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Ignition of a Binary Component FuelDroplet in a Rapid Compression Machine:Comparative AnalysisHyemin Kima, Seung Wook Baeka & Sang Heon Hanb

a Division of Aerospace Engineering, Korea Advanced Institute ofScience and Technology, Daejeon, Koreab Division of Ocean System Engineering, Korea Advanced Institute ofScience and Technology, Daejeon, KoreaAccepted author version posted online: 09 Sep 2014.Publishedonline: 20 Jan 2015.

To cite this article: Hyemin Kim, Seung Wook Baek & Sang Heon Han (2015) Ignition of a BinaryComponent Fuel Droplet in a Rapid Compression Machine: Comparative Analysis, Combustion Scienceand Technology, 187:4, 659-677, DOI: 10.1080/00102202.2014.960563

To link to this article: http://dx.doi.org/10.1080/00102202.2014.960563

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Combust. Sci. Technol., 187: 659–677, 2015Copyright © Taylor & Francis Group, LLCISSN: 0010-2202 print / 1563-521X onlineDOI: 10.1080/00102202.2014.960563

IGNITION OF A BINARY COMPONENT FUEL DROPLETIN A RAPID COMPRESSION MACHINE: COMPARATIVEANALYSIS

Hyemin Kim,1 Seung Wook Baek,1 and Sang Heon Han2

1Division of Aerospace Engineering, Korea Advanced Institute of Science andTechnology, Daejeon, Korea2Division of Ocean System Engineering, Korea Advanced Institute of Science andTechnology, Daejeon, Korea

The autoignition characteristics of a binary component fuel droplet inside a rapid compres-sion machine (RCM) were investigated. An RCM is an experimental setup for simulating asingle compression stroke of an internal combustion engine. During the experimental time,the temperature and pressure inside the reaction chamber were temporally varied, and theignition of a binary component droplet occurred when sufficient fuel vapor was generatedaround the droplet. n-Heptane was selected as the base fuel, and a specific volumetric ratioof iso-octane and n-decane was added as fuel additives. The droplet was placed at the cen-ter of the reaction chamber by suspending at the tip of a thermocouple, and its transienttemperature was measured. The initial droplet diameter was in the range of 450−1000 µm,and the temporal variation of droplet diameter was measured by analyzing images from ahigh-speed charge-coupled device array camera. In the single component experiments, amonotonic increase of the ignition delay with droplet diameter was observed, and the gra-dient was different for n-heptane and n-decane. The iso-octane droplet did not ignite inthe present experimental conditions. The addition of iso-octane in n-heptane increased theignition delay by dissipation of n-heptane vapor and an autoignition temperature change.The ignition delay and maximum droplet diameter ignition limit rapidly changed when thevolume fraction of iso-octane was approximately 0.5, and the effect of iso-octane on theignition delay was diminished for lower volume fractions. The ignition delay also increasedwith n-decane addition by suppressing the evaporation process. Compared to iso-octane, theincrease in the n-decane volume fraction resulted in a gradual change of the ignition delayand maximum droplet diameter ignition limit due to the different mechanism of the ignitionsuppression process.

Keywords: Autoignition; Binary component fuel; Droplet; Rapid compression machine; Transient condition

INTRODUCTION

The ignition characteristics of a fuel droplet in an internal combustion engine havelong been an important issue for researchers because they are closely related to engineperformance and pollutant emission (Ganesan, 2007). For better performance and lower

Received 20 May 2014; revised 1 August 2014; accepted 28 August 2014.Address correspondence to Seung Wook Baek, Division of Aerospace Engineering, School of Mechanical,

Aerospace and Systems Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291Daehak-ro (373-1 Guseong-dong), Yuseong-gu, Daejeon 350-701, Korea. E-mail: swbaek@kaist.ac.kr

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gcst.

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660 H. KIM ET AL.

pollutant emission, new-generation engines, such as the variable compression rate (VCR)engine and the homogeneous charge compression ignition (HCCI) engine, have been devel-oped. The optimal ignition and combustion mode for internal combustion engines were alsocarefully examined.

One of the crucial problems for engines during operation is unexpected autoignition,which is called ‘knocking’. When the temperature inside the engine cylinder is too highor low, ignition of the fuel-oxidizer mixture occurs faster or slower than intended. Manyresearchers have been dedicated to this problem because it decreases durability and engineperformance. It is also a critical obstacle to HCCI engine development. One of the solutionsis fuel reformation, which changes the ignition and combustion characteristics of the fuel.The performance of reformed fuel is compared with the binary component fuel, and it is cat-egorized by ‘octane-number’ and ‘cetane-number’ (Hollembeak, 2004). Thus, autoignitionresearch on binary component fuel would enhance the understanding of various fuels.

