Atomization and Evaporation Characteristics of Biodiesel and

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Atomization and Evaporation Characteristics of Biodiesel and Dimethyl Ether Compared to Diesel Fuel in a High-Pressure Injection System Hyung Jun Kim,† Su Han Park,† Hyun Kyu Suh,† and Chang Sik Lee*,‡ Graduate School of Hanyang UniVersity, and Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea ReceiVed September 24, 2008. ReVised Manuscript ReceiVed December 25, 2008

This paper experimentally studies and numerically analyzes the spray and atomization characteristics of biodiesel and dimethyl ether (DME) fuels as alternatives to diesel fuel. To analyze the macroscopic characteristics (such as axial/radial distance and spray cone angle and area), the macroscopic spray behaviors of three fuels (diesel, biodiesel, and DME) were measured using frozen images obtained from the visualization system. For the numerical analysis, the overall Sauter mean diameter (SMD) and the SMD contour plot of three fuel sprays were calculated using the KIVA-3V code with the addition of the fuel properties in the fuel library. The macroscopic characteristics of biodiesel spray, as the axial/radial distance, spray cone angle, and spray area, show a similar trend to that of diesel spray, and DME fuel has smaller axial and radial distances than the biodiesel and diesel fuels. The biodiesel and diesel fuels were influenced by the Weber number of droplets, but the DME fuel is influenced by the Reynolds number of droplets. It was also determined that the DME spray has superior breakup performance to that of the diesel and biodiesel sprays under similar injection conditions because DME fuel has a much lower viscosity and lower surface tension and is more volatile than the other two fuels. The accumulated vapor masses of the biodiesel and diesel sprays were extremely small, but that of the DME fuel spray continually increased after the start of the injection.

1. Introduction Spray characteristics, such as the atomization, penetration, and cone angle before impingement on the cylinder wall, have a great effect on combustion characteristics. This is due to the direct injection of fuel into the cylinder in the high-speed direct-injection (HSDI) diesel engine. Therefore, many investigations on the spray characteristics of diesel fuel have been conducted continuously since the development of the diesel engine. To reduce exhaust emissions from engine combustion, recent studies have been conducted to apply alternative fuels that contain large amounts of atomic oxygen in place of the diesel fuel in a diesel engine. Among various alternative fuels, biodiesel and dimethyl ether (DME) fuels have been recognized because these oxygenated fuels have lower exhaust emissions, such as soot and particulate matter, than a diesel engine running on diesel fuel under similar experimental conditions. However, biodiesel fuel presents problems regarding cold starts and acidification while in longterm storage. DME fuel presents other problems, such as low viscosity and its adverse reaction to rubber, which necessitate that the fuel supply system of the engine be modified. For these reasons, investigations into the differences of spray characteristics of biodiesel and DME fuels compared to diesel fuel have been continuously conducted by many researchers. * To whom correspondence should be addressed: Department of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Seongdonggu, Seoul 133-791, Korea. Telephone: +82-2-2220-0427. Fax: +82-2-22815286. E-mail: [email protected]. † Graduate School of Hanyang University. ‡ Department of Mechanical Engineering, Hanyang University.

Several studies have been performed that examine the emission characteristics of alternative fuels. Agrawal et al.1 conducted an experimental investigation to determine the emission characteristics of diesel and biodiesel fuels according to the rate of exhaust gas recirculation (EGR) in the directinjection diesel engine with two cylinders. They report that the combination of 20% biodiesel-blended fuel and 15% EGR improves thermal efficiency and reduces exhaust emissions, such as NOx, HC, and CO. Kim et al.2 study the spray and combustion characteristics of DME fuel in the diesel engine with the common-rail injection system. They suggest that the injection delay and spray penetration of DME fuel is shorter than that of diesel fuel. The combustion characteristics of DME fuel include advanced ignition timing and a shorter combustion period than that of diesel fuel. Tsolakis et al.3 report that smoke emissions decreased but NOx increased when ultralow sulfur diesel (ULSD) with 20% rapeseed methyl ester (RME) is used as a test fuel. The NOx emission can also be reduced by employing EGR, although the engine efficiency decreased in their studies. Kim et al.4 analyze exhaust emission characteristics of biodiesel fuel according to various injection conditions. They report that the maximum combustion pressure and heat release rate decrease more at the split-injection condition than in a single-injection condition. Finally, the split-injection condition causes a rapid (1) Agrawal, D.; Sinha, S.; Agarwal, A. K. Renewable Energy 2006, 31, 2356–2369. (2) Kim, M. Y.; Bang, S. H.; Lee, C. S. Energy Fuels 2007, 21 (2), 793–800. (3) Tsolakis, A.; Megaritis, A.; Wyszynski, M. L. Energy Fuels 2003, 17 (6), 1464–1473. (4) Kim, M. Y.; Yoon, S. H.; Lee, C. S. Energy Fuels 2008, 22 (2), 1260–1265.

