Atomization Characteristics of Dimethyl Ether Fuel as an Alternative

Energy & Fuels 2006, 20, 1471-1481

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Atomization Characteristics of Dimethyl Ether Fuel as an Alternative Fuel Injected through a Common-Rail Injection System Hyun Kyu Suh,† Sung Wook Park,† and Chang Sik Lee*,‡ Graduate School of Hanyang UniVersity and Department of Mechanical Engineering, Hanyang UniVeristy, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea ReceiVed December 15, 2005. ReVised Manuscript ReceiVed March 25, 2006

The objective of this work is to analyze the macroscopic development and atomization characteristics of dimethyl ether (DME) and diesel fuel spray injected through a common-rail injection system in a diesel engine. To investigate the spray behavior of DME fuel, the macroscopic and microscopic spray characteristics of DME were analyzed in terms of spray development, spray tip penetration, spray cone angle, impingement timing, mean droplet size, and the mean velocity. The process of macroscopic DME spray development was visualized according to various injection parameters such as injection duration, ambient and injection pressures, ambient temperature, and combustion chamber geometry via a spray visualization system composed of an neodymium-doped yttrium aluminum garnet (Nd:YAG) laser and an intensified charge coupled device (ICCD) camera. DME fuel atomization characteristics in a high-pressure spray chamber such as axial mean velocity and droplet size distribution were measured with a phase Doppler particle analyzing (PDPA) system. It was revealed that the DME spray has a shorter spray tip penetration and a wider spray cone angle than diesel fuel under identical test conditions. The surface area also was smaller, and the axial direction spray centroid of DME was higher than that of diesel fuel. In the case of incylinder spray, the DME spray developed slower and disappeared faster after the start of the injection. The comparison of spray profiles in both fuels shows that diesel fuel has a larger Sauter mean diameter (SMD) value than DME.

1. Introduction Diesel engines have many advantages over gasoline engines, which include higher thermal efficiency, lower fuel consumption, and lower CO2 gas emissions. However, exhaust emissions such as NOx and particulate matter (PM) from diesel engines can be a serious environmental problem. Therefore, the reduction of pollutant NOx and PM emissions from diesel engines is an important concern. It has long been recognized that oxygenated alternative fuels have less polluting combustion characteristics than diesel fuel. Among these various alternative fuels, dimethyl ether (DME) is the most attractive alternative fuel to solve the exhaust emissions problems of diesel engines. DME engines are expected to be nearly equal to diesel engines in both thermal efficiency and power output. In a compression ignition engine, the fuel is injected directly into the combustion chamber, where gas flow is highly turbulent. Fuel is injected in a very short period of time, and both liquid droplets and fuel vapor exist in the combustion chamber. The distribution of fuel droplets in a diesel engine combustion chamber is a dominant factor in governing the fuel/air mixture formation, the combustion process, and, ultimately, engine performance. This is why the characterization of liquid fuel atomization and vaporization is important, because the atomization and evaporation of the fuel spray directly affect the engine’s performance and emission characteristics. For this reason, diesel engine fuel injection systems were developed with atomization and fuel droplet evaporation in mind. * Corresponding author. Tel.: +82-2-2220-0427. Fax: +82-2-22815286. E-mail: [email protected]. † Graduate School of Hanyang University. ‡ Department of Mechanical Engineering.

Figure 1. Schematics of spray visualization and PDPA system.

The spray behavior and atomization performance of DME fuel seems to be different from that of diesel fuel because the different properties of DME fuel can influence spray development and droplet atomization. Thus, there is much experimental and theoretical research on the fuel spray and combustion characteristics of DME. Yoshio et al.1 reported that the spray tip penetration speed of DME was slower and the spray angle wider than diesel because DME fuel has a short breakup time and a fast evaporation process. They also conducted experimental investigations to clarify the effect of DME injection characteristics on heat release and exhaust emissions.2,3 Kim et al.4 analyzed (1) Yoshio, S.; Akira, N.; Li, J. SAE Tech. Pap. Ser. 2001, 2001-013635. (2) Yoshio, S.; Akira, N.; Li, J. SAE Tech. Pap. Ser. 2001, 2001-013634.

10.1021/ef050420f CCC: $33.50 © 2006 American Chemical Society Published on Web 05/24/2006

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Figure 2. Single-hole injector. Figure 4. Distance from the nozzle tip to the combustion chamber and ambient pressure.

