Combustion and Flame 173 (2016) 106–113

Contents lists available at ScienceDirect

Combustion and Flame

journal homepage: www.elsevier.com/locate/combustflame

Ignition characteristics of kerosene droplets with the addition of

aluminum nanoparticles at elevated temperature and pressure

Dong Min Kim, Seung Wook Baek

∗, Jisu Yoon

Department of Aerospace Engineering, School of Mechanical and Aerospace 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 27 February 2016

Revised 26 July 2016

Accepted 27 July 2016

Keywords:

Ignition

High pressure

High temperature

Kerosene droplet

Aluminum nano-particles

a b s t r a c t

The effects of various ambient pressure and temperature conditions on the ignition characteristics of

kerosene-based nanofluid droplets were investigated experimentally. Individual kerosene droplet contain-

ing 0.1 or 1% by mass was mounted on the tip of silicon carbide (SiC) fiber and exposed to ambient

temperatures in the range of 40 0–70 0 °C and ambient pressures in the range of 0.1–2.5 MPa under nor-

mal gravity. An increase in ambient pressure was observed to reduce the ignition location distance below

a droplet. The ignition delay times of pure kerosene droplets were also examined for comparison. The

results showed that the ignition delay time of Al NPs-laden kerosene droplets decreased exponentially

with increasing temperature, as did that of pure kerosene droplets. An ambient pressure increase from

0.1 to 2.5 MPa led to a decrease in the lowest ambient temperature for ignition from 800 to 400 °C. At

pressures greater than 1 MPa, the ignition delay times of droplets with 1% Al were shorter than those

of pure kerosene and kerosene with 0.1% Al. Furthermore, as the ambient pressure increased from 0.1 to

2.5 MPa, the ignition delay was found to decrease and then increase exceeding the limiting pressure.

© 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

n

s

t

e

c

t

a

w

c

o

T

o

f

t

n

s

r

A

n

8

A

1. Introduction

Purposely designed high-energy-density fuels are a compelling

need for future combustion and propulsion systems. The energy

density of available conventional fuels is sometimes a major lim-

iting factor for current liquid propulsion systems. Therefore, there

is an enormous need to enhance the energy density of both tra-

ditional and new synthetic fuels. The volumetric energy contents

of metals are normally higher than the energy densities of con-

ventional liquid fuels so that higher combustion energies can be

achieved using metallized liquid fuels. Because of this advantage

that metallized liquid fuels offer, many researchers have conducted

research on slurry fuels, in which micron-sized metallic particles

are suspended in liquid hydrocarbon fuels. The major disadvan-

tages that limit the practical application of such fuels are incom-

plete combustion, undesirable deposits of particle in the combus-

tion chamber and increased particle emissions in exhaust. Particle

agglomeration and longer burning time are the primary causes of

these defects [1] .

Nanofluid fuels are a stable suspension of energetic nanoparti-

cles (NPs) mixed with conventional liquid fuels. Various types of

∗ Corresponding author.

E-mail address: swbaek@kaist.ac.kr (S.W. Baek).

b

a

t

http://dx.doi.org/10.1016/j.combustflame.2016.07.033

0010-2180/© 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved

ano-sized energetic materials have been used as additives and

uspended in traditional liquid fuels for the purpose of enhancing

heir ignition and combustion characteristics. The sizes of agglom-

rates should be reduced in dealing with nanometer-sized parti-

les. Van Devener and Anderson [2] reported a catalytic combus-

ion of JP-10 at atmospheric pressure, using nanoparticulate CeO 2

s the catalyst. A substantial reduction in the ignition temperature

as observed. Hot plate ignition probability measurements were

arried out at atmospheric pressure on various volume fractions

f aluminum/diesel mixtures and aluminum-oxide/diesel mixtures.

he ignition probability of the nanoparticles/diesel mixtures was

bserved to be much higher than that of pure diesel [3] . The ef-

ect of nanoscale Al particle addition on the combustion charac-

eristics of gelled nitromethane was explored from both a deto-

ation and deflagration perspective using an optical pressure ves-

el up to 14.2 MPa [4] . The impact of energetic Al nanoparticles on

educing the ignition delay of ethanol and JP-8 was presented by

llen et al. The autoignition delay was measured for neat and Al

anoparticle-enhanced mixtures at compressed conditions of 772–

30 K and 12–28 bar in the RCM [5] . The heat of combustion of n-

l and n-Al 2 O 3 in ethanol was investigated using a modified static

omb calorimeter system [6] .