The autoignition characteristics of a binary component fuel droplet are affected byseveral parameters, including the composition of the fuel, the surrounding gas temperature,and the chemical kinetics of the fuel vapor. Moreover, the heat and mass diffusion pro-cesses in the vicinity of the droplets are also significant factors for ignition (Borghesi et al.,2011). It is, therefore, very important to consider each factor to understand the autoignitionbehavior of binary fuel droplets.

A number of studies have reported the ignition and combustion characteristics of abinary fuel droplet in various fuel blends and ambient conditions. Liu and Avedisian (2012)conducted an experimental study of the combustion behavior of n-heptane/toluene andn-heptane/iso-octane blends to find a proper replica of commercial grade gasoline droplets.They found that a 5% heptane and 95% toluene mixture droplet replicated the burning rateof gasoline well in the quasi-steady period. Moreover, none of the mixture fractions exam-ined for heptane/iso-octane or heptane/toluene blends matched the flame standoff or sootstandoff ratios of gasoline, with values being consistently higher than gasoline throughoutthe droplet burning period. Mikami et al. (1997) observed the combustion characteristics ofan n-heptane/n-decane mixture droplet at elevated pressure. In their research, the impor-tance of inert-gas dissolution in the liquid fuel near the critical point was evaluated, andnon-monotonic dependence of the dissolution on the initial fuel composition was demon-strated. In Khan’s (2007) research, an ignition delay of an n-heptane/n-hexadecane binarydroplet at elevated temperature and pressure was described. The result showed a decreasein the ignition delay at higher pressure and temperature conditions. In addition, there wasa certain point at which the ignition delay leveled off because the controlling fuel for theignition changed as the concentration of the fuel changed. Avedisian and Callahan (2000)performed an experimental study of nonane/hexanol mixture droplet combustion with-out convection flow. The droplet diameter change during combustion was measured. Theyfound that the addition of hexanol lowered the burning rate of the droplet by decreasing theflame temperature.

Bulk ignition and combustion of binary/multicomponent fuel spray in combustiondevices were also studied by several researchers. Carr et al. (2012) showed that the addi-tion of toluene to hexadecane causes a nearly identical relative increase in both the physicaland chemical ignition delay. Their model predicted trends in ignition delay that were sim-ilar to those observed in the experimental result, although the predicted ignition delayswere significantly lower than the observed values. Möller et al. (2013) conducted numeri-cal research on multi-component spray ignition in internal combustion engines. They foundthat optimization of the parameters of the multi-component spray model could improve the

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IGNITION OF A BINARY COMPONENT FUEL DROPLET 661

prediction results. Their results were compared with experimental results for validation.In the study of Kirsch et al. (1981), ignition of decane and blends of decane with lowercetane was performed. They found that the cetane number was insufficient to characterizethe ignition quality of fuels with significant aromatic content. Moreover, the effectivenessof additives on ignition enhancement was shown to depend on the physical conditions,including the temperature and the duration of the ignition delay.

Although these previous studies are valuable, they are limited with respect toobserving ignition characteristics of binary fuel droplets. In the case of a single dropletexperiment, most of the previous studies were conducted in steady ambient conditions, soit is difficult to consider the change in ambient pressure and temperature conditions. On theother hand, bulk ignition studies with spray combustion are not suitable for observing thespecific droplet behavior because the effects of neighboring droplets are significant. Thus,a study analyzing the ignition characteristics of an isolated binary fuel droplet in time-varying ambient conditions is essential for fundamental understanding of droplet behaviorin transient conditions.

The main goal of the study is to experimentally observe the ignition characteristics ofa binary component fuel droplet in a time-varying environment using an rapid compressionmachine (RCM). In this study, a single binary component fuel droplet was installed insidea reaction chamber by suspending it at the tip of a 50-µm diameter fine thermocouple, andthe temporal variation in the droplet diameter and bulk-temperature of droplet inside tem-perature were observed. The ignition delay was measured by imaging the droplet using ahigh-speed camera. n-Heptane was set as the base fuel, and iso-octane and n-decane wereselected as additive fuels. A comparison of the ignition characteristics between addition ofiso-octane and n-decane was performed. The present research provides basic informationregarding the ignition characteristics of a binary component fuel droplet at engine-like tran-sient conditions. Moreover, it may suggest ignition control strategies for diesel (DI)-HCCIengines, which is currently an important challenge (Ma et al., 2008).

EXPERIMENTAL SETUP

A schematic of the experimental apparatus is shown in Figure 1a. The experimen-tal setup is composed of an RCM, sensors, and an optical observation system, includinga high-speed charge-coupled device (CCD) array camera and a post-processing sys-tem. The storage of the sensors and camera data was performed by a linked personalcomputer (PC).

