Fuel 93 (2012) 539–548

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Fuel

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Liquid length and vapor penetration of conventional, Fischer–Tropsch, coal-derived,and surrogate fuel sprays at high-temperature and high-pressureambient conditions

Sanghoon Kook a,⇑, Lyle M. Pickett b

a School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney NSW 2052, Australiab Combustion Research Facility, Sandia National Laboratories, P.O. Box 969, MS 9053, Livermore, CA 94551, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 December 2010Received in revised form 27 September 2011Accepted 4 October 2011Available online 20 October 2011

Keywords:Jet fuelDiesel engineLiquid lengthVapor penetrationSpreading angle

0016-2361/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.fuel.2011.10.004

Abbreviations: ASI, after start of injection; CMOS,semiconductor (camera); CO, carbon monoxide;di-methyl ether; EGR, exhaust gas recirculation;maximum; GTL, gas-to-liquid; Nd:YAG, neodymiumgarnet; UHC, unburned hydrocarbon.⇑ Corresponding author. Tel.: +61 2 9385 4091; fax

E-mail addresses: s.kook@unsw.edu.au (S.(L.M. Pickett).

Considering liquid fuels from a variety of feedstocks and refinement processes, evaporating sprays of var-ious fuels are investigated in a constant-volume chamber at ambient and injection conditions typical of adiesel engine. For diesel sprays, the liquid phase of the spray reaches a maximum penetration distancesoon after the start of injection, while the vapor phase of the spray continues to penetrate downstream.This liquid-phase penetration distance or in short, the ‘‘liquid length’’, depends upon the ambient andfuel-injector conditions. To clarify fuel effects on liquid length and vapor penetration, in this study we per-formed simultaneous high-speed imaging of Mie-scattering and schlieren at approximately 50-kHz fram-ing rates. Experiments were performed at specified ambient gas temperature and density in anenvironment with 0% oxygen. This non-reacting condition was selected intentionally to isolate the mixingand vaporization processes from more complex effects of combustion. Fuels with a wide range of boilingpoints and densities were examined, including a conventional No. 2 diesel, low-aromatics jet fuel, world-average jet fuel, Fischer–Tropsch synthetic fuel, coal-derived fuel, and a two-component surrogate fuel.Results show that the liquid length increases with increasing fuel density for fuels of similar boiling-pointtemperature. Considering a mixing-limited vaporization process, entrained ambient mass per fuel mass isinversely proportional to the fuel density, and thereby, high fuel density causes a decrease in ambient-to-fuel ratio, which increases liquid length. Consequently, both boiling point and density of fuel affectliquid length. In contrast to the liquid-length result, no correlation between fuel density and vapor pene-tration was found. This is because the momentum flux, which governs the jet penetration, is not a functionof the fuel density. Although the liquid length varies, the similarity in vapor penetration for all fuels inves-tigated shows that the fuel density and boiling point do not significantly affect the total entrainment andmixing into the spray.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction biomass, and coal) will account for about 25% of the transportation

The depletion of conventional fossil fuels has prompted researchon alternative fuels for utilization in a wide range of vehicles. Ofparticular interest are paraffinic synthetic fuels manufacturedthrough Fischer–Tropsch and other processes and their usage inhigh-efficiency diesel engines [1–4]. For example, predictions showthat synthetic fuels produced from non-fossil feedstocks (e.g., gas,

ll rights reserved.

complementary metal–oxideCTL, coal-to-liquid; DME,

FWHM, full width at half-doped yttrium aluminum

: +61 2 9663 1222.Kook), LMPicke@sandia.gov

fuel supply in 2050 [5]. To use these fuels in conventional diesel en-gines, compatibility issues such as fuel lubricity, seals, and agingcharacteristics must be addressed. At the same time, to extractthe maximum benefits from new fuels through optimized enginedesign and calibration, a detailed understanding of spray and com-bustion characteristics of these fuels is essential.

In modern diesel engines, fuel in the liquid phase is directlyinjected into the cylinder by using an electronically-controlled com-mon-rail injection system. The fuel injector consists of several-holenozzles, a needle that initially blocks nozzle holes, and a needle-lifting system to allow fuel injection. The common-rail is an accumu-lator containing high-pressure fuel that holds this pressure duringthe fuel injection event. As a control solenoid or piezo is actuated,hydraulic pressures lift the needle, fuel injection begins, andliquid-phase fuel is issued into the hot ambient gases. After the start

540 S. Kook, L.M. Pickett / Fuel 93 (2012) 539–548

of injection, the liquid-phase fuel reaches a maximum penetrationlength (i.e., liquid length), that is nearly constant during the steadyperiod of injection, while the vapor-phase of the spray continuesto penetrate downstream. This liquid length is observed in typicaldiesel sprays and depends upon the ambient and fuel injection con-ditions [6–11]. Presenting data over a wide range of ambient andinjector conditions, Siebers [12] showed that this liquid length isultimately limited by mixing. In other words, sufficient ambientthermal energy and mass are mixed into the spray to produce localthermodynamic conditions where the fuel is vapor phase.