A detailed understanding of the vaporization and ignition char-

cteristics of an isolated droplet is essential to understanding

he operation of various practical liquid fuel combustion systems.

.

D.M. Kim et al. / Combustion and Flame 173 (2016) 106–113 107

I

e

t

n

c

o

e

a

a

d

t

[

i

c

T

t

b

p

g

Q

f

s

b

a

A

e

J

p

J

t

l

o

N

l

h

e

t

n

e

[

2

2

t

f

v

(

k

S

d

u

l

b

N

t

o

p

T

w

m

I

Fig. 1. Schematic of the experimental setup: (1) pressure vessel, (2) guide bar,

(3) furnace entrance, (4) electric furnace, (5) quartz glass window on furnace,

(6) temperature controller, (7) furnace lever, (8) nitrogen vessel, (9) quartz glass

window on pressure vessel, (10) light source, (11) Si–C fiber, (12) droplet, (13)

shock absorber, (14) droplet maker, (15) plunger micro pump, and (16) high-speed

camera.

m

A

w

2

a

v

s

a

s

d

e

n

p

T

p

d

a

t

t

w

s

t

0

i

gnition of an individual droplet is considered a classical subject,

xamination of which provides the opportunity to explore the in-

eractions of the physical and chemical processes involved. The ig-

ition of a nanofluid fuel droplet is a highly complex phenomenon

ompared with the ignition of a pure liquid fuel droplet because

f its multi-component, multi-phase and multi-scale nature. Sev-

ral physical and chemical processes occur during the ignition of

nanofluid fuel droplet. These processes are made up of mass

nd energy transport between phases and microexplosions. In ad-

ition, the thermo-physical properties of nanofluid fuels such as

heir thermal conductivity [8] , mass diffusivity [9] , surface tension

10] , and radiative properties [11,12] , introduce additional complex-

ty. Furthermore, these properties can vary with time because of

ontinual change in the concentration of NPs within a droplet.

herefore, experimental studies, which are very rare, are required

o obtain a basic understanding of the vaporization and ignition

ehaviors of nanofluid fuel droplets.

Sabourin et al . [13] examined the burning rate of monopro-

ellant nitromethane through the addition of 1% functionalized

raphene sheets, which reduced its ignition temperature. Gan and

iao [14] observed the burning behavior of ethanol and n-decane

uel droplets by loading nano and micro-sized Al particles at atmo-

pheric pressure. Gan et al. [15] examined the simultaneous com-

ustion of dilute suspensions of boron and iron NPs in n-decane

nd ethanol. Recently, the intense disruptive burning behavior of

l NP-laden heptane droplets, which is characterized by multiple

xpansions and ruptures or microexplosions, was investigated by

aved et al. [16] . Because of these intense and frequent microex-

losions, almost no residue from Al NPs remained on the fiber.

aved et al. [17] also reported that the addition of dilute concen-

rations of Al NPs to kerosene droplets reduced the ignition de-

ay time while lowering the ignition temperature. However, most

f these previous studies were conducted at atmospheric pressure.

o previous research has been reported on the effects of the NP-

oading rate on droplet evaporation and ignition behavior under

ot stagnant air in furnace at elevated pressure conditions.

The goal of this study was to enhance the understanding of the

ffects of the addition of Al NPs to liquid kerosene fuel on its igni-

ion characteristics at elevated pressures and temperatures under

ormal gravity. The ignition behavior of pure kerosene was also

xamined for comparison. Kerosene was adopted as the base fuel

18] , and Al NPs were selected as energetic additives.