RCM

An RCM is a device for simulating homogeneous charge compression conditionswith a single compression stroke. It is suitable for observing the autoignition characteris-tics of fuel because of the disregard of the combustion product effects and the availabilityof optical observation (Guibert et al., 2010). A sectional view of the RCM used in theexperiment is shown in Figure 1b, with a mark for each part, such as the reaction chamber,driving chamber, and driving piston. The detailed specifications of the RCM are describedin a previous article (Kim et al., 2014a). A crevice design in the reaction chamber pistonwas applied to reduce corner vortices (Lee and Hochgreb, 1998). Pressure sensors and athermocouple were installed at the wall of the RCM.

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662 H. KIM ET AL.

a)

Adjust wheel

Reaction chamberDriving chamber

ClearanceQuartz window

Syringe

25

24174

Crevice piston (Unit : mm)

b)

Figure 1 (a) Schematic of the experimental setup and (b) sectional view of the RCM with a mark for each part.

It is important to reproduce identical experimental conditions in each experimentbecause a discrepancy in the mechanical operation may generate deviations in the operationtime and peak pressure, which eventually lead to experimental errors. When the validationexperiment was conducted with the same operation conditions, a maximum 1.7% differencewas observed for the operation time and 1.2% for the peak pressure.

Thermocouple and Pressure Transducer

Several methods for single droplet installation inside a combustor have been sug-gested by previous researchers (Ghassemi et al., 2006; Honnery et al., 2013; Watanabeet al., 2010). Among these, the use of thermocouple was selected in the present study

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IGNITION OF A BINARY COMPONENT FUEL DROPLET 663

because of its several advantages, including stable droplet installation and measurementof the droplet temperature. A Ch−Al sheathed K-type thermocouple (Omega Engineering,Inc.) was used for droplet suspension. The cover tip of the thermocouple was removed, andan approximately 100-µm bead was formed by welding the two 50-µm inner wires. Thethermocouple was installed at the wall of the reaction chamber with a proper connector forhigh pressure. A tip of the thermocouple, where the single droplet was installed, was placedat the center of the reaction chamber. A 200-µm diameter surgery needle was used for thedroplet installation, and it was removed prior to each experiment. To avoid the effect of thefuel vapor from detached droplet or residue, the reaction chamber was properly purged withdry air. The detailed calibration procedure of the thermocouple is shown in the references(Kim et al., 2014a; Park and Ro, 1996). The standard error limit of thermocouple was about0.4% (Kim et al., 2014b).

The pressure inside the reaction chamber was measured using the pressure trans-ducers (Sensys, Inc., PMS) shown in Figure 1a, which can measure the pressure range of1–70 bar. A cooling accessory was provided to minimize the thermal shock effect.

Time synchronized pressure and temperature data were recorded with a resolutionof 1 ms. Data were acquired using a data acquisition (DAQ) system (IOtech Inc., PDAQ3000 series) and stored using a PC. The embedded program for noise reduction of thepressure and temperature data was used.

Optical Observation Setup

Optical access of the droplet was provided through 10-mm diameter quartz windowsinstalled in the reaction chamber. A high-speed CCD array camera was used to image thedroplets at 500 frames per second. The images were processed using a Visual Basic pro-gram to extract the temporal variation of the droplet diameter. A calibration of scale wasconducted using a known reference, and the scale factor for correction was inserted in theprogram. As shown in Figure 2, the droplet boundary in the image was determined from thedark pixels at the edge of the droplet. Preliminary experiments were conducted to find theoptimum color code for the detection of the droplet boundary. The area of the droplet wasmeasured by counting the number of pixels inside the boundary, and the effective diam-eter was calculated (Khan, 2007; Kim et al., 2014a, 2014b). Uncertainties of the dropletdiameter measurement were reported to be about ±3% (Kim et al., 2014a).

Ignition of the droplet was judged by observing a luminous yellow flame in thedroplet image. A detailed explanation of the ignition decision process is presented in thenext section. A light-emitting diode (LED) backlight with a filter was used to avoid thermalradiation effects and brightness oscillations of the light source. The color temperature ofLED was 5000 K.

Properties of the Fuels

n-Heptane, iso-octane, and n-decane were chosen as the experimental fuels. Amongthese, n-heptane was set as the base fuel, and specific volume fractions of iso-octane andn-decane were added for binary component fuel. The thermodynamic properties of thethree fuels at atmospheric conditions are presented in Table 1, and the change in the vari-ables with temperature and pressure is correlated in Poling et al. (2000). The propertiesof iso-octane are similar to n-heptane except for the autoignition temperature due to the

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664 H. KIM ET AL.

1mm

Figure 2 Image processing for the measurement of the droplet diameter.

