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Applied Thermal Engineering

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

The spray vaporization characteristics of gasoline/diethyl ether blends atsub-and super-critical conditions

Cheng Zhan, Shangqing Tong, Chenglong Tang⁎, Zuohua HuangState Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China

H I G H L I G H T S

• Visualization of vapor and liquid fuelis actualized at identical operatingconditions.

• Spray vaporization behaviors of gaso-line/ diethyl ether at sub-and super-critical conditions are investigated.

• Vaporization process is acceleratedwith diethyl ether addition at sub-critical condition.

• Diethyl ether addition effect on va-porization is opposite at supercriticalcondition.

G R A P H I C A L A B S T R A C T

A R T I C L E I N F O

Keywords:GasolineDiethyl etherVaporizationSupercritical sprayGCI

A B S T R A C T

The multi-hole spray vaporization characteristics of gasoline, diethyl ether (DEE) and their blends were in-vestigated at two pressures and elevated temperatures by using high speed schlieren photography and laser sheetpatternation. Results show that the effects of DEE addition on spray behaviors depend on the ambient conditionsthat may lead to sub- and supercritical vapor generation mechanisms: (a) for 0.5MPa ambient pressure at sub-critical state, spray plumes from different nozzle holes are separated. DEE addition results in higher vapor tippenetration, higher spray angle and decreased liquid cross-sectional area, indicating that DEE addition enhancesvaporization. However, this effect becomes weaker at higher temperature due to accelerated gasoline vapor-ization. (b) for 3.8MPa ambient pressure, spray plumes coalesce as a single plume-like structure due to strongeraerodynamic resistance to the axial penetration of each plume. In addition, opposite from the 0.5MPa ambientpressure case, DEE addition results in an inhibited vaporization because the fuel blends are more easily heated tosupercritical state with the addition of DEE. The thickened interface and reduced mean free path at supercriticalstate lead to restricted vapor phase generation. This phenomenon is also evidenced by the absence of large liquidblob on the patternation images.

1. Introduction

Diesel engines have been widely used in commercial vehicle trans-portation due to its high power output and thermal efficiency [1–3].Simultaneous reduction of PM and NOx emissions in diesel engines,however, remains as a primary challenge for more stringent emission

regulations [4]. Low temperature and highly diluted mixture prepara-tion are considered to be effective to inhibit NOx and PM formation [5].Recently, new combustion concepts like gasoline compression ignition(GCI) has been proposed by Kalghatgi [6,7] for the purpose of si-multaneous reduction of NOx and PM. The most common method toachieve GCI combustion mode is to introduce the direct injection of

https://doi.org/10.1016/j.applthermaleng.2019.114453Received 28 May 2019; Received in revised form 31 August 2019; Accepted 26 September 2019

⁎ Corresponding author.E-mail address: chenglongtang@mail.xjtu.edu.cn (C. Tang).

Applied Thermal Engineering 164 (2020) 114453

Available online 30 September 20191359-4311/ © 2019 Elsevier Ltd. All rights reserved.

T

gasoline into diesel engine with high compression ratio. Johanssonet al. studied the various gasoline-like fuels with different octanenumbers in a conventional diesel engine [8–11]. Their results show thatgasoline-like fuels increase fuel-air mixing time and create a partiallypremixed combustible mixture, leading to simultaneous reduction ofPM and NOx emissions.

Because the combustion phase control in compression ignition en-gines is of high correlation with injection timing, the gasoline sprayevolution and its vapor fuel generation should be deeply investigatedfor better understanding about in-cylinder mixture formation in GCIengine. At present, most of the research works about GCI target onengine combustion and emissions characteristics rather than its detailedin-cylinder spray and atomization processes [12,13]. Besides, the ma-jority of current research works about gasoline spray characteristics areconducted under non-vaporization and atmospheric pressure conditions[14,15]. For better control of the practical thermal process of the GCIengines, it is necessary to understand the vaporizing spray behaviorsunder elevated temperature conditions [16]. However, limited in-vestigations on vaporizing spray characteristics have been reported,which are more close to the realistic operating conditions. Guo et al.[17–19] studied the radial expansion of gasoline flash boiling jet inconstant volume vessel. However, their experiments is conducted at lowambient pressure and room temperature, which is away from thepractical conditions for typical GCI engines [20]. Payri et al. [21] andAllocca et al. [22] investigated the gasoline spray vaporization beha-viors at high temperatures, but the highest ambient pressure is around0.7MPa, which is also lower than the typical GCI engine ambientpressure at injection timing.

