remaining droplet expanded severaltimes (at 0.990, 1.080, 1.105, 1.215, 1.245 and 1.260 s) due to bub-ble 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 comparisonof the re-expansion of the droplets and their rupturing behaviorbetween the 5.0% and 7.0% Al NP suspension droplets reveals thatmore intense microexplosions occurred with high Al NP concentra-tions at high ambient temperatures. A fast internal pressure buildup caused by more HNSs provided by a high Al NP concentration athigh temperatures, results in more aggressive microexplosionbehavior such as a large expansion, severe bubble formation andmore disruptive fragmentation. The microexplosion phenomenonstarted earlier with an increase in the concentration of the AlNPs at the same temperature.
There are two types of microexplosions which were observedhere. One is droplet fragmentation without any prior dropletexpansion. The other is the repeated expansion and subsequentrupture of the droplet. The first kind of droplet reduction wasdue to the sudden escape of vapors from the droplet surface. Thesegaseous liquids were produced at or near to the droplet surface by
Fig. 7. SEM micrographs of the evaporation residue obtained at 800 �C for 2.5% Al NPs wimagnified view of surface, magnification = 30,000.
Fig. 6. SEM micrographs of the evaporation residue obtained at 600 �C for 2.5% AlNPs with 1.25% OA (a) overall view and (b) magnified view of surface,magnification = 30,000.
40 I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44
the HNSs provided by the NPs. The second kind of microexplosionwhich was observed with high concentration of Al NPs during thelatter part of the droplet lifetime at 400–700 �C was due to theaccumulation of NPs at the droplet surface. These NPs may gatherto form a shell at the droplet surface and simultaneously raise thedroplet surface temperature to close to the liquid local boilingpoint. The NPs present inside the droplet may also be heated be-yond the liquid local boiling point, providing multiple HNSs andconverting the surrounding liquid into vapor. This vapor expandedthe droplet while increasing its diameter. By receiving further heatfrom the NPs these vapors became superheated, an intense pres-sure buildup was caused, which resulted in the catastrophic frag-mentation of the droplet. An increase in the NPs concentration oran increase in the temperature resulted in an early accumulationor shell formation of the NPs at the droplet surface, which resultedin early onset of microexplosions. This is why microexplosionswere observed during the major portion of the droplet’s lifetimewith a greater intensity at 800 �C in droplets containing a relativelyhigh concentration of Al NPs. In the case of the 2.5% Al NPs, theadded Al NPs were not enough to form a shell at the droplet surfacewhich could raise its surface temperature. They only providedHNSs at or near to the droplet surface.
The residues obtained at the end of evaporation were collectedto investigate the effects of microexplosions on shell formation ofnanofluid fuel droplets as well as to confirm the physical state ofthe Al NPs present at the droplets’ surface. Residues of the 2.5%and 5.0% Al NP suspension droplets were obtained at two differenttemperatures (600 and 800 �C) and studied using field-emissionscanning electron microscope (FESEM) and energy-dispersive X-ray spectroscopy (EDX). The selected temperature values were be-low and above the melting temperature of the Al NPs (660 �C),respectively. The residues were collected very carefully using thecollection procedure as described in a recent study [8].
Figs. 6 and 7 show the FESEM images of the residues after evap-oration at 600 and 800 �C for a 2.5% Al NP suspension. An overallview of the residue (Fig. 6a) shows that it was soft, powdery and
th 1.25% OA (a) overall view, (b) close view of lump, (c) close view of surface and (d)
Fig. 9. SEM micrographs of the evaporation residue obtained at 800 �C for 5.0% Al NPs wiand (d) magnified view of surface, magnification = 30,000.
Fig. 8. SEM micrographs of the evaporation residue obtained at 600 �C for 5.0% AlNPs with 2.5% OA (a) overall view and (b) magnified view of surface,magnification = 30,000.