Table 1 Thermophysical properties of each fuel at atmospheric pressure

n-Heptane iso-Octane n-Decane

Density (g/mL) 0.680 0.692 0.730Boiling point (◦C) 98 99 174Specific heat (J/kg·mol) 224.64 242.49 315.46Heat of vaporization (kJ/kg·mol) 31,698 31,008 39,279Autoignition temperature (◦C) 223 396 210

different molecular structure. In the case of n-decane, the autoignition temperature is sim-ilar to n-heptane, but the other properties are different because of the heavier molecularweight.

It is necessary to consider the variation in the boiling temperature because it indicatesthe concentration of fuel vapor around the droplet, which is proportional to the ratio of theboiling temperature and the droplet surface temperature. To calculate the boiling temper-ature of the droplet in the RCM, the equation from the Korea Thermophysical PropertiesData Bank (KDB) was used. The vapor pressure and droplet boiling temperature are relatedas follows:

ln(pkPa) = A ∗ ln(Tboil) + B

Tboil+ C + D ∗ Tboil

2 (1)

where the coefficients of the equation are A = −14.12388, B = −8030.070, C = 108.1461,and D = 1.204855 × 10−5 for n-hepatne; A = −1.08858, B = −7041.769, C = 86.7458,and D = 8.872766 × 10−6 for iso-octane; and A = −7.76881, B = −8163.335, C =69.7646, and D = 2.620333 × 10−6 for n-decane. The temperature when the vapor andambient pressure are equivalent is set as the boiling temperature. The changes in the reac-tion chamber pressure and boiling temperature of the experimental fuels are presented

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IGNITION OF A BINARY COMPONENT FUEL DROPLET 665

0 100 200 300 4000

100

200

300

400

Boiling temperaturen-heptane

iso-octane

n-decane

Pre

ssu

re (

bar)

Tem

pera

ture

(°C

)

Time (ms)

Critical Point

0

10

20

30

40

50

Reaction chamber

pressure

TDC

Figure 3 Time variation of the pressure and boiling temperature of each fuel.

in Figure 3. The calculation was conducted at subcritical pressure only, and the criticalregion of each fuel was marked. This graph shows that the characteristics of the volatilityin the RCM conditions are similar for n-heptane and iso-octane because the variation inthe boiling temperature is similar. On the other hand, the boiling temperature of n-decanewas higher than the others by approximately 70◦C because of the relatively non-volatilebehavior of n-decane.

Even though the autoignition temperature is an important factor, there are no standardvalues because it is affected by several factors, such as the concentration of the fuel vaporand the dimensions of the container. Moreover, there are limited data on the autoignitiontemperature at high pressure conditions. Previous research qualitatively showed that theautoignition temperature of hydrocarbon fuel decreased as the ambient pressure increased(Brandes et al., 2005).

RESULTS AND DISCUSSION

Experimental Conditions and Measurement Techniques

The concentration of each fuel and initial droplet diameter were set as the experi-mental conditions. Prior to the binary component droplet experiment, single componentdroplet experiments were conducted for basic research. The volumetric mixing ratios of thebase fuel (n-heptane) and additive (iso-octane and n-decane) were 0.75:0.25, 0.5:0.5, and0.25:0.75. The effects of the initial diameter on droplet ignition were observed by changingthe droplet diameters over the experimental range of 450–1000 µm. A comparison of howthe ignition characteristics depend on the additive fuels is discussed in the following.

The ignition delay of the droplet is divided in two parts: physical delay and chemicaldelay. The physical ignition delay is the time spent for the droplet by heating, evaporation,and mixing with the surrounding air. The chemical ignition delay occurs due to the chemicalreactions of fuel/air mixture until the appearance of a hot yellow flame. The total ignitiondelay is the sum of these two ignition delays (Moriue et al., 2000).

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Following the procedure of previous studies (Jeong and Lee, 2008; Whang et al.,1997), the ignition point was determined by observing the appearance of a yellow flame inthe image, and the total ignition delay was calculated by counting the number of framesbetween the start of the experiment and the ignition point. Most of the droplets installed atthe tip of the thermocouple wire became detached during combustion due to the decreasedsurface tension in the high-temperature environment. Thus, the droplet temperature dataafter detachment of the droplets were removed.

The compression time of RCM was 215 ± 2 ms, and the peak pressure inside thereaction chamber was 28.6 bar. The variation in the pressure conditions inside the chamberis presented in Figure 3. The pressure rapidly increased during the compression stroke andstarted to decrease after top dead center (TDC) because of heat loss through the chamberwall. The estimation of peak temperature inside the reaction chamber was conducted by thecore gas hypothesis (Lee and Hochgreb, 1998), which is denoted by:

∫ T(t)

T0

γ

γ − 1

dT

T= ln

[P(t)

P0

](2)

The estimated temperature of the core was 470◦C in the present condition.The experiments were conducted at room temperature (i.e., 25◦C ± 1.5◦C) and atmo-

spheric pressure. Before each experiment, the reaction chamber was charged with dry air toremove the fuel vapor and combustion products remaining from the previous experiments.