To study the spray vaporization under high ambient pressures,Nishida et al. [23,24] utilized laser absorption-scattering (LAS) tech-nique to quantitatively determine the concentration of liquid and vaporphase spray in a high-pressure atmosphere (1.0MPa, 500 K). Ziganet al. [25–27] investigated the gasoline-like fuel spray vaporizationcharacteristics in a high-pressure atmosphere (1.5MPa, 473 K) viaschlieren method. Their results indicate that for vaporizing spray, thethermal process would transform from distillation-like mode into co-vaporization mode under high ambient pressure. Unfortunately, am-bient pressure of these experiments (less than 1.5MPa) still cannotreach the typical ambient conditions in actual GCI engine. Lopez et al.used an optical engine to visualize the actual spray in GCI operatingconditions [28] and the liquid penetration length was extracted throughthe macroscopic spray images using Mie-scattering techniques. Actu-ally, the spatial dispersion about vapor fuel is of high importance indetermining in-cylinder mixture preparation. Due to the limitations forvapor phase detection under high ambient pressure, Dahms et al.[29,30] theoretically conducted the thermodynamic calculations aboutvaporizing spray of hydrocarbon mixtures at near-critical (low ambientpressure) and supercritical conditions (high ambient pressure) to give adeeper insight about actual thermal process in GCI engines. They pro-posed that single-phase fuel injection, instead of classical spray break-up and vaporization process, may occur under late injection conditionsin GCI engines. Hence, it is necessary to conduct deeper spray

vaporization investigations for GCI combustion over a wide range ofoperating conditions, especially at high ambient temperature andpressure.

Except for more complicated ambient conditions about gasolinespray compared to conventional DISI gasoline engines, putting GCIengines into reality still faces myriad serious fundamental challenges.Limited operating conditions is one of the predominant problems be-cause of low CN of gasoline, resulting in cold-start difficulties andmisfire at high engine speed [31]. Introducing high reactivity fuel, suchas PODE and DTBP [32–34], into the GCI engines is considered as aneffective way to extend the engine operating conditions and adjustcombustion phase flexibly [35–37]. Wang et al. [38] investigate theeffects of fuel reactivity on combustion and emission characteristics ofGCI engines. They proposed that higher CN fuels (CN > 56) allowmore precise control on combustion phase and better engine perfor-mance in terms of power output and emissions.

Diethyl ether (DEE) is a biofuel with extremely high reactivity(CN > 125) and it improves engine cold start performance [39] due toits high physical volatility and chemical reactivity. In terms of enginesperformance, Rakopoulos et al. [40–42] investigated the combustionand emissions characteristics of DEE-diesel blends, and they found thatthe NOx and PM emissions of blended fuel were significantly reducedcompared to pure diesel. Other engine investigations about DEE fuelcan be found in recent review work [43]. Fundamental combustionchemistry studies on DEE have also been conducted [44,45]. Recently,several works on DEE spray and mixture formation have been reported.Vijayakumar et al. [46] found that the penetration of DEE spray issmaller than diesel at both vaporizing and non-vaporizing conditions.Balaji et al. calculated the spray characteristics of DEE and dieselthrough KIVA-4 CFD code, which were then compared with their ex-perimental results. They found that DEE fuel owns shorter spray tippenetration and better atomization behaviors due to reduced viscosity[47]. Zhan et al. [48] showed that addition of DEE into biodiesel ex-hibits the ability of reducing sauter mean diameter (SMD) and promotesfuel atomization process at room temperature. These works reflect thepotential of DEE as a promising fuel additive to improve engine cold-start performance and enhance its ignition reliability. However, no vi-sualization works about the effects of DEE addition on spray vapor-ization characteristics in actual GCI operating conditions are reportedand then the thermal process between DEE/gasoline fuel and environ-ment is still in lack of deep understanding.

Because DEE is a promising biofuel with high reactivity, it is ben-eficial for the precise combustion phase control for GCI engine.Understanding the spray behaviors of high reactivity fuel such as DEEand DEE/gasoline blends is important for developing the GCI techni-ques. We note that previous work on DEE spray primarily use the blendsof DEE/diesel (both fuels have high reactivity) at room temperature[47,48], no gasoline/DEE spray investigations have been reported. Inaddition, for those vaporizing spray studies of neat gasoline, the highestambient pressure is lower than the chamber pressure at top dead centerof typical GCI engines. One objective of this research work is to sys-tematically investigate the spray vaporization characteristics of DEE

Nomenclature

AOI area of interestCN cetane numberDEE diethyl etherDEE50 gasoline diethyl ether(50–50% vol)DISI direct injection spark ignitionDTBP di-tert-butyl peroxideECU electronic control unitGCI gasoline compression ignitionGDI gasoline direct injection