Figure 3. Schematics of (a) conventional and (b) narrow-angle combustion chamber geometry and visualization area. Table 1. Specifications of the Spray Visualization System and Phase Doppler Particle Measuring System light source wavelength laser power beam thickness resolution

Spray Visualization System Nd:YAG 532 nm 270 mJ ∼1 mm 1280 × 1024

Phase Doppler Particle Measuring System light source Ar-ion wavelength 514.5, 488 nm focal length 500 mm for transmitter 250 mm for receiver collection angle 30° Table 2. Specifications of the Test Nozzle

nozzle type number of holes hole diameter

free spray

incylinder spray

mini-sac 1 0.3 mm

mini-sac 5 0.167 mm

Figure 5. Spray characteristics and measurement points of the spray experiment. (a) Free spray and measuring point. (b) Impinged spray.

combustion characteristics of DME fuel such as combustion pressure, combustion duration, and pollutant levels as compared to LPG in a constant volume chamber. Also, No et al.5 studied DME spray characteristics such as spray tip penetration, spray angle, and Sauter mean diameter (SMD) under various injection conditions. The macroscopic spray characteristics of DME fuel in the combustion chamber were also investigated by Suh et al.6 They suggested that the use of DME can reduce combustion chamber wall wetting. Ikeda et al.7 measured and compared the injection rate of multiple injections in a common-rail injection (3) Yoshio, S.; Akira, N.; Takashi, S.; Yuichi, G. SAE Tech. Pap. Ser. 2000, 2000-01-1809. (4) Kim, T. K.; Im, M. H.; Jang, J. Y. Trans. KSAE 2003, 11 (5), 8388. (5) No, S. Y.; Hwang, J. S.; Kim, S. C.; Ha, J. S. JSAE ReV. 2003, 20030302. (6) Suh, H. K.; Park, J. H.; Park, S. W.; Lee, C. S. IPC-13 conference, 2005; pp 602-607 (7) Ikeda, T.; Ohmori, Y.; Takamura, A.; Sato, Y.; Li, J.; Kamimoto, T. SAE Tech. Pap. Ser. 2001, 2001-01-0527.

Figure 6. Image threshold process (Pinj ) 60 MPa, Pamb ) 0.1 MPa, tinj ) 1.0 ms). (a) Original image. (b) Threshold processing image.

system using diesel and DME fuel. They reported that the maximum injection mass of DME depends on the cavitation number. Teng et al.8 investigated the effect of injection pressure on the liquid DME viscosity. A review of recent developments (8) Teng, H.; James, C. M.; Jeffrey, B. S. SAE Tech. Pap. Ser. 2002, 2002-01-0862.

Characteristics of DME Fuel as an AlternatiVe Fuel

Figure 7. Comparison of spray development between diesel and DME (Pinj ) 60 MPa, Pamb ) 1 MPa, Tamb ) 293 K, tinj ) 1.0 ms).

Energy & Fuels, Vol. 20, No. 4, 2006 1473

Figure 8. Effect of ambient pressure on the DME spray development process (Pinj ) 40 MPa, Tamb ) 293 K, tinj ) 1.0 ms).

Table 3. Criteria for PDPA Measurement burst threshold mixer frequency filter frequency PMT voltage signal-to-noise ratio diameter subrange velocity subrange

0.5 mV 36, 40 MHz 40 MHz 500 V 65 2-100 µm -292-292 m/s

Table 4. Experimental Conditions injection system

common-rail Free Spray

injection pressure (Pinj) ambient pressure (Pamb) ambient temperature (Tamb) injection duration (tinj)

40, 60 MPa 0.1, 1, 2 MPa 293, 393, 493 K 1.0 ms

Figure 9. Effect of ambient temperature on spray development between diesel and DME (Pinj ) 40 MPa, Pamb ) 2 MPa, Tamb ) 493 K, tinj ) 1.0 ms).

Incylinder Spray injection pressure (Pinj) 40, 60 MPa piston distance (LW)1 mm-3.5 MPa-293 K ambient pressure (Pamb)4 mm-2.5 MPa-293 K ambient temperature (Tamb) injection duration (tinj) 0.5, 1.0 ms