10.1021/ef800811g CCC: $40.75  2009 American Chemical Society Published on Web 02/02/2009

Atomization and EVaporation Characteristics

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decrease of NOx emissions, and the split injection shows less sensitivity to injection timing. Besides studying the emission characteristics of biodiesel and DME fuel, many researchers have investigated the spray characteristics of alternative fuels as well. Kegl and Hribernik5 experimentally analyzed the spray characteristics of diesel and biodiesel fuels. They suggest that the injection timing advances with increasing amounts of biodiesel used in the engine. This leads to high combustion pressures and temperatures. Lee et al.6 experimentally investigate biodiesel-diesel blended fuel sprays using a common-rail injection system. Their results show different spray penetrations according to the blended ratio of biodiesel and also show that the Sauter mean diameter (SMD) of biodiesel-blended fuels is larger than that of diesel fuel. This larger SMD is due to the high viscosity of the biodiesel. Suh and Lee7 report that the developing shape of a diesel fuel spray is longer and wider than that of DME fuel and that the atomization performance of the DME spray is better than that of diesel fuel under similar injection conditions. Kim et al.8 experimentally and numerically investigate the spray characteristics of biodiesel, DME, and ethanol-blended biodiesel fuels. The experimental results of the macroscopic spray characteristics agree with the calculated results. In addition, the breakup characteristics of DME fuel indicate good performance. They carry out the investigation of the comparison between the diesel and DME fuel according to the chamber shape under the high ambient pressure condition and report that the vaporization of DME spray occurs remarkably more often than that of diesel spray.9 The different properties of alternative fuels, such as biodiesel and DME, present uncertainties compared to the use of the conventional diesel fuel. Also, detailed comparisons of the spray characteristics and droplet sizes according to the axial distance among diesel, biodiesel, and DME fuels under similar injection conditions are limited and have many uncertainties regarding the evaporation characteristics of the liquid phase in fuel sprays with high-pressure injection. Thus, the experimental research regarding the spray characteristics of the diesel, biodiesel, and DME fuels presented in this study were conducted to analyze the macro- and microscopic spray characteristics, such as the spray images, cone angle, axial/radial distance, SMD value, and droplet distributions. These characteristics were examined according to various elapsed times after the start of the injection at certain axial distances from the nozzle tip. In addition, the overall SMD, the contour plots of SMD distribution, and the evaporation characteristics of the three fuels were calculated and analyzed by the numerical method using the KIVA-3V code.