. Experimental setup

.1. Preparation of stable nanofluid fuels

This study began with the investigation of the physical methods

o be used to prepare homogeneous, stable, and durable nanofluid

uels. Stable kerosene-based nanofluid fuels were prepared with

arious concentrations of NPs using a two-step method. Al NPs

70 nm) were purchased from US Research Nanomaterials, Inc., and

erosene was purchased from JUNSEI (CAS NUMBER 8008-20-06).

urface modification was applied to the Al NPs to improve their

ispersion stability in kerosene. Oleic acid (OA) was selected for

se in surface coating of the Al NPs because of its low molecu-

ar weight and its boiling point of 360 °C, which is close to the

oiling point of the base fuel (180–270 °C). The surfaces of the

Ps were coated with OA using a planetary ball mill (PM100) so

hat oxide-free, ligand-protected and fuel-soluble Al NPs could be

btained [19–20] . An ultrasonic disruptor was then used to dis-

erse particles evenly in kerosene while avoiding agglomeration.

he sonicator, which generates a series of 5-s-long pulses 5 s apart,

as turned on for 20 min. The ratio of Al NPs to OA was opti-

ized at 2:1 in to obtain stable suspensions of NPs in kerosene.

n this manner, a homogeneous suspension was obtained, and re-

ained stable for 2 hours without sedimentation of the NPs. These

l/OA mixtures were blended in two proportions (0.1% and 1%)

ith kerosene.

.2. Experimental apparatus and procedure

The experimental apparatus used here was designed, fabricated

nd installed by our group in past and discussed in detail in pre-

ious studies [7,16,17,21–24] . The experimental procedure was de-

cribed in detail in Javed et al. [17] , along with the data reduction

nd analysis. A schematic view of the experimental apparatus is

hown in Fig. 1 . A droplet hanging on a fine SiC fiber (100 μm in

iameter) is subjected to a hot air environment by a freely falling

lectric furnace, resulting in combustion. This ignition unit with in-

er dimensions of 0.065 m ×0.065 m ×0.12 m is enclosed within a

ressure vessel with height of 0.8 m and inner diameter of 0.15 m.

wo glass windows with 0.05 m ×0.04 m ×0.01 m are installed to

ermit observation of the droplet ignition process. In order to re-

uce the time for gas temperature to rise before falling of furnace

nd to prevent the droplet from being preheated by the furnace,

he falling distance should be long. However, a long travel dis-

ance of the furnace would induce a strong impact on the base

hen it falls. The falling distance is set about 0.4 m in the present

tudy to satisfy the above requirements. Based on this design, this

ravel distance provides a droplet with entering time shorter than

.5 s. Impact shock due to collision of the furnace with bottom wall

s relieved by placing a couple of shock absorbers at the impact

108 D.M. Kim et al. / Combustion and Flame 173 (2016) 106–113

m

t

e

f

c

n

o

m

p

m

d

s

o

m

d

n

h

a

m

d

l

t

c

h

k

n

t

c

s

d

p

s

l

i

i

1

a

a

o

b

t

3

A

o

p

r

o

A

r

d

i

s

c

w

t

a

k

t

n

1

location, so that the droplet should remain stable. In order to pro-

duce a droplet around the bead at the tip of the SiC fiber, an injec-

tion system is installed. It produces a droplet as small as 1 mm in

diameter. It consists of a stainless steel pipe, micro needle, micro

pump and a lever as shown below and operates quite well even

at high-pressure environments. The needle is connected to the mi-

cro pump through a fine capillary tube. The movement of needle

is provided by a lever on which needle is mounted. The lever can

carry the needle very close to the suspended fiber by vertical dis-

placement and then, the liquid fuel is transferred to the bead by

turning the lever, thereby generating a droplet.

The burning process was observed using a high-speed camera

(operating at a rate of 200 frames per second). A single droplet

was suspended using a SiC fiber. The initial average diameter of

a droplet was approximately 1.0 ± 0.1 mm. Dry air was used as an

oxidizer for ignition. The ambient pressure was varied within the

range 0.1–2.5 MPa, and the ambient temperature was varied within

the range of 40 0–70 0 °C, which is higher than the ignition tem-

perature of kerosene (220 °C). Because a thin SiC fiber was used

for droplet suspension, the heat transfer between the droplet and

the fiber was minimized. The ignition delay time is defined as the

time to ignition, starting from the time at which a droplet of liq-

uid fuel is introduced into the hot air environment. Ignition is vi-

sually detected by the appearance of a visible flame, using high-

speed color photography. For each ignition data point, measure-

ments were taken at least five times to ensure reproducibility and

consistency. Average values were determined and displayed with

error bars that indicate the maximum and minimum values of the

data obtained. The variations around the average values resulted

mainly from the uncertainties associated with the temperature and

droplet diameter.

