An Experimental and Numerical Investigation of Atomization

Energy & Fuels 2008, 22, 2091–2098

2091

An Experimental and Numerical Investigation of Atomization Characteristics of Biodiesel, Dimethyl Ether, and Biodiesel-Ethanol Blended Fuel Hyung Jun Kim,† Hyun Kyu Suh,† Su Han Park,† and Chang Sik Lee*,‡ Graduate School of Hanyang UniVersity, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea, and Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea ReceiVed NoVember 19, 2007. ReVised Manuscript ReceiVed February 1, 2008

This paper investigates the experimental and numerical analysis of the macroscopic and microscopic spray characteristics of biodiesel, dimethyl ether (DME), and biodiesel-ethanol blended fuels in the common-rail injection system. For the experimental study of the macroscopic spray characteristics, spray developing process and spray tip penetration of the test fuels were visualized using a laser sheet method. In order to analyze the atomization characteristics of the three kinds of fuels, the microscopic characteristics such as the Sauter mean diameter (SMD) at the measuring point in the spray, frequency distribution of the number of droplets according to droplet size, and the relation of droplet diameter and Weber number were compared with numerical results obtained by the calculation. Based on the results of the investigation, the experimental results for the atomization characteristics of different fuels show a good agreement with the predicted results. In this work, the effects of different alternative fuels such as biodiesel, dimethyl ether, and biodiesel-ethanol blended fuel on the spray atomization characteristics were simulated by the KIVA code and the calculated results using the hybrid model combined with the primary and secondary breakup were compared and analyzed with the experimental results in terms of the shape of spray, spray evolution processes, and SMD distribution.

1. Introduction The diesel engine has been utilized as a power source for vehicles because it has many advantages such as high thermal efficiency, excellent performance of fuel consumption, and good durability. However, nitrogen oxide and particulate matter which are exhausted by the diesel engine have become major causes of environmental contamination and it has been known that the particulate matter is a harmful influence on the respiratory organs and the lungs of the human body. In order to deal with strong environmental regulations, many investigations such as highpressure injection, electronic control of the fuel spray system, development of a new combustion method for the reduction of emissions, and improvement of after-treatment devices have been actively investigated by many researchers. In regard to clean emissions from the compression ignition engine, many attempts have been made to apply alternative fuels to diesel engines to respond to restrictions of exhaust emissions and the high cost of new technical development for future vehicles. Among various alternative fuels, biodiesel and dimethyl ether (DME) fuels have become the focus of alternative fuels as diesel substitutes. First, biodiesel fuel, which is a recyclable fuel, can be produced from the oils of animals and plants and has been estimated to be oxygenated fuel that includes the oxygen molecule which diesel fuel does not have. Also, there are many kinds of biodiesel fuels such as canola, soybean, and palm oil according to the source of production. It has merit which is the * To whom correspondence should be addressed. Telephone: +82-22220-0427. Fax: +82-2-2281-5286. E-mail: [email protected]. † Graduate School of Hanyang University. ‡ Department of Mechanical Engineering, Hanyang University.

remarkably lower emissions compared to diesel fuel because biodiesel fuel does not contain the elements of aromatic compounds and sulfur. From the viewpoint of combustion characteristics, biodiesel fuel can directly apply to the existing diesel engine without modifications. It is suitable for the compression ignition engine due to the high cetane number and the similar properties of diesel fuel. But the viscosity of biodiesel fuel is higher than that of diesel fuel and the difficulty of a cold start is a weak point due to the crystallizing property of the fuel at low temperature. Also, pump and filter life in the fuel system is shortened by the acidification of fuel during longterm storage. Although the biodiesel fuel has many weak points, numerous attempts have been made to apply biodiesel fuel to the diesel engine because of its low emission characteristics. Many investigations of the spray and combustion characteristics of biodiesel blended fuels such as the diesel-biodiesel, biodiesel-ethanol, and biodiesel-DME fuel have been carried out by many researchers.1,2 Tsolakis3 conducted investigations on the particulate size distribution from the diesel engine fuelled by petroleum diesel and rapeseed methyl ester (RME) biodiesel fuel with EGR. In this work, experimental results confirmed that RME combustion reduces the smoke and particle mass but the oxides of nitrogen increase as well as the number of smaller emissions of particulate matter. Also, the atomization and combustion characteristics of biodiesel blended fuel were conducted by Lee (1) Tsolakis, A.; Megaritis, A.; Wyszynski, M. L.; Theinoi, K. Energy 2007, 32, 2072–2080. (2) Ying, W.; Longbao, Z.; Hewu, W. Atmos. EnViron. 2006, 40, 2313– 2320. (3) Tsolakis, A. Energy Fuels 2006, 20, 1418–1424.