al.10 conducted experimental comparisons between biodiesel and diesel fuel properties, such as density, kinetic viscosity, and dynamic viscosity according to temperature. They measured the specific gravity and dynamic viscosity of biodiesel fuel. Teng et al.11-13 performed numerical studies of the thermodynamic properties of liquefied DME. The equations for DME properties, such as density, surface tension, viscosity, vapor pressure, and latent heat with respect to temperature, were systematically developed to analyze the fuel spray and combustion of liquefied DME fuel on the basis of the existing molecular22,23 and chemical structure theories.24,25 The fuel libraries of biodiesel and DME fuels were created on the basis of this information. The calculations of the spray and evaporation characteristics of fuels were carried out after the fuel library of test fuels was inserted into the KIVA-3V code. 2.2. Atomization Model Using the Nozzle Flow Model. In this study, the nozzle flow model proposed by Sarre et al.14 was used to analyze the flow through an internal injector nozzle hole. In the common-rail injection system with high velocity and pressure, a cavitation appeared in the inside of the nozzle because of the transition of a fluid from a liquid to a vapor at low pressure. Moreover, the initial droplet diameter and velocity at the nozzle exit changed in the region under the influence of the cavitation. As a result of these observations, the initial droplet size and velocity at the nozzle exit were calculated using the nozzle flow model to postulate the initial condition in the breakup model. Also, the atomization characteristics of spray with the common-rail injection system were analyzed by an application of the hybrid breakup model combined with the primary and secondary breakup theories. The Kelvin-Helmholtz (KH) breakup theory,15 which is based on the growth of an initial perturbation of the liquid surface at the interface between the surrounding gas and the fuel with different densities, was used for the primary breakup model. The secondary breakup model used was the RayleighTaylor (RT) model,16 which is based on the RT instability that happens when the interface between a given fuel and ambient gas is accelerated to the gas with low density. The breakup in the KH-RT hybrid model suggested by Su et al.17 happened continuously with the competition between the KH and RT breakup on the atomization process. After the comparison of experimental results in this study, the size and time constant of the KH breakup model were regulated to be 8.0 and 40, respectively. The breakup constants, which are affected by the droplet size and time in the RT model, were chosen as 1.0 in the present study. In this study, it is assumed that a new parcel is formed when 0.3% of the parcel mass is emitted by the KH

2. Numerical Formulation

(10) Yoon, S. H.; Park, S. H.; Lee, C. S. Energy Fuels 2008, 22 (1), 652–656. (11) Teng, H.; McCandless, J. C.; Schneyer, J. B. Thermochemical characteristics of dimethyl ethersAn alternative fuel for compressionignition engines. SAE Tech. Pap. 2001-01-0154, 2001. (12) Teng, H.; McCandless, J. C.; Schneyer, J. B. Compression ignition delay (physical + chemical) of dimethyl ethersAn alternative fuel for compression-ignition engines. SAE Tech. Pap. 2003-01-0759, 2003. (13) Teng, H.; McCandless, J. C.; Schneyer, J. B. Thermodynamic properties of dimethyl ethersAn alternative fuel for compression-ignition engines. SAE Tech. Pap. 2004-01-0093, 2004. (14) Sarre, C. K.; Kong, S. C.; Reitz, R. D. Modeling the effects of injector nozzle geometry on diesel sprays. SAE Tech. Pap. 1999-01-0912, 1999. (15) Reitz, R. D. Atomisation Spray Technol. 1987, 3, 309–337. (16) Bellman, R.; Pennington, R. H. Q. Appl. Math. 1954, 12, 151– 162. (17) Su, T. F.; Patterson, M. A.; Reitz, R. D.; Farrel, P. V. Experimental and numerical studies of high pressure multiple injection sprays. SAE Tech. Pap. 960861, 1996.

2.1. Application of Biodiesel and DME Fuels to the KIVA-3V Code. To apply the diesel, biodiesel, and DME fuels to the KIVA-3V code, diesel fuel No. 2 (DF2) from the fuel library21 was used to calculate the diesel spray. In addition, the spray characteristics of biodiesel and DME fuels were calculated by inserting their fuel properties into the fuel library. Yoon et (5) Kegl, B.; Hribernik, A. Energy Fuels 2006, 20 (5), 2239–2248. (6) Lee, C. S.; Park, S. W.; Kwon, S. I. Energy Fuels 2005, 19 (5), 2201–2208. (7) Suh, H. K.; Lee, C. S. Fuel 2008, 87, 925–932. (8) Kim, H. J.; Suh, H. K.; Park, S. H.; Lee, C. S. Energy Fuels 2008, 22 (3), 2091–2098. (9) Kim, H. J.; Suh, H. K.; Lee, C. S. Energy Fuels 2008, 22 (4), 2851– 2860.