Ignition of a Single Component Droplet

Observation of the single component droplet behavior before the binary componentdroplet experiment was conducted provided additional information on the ignition behaviorof each fuel in the RCM condition. A detailed explanation of the ignition characteristicsof a single n-heptane droplet in RCM conditions was presented in the previous research(Kim et al., 2014b). Thus, a comparison of each single component droplet behavior washighlighted in this study.

Figure 4 shows the time variation of the droplet temperature and the normalizeddroplet diameter of each fuel for similar droplet diameters. The starting point of the droplettemperature was similar for n-heptane and iso-octane, but it was 11◦C higher for n-decanebecause the vapor pressure of n-decane is significantly lower than the others under equiva-lent ambient conditions, which causes a higher wet-bulb temperature of the droplet (Milleret al., 1998).

In the early stages of compression, the droplet temperature increased as the ambienttemperature increased. The increasing rate was similar for n-heptane and iso-octane butsmaller for n-decane because of the larger heat capacity of the n-decane droplet; moreheat was needed for n-decane droplet heating. The boiling temperature of each fuel alsoincreased with ambient pressure, so the temperature of the droplets remained far below theboiling temperature. The diameters of all droplets inflated during this period due to thermalexpansion (Honnery et al., 2013; Khan, 2007).

As the compression stroke proceeded, the increasing rate of the n-decane droplettemperature rose faster than the others. The main reason for this difference was the differentevaporation rates. Compared to the n-decane, the temperatures of n-heptane and iso-octanewere closer to their boiling temperatures. In this circumstance, the concentration of fuelvapor around the n-heptane and iso-octane droplet surface was higher, which resulted in a

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IGNITION OF A BINARY COMPONENT FUEL DROPLET 667

0 100 200 300 4000

100

200

300

400

No ignition

Droplet

Temperature

n-heptane

iso-octane

n-decane

Tem

pera

ture

(°C

)

Time (ms)

Ignition

TDC

0.9

1.0

1.1

n-heptane (575 µm)

iso-octane (582 µm)

n-decane (582 µm)

No

rmalized

dro

ple

t d

iam

ete

r

Boiling

Temperature

Figure 4 Time variation of the boiling temperature, droplet temperature, and normalized droplet diameter foreach fuel.

more active evaporation process. Thus, heat consumption for evaporation increased, and ithindered the increase of the droplet temperature. On the other hand, the droplet temperatureof n-decane was still far from the boiling temperature, so evaporation was relatively inactiveand a greater portion of heat from the ambient gas could be used for droplet heating. Thetemperature of all fuel droplets was lower than the boiling and critical point during thecompression period, so even though the ambient pressure exceeded the critical pressure ofthe fuels, they remained in the sub-critical region.

After compression, the air temperature inside the reaction chamber decreased dueto the heat loss through the cylinder walls. However, the temperature of the droplets stillincreased after compression because the droplet temperatures were below the boiling tem-perature and the ambient air temperature, even after the piston reached TDC. The diametersof all droplets also slightly increased during this period. The temporal variation in thedroplet temperature and diameter before ignition was almost identical for n-heptane andiso-octane due to the similarity in their thermophysical properties.

When the droplet temperature of n-heptane reached approximately 100◦C, ignitionoccurred, which shows that as the temperature of the n-heptane droplet reached 100◦C, asufficient fuel/air mixture was generated for ignition (Kim et al., 2014b). In the case ofn-decane, the droplet temperature at the ignition point was approximately 150◦C because ithas lower volatility than n-heptane. More time was required for the droplet to reach suffi-cient fuel vapor generation, and consequently, ignition was delayed. The result shows thateven though these fuels have similar autoignition temperatures, the difference in volatil-ity affects the physical ignition delay in the RCM conditions. In the case of the iso-octanedroplet, ignition did not occur during the experimental time, and a pure evaporation processwas observed. Considering that the evaporation characteristics of iso-octane droplet weresimilar to those of the n-heptane droplet, the high autoignition temperature of iso-octane is

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400 600 800 1000200

220

240

260

280

300

Ignitable

range

n-heptane

n-decane

Ign

itio

n d

ela

y (

ms)

Initial droplet diameter (µm)

TDC

n-heptane

n-decane

Figure 5 Ignition delay of single-component droplets for different initial droplet diameters.

the main factor of the lack of autoignition. After ignition, a rapid increase in the n-heptaneand n-decane droplet temperatures was detected by the heat feedback from the flame. Thedroplet temperature of iso-octane gradually increased until it reached approximately 176◦Cand started to fall. The diameters of the droplet also started to decrease after 300 ms byevaporation.