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44 41
had been broken during the SEM sample preparation. The magni-fied view (Fig. 6b) of the residue shows that most of the NPs pre-served their original spherical shapes at 600 �C. When thetemperature increased to 800 �C, a large and compact residuewas formed as shown in Fig. 7a. A closer view (Fig. 7b) shows thatthe residue was soft, crumbly and porous in nature. A further de-tailed view of the residue’s surface (Fig. 7c) revealed that it hasbeen punctured by the NPs coming out of the surface through mic-roexplosion. Fig. 7d shows a magnified view of the residue, whichclearly shows that most of the NPs loosen their shapes, while beingfused and stuck each other to form larger lumps.
Figs. 8 and 9 show the FESEM characterization micrographs ofthe 5.0% Al NP suspension residue collected at 600 and 800 �C.The droplet shrank and formed an elliptical shaped agglomerateon the SiC fiber as shown in Fig. 8a. This agglomeration seemedcompact and porous because during the preparation of the SEMsample the whole lump was detached from the SiC fiber and wasnot broken into pieces as happened to the 2.5% suspension residueat the same temperature (Fig. 6a). Fig. 8b shows a magnified viewof the residue, which disclosed that most of the NPs preserved theiroriginal shapes, whereas a few NPs fused, stuck together and loos-ened their original shapes. The agglomeration obtained at 800 �C(Fig. 9a) was squeezed, sticky and remained on the SiC fiber withholes at its surface. The effects of microexplosion on the evapora-tion residue were clearly noticeable here. A closer view of the res-idue (Fig. 9b) shows that its surface contained many small pores. Aclose up of the inside of a hole (Fig. 9c) shows the presence of wellseparated NPs. These NPs provided HNSs and produced enough va-pors which on rupturing resulted in holes in the droplet surface.Magnified views (Fig. 9d) of the residue show that the Al NPs weredensely packed; some of them fused a little and loosened their ori-ginal 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 residuewas formed with more perforations at the droplet’s surface due tomicroexplosions. However, evaporation rates for the 2.5% NP sus-
th 2.5% OA (a) overall view, (b) close view of lump, (c) close view of a hole at surface
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 thedroplet’s surface. In both cases the overall evaporation rate wassubstantially higher than the pure kerosene droplets’ evaporationrate at 800 �C.
EDX was conducted to analyze the composition of these resi-dues. The evaporative residues were mostly composed of Al andO, with small amounts of C. The O content was due to the oxidationof the Al NPs exposed to air after the vaporization and during thepreparation of the samples for FESEM. The O contents were5.38% and 20.7% (by weight) in the residues of the 2.5% NP suspen-sion, evaporated at 600 and 800 �C, respectively as shown inFig. 10. Similarly, Fig. 11 shows the O contents obtained from the5.0% NP suspension were 4.51% and 16.6% (by weight) in the resi-dues evaporated at 600 and 800 �C, respectively. This indicateshigh oxidation of the Al NPs to Al2O3. All of the residues werecooled down to room temperature in the presence of nitrogen be-fore being taken out into air; therefore, the high oxygen contentwas due to the presence of elemental Al at their surfaces, whichwas oxidized. The EDX results imply that the residue obtained at
Al
O
C
Energy (eV)
100 200
T=600oCT=800oC
Fig. 11. EDX spectrum of the evaporation residues (agglomerates) obtained at 600and 800 �C for 5.0% Al NPs in kerosene coated with 2.5% OA.
Al
O
C
Energy (eV)100 200
T=600oCT=800oC
Fig. 10. EDX spectrum of the evaporation residues (agglomerates) obtained at 600and 800 �C for 2.5% Al NPs in kerosene coated with 1.25% OA.
800 �C contained high amounts of elemental Al at its surface,which was thought to be the melted Al NPs or the Al NPs exposedthrough perforations in the droplet’s surface.
3.3.2. Evaporation rate of n-Al/kerosene dropletsThe evaporation rates of the n-Al/kerosene droplets containing
dense concentrations of Al NPs were calculated from their evapora-tion history curves through a least squares fitting in the linearevaporation stage and by a two point fit in the microexplosion part.For the microexplosion stage, the use of the least squares methodis not justified due to the droplet expansion and contraction.Therefore the rate of diminishment was simply estimated by mea-suring the slope between the two end points [34,35]. Fig. 12 showsa sample least squares fit and a two point estimation for n-Al/ker-osene droplets containing 5.0% Al NPs.