related to the use of DME in diesel engines was reported by Sorenson,9 who discussed the fields of engine performance, emission characteristics, fuel injection systems, and ignition delay. The atomization characteristics of DME blended with vegetable oil was investigated by Kim et al.10 at various injection pressures, ambient pressures, and mixing ratios. Other extensive experimental and numerical studies on the combustion, emissions, and injection characteristics of DME fuel have been carried out.11-13 However, most of these studies provide little fundamental understanding of the transient fuel spray and there are many uncertainties as to the atomization and microscopic characteristics of DME fuel spray. In this study, the macroscopic spray developments were investigated in terms of spray tip penetration, spray cone angle, spray area, and axial direction centroid using a spray visualization system under various experimental conditions. The transient atomization and microscopic characteristics of a single-hole DME spray were also investigated in terms of Sauter mean diameter and axial mean velocity distributions to clarify the spray’s time-dependent droplet formation process through a time-resolved analysis of droplet size data acquired by a phase Doppler particle analyzer (PDPA) system. The experimental DME atomization characteristic results were compared with those of diesel fuel under the same experimental conditions. (9) Sorenson, S. C. ASME spring technical conference, 2000; 2000ICE-292, pp 65-74. (10) Kim, I.; Goto, S.; Ehara, R. Proceeding of ILASS-ASIA, 2000; pp 127-132. (11) Mingfa, Y.; Zunqing, Z.; Sidu, X.; Maoling, F. SAE Tech. Pap. Ser. 2003, 2003-01-3194. (12) Xu, S.; Mingfa, Y.; Xu, J. SAE Tech. Pap. Ser. 2001, 2001-010142. (13) Kajitani, S.; Oguma, M.; Mori, T. SAE Tech. Pap. Ser. 2000, 200001-2004.

Figure 10. Effect of injection duration on the DME fuel spray development (Pinj ) 40 MPa, Pamb ) 0.1 MPa, Tamb ) 293 K, tinj ) 1.0 ms).

2. Experimental Apparatus and Procedure 2.1. Experimental Apparatus. The macroscopic spray structure including spray tip penetration and overall spray behaviors can be obtained from the spray images obtained using a spray visualization system composed of an Nd:YAG laser (Continuum, SL2-10), cylindrical lenses and mirrors, a digital delay/pulse generator (Berkeley Nucleonics Corp, model 555), an intensified charge coupled device (ICCD) camera (The Cooke Corp, Dicam-PRO), and a PC installed with an image grabber as shown in Figure 1. A Nd:YAG laser with a wavelength of 532 nm was used as a light source. Cylindrical lenses were used to illuminate the spray development by forming a laser sheet beam less than 1 mm. A high resolution ICCD camera was used to capture the images. A high-pressure injection system generated high pressure in the common-rail for fuel injection and was made in order to more easily control the injection pressure. To pressurize the common-rail for the spray injection, two high-pressure pumps (Haskel, HSF-300) operated by compressed air generated a high fuel pressure for injection and stored it in the common-rail. In this experiment, the fuel pressure in the common-rail was controlled by the quantity of air up to 200 MPa. A high-pressure spray chamber that can be pressurized up to 4 MPa was used to raise the ambient pressure with nitrogen gas. To maintain a constant temperature in the highpressure chamber, the temperature was controlled with a temperature regulator. The DME was pressurized to 1 MPa in the fuel tank by nitrogen gas to avoid vaporization in the fuel supply line. Table 1


Figure 11. Comparison of the spray tip penetration and spray cone angle between diesel and DME (Pinj ) 40 MPa, Pamb ) 0.1 MPa, Tamb ) 293 K, tinj ) 1.0 ms). (a) Spray tip penetration: (symbol) experiment; (line) empirical eq 14. (b) Spray cone angle.

shows the specifications of the microscopic spray visualization and measuring system. As also can be seen in Figure 1, the PDPA system was composed of a high-pressure fuel injection system, a transmitter, a receiver, and a signal synchronizer. A laser beam was split, and the two resulting beams crossed each other at a specified angle, the two beams being coherent to each other in the control volume of their intersection. If a particle traverses the control volume, it scatters the light, and the two components of scattered light have different Doppler signals. Therefore, at the surface of the photodetector, the two light components interfere, resulting in a pulsating light intensity. On the basis of the data rates and the signal intensity, the output of the Ar-ion laser and photomultiplier voltage were determined to be 700 mW and 500 V, respectively. The test injector used in this investigation was a mini-sac type injector designed for a common-rail injection system. For a free spray, it has a single-hole nozzle with a 0.3-mm internal diameter and a 0.8-mm hole length, as shown in Figure 2. The test injector was controlled by a peak current of 16.0 A and a hold current of 5.0 A. In this investigation, two different types of injectors were used. The specifications of the test injectors are illustrated in Table 2. 2.2. Experimental Procedure. The experiments were conducted in both free and combustion chamber conditions to compare the spray characteristics. In the case of free spray, experiments were carried out to analyze the effects of injection pressure, ambient pressure and temperature, and injection duration on the spray characteristics. To describe and investigate the effects of ambient conditions on spray behaviors, DME and diesel were injected into a high-pressure chamber pressurized by nitrogen gas. In this study, to analyze the spray development process in a combustion chamber, a different type of combustion chamber was used. Figure 3 shows

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Figure 12. Effect of injection duration on the (a) spray tip penetration and (b) spray cone angle of DME (Pinj ) 40 MPa, Pamb ) 0.1 MPa, Tamb ) 293 K).