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Figure 1. Schematics of the experimental apparatus and test injector.

wave instability mechanism. To prevent an excess of droplets merging in the calculation cell area, 5000 was determined to be a reasonable number of parcels. 2.3. Evaporation Model. In this study, evaporation characteristics, such as the number of droplets and vapor fuel mass in the air, were analyzed for each fuel using the basic model in the KIVA-3V code. This model shows the evaporation of a single droplet based on the lumped-body theory.18 In the energy ˙ gas-drop) from the balance equation, the heat-transfer rate (Q ambient gas to the droplet surface under given evaporation conditions is ˙ gas-drop ) m ˙L+Q ˙ in-drop Q ˙ fQ

(1)

˙ L are the mass-transfer rate from the droplet where m ˙ f and Q and the latent heat, respectively. The heat conduction rate ˙ in-drop) from the droplet surface into the internal droplet can (Q be expressed as ˙ in-drop ) h4πr2(Tamb - Tdrop) Q

(2)

In the above equation, h is the heat-transfer coefficient. Tamb and Tdrop are the ambient and droplet temperatures, respectively. Under the supposition that the heat and mass transfer occurs between a flowing fluid and a spherical droplet, the Nusslet and Sherwood numbers are calculated using the Ranz and Marshall correlation.19 The mass-transfer rate (m ˙ f) from a droplet in the steady state can be calculated using a correlation suggested by Frossling.20 m ˙ f ) 2πr(FD)air

Ys - Y∞ Sh 1 - Ys

(3)

Here, (FD)air is the fuel vapor diffusivity in the air. Ys and Y∞ are the mass fraction on the droplet surface and mass fraction of the fuel vapor in the free stream condition, respectively. Therefore, the heat conduction rate into the internal droplet can be expressed by the following equation under the assumption that the droplet is a lumped body with even temperature: dT ˙ in-drop ) Ffuel 4 πr3cfuel drop Q 3 dt

(4)

(18) Amsden, A. A.; O’Rourke, P. J.; Butler, T. D. KIVA-II: A computer program for chemically reactive flows with sprays. Los Alamos Report, Los Alamos National Laboratory, Los Alamos, NM, 1989; LA-11560-MS, pp 12-20.

Here, Ffuel is the density of the fuel, and cfuel is the specific heat of the fuel. The temperature of the droplet was determined using the iteration method in the KIVA-3V code. 3. Experimental Apparatus and Procedures 3.1. Visualization and Droplet Measuring System. To analyze the injected spray characteristics of diesel, biodiesel, and DME fuels, a visualization system was installed as illustrated in Figure 1a. The spray characteristics were understood by investigating the spray development process, axial and radial distances, spray cone angle, and spray area obtained from the spray images of various fuels under similar injection conditions. The spray visualization system used in this study was composed of a Nd:YAG laser (Continuum, SL2-10), a set of cylindrical lenses with a mirror, a digital delay/ pulse generator (Berkeley Nucleonics Corp, Model 555), an intensified charged couple device (ICCD) camera (The Cooke Corporation, Dicam-PRO), and a PC installed as an image grabber. An Nd:YAG laser was the light source, with a 532 nm wavelength. Cylindrical lenses formed a laser sheet beam less than 1 mm thick and were used to illuminate the spray evolution. A high-resolution ICCD camera was used to capture the spray images. The droplet measuring system [phase Doppler particle analyzer (PDPA)] was used to investigate microscopic spray characteristics, including the Sauter mean diameter (SMD), droplet mean velocity, and frequency distribution of the sizes of the droplets. The droplet measuring system consisted of an Ar-ion laser (INNOVA 70C, Coherent) with 0.7 W of laser output and a photomultiplier tube (PMT) voltage of 500 V, along with a transmitter, receiver, and signal analyzer. The laser output and PMT voltage of the Ar-ion laser were optimized for the data rate and signal intensity of the signal analyzer. To obtain time-resolved data, the signal analyzer was synchronized with an injector driver using the digital delay/pulse generator. The specifications of the spray visualization system and droplet measuring system are listed in Table 1. 3.2. Experimental Procedures. In this paper, conventional diesel fuel, biodiesel fuel derived from soybean oil, and DME were used to investigate the breakup characteristics of the common-rail injection system. The specifications of the test fuels are listed in Table 2. For the study of atomization characteristics, the three fuels were injected separately at an injection pressure of 60 MPa and with an energizing duration of 0.7 ms. This work used a diesel injector with a single hole that had a 0.3 mm nozzle diameter and a 2.67 length/diameter (L/D) ratio, as illustrated in Figure 1b. The (19) Ranz, W. E.; Marshall, W. R. Chem. Eng. Prog. 1952, 48, 141 (Part I) and 173 (Part II). (20) Faeth, G. M. Prog. Energy Combust. Sci. 1977, 3, 191–224.