LAS laser absorption-scatteringPID proportion integration differentiationPODE poly-oxymethylene dimethyl ethersPamb ambient pressurePc critical pressureRVP reid vapor pressureSOI start of ignitionSMD sauter mean diameterTamb ambient temperatureTc critical temperatureVTP vapor tip penetration

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and DEE/gasoline blends at elevated temperatures and ambient pres-sures as high as 3.8 MPa, which is close to actual GCI engine conditions.In addition, because most of the previous works on the spray char-acteristics use single hole nozzles, but for practical GCI engines, how-ever, multi-hole nozzle injectors are typically used and there may beinteractions among the spray plumes from different nozzle holes. Thusthe second objective of this work is to investigate the DEE and DEE/gasoline vaporizing spray behaviors by using an OEM six-holes injectorand the high speed schlieren techniques. Finally, to better understandthe vaporization and the interaction behaviors among different sprayplumes, spray patternation images, which represents cross-sectionalinformation of the multi-hole spray, will be collected and analyzed.This is realized by planar laser sheet illumination at a specific crosssection of the spray plumes. In the following, the experimentations interms of the fuel, the setup, the procedure and the data processing willbe firstly introduced. Then we will show the results of multi-hole DEE/gasoline spray vaporization behaviors, including the spray evolution bythe high speed schlieren images, and the spray cross-sectional in-formation by laser sheet patternation.

2. Experimental specifications

2.1. Test fuels and their properties

Three fuels, Gasoline (commercial 95# gasoline purchased fromChina National Petroleum Corporation), DEE (purity higher than99.5%), and their blend DEE50 (50%/50% vol. gasoline and DEEblends) were tested in this study. DEE shows good mixing with gasolineand the blend DEE50 shows no fuel stratification for almost threemonths, as shown in Fig. 1. As presented in Table 1, the density, visc-osity and surface tension of DEE are slightly smaller than that of ga-soline. The reid vapor pressure (RVP) of DEE at 293 K is higher thanthat of gasoline. In addition, DEE owns lower boiling point (307.6 K).Special attention should be paid on critical temperature Tc and criticalpressure Pc of the tested fuels. Because gasoline is a complex mixture ofhydrocarbons, it is impractical to measure its critical parameters. Tc andPc of gasoline are predicted according to the empirical correlationproposed in Ref. [49]. Pamb (3.8MPa) and the highest Tamb (516 K)investigated in this work exceeds the DEE critical temperature Tc(466.5 K). Therefore, for the ambient condition Pamb= 3.8MPa andTamb= 516 K, DEE spray may be gradually heated up to reach its su-percritical state. Gasoline has higher Tc (> 538 K), and its spray willonly be in subcritical state during the whole evolution process.

2.2. Experimental setup

The spray evolution is visualized from both the side view and thecross-sectional view. The side view high-speed camera with schlierenillumination and well aligned knife edge is used to record the spraypropagation as shown in Fig. 3. The cross-sectional view CCD camerawith a pulsed laser sheet illumination is used to record the spray pat-ternation as shown in Fig. 4. The schlieren method and optical patter-nation share the same fuel supply system, signal synchronizationsystem. The only difference lies in optical path arrangement. Fuelsupply system includes the fuel tank, filter, manual hydraulic pump andinjector. Filtered fuel is pressurized to 15MPa and transported to theinjector by a hydraulic pump. At least 200 cm3 fuel drainage is neededto ensure complete fuel discharge inside the filter, fuel pipe and injectorfor each fuel replacement procedure. An OEM six-hole GDI injector isadopted to discharge pressurized fuel and initialize the spray. We notethat though present injection pressure is much lower than the typicaldiesel fuel injection system, further increase in injection pressure doesnot lead to significantly reduced sauter mean diameter (SMD) for ga-soline [15]. Fig. 2 demonstrates the geometry of nozzle hole assemblyand typical spray plumes patternation. The injector mounting positionis carefully adjusted to assume a nearly axisymmetric distribution of

individual plumes. The outlet diameter of each nozzle hole is 0.5mm.The plume centroid distance (the distance on the patternation imagebetween the plume center of mass and the center of the nozzle) ofplume 3# and 5# is larger than those of other plumes. Injectionduration is fixed at 1.5ms. Average injection mass of total six nozzlesper single injection is measured to be 16.8 mg. Fuel temperature ismaintained at 293 K.

A constant volume vessel is used to provide a controllable ambientpressure and temperature, which is filled with N2 to the desired am-bient pressure. Two ambient pressures are tested (0.5 and 3.8MPa), andno leakage is guaranteed. The vessel temperature is monitored by a K-type thermocouple programmed by proportion integration differentia-tion (PID) strategy with an accuracy of± 1 K in conjunction with a2 kW heating system. Temperatures measured at 5 locations within thechamber have been compared and uniformity of temperature is guar-anteed. Four ambient temperatures are tested (349, 418, 466, and516 K) in this work.