the detailed geometry and shape of the combustion chamber. Figure 3a illustrates the conventional combustion chamber, with a 135° fuel injection area, and the narrow combustion chamber seen in Figure 3b had 74° of combustion chamber wall for fuel injection. The visualization region in this experiment is indicated with a dotted line in Figure 3. Diesel is injected at the compression stroke in a DI diesel engine. To analyze the spray behaviors of the in-combustion chamber conditions, incylinder conditions were determined in terms of distance from the nozzle tip to the combustion chamber (LW) and ambient pressure (Pamb). Among these different data types, two variables (distance from piston crown and ambient pressure) were used to decide the fuel injection timing. Figure 4 illustrates the distance from the injector nozzle tip to the piston upper surface and the ambient pressure. In this study, the spray tip penetration, spray cone angle, spray area, and centroid of spray were investigated under various experimental conditions. The definition of spray characteristics and the measurement points of the spray experiment are illustrated in Figure 5. The spray tip penetration was defined as the maximum distance from the nozzle tip that the injected spray reached. The experimental results can be compared with those of previous studies by estimating the results from the empirical equations. The empirical equation suggested by Hiroyasu and Arai14 to predict spray tip penetration was used for two cases: where the injection time was longer than and shorter than the breakup time. For injection times shorter than the breakup time tb, S ) 0.39tasoix(2∆P)/Ff

(tasoi e tb)

For injection times longer than the breakup time tb, (14) Hiroyasu, H.; Arai, M. SAE Tech. Pap. Ser. 1990, 900475.

(1)

Characteristics of DME Fuel as an AlternatiVe Fuel S ) 2.59xDtasoi(2∆P/Fg)0.25

(tasoi g tb)

Energy & Fuels, Vol. 20, No. 4, 2006 1475 (2)

The breakup time is given by the dimensionally correct equation tb )

28.65FfD

(3)

xFg∆P

To investigate the spray area and centroid of spray, the bright distribution of the original spray images were analyzed and the optimal threshold level had to be determined. Figure 6 shows the image threshold process of the original spray images. In this study, the bright distributions of the original image are analyzed and the optimal threshold level is determined. The optimal threshold level was 30. By determining the optimal threshold level, the spray tip penetration, spray cone angle, spray area, and centroid of spray could be obtained. Simultaneous measurements of droplet velocity and size were conducted in 5-mm intervals in the axial direction (Z) from the nozzle tip and in 2-mm intervals in the radial direction (R), shown in Figure 5. At each measurement point, approximately 30,000 droplets were captured and averaged. To obtain the time-resolved data, the signal analyzer was synchronized to the injector driver with a digital pulse/delay generator. The representative SMD of a spray droplet at a specific time was determined by averaging the captured droplets at all of the measurement points. To examine the time-dependent development process of the spray, a time-resolved analysis of the data acquired during the many injection events was conducted. For N injections, the Sauter mean diameter (SMD, D32) with respect to the time window, ∆t, was defined by ∆t

∆t

t+

t+ 2

D32(t) ) (

2

n

n

∑ ∑ D )/( ∑ ∑ D ) 3

2

i

i

∆t i)1

(4)

∆t i)1

t-

t2

Figure 13. Effect of injection pressure on the spray tip penetration and spray cone angle of DME (Pamb ) 0.1 MPa, Tamb ) 293 K, tinj ) 1.0 ms). (a) Spray tip penetration: (symbol) experiment; (line) empirical eq 14. (b) Spray cone angle.

2

The arithmetic mean diameter (AMD, D10) was defined by

experimental conditions of both free spray and incylinder spray are listed in Table 4.

∆t

3. Results and Discussions

∆t

t+

t+ 2

D10(t) ) (

n

n

2

∑ ∑ D )/( ∑ ∑ i) i

∆t i)1

t-

(5)

∆t i)1

t2

2

The mean velocity could then be determined with the following equation: ∆t t+

1 Mean velocity(t) ) [ n

2

n

∑ ∑ V] i

(6)

∆t i)1

t2

where n is the number of droplets for the specific time interval and Di and Vi indicate the droplet diameter and velocity, respectively. The PDPA measurement is performed based on many criteria such as the threshold voltage and signal-to-noise ratio. By adjusting these conditions, the data rate or the accuracy of the results can be improved. In this case, the criteria on the PDPA measurement were set to optimum values for the high-speed fuel spray as listed in Table 3. The experiments were conducted under both free spray and incylinder conditions. In the case of the PDPA system, the experiments were conducted at atmospheric conditions. The