In diesel engines, conventional wisdom dictates that fuel mustbe almost entirely vaporized before the liquid hits the piston bowlwall or cylinder liner [13]. This is because over-penetration of fuelsprays and cylinder-wall wetting can be a concern, particularly inlow-temperature environments [14] or at low-density in-cylinderconditions [10,11,15,16]. In fact, the dilution of fuel into the cylin-der wall oil film can increase piston-to-wall friction and compo-nent wear [16–18] and degrade the quality of bulk engine oil[19]. Wall-wetting also contributes to higher emission of unburnedhydrocarbons (UHC) and carbon monoxide (CO) and, hence,reduced combustion efficiency [20–22].

When alternative fuels are used at diesel conditions, there is astrong positive correlation between the liquid length and fuel boil-ing point temperature [23]. A low-boiling-point fuel evaporatesquickly and therefore the liquid length is shorter than that of ahigh-boiling-point fuel [23–25]. For the same reason, oil dilutionfrom liquid over-penetration of fuel sprays is less likely a concernfor fuels with low boiling points. These fuels evaporate more easilyand have a lower tendency to remain in the oil. As such, under-standing the effect of boiling point temperature on the liquidlength is needed as a basic parameter towards understanding over-all fuel spray evaporation.

In contrast to the boiling-point effect, the direct influence offuel density on liquid length is still in question. A scaling law forliquid penetration shows that lower fuel density causes decreasedliquid lengths [10,12] but fuel density and boiling point tempera-ture typically change simultaneously in conventional fuels andtherefore have confounding effects. However, future alternativefuels may have different trends with respect to fuel density andboiling point. To address this question, one may compare fuels thathave variations in both the boiling point and the density.

Vapor penetration is another important parameter to under-stand alternative fuel sprays at diesel conditions. As mentionedabove, the vapor fuel continues to penetrate downstream whilethe liquid-phase fuel penetration stays at the liquid length. Intui-tively, the vapor penetration rate is related to the total air entrain-ment rate and fuel–air mixing. For instance, at fixed injector andambient conditions, a higher vapor penetration rate may cause bet-ter air utilization, which affects diesel-like combustion processessignificantly. Some have reported that fuels with high volatility(e.g., di-methyl ether or DME in short) show better mixing as evi-denced by a higher spreading angle than a conventional diesel[26–28]. However, these observations were for non-evaporatingsprays that tend to have faster penetration and smaller spreadingangle than evaporating sprays [29,30]. One explanation for thistrend is that the evaporation cooling increases local gas density,which slows the newly injected fuel penetration [29]. The velocityslip between the droplet and gas jet is another cause. Also, vapor-ization contraction may contribute to a shorter penetration for theevaporating sprays [30,31]. These concerns mean that use of themeasured vaporizing penetration and dispersion at the high-temperature conditions typical of diesel injection is the best wayto study fuel effects on vapor fuel-ambient mixing. For example,a previous investigation in this laboratory showed that vapor-phase penetration and spreading angle of diesel and JP-8 were wellmatched despite the faster evaporation for JP-8 [32].

To clarify the liquid length and vapor penetration of alternativefuels, in this study simultaneous imaging of liquid spray and vaporfuel are performed at typical in-cylinder conditions of dieselengines. While ambient conditions and injection parameters arefixed, various fuels with a wide range of boiling points and densi-ties have been investigated. Fuel-to-fuel comparisons are made forthe alternative fuels as well as a conventional diesel fuel. The fun-damental insights gained in this study are useful for diesel enginedesign optimization with alternative fuels.

2. Experiments

2.1. Combustion chamber and operating conditions

Fuels sprays were studied in an optically accessible, constant-volume combustion chamber under simulated, quiescent dieselengine conditions. A schematic of the chamber along with the opti-cal setup is shown in Fig. 1. The chamber has a cubical shape,108 mm on each side. Sapphire windows are located in three sideports of the chamber to allow line-of-sight imaging of the injectedfuel spray. A common-rail injector with a single-hole, mini-sacnozzle is mounted in a metal side-port, directing a spray into thecenter of the chamber. More details about the chamber geometry,fuel injection system, and ambient condition generation are avail-able in [33].

The operating conditions and injector hardware are summa-rized in Table 1. Experiments were performed at the specifiedambient gas temperature and density in an environment with 0%oxygen. This non-reacting condition was selected intentionally toisolate the mixing and vaporization processes from more complexeffects of combustion. The ambient gas conditions were selected tomimic operating conditions for the latest generation of low-emission diesel engines. The 900-K temperature and 6-MPapressure therefore represent engine conditions with more cooledexhaust gas recirculation (EGR), slightly lower compression ratio,and more aggressive turbocharging (boost) than conventionalengines. The injector is a standard common-rail diesel injector(Bosch, second generation) with a single, 0.090-mm nozzle. Thenozzle is a mini-sac type and has a convergent, hydro-ground ori-fice (KS1.5/86 specification by Bosch). The common-rail (injection)pressure was fixed at 150 MPa throughout the experiments.