Table 1. Specifications of Visualization and Droplet Measuring Systems light source laser power (mJ) wavelength (nm) beam thickness (mm) resolution

Visualization System Nd:YAG laser maximum of 270 532 ∼1 1280 (height) × 1024 (vertical)

Droplet Measuring System light source Ar-ion laser wavelength (nm) 514.5, 488 laser beam diameter (mm) 1.4 beam expander ratio 0.5 focal length (mm) transmitter 500 receiver 250 collection angle (deg) 30 filter frequency (MHz) 40 PMT voltage (V) 500 diameter subrange (µm) 2-80 velocity subrange (m/s) from -292 to 292

Table 2. Properties of the Test Fuels

fuel property

diesel

biodiesel (soybean oil)

carbon content (wt %) hydrogen content (wt %) oxygen content (wt %) density (kg/m3) viscosity (mm2/s) surface tension (kg/s2) boiling point temperature (°C) flash point temperature (°C) cetane number

87 13 0 828 2.835 0.027 180-340 60-80 40-55

77 12 11 884 4.022 0.028 315-350 100-170 48-65

DME (dimethyl ether) 52 13 35 660 0.12-0.15 0.012 -23 -42 68

experiments on visualization and droplet measuring were conducted under atmospheric conditions, such as an ambient pressure of 0.1 MPa and an ambient temperature of 293 K. The subrange of the diameter, which is the effective range of the PDPA signal analyzer, was set from 2 to 80 µm because the nozzle diameter was 300 µm, and approximately 20 000 droplets were collected and averaged at each measurement point. In addition, the fuel droplet size and velocity were measured every 10 mm (from 5 to 35 mm) along the axial direction.

4. Results and Discussion 4.1. Spray Behaviors of Diesel, Biodiesel, and DME Fuels. Images of the three test fuels were obtained using the visualization system with a Nd:YAG laser light source. To make possible the comparison of the spray behaviors among the test fuels, observations were made according to the time after the start of the injection. For the greatest accuracy in the experimental results of the axial distance, maximum radial distance, and spray angle, an optimal threshold level of 190 was chosen as a result of comparing various threshold levels according to the brightness of images. The original images were converted by the same threshold level. In addition, the spray images changed the cell points because the location and the spray area were calculated by a sum of all cell points. The image conversion processes that were used are illustrated in Figure 2a. In this work, numerical investigations were carried out by measuring the experimental injection rate of three fuels. The injection rates were also measured with an injection rate measuring system based on the Bosch method.26 It was determined that the biodiesel and diesel fuels showed similar (21) Amsden, A. A. KIVA-3: A KIVA program with block-structured mesh for complex geometries. Los Alamos Report, Los Alamos National Laboratory, Los Alamos, NM, 1993; LA-12503-MS, pp 33-38.