3.1. Spray Macroscopic Characteristics. 3.1.1. Free Spray. Figure 7 shows the comparison of spray development between diesel and DME fuels at 60 MPa of injection pressure, 1 MPa of ambient pressure, and room temperature. First, the time after start of injection (tasoi) was set to zero when the spray showed the first appearance of the liquid phase at the nozzle tip. The spray development process of both fuels was marked in proportion to the elapsed time after the start of injection. The comparison of the spray development process between diesel and DME shows that the diesel spray development was thinner and longer than that of DME under identical experimental conditions. The effect of ambient pressure on the DME spray development process is illustrated in Figure 8. DME spray development at high ambient pressure was slower and the spray angle was wider than under atmospheric conditions. The evaporating process that can be seen in the outer spray region at Pamb ) 0.1 MPa disappears at high ambient pressure. It can be said that the amount of the liquid phase of a DME spray increases at the higher ambient pressures due to decreased evaporation. Figure 9 shows the effect of ambient temperature on the spray development process. As the ambient temperature increased, the liquid phase of both fuels became shorter and the spray wider due to the improvement of evaporation and the enhanced momentum of ambient gas at the increased ambient temperature.


Figure 14. Effect of ambient pressure on the spray tip penetration and spray cone angle of DME (Pinj ) 40 MPa, Tamb ) 293 K, tinj ) 1.0 ms). (a) Spray tip penetration: (symbol) experiment; (line) empirical eq 14. (b) Spray cone angle.

The diesel spray shapes were longer and wider as the ambient temperature increased. In the case of DME fuel, it can be seen that the evaporating processes of DME were more rapid at high ambient temperatures because DME fuel remains in the vapor phase at room temperature. The effect of the injection duration on DME fuel spray development was illustrated in Figure 10. The injection duration of DME fuel was varied from 0.8 to 1.4 ms in steps of 0.2 ms and the injection pressure, ambient pressure, and ambient temperature were 40 MPa, 0.1 MPa, and 293 K, respectively. In the case of tinj ) 0.8 ms, the spray development of DME was shorter because the injector nozzle was not fully opened. However, the spray developments of the other cases are all much the same. From the spray images captured by the spray visualization system, the spray tip penetration and spray cone angle were compared for both diesel and DME fuel as shown in Figure 11. The error bar on the data is added to represent the experimental uncertainty, as shown in Figure 11a and b. In this experiment, 10 images were captured for determining the spray characteristics and spray behaviors. The predicted spray tip penetration from the empirical equation of Hiroyasu and Arai14 is also shown with the experimental results for reference. As can be seen in the figure, the spray tip penetration of DME fuel was shorter and the spray cone angle was wider than that of diesel fuel in the experimental results because the DME droplets evaporated more quickly. However, the empirical equation result is somewhat different because the empirical equation results before the breakup time depend on the fuel

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Figure 15. Comparison of the (a) spray tip penetration and (b) spray cone angle of DME according to the ambient temperature (Pinj ) 60 MPa, Pamb ) 1 MPa, tinj ) 1.0 ms).

Figure 16. Comparison of spray tip penetration between diesel and DME according to the ambient temperature (Pinj ) 60 MPa, Pamb ) 2 MPa, tinj ) 1.0 ms).

density. In the case of diesel spray tip penetration, the empirical equation underestimated the spray tip penetration under atmospheric conditions. As for the breakup time, the DME fuel breakup time is quicker than diesel fuel by about 0.198 ms, as can be seen in the empirical spray tip penetration results. On the basis of these results, it can be said that the momentum of DME droplets injected from the nozzle tip was lost more quickly than that for diesel under the same ambient conditions. As illustrated in Figure 11, the captured image data shows that the


Figure 17. Comparison of the spray area and axial direction centroid of spray (Pinj ) 60 MPa, Pamb ) 2 MPa, tinj ) 1.0 ms). (a) DME (Tamb ) 293 K). (b) DME (Tamb ) 493 K). (c) DME (Tamb ) 393 K). (d) Diesel (Tamb ) 393 K).


Figure 19. Spray development process in (a) conventional or (b) narrow-angle combustion chamber (Pinj ) 60 MPa, Pamb ) 3.5 MPa, Tamb ) 293 K).

Figure 20. Spray development process in combustion chamber (Pinj ) 60 MPa, Tamb ) 293 K).