A flexible image-processing method was developed using the

Fortran language to obtain the droplet diameter. The procedure for

calculating the droplet diameter from the captured images was de-

scribed in our previous study [21–24] . Briefly, in this method, a

threshold value for the pixel gray level was carefully set to count

the pixels in the droplet zone. Then, the area of a circle having the

same number of pixels, which the droplet actually represents, is

calculated. This way in turn yields the droplet diameter using the

law of proportions with reference to the diameter of a SiC fiber

(100 μm). In some cases, the droplet is distorted during the ignition

process. However, using this approach, the diameter of an irregu-

larly shaped droplet can be calculated with reasonable accuracy.

This method was executed iteratively for each image to determine

the temporal variation of the droplet diameter during the ignition

process.

3. Results and discussion

3.1. Droplet ignition

Typical ignition characteristics may be described in terms of

the ignition delay which is determined as a function of the initial

ambient gas temperature. The effects of other variables including

the pressure, equivalence ratio, droplet size, and fuel type, may be

included via parametric investigation. The ignition delay was de-

fined as the time interval from the instant at which the droplet

was exposed to the high-temperature environment to the instant

at which a visible flame was observed. When an isolated single

droplet with a certain initial temperature is suddenly exposed to

a high-temperature environment at a pressure in the range of 0.1–

2.5 MPa, vaporization starts on the droplet surface after an initial

heating-up period.

The fuel vapors generated are diffused radially outward and

downward as time passes, because kerosene vapor is heavier than

air, while mixing with air in the ambience. When a fuel vapor-air

ixture of appropriate proportion is produced and the fuel activa-

ion energy is overcome, ignition takes place. The time lag between

xposure to a high temperature and when a sufficient amount of

uel vapor has been produced for ignition to occur is the physi-

al delay. The time lapse between when a sufficient amount of ig-

itable fuel vapor is available and when the activation energy is

vercome is the chemical delay. Note that in the ignition of a ho-

ogeneous mixture of a fuel and an oxidizer, there is almost no

hysical delay, unlike in the case of an inhomogeneous mixture, as

ixing of the fuel and air is not required. Therefore, the ignition

elay associated with premixed gaseous mixtures is significantly

horter than associated with non-premixed cases under the same

perating conditions. In other words, the ignition delay for a pre-

ixed fuel and oxidizer consists almost entirely of the chemical

elay.

The experimental apparatus used in this study is based on a

on-premixed configuration, so the ignition delay times described

ere should only be compared with the other data obtained using

similar experimental arrangement. These data provide a funda-

ental understanding of the overall ignition characteristics of fuel

roplets, which are important in the optimal design of practical

iquid–fuel combustors. The experimental apparatus employed in

his study is relatively easy to use and is appropriate for use in a

omparative study of the effects of the addition of NPs to a liquid

ydrocarbon fuel on the fuel’s overall ignition characteristics. The

erosene used as the fuel in this study is a complex multicompo-

ent fuel. Vaporization of the various components affects the igni-

ion process and makes it completely different than that of single-

omponent hydrocarbon fuels.

The effects of the addition of dilute concentrations of nano-

ized Al particles on the ignition characteristics of kerosene

roplets were investigated at various elevated temperatures and

ressures. The ignition delay time of pure kerosene droplets was

tudied first as a baseline for comparison with others. This al-

owed us to investigate the effects of the addition of NPs on the

gnition behavior of kerosene-based nanofluid fuel droplets. The

nitial diameter of the droplets was selected to be approximately

.0 ± 0.1 mm to minimize the effects of variation in the droplet di-

meter on the ignition delay. The ignition delay time was defined

s the time from the entry of the droplet into the furnace to the

nset of ignition. The occurrence of ignition was visually identified

y the appearance of a visible flame, using high-speed color pho-

ography.

.2. Ignition delays for various ambient pressures

Figure 2 (a)–(f) compares the ignition delay times of n-

l/kerosene droplets containing dilute concentrations (0.1% and 1%)

f Al NPs with those of pure kerosene droplets for various ambient

ressure conditions. The ignition delay time is plotted on a loga-

ithmic scale against the inverse ambient temperature. This type

f representation is usually adopted for the case directed by the

rrhenius rate law. The activation energy of the system can be de-

ived from the slope of the line. The ignition delay time is normally

ependent on the chemistry of the ignition as well as on the phys-

cal process of droplet heating. Nonetheless, the ignition delay data

till lie on a straight line for the range of experimental conditions

onsidered.