2.2. Optical diagnostics

Two high-speed movies were collected simultaneously as shownin Fig. 1. One was a Mie-scattering movie to analyze liquid-phasespray penetration. A continuous-wave Nd:YAG laser (532 nm) oper-ating with 2-W power was the light source. The laser beam wasformed into a 100-mm wide and 20-mm laser sheet to illuminatethe spray. A high-speed CMOS camera (IDT X-Vision) fitted with a532-nm band-pass filter (10-nm FWHM) and a 50-mm focal length,f/1.2 lens collected Mie-scattered light from fuel droplets at 50,000frames per second. The lens aperture (f/1.2) and camera exposuretime (17 ls) were selected to intentionally saturate pixels in denseregions of the spray, thus providing the sensitivity needed to identifylow-intensity regions where liquid completely vaporizes.

The other high-speed movie acquisition used a schlieren sys-tem, capable of detecting refractive index gradients at the bound-ary between vaporized fuel and hotter ambient gases. A 150-W(electrical power) mercury-xenon arc lamp was the white-lightsource. To obtain a high-quality point source, the emitted beampassed through a 50-mm focal length f/1.2 Nikkor lens and1-mm aperture. The beam was then collimated by an f/8 mirrorwith a 115-mm diameter, passed through the chamber windowsand then refocused to another high-speed CMOS camera (Vision

Mixing fan

Xe Arc Lamp

1 mm Aperture

Parabolic Mirror115 mm diameter, f/8

Parabolic Mirror115 mm diameter, f/8

SchlierenHigh-SpeedCMOS Camera

Nikkor Lensf=50 mm+Filters

Mie-Scattering

High SpeedCMOS-Camera

Nikkor Lensf=50 mm, f/1.2

2 W532 nmLaser Beam20x100 mm

Beam Block

Diesel Spray(front view)

f/1.2 Nikkor Condensing Lens

Constant-Volume

Fig. 1. Optically-accessible constant-volume chamber. Also shown are schlieren and Mie-scattering imaging setup.

Table 1Operating conditions.

Ambient conditionsO2 0%N2 89.71%CO2 6.52%H2O 3.77%Molecular mass 28.68 g/molPressure 6 MPaTemperature 900 KDensity 22.8 kg/m3

Injector conditionsType Bosch common-rail, 2nd generationNozzle Single-hole, KS1.5/86, mini-sacNozzle diameter 90 lmInjection pressure 150 MPa

S. Kook, L.M. Pickett / Fuel 93 (2012) 539–548 541

Research Phantom v7.1) equipped with a 50-mm focal length Nik-kor lens. Images were collected at approximately 50,000 framesper second and with 256 by 96 pixel resolution. A zero-incidence532-nm mirror blocked laser scattering from liquid droplets.

2.3. Fuels

A total of six fuels were tested at the fixed injection and ambi-ent conditions. The fuels consisted of the following: (1) the No. 2diesel (‘‘D2’’), a reference fuel with distinctly higher boiling pointthan the other fuels; (2) a conventional JP-8 (‘‘JC’’), a sample toassess aviation fuel sprays using diesel engine hardware at dieselengine conditions; (3) another conventional aviation fuel calledthe world-average jet fuel (‘‘JW’’) which is an equal blend of fiveJet-A fuel samples from different US manufacturers; (4) aFischer–Tropsch fuel (‘‘JS’’) characterized as a low-density fuel;(5) a coal-derived fuel (‘‘JP’’) with higher density than any otherfuel tested; and (6) a surrogate fuel (‘‘SR’’), a mixture of 23%m-xylene and 77% n-dodecane by volume with a low boiling point.Physical properties for the fuels are outlined in Table 2. Note thatthe fuel properties are given at room temperature, which is lowerthan the 100 �C fuel temperature used in the experiments. Addi-tional information can be found in Ref. [34] in which Holley andco-workers performed an ASTM D2425 standard analysis for JW,JS, and JP fuels and provided a detailed composition data. Fig. 2

shows the distillation curves for diesel and jet fuels used in thisstudy. For the surrogate SR, the boiling point temperature of twosingle component fuels, n-dodecane (C12H26) and m-xylene(C8H10), are also indicated. It is notable that the boiling point ishigher for diesel compared to jet fuels for all recovery ratios. Theliquid length is therefore expected to be longer for diesel.