A high speed camera (Phantom V611, 512×512 pixel resolution,20,000 fps and 10 μs exposure time in this work) is in perfect alignmentwith the incident point light emitted from LED light source and aperturein Fig. 3. The convex lens with effective diameter 100mm then convertsincident point light into uniform parallel light beam and illuminates thechamber through two high transmittance quartz glasses. The spatialresolution of schlieren image is calibrated to be 177 μm/pixel. Each testcondition is repeated at least three times and the recorded high-speedvideos are very reproducible (see supplemental materials).

Patternation images provide the cross-sectional information of dif-ferent spray plumes, which is especially useful for multi-hole sprayvisualization. The patternation imaging system is sketched in Fig. 4. Asingle pulsed Nd: YAG laser (Litro Bernoulli), pulse energy of 200mJ,and wavelength of 532 nm is used. Laser beam is transformed into anapproximately 1mm thickness laser sheet through cylindrical diver-gence lens with the focus length of −20mm. The minimum laser pulsewidth is 3 ns, which is negligibly small compared to the injection evensuch that highly time-resolved images can be obtained. The laser sheetthen illuminates the spray cross-sectional area at a specific distancedown the injector tip center (30mm in this work). The scattering lightfrom the laser sheet illuminated spray is captured by a 12-bit CCDcamera (Imager ProSX 5M) perpendicular to the orientation of the lasersheet. A narrow-band light filter (532 ± 10 nm) is mounted in front ofcamera lens to eliminate interference light. The exposure time is set as100 μs and spatial resolution of laser sheet images is calibrated to be91 μm/pixel. Each operating condition is repeated twenty times assuggested in Ref. [50]. We note that the two optical setups (schlieren

Fig. 1. State of three blended fuels after standing for three months withoutexternal interference.

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and patternation imaging) work separately for identical conditions.Electronic control unit (ECU) is designed to monitor the time se-

quence of the injection process, laser triggering and camera capturing,as shown in Fig. 5. The energizing current generated from ECU liftsneedle valve and initiate the injection. The motion of the needle valvewill postpone the injection process by 500 μs (t1) approximately, as wetested. Generally, for injection pressure of 15MPa, the needle valvelifting time t1 almost equals to its seating time t2, which means that theactual injection duration is nearly equal to energizing time with anerror less than 50 μs. The laser trigger signal is also generated by ECU toactivate the laser pulse. Its time sequence t3, relative to energizingsignal, can be adjusted to capture spray patternation information for aspecific delay time. However, there will always exists a delay time t4 forlaser from receiving trigger signal till emitting laser beam. This delaytime t4 is embedded in laser circuit configuration and is set at 300 μs inthis study. The camera waiting time is long enough (3 s) to ensure thatall the whole spray evolution process is captured. Time after start ofinjection (ASOI), describes the laser shooting moment with respect tothe injection initiation, and it can be obtained by subtracting injectiondelay t1 from the sum of laser trigger delay t3 and laser emitting delayt4.

2.3. Image processing procedure

2.3.1. Schlieren image processing for spray parameter determinationFig. 6 shows the raw schlieren images and post-processing proce-

dure for spray macroscopic parameters determination. The original 8-bit RGB spray image Fig. 6(b) and the background image Fig. 6(a) areboth converted into corresponding gray level images. Fig. 6(c) is thesubtraction result of background image from the raw spray image.Because of the knife cutting in schlieren method, there is an in-homogeneity background illumination, i.e., left half side of the windowpossesses higher pixel gray level, as shown in Fig. 6(e). As a con-sequence, the identification of the spray (which has lower gray level)edge is affected by the background of the right half side. Therefore, a

self-develop algorithm is developed based on the statistical gray leveldifference to enhance the image contrast so as to compensate illumi-nation light loss due to knife cutting. Typical refined spray image isthen shown in Fig. 6(d). As can be seen, the spray edge is better iden-tified, compared to Fig. 6(c). Subsequently, a median filter function isutilized with an interrogation window of 5×5 pixel to eliminate theimage noise disturbance, and the spray image is then converted tobinary image using threshold value of 0.05, which means that the pixelwith gray level lowed than 5% of maximum gray level is set as zero. Thespray edge is extract with adoption of Canny edge detection algorithm[51] for geometrical parameters calculation like vapor tip penetration(VTP) and spray angle.