patterns, but the injection rate of DME fuel is lower than those of other fuels under similar injection conditions, as illustrated in Figure 2b. Figure 3 shows the overlapping spray evolution processes of the three fuels at an injection pressure of 60 MPa, at 0.7 and 1.2 ms after the start of the injection. As seen in Figure 3a, the spray evolution processes of the fuels show that the spray size in order of greatest to smallest is diesel, biodiesel, and DME fuel. The spray radial distance and axial distance of the DME fuel are shorter than those of the other two fuels because of the vaporization property of DME fuel in atmospheric conditions. At 1.2 ms after the start of the injection, it was found that the DME has the narrowest spray distribution in the radial direction. This result indicates that the breakup and evaporation of DME spray occurred more in the outer spray region than in the center region because of the drag force created by the friction between the fuel and surrounding gas, as illustrated in Figure 3b. The spray radial distance of diesel and biodiesel increased more than that of DME spray at the 0.7 ms after the start of the injection. This increase was most likely due to the loss of momentum in the droplets after the breakup caused by air friction in the outer spray region. After the breakup, the droplets moved in a radial direction and advanced to the axial direction. Figure 4 shows the axial distance and maximum radial distance of the three fuels according to the time after the start of the injection. The axial distance of diesel and biodiesel fuels showed patterns of continuous increase. In addition, the increases in rates of diesel and biodiesel sprays in the axial distance have slowed after 0.6 ms because the progress of the spray development was disturbed by the surrounding gas in the downstream spray. On the other hand, the pattern of decrease in the axial distance of the DME fuel appeared at 1.0 ms after the start of the injection. The maximum radial distance of the diesel and biodiesel fuels showed a decreasing trend at 1.0 ms after the start of the injection. It is assumed that the droplets at the outer spray region rolled through the vortex produced by the surrounding air. In the DME fuel, a continuous decrease of maximum radial distance was observed from 0.6 ms after the start of the injection. This decrease was due to the evaporation property of the DME fuel. The spray cone angle and spray area of the three fuels according to the time after the start of the injection are illustrated in Figure 5. The spray cone angle of the diesel and biodiesel fuels rapidly decreased as the axial distance increased until 0.4 ms. At the same time, the spray area was increased by the diffusion of spray as the time elapsed after the start of the injection. In contrast to the other two fuels, the spray cone angle of the DME fuel decreased from 0.6 ms after the start of the injection because of the vaporization of droplets at the outer spray region. Therefore, the increased rate of spray area in the DME fuel slowed as the spray cone angle decreased. 4.2. Atomization Characteristics in the Experimental Results. To analyze the atomization characteristics according to the axial distance from the nozzle tip, the droplets at axial distances of 5-35 mm from the nozzle tip were measured using (22) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987. (23) Hirschfelder, J. O.; Curtiss, C. F.; Bird, R. D. Molecular Theory of Gases and Liquids; John Wiley and Sons: New York, 1954. (24) Lide, D. R. Handbook of Chemistry and Physics, 80th ed.; CRC Press: Boca Raton, FL, 1999. (25) Masterton, W. L.; Slowinski, E. J. Chemical Principles; W.B. Saunders Company: Philadelphia, PA, 1977. (26) Bosch, W. The fuel rate indicator: A new measuring instrument for display of the characteristics of individual injection. SAE Tech. Pap. 660749, 1966.


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Figure 2. Spray image conversion process and injection rates of the three fuels.

Figure 4. Axial distance from the nozzle tip and maximum radial distance according to the time after the start of the injection.

Figure 3. Spray evolution processes of the three fuels according to the time after the start of the injection (Pinj ) 60 MPa).

a PDPA. Figure 6 shows the effect of the axial distance on the frequency distribution curve of three fuels according to the droplet size. The gradient of frequency distribution curves of droplets with diameters of 15 µm is steep as the axial distance from the nozzle tip increases. Therefore, many small droplets with diameters under 15 µm were distributed far from the nozzle tip, but those droplets showed a small distribution near the nozzle tip. In addition, the order of the droplet distribution curves for all fuels reversed when the droplet diameter was between 10 and 15 µm, and these points defined the cross points seen in Figure 6. The frequency distribution curves of biodiesel spray according to the axial distance are similar to those of the other two fuels, as shown in Figure 6b. From this result, it can be known that the atomization of biodiesel spray progresses slowly

Figure 5. Spray cone angle and spray area according to the time after the start of the injection.

until the axial distance reaches 35 mm because of the high viscosity and surface tension. The frequency distributions of DME spray at the axial distances of 25 and 35 mm show almost the same curves as those illustrated in Figure 6c. This is because many droplets with diameters less than 10 µm vaporized at the axial distance of 35 mm because of the volatile characteristics of DME fuel. Figure 7 illustrates the effect of the axial distance from the nozzle tip on the SMD according to the time after the start of the injection. The SMD of all fuels tended to decrease as the axial distance from the nozzle tip increased. Also, the SMD according to the axial distance was somewhat increased as time



Figure 6. Effect of the axial distance on the distribution ratio curves of the three fuels according to the droplet size.

elapsed after the start of the injection because the floating droplet after the injection may have caused the collisions and coalescences of injected droplets. The rate of increase of SMD in the biodiesel fuel is greater than that of the diesel fuel because of the high viscosity of biodiesel fuel. The SMD range of biodiesel and diesel fuels is 30-70 µm, and the SMD range of the DME fuel is 10-40 µm. From these results, we can state that the atomization performance of the DME fuel is better than that of both biodiesel and diesel fuels because of the greater volatility and lesser density of DME fuel. The relation between the Weber and Reynolds numbers for the three fuels is illustrated in Figure 8. The Weber and Reynolds numbers of the droplets were calculated using the droplet