3. Results and discussion

3.1. Liquid length

Fig. 3 shows an example of high-speed liquid imaging for thetemporal development of the spray. The start of injection wasdetermined in the movie by one frame prior to the first appearanceof spray. At the image top-right, time after the start of injection(ASI) is given. The result is for D2 fuel and from an instantaneouscycle. Images at the top of the figure show a sequence of Mie-scattered light from liquid droplets with a bright, saturated regionin the dense part of the spray and decreasing intensity at the near-nozzle region and tip of the spray. To aid in visualization of theliquid-phase boundary, a contour at 26/256 (i.e., about 10% of themaximum image count) intensity counts is overlaid on the image.Using this contour, the maximum liquid penetration is defined as adistance from the nozzle to the spray tip. The lens aperture andcamera exposure was set to intentionally saturate the camera inthe dense region of the spray while leaving sufficient dynamicrange to identify regions of the spray where liquid completelyevaporates. However, changes in the image threshold level didnot significantly affect the measurable liquid penetration, similarto Refs. [24,35]. For example, a change of six image counts in thethreshold level (i.e., 10 ± 2% of the maximum image count) causesonly ±0.3 mm variations (or ±2% of 16 mm liquid length), whichare within the turbulent fluctuations during the quasi-steady per-iod of injection. The vertical dashed lines drawn on the images arethe time-averaged liquid length, which is determined in the quasi-steady region after the initial transient (approximately 200 ls ASI)and prior to the end of injection. At the bottom of Fig. 3, the max-imum liquid penetration distances corresponding to this imagesequence are given. To illustrate, circle symbols are provided onthe liquid penetration trace for each image time. The liquid lengthis also given as horizontal dashed lines.

Table 2Property of the fuels studied.

Fuels D2 JC JW JS JP SRNo. 2diesel

ConventionalJP-8

World average Jet Ablend

Fischer–Tropschfuel

Coal-derivedfuel

Surrogate fuel (23% m-xylene, 77% n-dodecane)

T10 (�C) 211 170 180 167 192T90 (�C) 315 240 251 254 243T100 (�C) 350 266 274 276 270 216Cetane index 47 39 – – –Cetane number 46 38 46 62 34 70 (estimated)Lower heating value (MJ/

kg)42.975 43.228 43.2 44.138 42.801 43.33

Density at 15 �C (kg/m3) 843 812 806 755.9 870.2 778.9Aromatics (vol.%) 27 11 19 0.4 1.9 23H2 (mass%) 13.28 13.9 – 15.4 13.2Sulfur (ppm) 9 1 0 0 0Kin. visc. (�20 �C) (mm2/

s)– 4.465 5.2 4.6 7.5

Kin. visc. (40 �C) (mm2/s) 2.35 � 1.4Freeze point (�C) – �57 �48 �51 �65Flash point (�C) 73 47 51 45 61

542 S. Kook, L.M. Pickett / Fuel 93 (2012) 539–548

The liquid penetration history shows that after the start ofinjection there is an initial increase, which is nearly linear and lastsapproximately 200 ls. After this initial increase, the maximumliquid penetration reaches the liquid length during the quasi-steady period of injection. Analysis of the extent of this fluctuationnear the liquid length shows that the standard deviation in maxi-mum liquid penetration divided by the mean value is less than5%. While the liquid region is connected from the nozzle to thespray tip, this variation in maximum penetration, caused by turbu-lent fluctuations, is important to understand the probability ofliquid impingement upon in-cylinder surfaces.

The contour line obtained from Fig. 3 is superimposed on theschlieren images in Fig. 4. To illustrate, the liquid length is againdrawn as vertical dashed lines. Note that the schlieren back-ground has a non-uniform small-scale structure in the ambientgas. This non-uniformity is caused by temperature gradients inthe ambient gases and in boundary layers at the windows ofthe chamber, and is therefore unavoidable. To improve the clarityof the vapor region in the schlieren images, a background correc-tion was implemented using the method similar to Ref. [36].Images at the right of the figure show the difference between suc-cessive images (In � In�1), compared to the raw schlieren (In) atthe left. As demonstrated, this method helps to reduce schliereneffects that exist in the ambient gases while identifying real vaporportions of the penetrating jet. However, difficulties for jet detec-tion exist when jet gradients are small or when there is littlechange in successive images. Later in Section 3.2, we will furtherdiscuss the method for the vapor boundary detection used in thisstudy. Here, the image sequence is only used to show an entirehistory of liquid penetration, vaporization, and vapor penetrationbeyond the liquid length.

Fig. 4 shows that up to 103 ls ASI, the liquid and vapor phasesare coincident. Soon afterward, however, a separation occursbetween liquid and vapor regions. The liquid reaches a liquidlength of 16 mm, which stays the same during the remainder ofthe injection. Meanwhile, the vapor-phase fuel continues to pene-trate across the chamber. The schlieren images are shown up to thetime that the head of the vapor jet goes out of the camera viewfield. As the fuel injection continues, the vapor jet will impingeupon the back wall, forming a wall jet. Eventually, the jet wouldwrap around and influence air entrainment of the free jet, butthe injection duration is too short, and the chamber volume toolarge, for there to be any interaction in this study. While not dis-counting the importance of the jet-wall interaction in a real engine,it is not an objective of this study, which is to compare free jets for

various fuels. Also, the transient behavior of the vapor jet after theend of injection is beyond the scope of this work. During the end-of-injection transient, the fuel injection velocity and mass flow ratedecline as the injector needle valve closes. Detailed discussionsabout how this injection ramp-down affects air–fuel mixing andvaporization are found in previous investigations conducted in thislaboratory [37–39].