At atmospheric pressure, the ignition of a pure kerosene droplet

as not observed in the atmospheric pressure ambient tempera-

ure range from 400 to 700 °C, whereas ignition occurred at 800 °Cs shown in Fig. 2 (a). Even when Al NPs were added to the

erosene, ignition did not take place at atmospheric pressure at

emperatures within the range of 400 to 700 °C. At 800 °C the ig-

ition delay for the pure kerosene droplet was observed to be

s, while it is 1.16 s for 0.1% addition of Al NPs and 1.09 s for

D.M. Kim et al. / Combustion and Flame 173 (2016) 106–113 109

Fig. 2. Comparison of ignition delay time of n-Al/kerosene droplets containing dilute concentrations (0.1% and 1%) of Al NPs with those of pure kerosene droplets for various

ambient pressure conditions.

110 D.M. Kim et al. / Combustion and Flame 173 (2016) 106–113

d

i

t

t

F

s

A

t

s

t

i

t

t

h

f

p

1.0% addition of Al NPs. These results show that the addition of

Al NPs leads to a slight increase in ignition delay at atmospheric

pressure. A comparison of Fig. 2 (a) with Fig. 2 (b)–(d) shows that

n-Al/kerosene droplets and pure kerosene droplets are ignited at

temperatures between 500 and 700 °C when the pressure is in the

range of 0.5 and 1.5 MPa, but they are not ignited at 400 °C. Fur-

thermore, as seen in Fig. 2 (e) and (f), both n-Al/kerosene droplets

and pure kerosene droplets are ignited at temperatures between

40 0 and 70 0 °C when the pressure is between 2.0 and 2.5 MPa.

These findings suggest that the minimum ambient temperature for

onset of ignition was lowered from 800 to 400 °C, as the ambi-

ent pressure increased from 0.1 to 2.5 MPa. The autoignition tem-

perature of bulk kerosene fuel is listed as 220 °C, according to the

manufacturer. Because the ambient density also increases as the

ambient pressure increases, the kerosene fuel vapors experience a

diffusional resistance in outward mass diffusion at higher ambient

pressures, which increases the residence time of the fuel vapor and

hence increases the Damkohler number, which is the ratio of mass

transfer time to characteristic chemical reaction time. o

Fig. 3. Comparison of ignition delay times of n-Al/kerosene droplets containing 1% of Al

2.5 MPa and various ambient temperature conditions.

When the pressure increases to 0.5 MPa, the n-Al/kerosene

roplets are ignited faster than pure kerosene droplets as shown

n Fig. 2 (b). This is because the addition of Al particles enhances

he evaporation rate of kerosene, because of the increased effec-

ive thermal conductivity of the droplet.

For ambient pressures in the range of 1.0 MPa to 2.5 MPa in

ig. 2 (c)–(f), the ignition delay of 1.0% Al/kerosene droplets was

horter than the others, whereas the ignition delay of 0.1%

l/kerosene droplets was longer than the others. Higher effec-

ive thermal conductivity of the droplet with 1.0% Al nanoparticles

eemed to help transfer the heat energy from the environment to

he droplet. Although the kerosene in the droplet reaches its boil-

ng temperature of 180–270 °C together with the nanoparticles, af-

erwards the nanoparticle temperature increases even higher than

he kerosene boiling temperature. Thereby, nanoparticles become

eat supplier to the liquid phase. That is why the evaporation rate

or the case with 1.0% of Al nanoparticles is higher than those of

ure kerosene and 0.1% Al nanoparticles droplets. Consequently, its

verall 1.0% Al/kerosene droplet heating is enhanced in comparison

NPs with those of pure kerosene droplets for ambient pressures between 0.1 and

D.M. Kim et al. / Combustion and Flame 173 (2016) 106–113 111

t

O

N

1

r

d

p

3

s

t

t

o

o

i

A

p

4

e

t

a

i

d

f

4

0

t

n

p

(

e

n

t

a

d

a

a

t

c

s

d

a

i

3

p

i

i

a

o that of pure kerosene droplets, which leads to faster ignition.

n the other hand, in the case of 0.1% Al/kerosene droplets, Al

Ps are distributed sparsely inside the droplet, in comparison with

% Al/kerosene droplets, so Al NPs become heat absorber, which

etards its droplet heating in comparison with the pure kerosene

roplets. Therefore, 0.1% Al/kerosene droplets are ignited later than

ure kerosene droplets.