Vapor tip penetration and spray angle are extracted from sprayimages under SAE J2715 GDI spray definition standard [52]. As shownin Fig. 7(a), spray tip penetration for given image is the length pro-jection along the injector axis from injector tip to spray tip (red line inFig. 7(b)). Spray angle is measured at 1.5ms ASOI images by settingreference points at two different axial cross section (5 and 10mmdownstream from nozzle tip), as shown by the angle of the two greenlines in Fig. 7(b). Plume bent angle θB is defined as the intersectionangle between plume axis and injector axis.

2.3.2. Patternation image processing for spray patternationThe raw patternation image is the arithmetic average results of

twenty individual images at the same operation conditions as shown byFig. 8(a). The Fig. 8(b) is calculated by in-house Matlab code containingbackground subtraction, boundary subtraction and threshold separa-tion. The boundary subtraction is implemented to eliminate unwantedwindow edge. The separation threshold is set as 0.3 to separate eachindividual spray plumes, from plume 1# to plume 6# in counter-clockwise direction. Then the information about centroid position lo-cation and liquid cross-sectional area of each individual spray plume isextracted from Fig. 8(b). Finally, the pseudo-color laser sheet image inFig. 8(b) is converted into the corresponding contour map Fig. 8(c) withcalibrated coordinate axis and grid.

Fig. 2. Nozzle geometry structure and typical spray plumes distribution.

Table 1Physical properties of test fuels.

Fuel type Density/(g·ml−1) Viscosity/(mPa·s) Surface tension/(mN·m−1) RVP/kPa Boiling point/K Tc/K Pc/MPa

Gasoline 0.734 0.770 21.5 ~45a T10%=343a

T50%=393a

T90%=463a

538–586b 2.6–3.2b

DEE50 0.717 0.647 19.2 —— —— ——DEE 0.698 0.563 17 58.9c 307.6c 466.5c 3.64c

a Chinese National Standard GB 17930-2016 (Gasoline for motor vehicles).b Empirical prediction formula of Sim et al. [49].c DEE properties from https://webbook.nist.gov/chemistry/.

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The centroid position location algorithm is elaborately programmedbased on the Eq. (1) [52]. Xc and Yc represents respectively the hor-izontal coordinate and vertical coordinate of plume centroid position,dA is the incremental area related to each interpolated data point, x, yand δm is respectively the horizontal and vertical coordinate, and massdistribution at each interpolated data point. Since the scattering lightintensity is proportional to local liquid mass distribution, δm derivedfrom the pixel gray level represents the local liquid mass fraction. Aftercalculating the centroid coordinate of each spray plume, plume cen-troid distance is derived and liquid cross-sectional area of each plume isalso calculated through calibrated patternation images.

∫

∫

∫

∫= =X

x δm dAδm dA

y δm dAδm dA

· ··

Y· ·

·c c(1)

3. Results and discussion

3.1. Macroscopic multi-hole spray behaviors

3.1.1. Morphology analysisFig. 9(a) displays the typical raw high speed camera schlieren

images for three tested fuels at relatively low ambient pressure andtemperature (0.5MPa and 349 K). Generally, the spray plumes from sixnozzle holes are separated, though only three plumes are imaged be-cause the side view schlieren images basically record the projectedspray plumes. For all the spray plumes at this ambient pressure, thedark region (liquid fuel) is wrapped by the brighter region (vapor fuel).At early injection period (0.05 ms), the raw images for all test fuelsshow very similar spray profiles. Liquid core represented by dark pixel

Fig. 3. Schematic diagram of schlieren method.

Fig. 4. Schematic diagram of optical patternation.

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triangle comes out of injector. Then from 0.5 ms to 1.5ms, the amountof liquid core gradually increases because of continuous fuel injection.In addition, more vapor fuel fringe is observed at the outline of liquidcore for pure DEE compare to gasoline and DEE50. After injectionfinishes (1.5 ms injection duration), more obvious vapor fuel and re-duced liquid fuel are observed for pure DEE, as shown by the images at2.0 ms. Due to DEE addition in test fuels, the fuel vaporization processis favored for DEE50 at this condition. This is reasonable since DEEowns higher reid vapor pressure and lower boiling point, DEE quicklyreaches its vapor state at Tamb= 349 K, which is above its boiling point(307.6 K). However, for gasoline, 10% distillation temperature (T10%) is343 K, it is then expected that only a small amount of liquid fuel is

vaporized at this condition. At 516 K, however, the vaporization en-hancing effect with DEE addition is not as evident as lower temperaturecondition, as shown in Fig. 9(b). This is because Tamb= 516 K is higherthan 90% distillation temperature (T90%=463 K) of gasoline, the va-porization of gasoline at this temperature is significantly accelerated.