To ensure that observations of the fuel-to-fuel comparison areresults of fuel properties rather than injection-to-injection varia-tions, an ensemble average of multiple injections is calculated foreach fuel. For example, Fig. 5 shows liquid penetrations of D2 fuelfor five injections and a cycle mean of these individual penetrationcurves. While the overall trend seen in Fig. 3 remains the same,there are certainly injection-to-injection variations. However, theuncertainty of the time-averaged liquid lengths is measured at lessthan 3% (e.g., 16.3 ± 0.47 mm for D2). The indicated error range cor-responds to �2r=

ffiffiffiffiNp

where r is a standard deviation of N samples.By presenting the ensemble-averaged liquid penetration, we there-fore show differences in liquid penetration originated from the fuelproperty, which exceed the variations between repeated injections.

The general spray penetration trend for all fuels studied isshown in Fig. 6. The figure shows plots of penetration versus time,with ensemble averages of five cycles shown for each fuel. We later(Fig. 7) show time-averages of this data during the quasi-steadyperiod of fuel injection for each fuel. The first noticeable trendshown by the data in Fig. 6 is the lower liquid length of jet fuelscompared to D2 fuel. This trend of low liquid length for high vola-tility fuels has been observed by others [23–25]. Among alternativefuels, the coal-derived JP shows the longest liquid length while theconventional JC shows the second-highest penetration rate. Thepenetrations for the other jet fuels are not clearly discernablewhile the liquid length in decreasing order appears to beD2 > JP > JC > JW > JS = SR.

As previously mentioned, it is well known that the boiling pointaffects the liquid length. Fig. 7 illustrates the correlation betweenthe liquid length and fuel boiling point (90% distillation tempera-ture) for all fuels tested. A choice of 90% distillation temperatureis consistent with previous studies [23,24] where the liquid lengthsfor multi-component fuels are successfully predicted using the 90%distillation temperature. The data points plotted in Fig. 7 representthe average of the liquid length data. An uncertainty band is alsodrawn as a plus sign for each fuel. In Fig. 7, there is a distinctdifference in boiling point by about 80 �C and associated liquidlengths between diesel (D2) and alternative fuels. This measurabledifference is expected because more energy is required to heat and

Fig. 2. Distillation curves for fuels used in this study. The atmospheric-pressureboiling points of single-component fuels n-dodecane (C12H26) and m-xylene(C8H10) are also given. 1ASTM D975-94, 2006 specification for diesel,2MIL-DTL-83133E for jet fuel.

Fig. 3. Mie-scattering liquid imaging time sequence ASI for D2 fuel (top). The meanquasi-steady liquid length is overlaid as a vertical dashed line. Also shown is themaximum liquid penetration distances determined from the liquid images(bottom).

S. Kook, L.M. Pickett / Fuel 93 (2012) 539–548 543

vaporize the less-volatile D2 than the other fuels. In other words,the spray entrainment length to supply the additional energy isincreased, which leads to longer liquid length. However, the boilingpoint cannot fully explain the differences in the liquid length. Forexample, the liquid lengths of JW and JS fuels are shorter than JCand JP fuels, despite having a higher boiling point. One might arguethat a choice of 90% distillation temperature is arbitrary and differ-ent boiling points may explain this discrepancy. For instance,Cannan et al. [23] reports that the mid-boiling point (50% distilla-tion temperature) correlates with the liquid length slightly betterthan the 90% distillation temperature for multi-component fuels.However, the distillation curves (Fig. 2) shows that the mid-boilingpoint of JW and JS fuels are also higher than JC and JP fuels. In otherwords, the 50% distillation temperature also does not explain thedifferences in liquid length. The shape of the distillation curve ismore flat for JP and its low-boiling point temperature (e.g., 10% dis-tillation temperature) is higher than the other jet fuels, but onceagain, the ordering in measured liquid length does not match thedistillation temperature order.

If the mixing-limited vaporization applies to the fuels examinedin this study, this discrepancy may be explained by effects of fueldensity. This is because the entrained ambient mass per fuel massis inversely proportional to fuel density. Specifically, since not onlythe fueling rate ( _mf ) but also the ambient entrainment ( _ma) areproportional to the fuel density (qf), the resulting ambient-fuelratio is inversely proportional to the square root of the fuel density[29], i.e.,

_ma= _mf /ffiffiffiffiffiffiqa

qf

stanðhsÞ

dnx ð1Þ

where qa is the air density, dn is the nozzle diameter, hs is spreadingangle, and x is axial distance from the nozzle. A high fuel densitycauses a decrease in _ma= _mf and, thereby, an increased liquid length.

The correlation between liquid length and fuel density is dem-onstrated in Fig. 8. For the most part, the correlation is linear anddemonstrates the important influence of fuel density on liquidlength and associated evaporation processes. D2 is far off the linearcorrelation because of its distinctively high boiling point, as dis-cussed in Fig. 7.