Figure 18. Comparison of the (a) spray area and (b) axial direction centroid (Pinj ) 40 MPa, Pamb ) 2 MPa, tinj ) 1.0 ms).

spray macroscopic characteristics and errors were not so large. Therefore, the error bars on the data of other figures are omitted. Figure 12 illustrates the effect of injection duration on the DME fuel macroscopic spray development process. For tinj ) 0.8 ms, the spray tip penetration was shorter than that for other cases. This pattern can also be found in the visualized spray

development process images. On the basis of these results, it can be postulated that the injector nozzle was never fully opened at tinj ) 0.8 ms; therefore, it can be said that the spray never developed completely due to the lower fuel mass injected. As for the spray cone angle, as the injection duration increased, the spray cone angle decreased. Figure 13 illustrates the effect of injection pressure on DME fuel spray development parameters such as spray tip penetration and spray cone angle. As mentioned above, both the spray tip penetration and spray cone angle increased as the injection pressure increase. The effect of injection pressure on the breakup of DME fuel shows that as the injection pressure increased, the breakup time decreased. Figure 14 shows the effect of ambient pressure on the DME fuel spray development parameters spray tip penetration and spray cone angle. As the ambient pressure increased, the predicted spray tip penetration by empirical equations showed good agreement with the experiments. The spray tip penetration decreased with the ambient pressure increase due to the slow spray development at high ambient pressures. In the case of the spray cone angle, an increase in ambient pressure induced a decrease of spray momentum in the axial direction. Therefore, droplets could not spread in the axial direction and, thus, stagnated and spread in the radial direction. This is why the DME spray cone angle increased at high ambient pressures. Figure 15 shows the effect of ambient temperature on the DME spray tip penetration and spray cone angle at constant injection pressure (Pinj ) 60 MPa) and ambient pressure (Pamb


Figure 21. Spray tip penetration and impingement timing in (a) conventional or (b) narrow-angle combustion chamber (Pinj ) 60 MPa, Pamb ) 3.5 MPa, Tamb ) 293 K).

) 1 MPa). As can be seen in the figure, the spray tip penetration increased almost linearly as the ambient temperature increased. It can be conjectured that the downstream droplet momentum increased due to the high ambient gas density as the ambient temperature increased. However, the spray cone angle decreased because of the rapid evaporating processes in the spray’s outer region at high ambient temperatures. The effect of ambient temperature on spray tip penetration between diesel and DME fuel is illustrated in Figure 16. The macroscopic spray characteristics of both fuels show that the spray tip penetration of diesel was longer than that of DME, regardless of the ambient temperature. To compare the spray behavior of diesel and DME fuel, the spray area and axial direction centroid of spray were investigated. The spray area is the cross-sectional area of the spray, and the axial direction centroid of spray indicates how the spray is distributed around the spray center. Figure 17 shows the comparison of the DME spray area and axial direction centroid using the image threshold process. As mentioned above, the optimal threshold level of the original images was 30. As the ambient temperature increased, the spray area of DME fuel increased and the axial direction centroid of DME spray moved to downstream. It can be guessed that higher ambient temperatures increased the spray area due to the rapid evaporation of DME. The comparison between DME and diesel is shown in Figure 17c and d; the diesel has a larger spray area than DME at high ambient temperatures because the less volatile

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Figure 22. Comparison of the spray area in both (a) conventional and (b) narrow-angle combustion chambers (Pinj ) 60 MPa, Pamb ) 3.5 MPa, Tamb ) 293 K).

diesel increases the diesel spray cone angel, and thus, the spray area is larger than that of DME fuel. The comparison results of the spray area and axial direction centroid in both fuels are illustrated in Figure 18. The spray area of diesel fuel was larger, as shown in Figure 18a. However, the comparison of the centroids shows that the DME fuel has a larger value. It can be conjectured that a large spray area and small centroid value in the diesel spray means that there is a good distribution of spray around the spray center under ambient conditions. 3.1.2. Incylinder Spray. In this study, the macroscopic spray behavior of DME fuel compared to diesel fuel in the combustion chamber was analyzed in terms of spray tip penetration, impingement timing, and spray area. The spray development process of diesel and DME fuel in a conventional combustion chamber is shown in Figure 19a. In a conventional combustion chamber spray, the spray development of DME was slower and disappeared more rapidly than the diesel at the same injection duration. It can be expected that, due to the evaporation characteristics of DME fuel, the wall wetting problem was reduced when fuel was injected into the combustion chamber. As the DME fuel injection duration was increased by 1.0 ms, the spray development became similar to that of diesel spray. So, to obtain the same spray development and distribution, the injection duration of DME fuel needed to be at least 1.0 ms longer. A similar spray development trend for the narrow-angle combustion chamber can be seen in Figure 19b. The diesel spray


Figure 23. Comparison of (a) SMD and (b) axial mean velocity according to the axial distance from the nozzle tip (Pinj ) 60 MPa, Pamb ) 0.1 MPa, Tamb ) 293 K, tinj ) 1.0 ms).