The evaporation rates of these two stages were obtained usingthe above methodology and are plotted in Fig. 13a and b for com-parison with the evaporation rates of pure kerosene droplets.Fig. 13a shows that at low temperatures (400–600 �C), the linearevaporation stage is dominant in the droplet evaporation lifetime,therefore their evaporation rates are close to the evaporation ratesof pure kerosene droplets at these temperatures. Whereas at hightemperatures, this stage comprises a small portion of the dropletlifetime and shows lower evaporation rates than the evaporationrates of pure kerosene droplets.
In contrast to the second stage, the third stage (the microexplo-sion stage) comprises less of the evaporating droplets’ lifetime atlow temperatures due to the low intensity and tendency for mic-roexplosions at these temperatures. Even for the 2.5% Al NP sus-pension, no microexplosion was observed at these temperatures.However at high temperatures, most of the droplet evaporationlifetime consists of microexplosions with a greater intensity so thatthe diminishment rate of this stage at these temperatures was con-siderably higher than with the evaporation rates of pure kerosenedroplets as shown in Fig. 13b.
As discussed above, at low temperatures (400–500 �C) the sec-ond stage evaporation rates represented the evaporation rates of n-Al/kerosene droplets whereas at high temperatures (600–800 �C) itwas the third stage evaporation rates. Therefore, these were plot-
Fuel: Kerosene+2.5% OA+5.0% Al NPsP= 0.1 MPa
3rd stageMicroexplosionsTwo Point Fit
2nd stageLinear evaporationLeast Square Fit
02 (s/mm2)
d2 /d02
0 1 2 3 4 5 6 70
0.5
1
T=400 oCT=500 oCT=600 oCT=700 oCT=800 oC
Fig. 12. Normalized temporal variations of the diameter squared of evaporatingstabilized kerosene droplets containing 5.0% Al NPs with each least-squares fit tothe linear evaporation part (second stage) and two-point estimation to themicroexplosion part (third stage) at various ambient temperatures (400–800 �C)and constant pressure of 0.1 MPa.
P=0.1MPa
Cv (m
m2 /s
)
400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
KeroseneKerosene+1.25% OA+2.5% Al NPsKerosene+2.50% OA+5.0% Al NPsKerosene+3.50% OA+7.0% Al NPs
P=0.1MPa
Temperature (oC)
Temperature (oC)
Cv (m
m2 /s
)C
v (m
m2 /s
)
400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
KeroseneKerosene+1.25% OA+2.5% Al NPsKerosene+2.50% OA+5.0% Al NPsKerosene+3.50% OA+7.0% Al NPs
(a)
(b)
(c)P=0.1MPa
Temperature (oC)400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
KeroseneKerosene+1.25% OA+2.5% Al NPsKerosene+2.50% OA+5.0% Al NPsKerosene+3.50% OA+7.0% Al NPs
Linear evaporationstage
Microexplosion stage
Fig. 13. A comparison of the evaporation rates of kerosene-based nanofluid fueldroplets containing dense concentrations (2.5%, 5.0% and 7.0%) of Al NPs with purekerosene droplets under different ambient temperatures; (a) evaporation rateconstants of linear evaporation part (second stage), (b) evaporation rate constantsof microexplosion part (third stage) and (c) true representative evaporation rateconstants.
I. Javed et al. / Experimental Thermal and Fluid Science 56 (2014) 33–44 43
ted together in Fig. 13c. This figure shows that the trend of theevaporation rate curves for the n-Al/kerosene droplets containingdense concentrations of Al NPs is similar to those of pure kerosene,i.e., the evaporation rate increased monotonically as the tempera-ture increased from 400 to 800 �C. However, the trend for theincreasing evaporation rate of pure kerosene was represented bya power-law fit, whereas the evaporation rate of the kerosenebased nanofluid fuels increased exponentially with the tempera-ture. This figure also clearly shows that the addition of dense con-centrations of Al NPs to the kerosene enhanced the evaporationrates substantially at high temperatures (600–800 �C) through in-tense microexplosions in the nanofluid fuel droplets. However atlow temperatures (400–500 �C), evaporation rates were slightlyenhanced with 2.5%, remained the same with 5.0% and slightly re-duced with 7.0% Al NP suspension droplets. The reduction in theevaporation rates of the n-Al/kerosene droplets with 7.0% Al NPsfrom 400 to 500 �C was due to the delayed onset and lower inten-sity of the microexplosions at these temperatures.