.3. Ignition delays for various ambient temperatures

The initial heating-up period of the n-Al/kerosene droplets was

horter than that of the pure kerosene droplets at higher tempera-

ure, regardless of the NP loadings because of the increased effec-

ive thermal conductivity of the droplet. The corresponding evap-

ration period of the n-Al/kerosene droplets was reduced because

f the increased evaporation rate. Figure 3 (a)–(d) shows a compar-

son of the ignition delays of n-Al/kerosene droplets containing 1%

l NP loading with that of pure kerosene droplets, as the ambient

ressure increases for ambient temperature conditions between

0 0 and 70 0 °C. For the range of temperature conditions consid-

red, the ignition delay of the droplets with 1% Al was shorter

han that of pure kerosene droplets in the pressure range of 0.5

nd 2.5 MPa. Both pure and n-Al/kerosene droplets experienced an

nitial heating-up period, followed by evaporation governed by the

2 -law. Droplets with 1% Al heated up more quickly, resulting in

aster evaporation and ignition. However, as shown in Fig. 3 (a) at

00 °C, neither type of droplets ignited at pressures in the range of

Fig. 4. Onset of ignition at 700 °C for various ambient pressure conditions. (a) Pure kero

.1 and 1.5 MPa. At 0.1 MPa, the onset of droplet ignition did not

ake place at temperatures up to 700 °C, whereas they were all ig-

ited at 800 °C at 0.1 MPa.

Figure 3 (a)–(d) illustrates a very interesting effect of ambient

ressure on the ignition delay. At 500 and 600 °C, as Fig. 3 (b) and

c) shows, the ignition delay is observed to decrease as the ambi-

nt pressure increased from 0.1 MPa to 1.0 MPa, neither pure nor

-Al/kerosene droplets ignited at all at 0.1 MPa, which means that

heir ignition delays were infinite. A decrease in the ignition delay,

s the ambient pressure increases, is considered to result from the

ecrease in the heat of vaporization of liquid. However, when the

mbient pressure increased further from 1.0 MPa to 2.5 MPa at 500

nd 600 °C, the ignition delay increased. This observed increase in

he ignition delay with increasing ambient pressure in excess of a

ertain limiting pressure was also observed at 400 and 700 °C as

hown in Fig. 3 (a) and (d), even though the rate of increase was

ifferent. The difficulty in diffusion and mixing of fuel vapor and

ir at higher pressure is the main reason for the increase in the

gnition delay.

.4. Effect of ambient pressure on ignition behavior

The ignition always began below the kerosene droplet as re-

orted by Ghassemi et al. [22] , because kerosene vapor is heav-

er than air and tends to fall downward. Whang et al. [25] stud-

ed the effect of the ambient temperature on the ignition location

nd distance from the droplet at atmospheric pressure. They found

sene, (b) kerosene + 0.05% OA + 0.1% Al NPs, (c) kerosene + 0.5% OA + 1.0% Al NPs.

112 D.M. Kim et al. / Combustion and Flame 173 (2016) 106–113

Fig. 5. Sequential photographs showing the ignition behavior of a droplet with kerosene + 0.5% OA + 1.0% Al NPs at 2.0 MPa at (a) 400 °C, (b) 500 °C, (c) 600 °C, (d) 700 °C.

D.M. Kim et al. / Combustion and Flame 173 (2016) 106–113 113

t

c

c

f

n

A

s

s

c

0

t

e

w

s

o

t

I

t

n

3

s

t

4

d

e

t

2

i

d

p

a

F

4

f

g

c

p

c

o

A

L

R

[

[

[

[

[

hat the distance between the droplet and the ignition location de-

reased as the ambient temperature increased because of an in-

rease in the Damkohler number.