At high ambient pressure (3.8MPa) and temperature (516 K) asshown in Fig. 10, the spray evolution becomes much slower, comparedto the Pamb= 0.5MPa cases in Fig. 9(b). In addition, the spray plumesfrom six nozzle holes cannot separate, and they just coalesce as a singleplume-like structure. The three tested fuels show similar spray propa-gation behavior initially (before 1.5ms) where liquid core is wrappedby vapor fuel fringe. At 2.5 ms the dark (liquid fuel) region is located atspray tip, which is different from the cases at 2ms in Fig. 9(b). This isbecause at high ambient pressure, the shear resistance from the ambientgas to the spray plumes is larger than that at low ambient pressure. Assuch, liquid phase with higher inertia can penetrate more than thevapor phase that has lower inertia, resulting in the dark region at thespray tip at 2.5 ms. In addition, Musculus et al. proposed in their reviewpaper [4] that entrainment wave at the end of injection will be gen-erated and propagates downstream, which then reinforces the fuel-gasinteraction and spray vaporization at upstream location. This may alsolead to the observed liquid phase on the spray tip.

Compared with gasoline and DEE50, more dark region is observedfor pure DEE. At 3.5 ms, the liquid core for all three fuels disappear forgasoline and DEE50, while there is still some dark liquid region for pureDEE. As mentioned previously, the vaporization enhancing effects ofDEE is more prominent at low ambient temperature. Therefore, forTamb= 516 K, gasoline and DEE50 show similar vaporization extent.However, for pure DEE, the existed dark region (liquid fuel) is causedits different phase change mechanism compared with gasoline at thishigh pressure and temperature condition. The critical temperature Tcand critical pressure Pc for DEE has been reported to be 466.5 K and3.64MPa in Table 1. At 2.5 ms, pure DEE spray can be heated by this516 K ambient gas to its supercritical state. However, for gasoline,

Fig. 5. Time sequence relationship of injection, laser and camera.

Fig. 6. Schlieren image post-processing procedure.

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during the whole spray penetration process, the ambient environment islower than its critical point. Therefore, for gasoline and DEE50, phasechange happens in subcritical state, and this phase change is primarilyaffected by heat transfer at the liquid interface between the spray andambient gas. However, for pure DEE at the later stage, the phase changehappens at supercritical condition. The fuel surface tension and meanfree molecular path gradually decreases and the interface becomesprogressively thicker, which then reduces the difference between liquidand vapor fuel [53]. As such, for pure DEE under this condition, be-cause of its vanish of surface tension at supercritical state, the mole-cular diffusion is the main mechanism for its phase change. The reducedmolecular mean free path and thicker interface greatly restricts themolecular diffusion such that phase change decelerates and there stillexists some dark liquid region even at 3.5 ms. More evidence about thisphase change mechanism is presented in the following.

3.1.2. Vapor tip penetrationFig. 11 shows the vapor tip penetration evolutions (VTP) of three

test fuels at different ambient conditions. For Pamb= 0.5MPa andTamb= 349 K condition in Fig. 11(a), VTP curves of three fuels beforet=0.4ms overlap. For t > 0.4ms, VTP of DEE becomes the largest,followed by DEE50 and gasoline has the smaller vapor tip penetration.This is because of fast vaporization of DEE at this subcritical condition.For t < 0.4ms, fuel is still cold and, the heating up of the spray takestime such that the difference in vaporization rate of three fuels is notyet to be distinguished at the early stage. The similarity of physicalproperties such as the density, viscosity and surface tension of three testfuels then leads to the overlapped VTP curves. After 0.4ms when theliquid spray is heated up, vaporization starts to affect the VTP evolu-tion. For DEE spray with rapid vaporization nature, more vapor phase isgenerated at the spray tip, which tends to expand and leads to thick-ened vapor layer. As a consequence, the vapor tip penetration of DEE islarger than that of gasoline in Fig. 11(a). When ambient temperature

Fig. 7. Definition of spray tip penetration, spray angle for multi-hole GDI spray (a) and post-processing example (b).

Fig. 8. Image post-processing procedure of laser sheet images.

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reaches 516 K in Fig. 11(b), similar vapor tip penetration is observed fordifferent fuels because of the vaporization rate of gasoline at thistemperature (higher than T90%, 463 K, of gasoline) is comparable toDEE.

At Tamb= 516 K and Pamb= 3.8MPa in Fig. 11(b), however, all thethree fuels show much smaller VTP, compared to the case in Fig. 11(a).In addition, the DEE addition effect on VTP is not as significant as thatin Fig. 11(a). VTP curves for all three tested fuels almost overlap duringthe whole penetration process. At this ambient condition, the vapor-ization enhancing effects of DEE is not as evident as lower temperatureconditions (mentioned in Section 3.1.1). Moreover, the high density gasgreatly restricts the vapor generation process on the spray surface, thusthe thickness and expansion of vapor phase layer are reduced at highambient pressure conditions. Due to the thinner vapor phase layer andrestriction of high density gas at high ambient pressure conditions,those influence factors like ambient temperature and fuel type showweak effects on VTP evolution at high pressures. Besides, at 3.8 MPaambient pressure, the difference among the vapor tip penetration of the

three fuels are negligibly small as shown in Fig. 11(c).