The ignition delays of the fuel droplets for different diameters are presented inFigure 5. The iso-octane droplet did not ignite in the range of the experimental dropletdiameters. The result showed monotonic increment of the ignition delay with droplet diam-eter due to the larger heat capacity, as was discussed in previous study (Kim et al., 2014b).In the case of n-decane, the increasing rate of ignition delay was much steeper than that ofn-heptane, and it was mainly due to the smaller volatility compared to n-heptane. Ignitiondid not occur when the diameter of the n-decane droplet exceeded about 590 µm, whereasit exceeded 1000 µm in the case of n-heptane, which shows that the delay of fuel vaporformation leads to a narrower ignitable range.

Ignition of an n-Heptane/iso-Octane Droplet

To evaluate the effect of the iso-octane concentration on ignition delay, a portion ofiso-octane was added to n-heptane. The temporal variation in the droplet temperature forvarious iso-octane volume fractions is shown in Figure 6. In all cases, the droplet temper-ature started to increase from the start of compression. The increasing rate of the droplettemperature was almost identical regardless of the iso-octane concentration, mainly dueto the similar thermophysical properties of n-heptane and iso-octane. However, distinctivedifferences are shown in the ignition point. When the volume fraction of iso-octane was0.25, ignition occurred at approximately 100◦C, whereas it occurred at 130◦C when theiso-octane portion was 0.75. After the ignition, a rapid increase in the droplet tempera-ture was observed due to the heat from the chemical reaction. The temporal changes in the

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IGNITION OF A BINARY COMPONENT FUEL DROPLET 669

iso-octane volume fraction

0.75 (633 µm)

0.5 (625 µm)

0.25 (623 µm)

0 100 200 300 4000

100

200

300

400

iso-octane B.T.

Te

mp

era

ture

(°C

)

Time (ms)

Ignition

TDC

B.T. = Boiling temperature n-heptane B.T.

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Figure 6 Time variation of the boiling temperature, droplet temperature, and normalized droplet diameter fordifferent iso-octane volume fractions.

a) b) c) d)

Figure 7 Location of the ignition for a binary component droplet when the iso-octane volume fraction was(a) 0.75, (b) 0.5, (c) 0.25, and (d) 0.0.

droplet diameter for each case are also shown in Figure 6. In all cases, similar variation inthe droplet diameter was observed during the experimental time.

Figure 7 shows the location of ignition for different iso-octane volume fractions. Forthe iso-octane volume fraction of 0.75, the flame was generated far downstream of thevapor, which is in conflict with the characteristics of droplet ignition in a high-pressureenvironment (Stauch et al., 2006). This may due to a combination effect of the ignitiondelay and the Stefan flow. Moriue et al. (2000) showed that relatively strong Stefan flow isgenerated for mixtures of similarly volatile fuels because of the simultaneous evaporation

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of both fuels. Moreover, a negative effect of the Stefan flow was also observed for igni-tion because of the reduced concentration of the reactants. In our case, ignitable n-heptanevapor moved downstream due to the high Stefan flow. In addition, the high concentrationof iso-octane vapor acted as an inert gas in the present RCM conditions and influenced thenegative effect on droplet ignition by dissipating the n-heptane vapor around the dropletsurface. Due to the above effects, the ignition was delayed and a disparity in the ignitionlocation eventually occurred. The flame that was generated downstream flowed backwardsand covered the whole droplet within 4 ms.

As the volume fraction of iso-octane decreased, the ignition point approached thedroplet surface because the concentration of iso-octane vapor was so low that a sufficientconcentration of n-heptane vapor could be generated in the vicinity of the droplet surface.When the volume fraction of iso-octane decreased to 0.25, the ignition delay recoveredclose to the pure n-heptane case. In all cases, a luminous flame covered the whole dropletbecause of soot formation.

The ignition delay for different fuel volume fractions and initial droplet diametersis presented in Figure 8. In all cases, the ignition delay was increased with initial dropletdiameter, as discussed previously. When the volume fraction of iso-octane was 0.75, thenegative effect of the iso-octane vapor on ignition was severe. The ignition delay remark-ably increased in this case, and ignition did not occur for droplet diameters over 700 µm.On the other hand, when the volume fraction of iso-octane decreased further to 0.5 and0.25, a similar trend of ignition delay was observed, except for a small increase at the vol-ume fraction of 0.5. It implies that a critical change in the ignition delay occurred at aniso-octane volume fraction between 0.75 and 0.5 and that the effect of iso-octane becameminor with further decrease. Detailed experiment was conducted in the range of criticalchange where the volume fraction of n-heptane was 0.25, 0.3, 0.4, and 0.5. The initialdroplet diameters were 600 ± 20 µm, and five times of experimental data were averaged

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Initial droplet diameter (µm)

iso-octane

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0.75

0.5

0.25

0.75

0.5

0.25

Figure 8 Ignition delay for different initial droplet diameters and iso-octane volume fractions.