development made rapid progress and flowed smoothly along the piston wall after the spray impinged on the wall. The spray swirled due to changes in the direction of the wall. The swirl intensity of the diesel fuel spray was large. It can be presumed that the droplet kinetic energy and momentum in the diesel spray was large and that low energy loss produced a larger spray swirl along the piston wall. However, it is different from conventional chamber spray development in that the DME spray disappeared rapidly at 1.0 ms of injection duration in the narrow-angle combustion chamber spray. The effect of the nozzle-wall distance on the spray development of DME in a conventional combustion chamber is illustrated in Figure 20. As shown in the figure, the fuel spray spread all along the upper side of the combustion chamber because the nozzle-wall distance was far. This may indicate that the fuel induces the cylinder wall wetting problem. Figure 21 illustrates the comparison of the spray tip penetration and impingement time for the diesel and DME fuel in combustion chambers. As shown in Figure 21a, the spray tip penetration of diesel was larger than that of DME fuel before 0.6 ms after the start of injection. After 0.7 ms of elapsed time, the spray tip penetration was almost the same, regardless of the fuel or the injection duration. In the case of impingement timing, the diesel fuel impinged on the wall quicklysat about 0.2 mssbecause of rapid spray propagation. However, the spray tip penetration and impingement timing of the narrow angle combustion chamber show a similar trend of elapsed time after the start of injection as that seen in Figure 21b.


Figure 24. Effect of radial distance on SMD. Z ) (a) 30 or (b) 60 mm.

A comparison of the spray area between diesel and DME fuel in both combustion chambers is shown in Figure 22. In the conventional combustion chamber in Figure 22a, the diesel spray area was larger than that of DME fuel at the same injection duration. Moreover, the diesel spray area rapidly increased at initial injection times, as shown in this figure. It can be presumed that the diesel spray spreads well at the same injection duration because diesel fuel had higher droplet momentum compared to DME. However, as the injection duration of DME fuel was increased to 1.0 ms, the spray area of DME fuel became similar after 0.9 ms after the start of injection. The spray area of diesel and DME fuel in the narrow angle combustion chamber is illustrated in Figure 22b. The diesel spray area was also larger than the DME spray area at the same injection duration. However, as the injection duration is increased beyond 1.0 ms, the spray area of DME becomes larger than the area of diesel spray, as shown in Figure 22b. 3.2. Spray Atomization Characteristics. The comparison of SMD and axial mean velocity with axial distance is illustrated in Figure 23. After showing its maximum value in the vicinity of the nozzle tip, the SMD in both fuels gradually decreased downstream. It indicates that the spray internal structure was dramatically changed within the near field of the nozzle tip where the atomization processes of droplet actively takes place. The comparison results of spray profiles in both fuels show that the diesel fuel has a larger SMD than that of the DME fuel. It can be guessed that the low kinetic viscosity of DME, about 20% of diesel fuel’s, enhanced the droplet atomization under the same injection conditions. On the other hand, the axial mean


Figure 25. Time-resolved evolution of droplet diameter distributions in (a) diesel fuel and (b) DME fuel.

velocities of both fuels showed similar trends with an increase of the axial distance. Analyzing the distribution of the SMD and axial mean velocity, it can be said that the DME fuel is superior to the diesel fuel in its fuel injection and atomization characteristics. In Figure 5, Z and R indicate the axial distance from the nozzle tip and the radial distance from the spray centerline. Figure 24 shows the SMD distributions with radial distance at the two axial distances, Z ) 30 and 60 mm. In the case of Z ) 30 mm, the SMD decreases with the increase of the radial distance. The diesel spray SMD reaches approximately 38 µm at the nozzle axis. In addition, the SMD decreases rapidly with processing to the outer region of the spray. However, the DME spray has an almost constant SMD value in the radial distance range 23∼26 µm from the nozzle axis to 6 mm. In the case of Z ) 60 mm, the SMD of both fuels decreased gradually with the increase in the radial distance. Analyzing the effects of axial distance on SMD distributions, the diesel spray shows a much lower value with the increase of the axial distance; however, the DME spray was almost constant, regardless of the axial distance. Figure 25 shows the time-resolved evolution of the droplet diameter at the axial distance of 30 mm and at the spray centerline of the axial direction. The spray arrival time, the time in which the spray edge reaches the measuring position of both fuels, was about 0.25 ms. Both fuels showed many large droplets detected near the starting point of the spray. However, many diesel droplets were detected throughout the entire elapsed time after the start of injection; detected DME droplets decreased 3

Suh et al.

Figure 26. Comparison of axial mean velocity according to the time after the start of injection. (a) Diesel fuel. (b) DME fuel.