3.3.3. Effect of Al NP concentration on the evaporation rateFig. 13c indicates that the evaporation rates of the n-Al/kero-
sene droplets containing dense concentrations of Al NPs were alsoaffected by the concentration of NPs. As the NP concentration in-creased from 2.5% to 7.0%, the evaporation rates obtained at lowambient temperatures (400–500 �C) went from being slightly high-er to being slightly lower than the evaporation rates of pure kero-sene droplets. This is because an increase in the concentration ofNPs in a droplet also increased their agglomeration in the final por-tion of its lifetime.
At relatively high ambient temperatures (700 and 800 �C), theevaporation rates were substantially increased with the NP con-centration. The maximum increase was obtained with the 2.5% AlNP suspension. A further increase in the NP concentration de-creased the enhancement in the evaporation rates but kept themhigher than the evaporation rates of pure kerosene droplets atthese temperatures. The enhancement in the evaporation rateswas due to microexplosions observed in the n-Al/kerosene dropletsat these temperatures and it is believed that these microexplosionsoccurred due to the presence of NPs in the droplets. In relativelyhigher NP suspension density (5.0% and 7.0%), the NPs accumu-lated at the droplet surface and formed a shell which controlsthe droplet’s surface temperature. Due to this shell, microexplo-sions started earlier in the droplet lifetime and occurred severaltimes with an expansion and subsequent contraction each time.In addition, due to the presence of this shell, the droplet takes alonger time to shatter into smaller droplets and particles. Conse-quently, a low enhancement in the droplet diminishment rateswas obtained for high NP suspension droplets. Whereas, for low(2.5%) NP loading rates, such a shell was not formed and the drop-let easily ruptured into smaller sized droplets which resulted inhigher evaporation rates.
4. Conclusions
The evaporation characteristics of kerosene based nanofluiddroplets with dense concentrations of Al NPs (2.5%, 5.0%, and7.0%) were studied at various elevated temperatures (400–800 �C) under normal gravity. The main conclusions of this exper-imental study are summarized as follows.
1. With high loadings of Al NPs, the kerosene based nanofluiddroplet vaporized in a different way as compared to the purekerosene droplet and exhibited an additional stage known as‘microexplosion’. The classical d2-law was not valid during thisstage 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 testedtemperatures (400–800 �C) in kerosene based nanofluid fueldroplets containing dense concentrations of Al NPs. The absenceof microexplosions in the pure and stabilized kerosene dropletsindicates that this phenomenon was based solely on the pres-ence of the Al NPs.
3. The microexplosions occurred earlier in the droplet’s lifetimeand with a much greater intensity with either an increase inthe ambient temperature or an increase in the NP loading rateat the same temperature. However, the effect of an increase inthe ambient temperature on the intensity and onset of microex-plosions was substantially higher than the NP loading rate.
4. The ambient temperature significantly affected the evaporationrates of the kerosene based nanofluid fuel droplets. Regardlessof the Al NPs concentration, the intense microexplosions at700 and 800 �C led to a substantial enhancement in the evapo-ration rate of kerosene droplets. However, at low temperatures(400–500 �C), due to the delayed onset and lower intensity ofthe microexplosions, the evaporation rate of the nanofluiddroplets was not significantly affected and remained similarto that of pure kerosene droplets.
5. The evaporation rate of nanofluid fuel droplets was also affectedby the concentration of the NPs. The evaporation rate enhance-ment increased with the NP concentration up to 2.5%, afterwhich it decreased. The maximum increase was obtained at800 �C with the 2.5% NP suspension, where the evaporation ratewas increased by 48.7%.
Acknowledgment
This work was supported by the Midcareer Researcher Programthrough 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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