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0.3 0.4 0.5200

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Ign

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n-heptane volume fraction

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Figure 9 Ignition delay for n-heptane volume fraction of 0.25, 0.3, 0.4, and 0.5.

for each volume fraction. The result shown in Figure 9 presents a decrease of ignition delayas the volume ratio of iso-octane decreased.

However, when the iso-octane volume fraction decreased further than 0.5, the concen-tration of n-heptane vapor at the droplet surface was sufficient that the role of iso-octanevapor as an ignition obstructer was weak. A previous study (Zabetakis et al., 1954) sup-ports the result that when the concentration of iso-octane was 70%, a rapid change of theautoignition temperature of n-heptane/iso-octane mixture was measured. Ignition did notoccur when the initial droplet diameter exceeded 930 µm for an iso-octane volume fractionof 0.5, and it increased to 1000 µm for a volume fraction of 0.25.

Ignition of n-Heptane/n-Decane Droplet

Figure 10 represents the temporal variation in the droplet temperature for vari-ous n-decane volume fractions. As explained previously, the initial droplet temperatureincreased as the volume fraction of n-decane increased because of the higher wet-bulb tem-perature. When the volume fraction of n-decane was 0.75, the initial droplet temperaturewas 23◦C, and it decreased to 17◦C for the volume fraction of 0.25. The increase in the wet-bulb temperature for higher n-decane volume fraction droplets is strong evidence of lowervolatility.

The droplet temperature increased during the compression stroke and after TDCbecause of the heat feedback from the ambient gas. The increasing rate of the droplet tem-perature was slightly faster for the low n-decane volume fraction case, but the differenceswere not significant during the experimental time, which can be understood by consider-ing the evaporation process. Although the heat capacity for the higher n-decane volumefraction droplets was larger, a greater portion of the heat from the ambient air could beused for droplet heating due to the suppression of evaporation, and it may reduce the differ-ences in increasing rate of droplet temperature. This outcome shows that multiple factorsare involved in the droplet temperature change during the transient heating process. In all

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n-decane volume fraction

0 100 200 300 4000

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n-heptane B.T.

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0.5 (625 µm)

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Figure 10 Time variation of the boiling temperature, droplet temperature, and normalized droplet diameter fordifferent n-decane volume fractions.

cases, the diameter of the droplet increased until autoignition because sufficient dropletheating was not reached.

Ignition first appeared for the n-decane volume fraction of 0.25 when the droplettemperature at ignition was 100◦C, which is similar with the pure n-heptane case. However,as the volume fraction of n-decane increased to 0.75, the droplet temperature at ignitionincreased to 140◦C. Because the autoignition temperatures of n-heptane and n-decane aresimilar, the change in volatility is a principal factor for the delay of ignition. Additionaltime for the droplet heating process was needed for droplets with high n-decane volumefraction.

Figure 11 represents the location of ignition at different n-decane volume fractions.Unlike the case of iso-octane, there were no distinctive differences in the ignition location;ignition occurred in the vicinity of the droplet surface, which is a typical characteris-tic of droplet ignition at high pressure. Previous research (Moriue et al., 2000) showedthat fractional distillation phenomena occurred for fuel mixtures with different volatilities.In this circumstance, n-heptane vapor was mainly generated, and it accumulated in thevicinity of the droplet surface due to the relatively lower Stefan flow. Ruszalo and Hallett(1992) explained that the more volatile component controls the ignition delay. Moreover,the droplet may be ignited as soon as there is sufficient fuel vapor generation becauseof the low autoignition temperature. Thus, in the case of the n-heptane/n-decane binarycomponent droplet, the formation of n-heptane fuel vapor is a key factor for ignition, andn-decane hindered the ignition by lowering the evaporation rate. The results showed thatthe role of n-decane is an ignition disrupter, which is the same as for the iso-octane case,but the mechanism was completely different.

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a) b) c) d)

Figure 11 Location of ignition for a binary component droplet when the n-decane volume fraction was (a) 0.75,(b) 0.5, (c) 0.25, and (d) 0.0.

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n-decane

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0.25

Figure 12 Ignition delay for different initial droplet diameters and n-decane volume fractions.

The ignition delay for different n-decane volume fractions and initial droplet diam-eters is shown in Figure 12. When the volume fraction of n-decane was 0.75, the delayof ignition was the largest, and it gradually decreased with n-decane volume fraction. Theeffect of n-decane became minor for the n-decane volume fraction of 0.25. In the case ofan n-decane volume fraction of 0.5, the result for the droplet diameter less than 650 µmwas similar to the 0.75 case due to the faster increase in the droplet temperature. The rela-tively higher droplet temperature of the smaller droplet generated sufficient n-heptane vaporfaster. However, the differences in the ignition delay increased when the droplet diameterexceeded 650 µm because of the lower droplet temperature. The maximum ignitable diam-eter for the n-decane volume fraction of 0.75 was 670 µm, and it increased to 1000 µm fora volume fraction of 0.75. Compared to the case of iso-octane, the addition of n-decane

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showed more gradual variation of ignition delay at different additive volume fractions.A detailed explanation is discussed in the next section.