Figure 7. Effect of the axial distance from the nozzle tip on the SMD according to the time after the start of the injection.

velocity, droplet diameter, and the density and surface tension at 20 °C according to the fuels, as defined by the following equation: Wedroplet )

FfuelUdroplet2Ddroplet , σfuel

Redroplet )

FfuelUdropletDdroplet µfuel (5)

To determine We and Re of droplets, the mean velocity of droplets (Udroplet) was calculated from the velocities of droplets at the axial and radial distances. In the comparison of the


Figure 8. Relation between the Weber and Reynolds numbers for the three fuels.

Reynolds and Weber numbers, droplets were affected by the viscosity and surface tension. The Weber and Reynolds numbers have large values for low surface tensions and fuel viscosities. Also, the high velocity and large size of the droplets increased their We and Re values. However, the distribution gradient between the Weber and Reynolds numbers of the fuel droplets

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is more affected by the surface tension and viscosity of the fuel than the velocity and size of the droplets. The distribution gradient of biodiesel fuel with the largest viscosity has a steep slope and a narrow width compared to that of the DME fuel. As shown in Figure 8c, the distribution gradient of the DME fuel has a gentle slope because the rate of increase of the Reynolds number is greater than that of the Weber number because of the low viscosity of the DME fuel. In addition, DME fuel with a very low viscosity has a large value of Re, and the biodiesel with high viscosity has a small value of Re. The diesel and biodiesel fuels have large values of We because of the high surface tension compared to the DME fuel. Therefore, we hypothesize that the breakup factor of the biodiesel and diesel fuels was influenced by the Weber number of droplets, but the Reynolds number of droplets also has a great effect on the main breakup factor of the DME fuel. 4.3. Numerical Analysis of Breakup and Evaporation Characteristics. In addition to the experimental investigation of the diesel, biodiesel, and DME fuels, numerical studies were conducted using the KIVA-3V code for a detailed analysis of atomization characteristics and evaporation properties. Figure 9 shows the calculated SMD according to the axial distance from the nozzle tip as time elapsed after the start of the injection. The SMD of the three fuels decreased as the axial distance from the nozzle tip increased, as stated in the SMD results seen in Figure 7. It is difficult to compare the experimental result in Figure 7 and the numerical result in Figure 9 because the measuring methods of the experiment and calculations are different. In the experiment, fuel was continuously injected until approximately 20 000 droplets were collected at each of the measurement points. Therefore, the representative SMD at a specific time was determined by averaging the captured droplet from all of the measurement points. On the other hand, the SMD in the numerical results was calculated from the total droplet of injected spray at one time. The numerical results in Figure 9 show the calculated SMD in the duration from the start of the injection to the end of the injection. The calculated SMD tended to increase with time after the start of the injection because many floating droplets after breakup collided and coalesced in the center region of the spray. In addition, the calculated SMD of the diesel fuel showed a pattern similar to that of the biodiesel fuel, which has the region from 10 to 80 µm. The calculated SMD values of the DME fuel ranged from 10 to 50 µm because of the low viscosity and rapid vaporization of the DME fuel. The contour plot of the calculated SMD distribution of fuels according to the axial and radial distances is shown in Figure 10. As seen in Figure 10, the values of SMD decreased in the direction of the outer spray region, while the SMD distribution near the nozzle tip is greater than that in the other regions. The decreasing pattern of the SMD distribution in the biodiesel spray along the axial distance from the nozzle tip is slower than that of the diesel spray because of the retardation of droplet atomization caused by the high viscosity and surface tension of biodiesel fuel. Also, the whole SMD distribution of the DME spray shows a lower SMD distribution than the other two fuels because of the rapid evaporation of small droplets after breakup and the low viscosity of DME. Figure 11 shows the calculated number of droplets and accumulated vapor fuel mass according to the fuel. The number of droplets after the breakup was remarkably increased by the fast velocity because of high pressure at



Figure 10. Contour plot on the calculated SMD distribution of fuels according to the axial and radial distance.