3.1.3. Spray angleFig. 12 shows the spray angle of three tested fuels at different am-

bient conditions. The spray angle is the key parameter to evaluate sprayradial dispersion and measured at 1.5ms ASOI based on the definition(θL+ θR, Fig. 7). We note that the thermal process inside the practicalengines such as the phase change, the mixture formation and transportunder different conditions can be characterized by the spray angle andVTP over wide range of Pamb and Tamb [17,18,54,55]. For a specific fuel,increasing ambient temperature and pressure increases the spray angle.However, with the increase of DEE blending ratio, the spray angleshows different behavior at different ambient conditions. ForTamb= 349 K and Pamb= 0.5MPa, the spray angles increase as moreDEE is added. The enlarged spray radial dispersion is caused by thevaporization enhancing effects of DEE at this condition. With the in-crease of DEE blending, more vapor fuel is generated during injectionperiod and then thickens the vapor fuel layer. We note that at 349 K and

Fig. 9. Raw schlieren images for three test fuels at fixed ambient condition.

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0.5MPa, DEE addition at this condition leads to both increased vaportip penetration shown in Fig. 11(a) and enlarged spray angle in Fig. 12.This is reasonable because strong DEE vaporization enhancing effectleads to vapor expansion in both azimuth and axial direction for thepresent multi hole spray. At higher ambient temperature, for both lowand high ambient pressure, the DEE blending effect on spray anglebecomes negligibly small, within the measurement uncertainty. This isbecause vaporization enhancing effects of DEE addition is not obviousat higher ambient temperature and the thickness of vapor fuel layer is

similar for three test fuels, as discussed previously in Section 3.1.2.

3.2. Multi-hole spray patternation characteristics

3.2.1. Images of spray patternation for different fuelsFig. 13 shows typical cross-sectional patternation images of three

test fuels at two ambient conditions. The red square marked areas onthe images are enlarged to show more detailed spray patternation in-formation. For Tamb= 349 K and Pamb= 0.5MPa conditions in

Fig. 10. Raw schlieren images for three test fuels at fixed 3.8MPa Pamb, 516 K Tamb condition.

Fig. 11. VTP of three test fuels at different ambient conditions.

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Fig. 12. Spray angle of three test fuels at different ambient conditions.

Fig. 13. Typical patternation images for three test fuels at two ambient conditions.

Fig. 14. Plume centroid distance for three test fuels at different Tamb.

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Fig. 13(a), at 0.9ms, most spray plumes have not reached the lasersheet plane and low light intensity images are captured. At 1.5 ms, allsix spray plumes reach the laser sheet plane. The six spray plumes areclearly separated, which is in consistence with the schlieren imagesshown in Fig. 9(a). The light intensity of plume 3# and plume 5# issignificantly weaker than other four plumes. This is because of thenozzle geometry design. The bent angles of plume 3# and 5# are largerthan those of other plumes. Hence the two plumes take more time toreach laser sheet plane and less liquid fuel is reserved due to longer fuelvaporization duration time. At 2.8 ms, the six spray plumes almostmerge and the image of pure DEE owns lower light intensity. Thisfurther validates the previous conclusion that the DEE addition intogasoline favors vapor fuel generation. In addition, some large blobs(representing bulk of liquid) are observed in the enlarged patternationimages for all test fuels.

For high ambient temperature (516 K) and pressure (3.8 MPa)conditions as shown in Fig. 13(b), the patternation images show sig-nificant difference compared to those images at low pressure conditionshown in Fig. 13(a). At 1.5 ms, no liquid fuel reaches the laser sheetplane, while all six spray plumes reach sheet plane in Fig. 13(a). This isbecause the tip velocity of spray plume is decreased due higher re-sistance at higher ambient gas condition. In addition, at 2.8 and 3.8ms,only single plume is observed for three test fuels at this high ambientpressure condition. This is because those original six plumes coalescedue to the extrusion of higher density gas. Furthermore, the pure DEEimages own higher light intensity, indicating that there will be moreliquid fuel for pure DEE. This further confirms our statement in Section3.1.1 on different phase change mechanism at different thermodynamicconditions. As to those magnified images, several large blobs are ob-served at the periphery of the coalesced spray for gasoline and DEE50with a typical size of around 0.6mm. However, for DEE, no such kind ofliquid blobs are observed. We note that for each test condition, we havecaptured the patternation image for more than 20 times and the liquidblobs behavior at given condition are very repeatable, indicating thatthe absence of liquid blobs for DEE at 516 K and 3.8MPa is not anoccasional coincidence by imaging. Instead, it reflects the phase changemechanism at supercritical condition, which is the sharp decrease ofsurface tension when thermodynamic condition is beyond supercriticalpoint [53].