Maximum Droplet Diameter Ignition Limit for Different Additives

The definition of the maximum droplet diameter ignition limit, which was introducedby Kim et al. (2014b), is the maximum droplet diameter that is auto-ignitable in the RCMconditions. If the droplet diameter exceeded the maximum droplet diameter ignition limit,the droplet did not ignite due to the delay in droplet heating and fuel vapor generation.

The maximum droplet diameter ignition limit for different iso-octane and n-decaneconcentrations are presented in Figure 13. At the n-heptane volume fraction of 0.25, bothmixtures showed reduced maximum droplet diameter ignition limits approximately around700 µm, which verifies the roles of iso-octane and n-decane as ignition disrupters with themechanisms shown previously.

The gap in the maximum droplet diameter ignition limit between the two binarycomponent fuels expanded at the n-heptane volume fraction of 0.5. A rapid increase inthe maximum droplet diameter ignition limit was observed in the iso-octane case, whereasa gradual variation was shown with the n-decane addition. The main reason for this is thedifferences in the ignition control mechanism. In the case of n-decane, fractional distillationphenomena cause the liquid concentration of n-heptane at the droplet surface to shrinkrapidly because the liquid n-heptane cannot be supplied to the surface area from insidein a short time by the low mass diffusivity of liquid (Moriue et al., 2000; Takei et al.,1993). Thus, only liquid n-heptane at the droplet surface was involved in the autoignitionprocess, and it changed the ignition delay and the maximum droplet diameter ignition limitgradually.

On the other hand, iso-octane also evaporated as much as n-heptane in the case ofn-heptane/iso-octane binary fuel. Therefore, liquid n-heptane inside the droplet was sup-plied for ignition by the vigorous evaporation process. Indeed, a high level of n-heptane

0.25 0.50 0.75

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Figure 13 Maximum droplet diameter ignition limit of each binary component droplet for different n-heptanevolume fractions.

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fuel vapor was generated in the vicinity of the droplet surface in the case of the n-heptanevolume fraction of 0.5. The change in the autoignition temperature for different iso-octanevolume fractions discussed previously was another factor for the rapid rise in the maximumdroplet diameter ignition limit.

In the case of an n-heptane volume fraction of 0.75, the maximum droplet diameterignition limit was reached at 1000 µm for additives of both n-decane and iso-octane, whichis similar to the case of pure n-heptane. This shows that sufficient n-heptane vapor forignition was generated regardless of the addictive fuel type. At this concentration, the effectof the fuel additive did not significantly influence the maximum droplet diameter ignitionlimit.

The present research shows the ignition delay change of a binary component dropletin RCM conditions when iso-octane or n-decane was added to n-heptane. The resultsstrongly present that the ignition characteristics for each ignition condition must becarefully considered when spray combustion devices are designed.

CONCLUSIONS

The autoignition characteristics of an n-heptane/iso-octane and n-heptane/n-decanebinary component fuel droplet at RCM conditions were experimentally investigated. Thevolume fraction of each fuel additive and the initial droplet diameter were considered asthe experimental parameters, and a comparison study of the effect of each fuel additive wasperformed. The major findings of this study are summarized as follows:

1. For a single component droplet, a monotomic increase in the ignition delay with theinitial droplet diameter was observed. In the case of n-decane, the increasing rate of theignition delay was larger than that of the n-heptane droplet due to its lower volatility.The pure iso-octane droplet did not ignite in the experimental range due to its highautoignition temperature.

2. The addition of iso-octane increased the autoignition temperature of the n-heptanedroplet. Ignition was delayed when the volume fraction of iso-octane was 0.75 due tothe dissipation of n-heptane vapor. The effect of iso-octane was reduced as the volumefraction decreased below 0.5 because of the sufficient supply of n-heptane vapor by theactive evaporation process and the rapid change in the autoignition temperature. A rapidincrease in the maximum droplet diameter ignition limit was also detected between theiso-octane concentrations of 0.25 and 0.5.

3. The addition of n-decane increased the ignition delay by suppressing the evaporation ofthe binary component fuel. Gradual variation in the ignition delay and maximum dropletdiameter ignition limit at different n-decane volume fractions was observed due to thefractional distillation phenomena of liquid fuel at the droplet surface. A limited effect ofthe liquid n-heptane fuel inside the droplet was also discussed.

FUNDING

This work was supported by the National Research Foundation of Korea (NRF) grantfunded by the Korea government (MEST) (No. 2014R1A2A2A01007347).

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