In contrast to that study above, this study examined the ef-

ect of an ambient pressure range of 0.1 and 2.5 MPa on the ig-

ition location at the onset of ignition for pure kerosene and n-

l/kerosene droplets at 700 °C. Figure 4 (a)–(c) illustrates the re-

ults for pure kerosene, 0.1% Al/kerosene and 1% Al/kerosene, re-

pectively. These figures clearly show that the ignition takes place

loser to the droplet as the ambient pressure increases from

.5 to 2.0 MPa, regardless of the Al loadings. At 2.5 MPa, igni-

ion is even observed above the droplet. An increase in ambi-

nt pressure seems to increase the Damkohler number in two

ays. Because the ambient density increases as the ambient pres-

ure increases, kerosene vapor experiences resistance to radial

utward diffusion at high ambient pressures, which increases

he residence time and hence increases the Damkohler number.

n addition, an increase in the ambient pressure may decrease

he chemical reaction time and hence increases the Damkohler

umber.

.5. Effect of ambient temperature on ignition behavior

Figure 5 (a)–(d) presents typical sequential photographs that

how the ignition behavior for various ambient temperatures be-

ween 400 and 700 °C for 1.0% Al/kerosene droplets at 2.0 MPa. At

00 °C, as Fig. 5 (a) shows, the flame was initiated far below the

roplet at 4.74 s, at which time the kerosene droplet was almost

vaporated. The flame was then observed to propagate upward. On

he other hand, at 500 °C as Fig. 5 (b) shows, ignition occurred at

.485 s, at which time the droplet was evaporating. Consequently,

t can be deduced that the onset of ignition occurs closer to the

roplet as the ambient temperature increases. As the ambient tem-

erature decreases, the ignition occurs farther below the droplets,

nd the flame propagates upward toward the droplet, as shown in

ig. 5 (a)–(c).

. Conclusions

In this study, the ignition behaviors of individual kerosene

uel droplet with different concentrations of Al NPs were investi-

ated experimentally at various ambient temperature and pressure

onditions. The main objective of this work was to gain an im-

roved understanding of the ignition characteristics of these multi-

omponent hydrocarbon-based nanofluid fuels. The major results

f this work can be summarized as follows.

1. Ignition is primarily controlled by the thermal time at low

temperatures of 400 °C whereas it is controlled by diffusion

and mixing time at higher temperatures between 500 and

700 °C.

2. The lowest ambient temperature for the onset of ignition de-

creases from 800 to 400 °C, as the ambient pressure increases

from 0.1 to 2.5 MPa.

3. At pressures greater than 1 MPa, the ignition delays of droplets

with 1% Al are shorter than those of droplets of pure kerosene

and those with 0.1% Al. The latter had the longest ignition

delays.

4. As the ambient pressure increases up to the limiting pressure,

the ignition delay decreases. However, the ignition delay in-

creases, as the ambient pressure increases beyond the limiting

pressure.

5. The ignition location distance from the droplet decreases as

the ambient pressure increases because of the increase in

Damkohler number.

cknowledgments

This work was supported by the RoKST&R Program funded by

ockheed Martin (Research Agreement P14-058 ).

eferences

[1] P. Roy Choudhury , Slurry fuels, Prog. Energy Combust. Sci. 18 (1992) 409–427 .

[2] B. Van Devener , S.L. Anderson , Breakdown and combustion of JP-10 fuel cat-

alyzed by nanoparticulate CeO 2 and Fe 2 O 3 , Energy Fuels 20 (2006) 1886–1894 .[3] H. Tyagi , P.E. Phelan , R. Prasher , R. Peck , T. Lee , J.R. Pacheco , P. Arentzen , In-

creased hot-plate ignition probability for nanoparticle-laden diesel fuel, NanoLett. 8 (2008) 1410–1416 .

[4] J.L. Sabourin , R.A. Yetter , B.W. Asay , J.M. Lloyd , V.E. Sanders , G.A. Risha , S.F. Son ,Effect of nano-aluminum and fumed silica particles on deflagration and deto-

nation of nitromethane, Propellants Explos. Pyrotech. 34 (2009) 385–393 .

[5] C. Allen , G. Mittal , C.J. Sung , E. Toulson , T. Lee , An aerosol rapid compressionmachine for studying energetic-nanoparticle-enhanced combustion of liquid

fuels, Proc. Combust. Inst. 33 (2011) 3367–3374 . [6] M. Jones , C. Li , A. Afjeh , G. Peterson , Experimental study of combustion char-

acteristics of nanoscale metal and metal oxide additives in biofuel (ethanol),Nanoscale Res. Lett. 6 (2011) 246 .