3.2.2. Patternation parameters: spray plume distance and cross-sectionalarea

Fig. 14 shows typical plume centroid distance at 1.5ms ASOI, and0.5 MPa for three tested fuels at different Tamb. The standard deviationof the plume centroid distance is found to be 10% by repeated mea-surements for 20 times Different plumes have different plume centroid

distance due to the nozzle hole geometry, as expected. For given nozzlehole, the ambient temperature and fuel type exhibit negligible influenceon plume centroid distance. This reflects that fuel vaporization does notaffect the plume centroid location for multi-hole spray. In addition, asshown in Fig. 14, the scattering light intensity of plume 3# and 5# areso weak that their plume centroid distance cannot be determined, in-dicating complete vaporization of spray from nozzle hole 3# and 5# atthis condition.

Fig. 15(a) shows the liquid cross-sectional area of typical plume(4#) for three test fuels at different Tamb and 0.5 Pamb conditions. Forgiven temperature, with the increase of DEE blending ratio, the liquidcross-sectional area decreases significantly because of enhanced va-porization of DEE. The liquid cross-sectional area decreases with theincrease of ambient temperature. Higher ambient temperature in-creases the heat transfer between the environment and the liquid spray,and vaporization is favored, the remaining liquid phase that passesacross the intersection between the plane and the spray plumes is thendecreased. As a consequence, the liquid cross-sectional area reduces.Similar cross-sectional area dependence on fuel type and ambienttemperature is also observed for plumes 1#, 2#, and 6#. For plumes 3#and 5#, the scattering light are too weak for the cross-sectional areadetermination because these two plumes have already completelyevaporated before they reach the intersection plane, as also shown bythe images at 1.5ms in Fig. 13(a).

However, at the ambient pressure of 3.8 MPa and 516 K as shown inFig. 15(b), the gasoline and DEE50 shows similar liquid cross-sectionalarea, while the cross-sectional area of DEE is evidently higher than thatof other two fuels. This further indicates that more liquid fuel is re-served for pure DEE owing to different vapor generation mechanism, asdiscussed in Section 3.1.1.

4. Conclusion

Systematic experiments on spray vaporization characteristics ofthree tested fuels (gasoline, DEE50 and DEE) have been conducted byusing an OEM multi-hole injector. Front view high speed images andcross sectional view of the patternation images were recorded and va-porization spray parameters like vapor tip penetration, spray angle andliquid cross-sectional area are extracted. Major conclusions are sum-marized as follows.

a) For subcritical conditions under low ambient pressure, all the sprayplumes for three tested fuels are separated, and that means the moreenlarged spatial dispersion to promote air-fuel mixing level.However, for supercritical conditions under high ambient pressure,all spray plumes coalesce due to high gas density for three tested

Fig. 15. Liquid cross-sectional area for three test fuels at (a) sub and (b) super-critical ambient pressure.

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fuels, and thus restrains spray evolution process. To avoid the oc-currence of supercritical conditions in realistic GCI engines, too lateinjection timing is not recommended;

b) At subcritical condition, the addition of DEE facilitates fuel vapor-ization process and more vapor region exists in the recorded images,which is also evidenced by increased vapor tip penetration, in-creased spray angle and reduced liquid cross-sectional area. Thisvaporization enhancing effects of DEE will be of much help to solvethe cold start difficulty of GCI engines. In addition, with the increaseof ambient temperature, the vaporization enhancing effects becomeweaker because of increased vaporization rate of gasoline;

c) At supercritical condition, effects of DEE addition on vaporizationbehaviors is totally different: DEE addition shows a tendency toincrease the liquid cross-sectional area. In addition, large liquidblobs are absent for DEE spray. This is because phase change undersupercritical condition is dominated by molecular diffusion andthicker interface exists at supercritical condition.

Acknowledgement

This work is supported by the Basic Science Center Program forOrdered Energy Conversion of the National Natural Science Foundationof China (51888103, 51722603, 51876163), the Fundamental ResearchFunds for the Central Universities and the Key Laboratory of HighEfficiency and Low Emission Engine Technology, Ministry of Industryand Information Technology, Beijing Institute of Technology.Experimental support from Ms. Rui Yang is acknowledged.

Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.applthermaleng.2019.114453.

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