Certainly, other fuel properties such as heat of vaporization,surface tension, and viscosity may affect the liquid length [40]. Fur-ther study, including more detailed measurements of fuel proper-ties in these multi-component fuels, will be required to clarify

these effects on the liquid length. However, as single-componentfuels of similar chemical structure to the fuels studied have similarheat of vaporization, we do not expect that heat of vaporizationdifferences have a major effect on liquid length in this study. Theabove results reinforce that the boiling point and fuel density areimportant parameters affecting the liquid length under dieselengine conditions. To the best of our knowledge, the fuel densityeffect on the liquid length has not been explicitly studiedpreviously. Therefore, the results confirm a prediction by the scal-ing law [10,12] that the liquid length increases with increasing fueldensity.

Fig. 4. Liquid-phase spray boundaries (from Fig. 3) overlaid on the raw schlierenimages (left) and background-subtracted images for D2 fuel. Vertical dashed linesshown are quasi-steady liquid lengths.

0 1 2 3 4 5 60

5

10

15

Time after Start of Injection [ms]

Liqu

id S

pray

Pen

etra

tion

[mm

] D2

JC JW JS

JP

SR

Fig. 6. The ensemble-averaged maximum liquid penetrations for all fuels tested.

Fig. 7. Effect of boiling temperature on liquid length. The plus signs are uncertaintybands.

544 S. Kook, L.M. Pickett / Fuel 93 (2012) 539–548

3.2. Vapor penetration

The high-speed schlieren images were analyzed to obtain thevapor penetration. Previously in Fig. 4, the raw schlieren imagesshowed a dark vaporized jet above a bright background, but diffi-culties for image processing existed because the background wasnot uniformly bright. In other words, there were many dark regionsin the ambient, despite a desired bright field. This presents chal-lenges for the vapor boundary detection such that a simple grayscale cut-off cannot be used to find the boundary of the vapor. Inthis study, some samples performed well when the backgroundis corrected either by subtracting the background with no spraysor two successive images. However, other stages of injectionrequired further background correction and, often, the back-ground-subtracted images did not easily reveal the entire jet (seeFig 4, for example). Therefore, a computer-based image processingtool has been developed for this analysis. The idea is that thetexture of the fuel spray can be differentiated from the non-uniformbackground of the schlieren images [29,41]. Fig. 9 shows examplesof this processing, with the computer-derived jet boundaries over-laid on schlieren images for all fuels tested. The schlieren images

Fig. 5. The maximum liquid penetrations for five different injections at fixedambient and injector conditions (D2 fuel). Also shown is an ensemble average.

are shown for a fixed imaging time of about 1 ms ASI when thevapor travels approximately a half of the camera view-field. Theimages show that, even with no background corrections, the tex-ture difference enables vapor boundary detection.

Using the obtained vapor boundaries, the vapor penetration iscalculated using a method similar to Refs. [29,41]. Firstly, aninstantaneous vapor spreading angle is determined using the por-tion of the fuel jet that is between the injector and one-half thevapor penetration distance to avoid artifacts of the vapor headwhere the penetrating jet is not fully developed. The area of the va-por boundary in this upstream region is fit to a triangle having thesame area. The angle of this triangle is defined as the instantaneousspreading angle (hs). The vapor penetration distance is obtainedusing the vapor boundary within the inner half of the spreadingangle (hs/2). Using this angle, the vapor penetration distance (Lv)is defined as the distance along the spray axis where one-half ofthe pixels across this inner angle have been identified as vaporizedfuel. This definition accounts for fluctuations or asymmetry in pen-etration at the jet head to provide a more consistent definition forthe vapor penetration than the maximum vapor boundary. Forexample, the selected image at the left of Fig. 9 shows longer pen-etration at the top of the jet. The computed penetration distancegives a better representation of the average vapor penetration,including both the top and the bottom of the jet.

S. Kook, L.M. Pickett / Fuel 93 (2012) 539–548 545

The first noticeable trend from Fig. 9 is the similarity in thevapor penetration and spreading angle for all fuels examined. Likethe liquid-phase spray penetration (Fig. 5), the same vapor pene-tration measurements were repeated for 5 injections, and theiraverage is plotted in Fig. 10. The liquid penetrations from Fig. 6are also shown again to demonstrate relative distance betweenmaximum liquid distances and vapor penetrations at a given timeASI. Also shown at the bottom of Fig. 10 is the uncertainty in thisaverage. In addition to unavoidable injection-to-injection varia-tions, potential differences in image analysis can add additionalerrors in the vapor penetration data. Along with this uncertaintyband drawn for all fuels, a time average of the uncertainty is alsoannotated in the figure legend.

An immediate conclusion from Fig. 10 is that the vapor penetra-tions are not significantly different for each fuel. The fuel-to-fuelfluctuation seen in the viewgraphs is within the error range of eachfuel and no correlation with fuel properties is found. For example,error bars that are drawn for SR fuel in the vapor penetration over-lap with all the penetration data. For the sake of clarity, the errorbars for other fuels are not drawn but from the time-averageduncertainty, more variations are predicted for fuels like JP and D2.

Since there is a 15% difference in fuel density between JP and JS(see Table 1), the same vapor penetration might be unexpected.However, an analysis of the momentum flux (the force that gov-erns the vapor head penetration) explains why fuel density (or vol-atility) does not affect penetration.