Figure 11. Calculated number of droplets and accumulated vapor fuel mass according to the fuel.

breakup, the small droplets were quickly volatilized by the evaporation property of DME fuel at atmospheric conditions. 5. Conclusions

Figure 9. Calculated SMD of fuels according to the time after the start of the injection.

the nozzle exit until 0.8 ms after the start of the injection. Meanwhile, the increasing rates of diesel and biodiesel sprays slowed from the end of the injection. On the other hand, the number of droplets of the DME spray decreased by evaporation from 0.8 ms after the end of the injection. The previously decreasing rate of the DME spray then increased from 1.0 ms after the start of the injection. The accumulated vapor fuel masses of the diesel and biodiesel sprays were extremely small, but that of the DME spray continually increased after the start of the injection. This may occur because, after

To compare the overall spray characteristics of biodiesel and DME fuels, experimental and numerical investigations of the atomization characteristics were conducted for alternative fuels, such as biodiesel, and diesel fuels. In this work, the numerical analysis was also performed using the KIVA-3V code for the detailed study of atomization characteristics and evaporation properties. The conclusions are summarized as follows: (1) In the spray evolution processes, the radial and axial distances of the DME spray were shorter than those of biodiesel and diesel sprays. The spray radial distance of the DME fuel had the greatest difference compared to that of biodiesel and diesel fuels. The spray cone angles of diesel and biodiesel fuels decreased rapidly as the axial distance increased until 0.4 ms, but the spray cone angle of the DME fuel decreased from 0.6 ms after the start of the injection. Also, the rate of increase of the spray area in the DME fuel slowed as the spray cone angle decreased. (2) In the frequency distribution curve according to the droplet


size, there are cross points that are inverted with regard to the order of the frequency distribution curve of droplets with diameters in the interval between 10 and 15 µm. The SMD of fuels decreased as the axial distance from the nozzle tip increased. (3) The atomization performance of DME fuel was better than that of biodiesel and diesel fuels because DME fuel has much lower viscosity and lower surface tension and is more volatile than the other two fuels. In the comparison of the Reynolds and Weber numbers of droplets, the breakup factor of the biodiesel and diesel fuels was influenced by the Weber number of droplets. Meanwhile, the Reynolds number of droplets had a great effect on the main breakup factor of the DME fuel. (4) In the numerical results, the calculated overall SMD of biodiesel fuel according to the axial distance from the nozzle tip showed a pattern similar to that of diesel fuel. DME fuel had a different droplet size distribution compared to those of the diesel and biodiesel fuels. The number of droplets in the diesel and biodiesel sprays continuously increased, but the number of droplets in the DME spray decreased from the end of the injection. (5) The accumulated vapor fuel mass of the biodiesel and diesel spray was extremely small, but that of the DME spray increased after the start of the injection because of the evaporation characteristics of the DME fuel. Acknowledgment. This study was supported by the Center for Environmentally Friendly Vehicles (CEFV) of the Eco-STAR project from the Ministry of Environment (MOE), Republic of

Kim et al. Korea. Also, this work was supported by the Second Brain Korea 21 Project in 2007.

Nomenclature cfuel ) specific heat of fuel Ddroplet ) droplet diameter h ) heat-transfer coefficient L ) latent heat m ˙ f ) mass-transfer rate ˙ gas-drop ) heat-transfer rate from the ambient gas to the droplet Q surface ˙ in-drop ) heat conduction rate from the droplet surface into the Q internal droplet ˙ L ) latent heat Q r ) droplet radius Redroplet ) Reynolds number of the droplet Sh ) Sherwood number Tamb ) ambient temperature tasoi ) time after the start of the injection Tdrop ) droplet temperature Udroplet ) droplet velocity Wedroplet ) Weber number of the droplet Ys ) mass fraction of fuel vapor on the droplet surface Y∞ ) mass fraction of fuel vapor in the free stream condition µfuel ) fuel viscosity Ffuel ) fuel density σfuel ) fuel surface tension (FDair) ) fuel vapor diffusivity EF800811G

Atomization and Evaporation Characteristics of Biodiesel and

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