Figure 27. Effect of injection pressure on the SMD of DME fuel (Pamb ) 0.1 MPa, Tamb ) 293 K, tinj ) 1.0 ms).

ms after the start of injection. It can be conjectured that the low surface tension of the DME droplets due to low kinetic viscosity promotes droplet atomization and evaporation with elapsed time after the start of injection. This can be seen in the SMD distributions, as illustrated in the figure. The SMD of diesel fuel was decreased by stages through the entire time after the start of injection; on the other hand, the SMD of DME fuel decreased rapidly 3 ms after the start of injection. Figure 26 shows the comparison of the axial mean velocity according to the time after the start of injection. The axial mean velocity of both fuels reaches a maximum value immediately after the start of injection. Then, the axial mean velocity decreased rapidly and finally reached an equilibrium state after



than that of DME fuel. Then, the overall SMD of both fuels decreased gradually as time elapsed after the start of injection. 4. Conclusions

Figure 28. Measured droplet size distributions (Pinj ) 60 MPa, Pamb ) 0.1 MPa, Tamb ) 293 K, tinj ) 1.0 ms).

Figure 29. Overall SMD between diesel and DME (Pinj ) 40 MPa, Pamb ) 0.1 MPa, Tamb ) 293 K, tinj ) 1.0 ms).

approximately 1.5 ms. In the case of diesel fuel, many rapid droplets were detected. However, DME droplets were found in a much smaller amount than diesel. It is expected that the low surface tension of DME fuel prevented the occurrence of spherical-shaped droplets, as mentioned above, and disturbed the passage of the Doppler signal to the receiver. Figure 27 illustrates the effect of injection pressure on the SMD of DME fuel. As the injection pressure increased, the SMD at both injection pressures decreased. In the case of Pinj ) 60 MPa, the SMD dramatically decreased between 15 and 20 mm of axial distance from the nozzle tip. It can be guessed that this is due to the atomization effect. Judging from this result, the 60 MPa of injection pressure enhances the droplet atomization process. The microscopic characteristics of fuel sprays such as the arithmetic mean diameter (D10) and Sauter mean diameter (D32) were investigated in order to reveal the atomization processes. Figure 28 shows the measured mean droplet size distributions as a function of the time after the start of injection when the injection pressure is 60 MPa. In this figure, the arithmetic mean diameter (D10) is almost constant around 15 µm, regardless of the time after the start of injection. However, the Sauter mean diameter (D32) decreased gradually as the spray atomized. In this study, the comparison of the overall SMD between diesel and DME was investigated, and the results are shown in Figure 29. In this figure, it is illustrated that the overall SMD of diesel under the same injection conditions is much higher

In this work, the macroscopic spray development and transient atomization characteristics of a single-hole DME spray were investigated experimentally. The following conclusions were obtained through this investigation. (1) The spray tip penetration of DME fuel is shorter and the spray cone angle is wider than those of diesel fuel because DME evaporated quickly under atmospheric conditions. Under the ambient conditions, the spray tip penetration of DME decreased and the spray cone angle increased with the increase in ambient pressure. (2) The spray area of diesel fuel is larger than that of DME under the same conditions. However, the comparison of the axial direction centroid shows that the DME fuel has the larger value. (3) The spray tip penetration and impingement time in a conventional combustion chamber show that the spray tip penetration of diesel is larger than that of DME before 0.6 ms after the start of injection and that diesel spray impinged quickly at about 0.2 ms on the wall. However, the narrow angle combustion chamber shows a similar trend for spray tip penetration and impingement timing. (4) The comparison results of spray profiles of both fuels showed that diesel fuel has a larger SMD value than DME. On the other hand, the axial mean velocities of both diesel and DME fuels show a similar trend with the increase of axial distance. (5) Many diesel droplets are detected throughout the entire elapsed time after the start of injection; detected DME droplets decreased 3 ms after the of start of injection. Acknowledgment. This study was supported by the CEFV (Center for Environmentally Friendly Vehicles) of ECO-STAR project from MOE (Ministry of Environment, Republic of Korea). Also, this work is financially supported by the Ministry of Education and Human Resources Development (MOE), the Ministry of Commerce, Industry and Energy (MOCIE), and the Ministry of Labor (MOLAB) through the fostering project of the Lab of Excellency.

Nomenclature D ) diameter L ) distance N ) number of injections n ) number of droplets P ) pressure R ) radial distance from the spray centerline S ) spray tip penetration T ) temperature t ) time V ) velocity of droplet Z ) axial distance from the nozzle tip Greek Φ ) diameter θ ) angle F ) density Subscripts amb ) ambient asoi ) after start of injection b ) breakup f ) fuel g ) gas inj ) injection w ) wall EF050420F

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