While the jet velocity Vf can be calculated using the pressuredrop (DP) and fuel density (qf), i.e., Vf /

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2DP=qf

q, the momentum

flux is _M ¼ _mf Vf , and hence _M ¼ qf V2f A / 2DPA. This shows that

the fuel density is not correlated with the momentum flux for afixed pressure drop and nozzle area. As a result, a change in fueldensity has no effect on the vapor penetration. This discussion isconsistent with the momentum flux measurement performed inthis laboratory for a biodiesel study [15] where the higher densityof biodiesel compared to a diesel caused an increase in the massrate-of-injection but the measured momentum flux was essentiallythe same for the two fuels. Further analysis showed that estimateddischarge (Cd) and area contraction (Ca) coefficients for a single-spray, axially-drilled, 0.108 mm nozzle (similar to the nozzle usedin this study) were equivalent within the measurement uncer-tainty i.e., the same Ca for the two fuels with different densities.Coupled with the fixed DP, this is in agreement with the samemeasured momentum flux.

Another interesting finding from Fig. 10 is that there is no cor-relation between the liquid length and vapor penetration. As dis-cussed in the Introduction, there is a perception that fast

Fig. 8. Effect of fuel density on liquid length. The plus signs are uncertainty bands.

evaporation of fuel can increase the percentage of fuel that ismixed with the ambient before the start of combustion. This maylead to an increased combustible mixture volume, thereby result-ing in more intense heat release. However, results in this studysuggest that high volatility does not increase the amount of oxygenmixed with the fuel.

Prior to drawing final conclusions, the spreading angle must beconsidered as it is proportional to the rate of ambient entrainmentaccording to Eq. (1). The images in Fig. 9 show that a vapor borderis identified, which may be used to measure hs. However, careshould be taken when schlieren images are used to measure thevapor spreading angle. A recent study conducted in our laboratory[41] showed that sensitivity of the schlieren setup causes a markedchange in the vapor spreading angle. Instead of measuring thespreading angle directly from the schlieren images, it was prefera-ble to use the measured vapor penetration, coupled with an appro-priate model input for spreading angle, to determine the rate ofambient entrainment. This is based upon the fact that the diagnos-tic uncertainty is much less of an issue for detecting the leadingedge of the vapor-phase spray, where the vapor border is markedby sharp gradients and is more easily observed and measured.Indeed, the vapor penetration was much easier, and more consis-tent to measure than spreading angle using the image analysis toolin this study.

Therefore, the same approach of Ref. [41] was employed todetermine the spreading angle and its variation with fuel type.To summarize, calibration of the Musculus and Kattke jet-mixingmodel [42,43] was conducted such that model predictions forspray penetration were repeated until the selected spreading angleresulted in the same penetration as the measurement. This modelincorporates multiple control volumes and a realistic radial distri-bution of fuel mass and therefore allows prediction of penetrationand mixing for variable rates of injection. For example, the modelwas successfully used to study transient spray behavior duringthe end-of-injection transients [38,39]. The spreading angle (hs)and area contraction coefficient (Ca) are required as inputs to themodel.

Prior to hs calculation, the effect of fuel density on jet mixingwas assessed. Fig. 11 shows the _mf = _ma corresponding to vaporboundaries for the JS and JP fuels, which have the lowest densityof 756 kg/m3 and the highest density of 870 kg/m3 respectively. Inthe model, the vapor boundary is calculated using a threshold(e.g., 5%) of maximum momentum flux, which is approximatelyconstant along the jet axis. The hs was fixed at 21� to examinewhether a variation in fuel density causes different vaporpenetrations. For the same reason, the Ca was fixed at 1 as thisvalue was measured in a similar nozzle [15]. The predictedtrend by the model follows that observed in the experiments. Inother words, fuel density does not affect the vapor penetration.

Finally, Fig. 12 shows the vapor penetrations for all fuels testedas an uncertainty band (gray region). This is reproduced fromFig. 10 in order to compare measured penetrations to predictedpenetrations for various hs and Ca. The figure shows that at a fixedCa = 1, a selection of 21� for the hs makes the predicted penetrationsfall into the center of the uncertainty band of the measured pene-tration. A slight change of hs reflects the uncertainty in the vaporpenetration measurement and the subtle variation between fuels.An uncertainty band of only 2� is enough to capture the variationin fuels. There are also consistent differences for each fuel, wheresmaller hs (20�) match better during the earlier stage of the fuelinjection and larger hs (22�) match better in the later stage. Theseresults suggests only small variations in spreading angle for allfuels investigated, consistent with visual inspections in the schlie-ren images (Fig. 9).

Any significant variation in the area contraction coefficientbetween fuel to fuel can also be addressed in the penetration

Fig. 9. Definition of vapor penetration (Lv) and spreading angle (hs). Vapor boundaries are detected and overlaid on schlieren images for each fuel.

Fig. 10. Vapor penetration distance determined (top) and its uncertainty (bottom)from the schlieren images. The penetration is a cycle average of five injections. Theuncertainty bands for SR fuel are shown at the top of the figure. Also shown areliquid penetrations for the same operating conditions (from Fig. 6).