[7] I. Javed , S.W. Baek , K. Waheed , Effects of dense concentrations of aluminum

nanoparticles on evaporation behavior of kerosene droplet at elevated temper-atures: phenomenon of microexplosion, Exp.Therm. Fluid Sci. 56 (2014) 33–44 .

[8] X. Wang , X. Xu , S.U.S. Choi , Thermal conductivity of nanoparticle–fluid mix-ture, J. Thermophys. Heat Transf. 13 (1999) 474–480 .

[9] S. Krishnamurthy , P. Bhattacharya , P.E. Phelan , R.S. Prasher , Enhanced masstransport in nanofluids, Nano Lett. 6 (2006) 419–423 .

[10] R.H. Chen , T.X. Phuoc , D. Martello , Surface tension of evaporating nanofluiddroplets, Int. J. Heat Mass Transf. 54 (2011) 2459–2466 .

[11] H. Tyagi , P.E. Phelan , R.S. Prasher , Predicted efficiency of a nanofluid-based di-

rect absorption solar receiver, ASME Energy Sustainability Conference (2007),pp. 729–736 .

[12] Y. Gan , L. Qiao , Optical properties and radiation-enhanced evaporation ofnanofluid fuels containing carbon-based nanostructures, Energy Fuels 26

(2012) 4224–4230 . [13] J. Sabourin , D. Dabbs , R. Yetter , F. Dryer , I. Aksay , Functionalized graphene

sheet colloids for enhanced fuel/propellant combustion, ACS Nano 3 (2009)

3945–3954 . [14] Y. Gan , L. Qiao , Combustion characteristics of fuel droplets with addition

of nano and micron-sized aluminum particles, Combust. Flame 158 (2011)354–368 .

[15] Y. Gan , Y.S. Kim , L. Qiao , Combustion of nanofluid fuels with the addition ofboron and iron particles at dilute and dense concentrations, Combust. Flame

159 (2012) 1732–1740 .

[16] I. Javed , S.W. Baek , K. Waheed , Autoignition and combustion characteristicsof heptane droplets with the addition of aluminum nanoparticles at elevated

temperatures, Combust. Flame 162 (2015) 191–206 . [17] I. Javed , S.W. Baek , K. Waheed , Autoignition and combustion characteristics of

kerosene droplets with dilute concentrations of aluminum nanoparticles at el-evated temperatures, Combust. Flame 162 (2015) 774–787 .

[18] T. Edwards , Liquid fuels and propellants for aerospace propulsion: 1903–2003,

J. Propuls. Power 19 (2003) 1089–1107 . [19] W. Yu , H. Xie , A review on nanofluids: preparation, stability mechanisms, and

applications, J. Nanomater. 1 (2012) 1–17 . 20] B. Van Devener , J.P.L. Perez , J. Jankovich , S.L. Anderson , Oxide-free, catalyst–

coated, fuel-soluble, air-stable boron nanopowder as combined combustioncatalyst and high energy density fuel, Energy Fuels 23 (2009) 6111–6120 .

[21] H. Ghassemi , S.W. Baek , Q.S. Khan , Experimental study on binary droplet evap-

oration at elevated pressures and temperatures, Combust. Sci. Technol. 178(2006) 1031–1053 .

22] H. Ghassemi , S.W. Baek , Q.S. Khan , Experimental study on evaporation ofkerosene droplets at elevated pressures and temperatures, Combust. Sci. Tech-

nol. 178 (2006) 1669–1684 . 23] Q.S. Khan , S.W. Baek , H. Ghassemi , On the autoignition and combustion charac-

teristics of kerosene droplets at elevated pressure and temperature, Combust.

Sci. Technol. 179 (2007) 2437–2451 . 24] H. Ghassemi , S.W. Baek , Q.S. Khan , Experimental study on binary droplet evap-

oration at elevated pressure and temperature, 43rd Aerospace Sciences Meet-ing and Exhibit (2005) .

25] J.J. Whang , C.Y. Yukao , J.T. Ho , S.C. Wong , Experimental study of the ignition ofsingle droplets under forced convection, Combust. Flame 110 (1997) 366–376 .