Fig. 11. Time sequence of vapor-phase spray from modeling for JS (756 kg/m3) andJP (870 kg/m3) fuels. Time after start of injection (ASI) is given at the image right.The spreading angle was set at 21�.

546 S. Kook, L.M. Pickett / Fuel 93 (2012) 539–548

measurement and prediction. Reducing Ca by 0.05, which issignificant and outside of the measurement uncertainty [15],causes a reduction in penetration similar to a 1� increase in hs.Once again, this change in penetration is within the uncertaintyband of the measured penetration. These observations supportthe idea that variations in both the vapor penetration andspreading angle are minor for fuels with various density andvolatility.

3.3. Engine considerations

Our new findings on the liquid length and vapor penetration fordifferent fuels have important implications for direct-injection die-sel engines. For example, liquid impingement on the piston wall or

a cylinder liner may be different for fuels with different liquidlengths. Knowing that fuels with low boiling point and densityhave short liquid lengths, wall wetting and fuel-in-oil dilutionissues would be less for the studied alternative fuels than diesel.Among the alternative fuels, the Fischer–Tropsch JS having mark-edly low density and boiling point may help reduce the wetting,particularly when the steady period of fuel injection comprises asignificant part of the entire injection event. If the injection dura-tion is very short (e.g., light load condition or pilot/pre/after/postinjections), the decreased wetting of low density fuels may notbe as significant. Also, the data in this study show that liquid-fuelwall impingement will not occur at these type of diesel-like ambi-ent and injector conditions, even for engines with small combus-tion bowls (typically �20–30 mm in diameter). Since wall

Fig. 12. Vapor penetration uncertainty band (gray area) from the measurement(reproduced from Fig. 10). Lines are modeling results for spreading angle and areacoefficient variations.

S. Kook, L.M. Pickett / Fuel 93 (2012) 539–548 547

wetting becomes more problematic for advanced low-tempera-ture-combustion regimes, however, findings in this study may bebeneficial for fuel-flexible diesel engines in some respects. The va-por penetration results also leave important messages for enginedevelopers. If ambient and injector conditions are fixed, the vaporpenetration and spreading angles are found to be nearly the sameregardless of fuel density and boiling point temperature. The over-all spray penetration pathway shows that fuels evaporate differ-ently resulting in different liquid penetration rates, however,total entrainment and mixing are not significantly affected by fuelproperty variation.

4. Conclusions

To clarify the liquid length and vapor penetration of variousfuels at diesel engine conditions, simultaneous imaging of liquidspray and vapor fuel was performed at typical in-cylinder condi-tions of diesel engines. Experiments were performed at the speci-fied ambient gas temperature (900 K) and density (22.8 kg/m3) inan environment with 0% oxygen. Six different fuels were investi-gated including the No. 2 diesel (‘‘D2’’), a conventional JP-8(‘‘JC’’), another conventional aviation fuel called the world-averageJet-A blend (‘‘JW’’), a Fischer–Tropsch fuel (‘‘JS’’), a coal-derivedfuel (‘‘JP’’), and a surrogate fuel (‘‘SR’’). The major findings from thisstudy are as follows:

1. D2 shows the longest liquid length due to its high boiling-pointcharacteristics. Among alternative fuels, the coal-derived JPshows the longest liquid length due to its high density. Theliquid length in decreasing order appears to be D2 > JP >JC > JW > JS = SR.

2. Under diesel engine conditions in this study, trends consistentwith mixing-limited vaporization apply, explaining both boil-ing-point-temperature and fuel-density effects on liquid length.

3. The variations in vapor penetrations are within the uncertaintyof schlieren image analysis for all fuels studied. From a momen-tum-flux analysis, it is found that fuel density is not proportionalto the vapor penetration.4. The measured vapor penetration is used to determine the

appropriate inputs for spreading angle and area contractioncoefficient for jet mixing model. A spreading angle of 21� with±1� follows the measured uncertainty in vapor penetration forall fuels investigated. Also, only 0.05 variations in the area con-traction coefficient are enough to capture the variation in pen-etration between fuels.

5. No correlation between the fuel–air mixing and fuel volatility isfound as evidenced by the similar vapor penetration andspreading angle for fuels with different liquid lengths.

6. In summary, when fuels with different densities and boilingpoints are used in diesel engine, fuels may evaporate differentlyresulting in different liquid penetration rates. However, thesame vapor penetration and spreading angle are expected indi-cating virtually the same total ambient entrainment andfuel–air mixing.

Acknowledgments

Support for this research was provided by Chevron Corporationand in part by Strategic Environmental Research and DevelopmentProgram (SERDP) Project WP-1578, with facilities supported by theDOE Office of Vehicle Technologies. The research was performed atthe Combustion Research Facility, Sandia National Laboratories,Livermore, California. Sandia is a multiprogram laboratory oper-ated by Sandia Corporation, a Lockheed Martin Company, for theUnited States Department of Energy’s National Nuclear SecurityAdministration under contract DE-AC04-94AL85000.

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