10.1021/ef700692w CCC: $40.75  2008 American Chemical Society Published on Web 04/01/2008

2092 Energy & Fuels, Vol. 22, No. 3, 2008

Kim et al.

Figure 1. Phase Doppler particle analysis system and particle motion analysis system.

et al.4 They reported that the more the biodiesel blending ratio increases, the more the combustion pressure and NOx increase, but soot in the emission shows a decrease of about 70% compared with the diesel fuel. Park et al.5 investigated a breakup mechanism of monodispersed droplets according to the mixing ratio of biodiesel fuel. They showed that the deformation of droplets decreased as the mixing ratio of biodiesel increased at the first breakup stage and the atomization performance of diesel fuel is superior to the biodiesel fuel. The macroscopic and microscopic characteristics of biodiesel fuel spray were conducted by Suh et al.6 They suggested that the biodiesel spray has a lower velocity and larger droplet diameter than the diesel spray and there is a great difference in the high blending ratio of the biodiesel. Another alternative fuel, DME fuel containing oxygen atoms in the fuel, is colorless and gaseous under normal conditions and it can easily change to a liquid state by pressurization above the saturation pressure. Also, DME fuel can be operated through the diesel cycle and a similar thermal efficiency can be obtained as with diesel fuel due to the high cetane number and good self-ignitability. DME fuel is estimated to be an environmentally friendly fuel because smoke and particle matter are remarkably lower than diesel fuel and it may lessen the destruction of the ozone layer and global warming because of characteristics of good resolvability in the atmosphere. However, DME fuel is prone to leakage of the fuel in the case of its direct application to the spray system of the diesel engine by low viscosity and it has to have a lubricant added because of its bad lubrication property. Also, DME has melting properties by reaction with rubber and so sealing materials of the fuel system must be changed for application to the diesel engine. Owing to many different properties of DME fuel compared with diesel fuel, many investigations into the spray characteristics of DME fuel have been studied by researchers. Wu et al.7 conducted an experimental study on the velocity field and atomization performance of oxygenated fuel spray. Their results indicated that the spray of oxygenated fuel showed different shape compared to the structure of diesel spray and (4) Lee, C. S.; Park, S. W.; Kwon, S. I. Energy Fuels 2005, 19, 2201– 2208. (5) Park, S. W.; Kim, S.; Lee, C. S. Energy Fuels 2006, 20, 1709– 1715. (6) Suh, H. K.; Park, S. W.; Kwon, S. I.; Lee, C. S. Trans. KSAE 2004, 12, 23–29. (7) Wu, Z.; Zhu, Z.; Huang, Z. Fuel 2006, 85, 1458–1464.

Table 1. Test Fuels and Properties of Fuels fuel density (kg/m3) viscosity (cP) surface tension (kg/s2)

biodiesel (soybean oil)

80% biodiesel + 20% ethanol (BDE20)

dimethyl ether (DME)

870 6.05 0.028

860 3.68 0.027

660 0.16 0.012

droplet size of oxygenated spray was smaller than that of diesel fuel. Park et al.8 investigated diesel fuel compared with DME fuel through the common-rail system and the results showed that the spray tip penetration and spray angle of DME fuel were shorter and wider than those of diesel fuel. Comparison of spray structures using the visualization system and atomization characteristics between DME and diesel spray in the highpressure chamber was conducted by Suh et al.9 Despite previous researches, systematic investigations into comparing biodiesel, DME, and biodiesel-ethanol blended spray characteristics are required because the comparative advantages and disadvantages are not yet well understood. The purpose of this study is to investigate the experimental and numerical investigations of macroscopic and microscopic characteristics of biodiesel, dimethyl ether, and biodiesel-ethanol blended fuels through the common-rail system. The hybrid breakup model was utilized to obtain numerical results by KIVA code. Also, the calculated results of the three fuels were compared with experimental results such as spray development process, SMD and mean diameter distribution according to the axial distance, and overall SMD after the start of the injection. 2. Experimental Apparatus and Procedures 2.1. Experimental Apparatus. The experimental apparatus to investigate the spray characteristics of the three fuels is composed of a spray visualization system, droplet size and velocity measuring system, and high-pressure pumps with a common-rail system for the high-pressure injection as shown in Figure 1. For the investigation of atomization characteristics, this work used a diesel injector with a single hole which has 0.3 mm nozzle diameter and 2.67 as the length-diameter ratio. An injector driver and digital delay/ pulse generator (Berkeley Nucleonics Corp, Model 555) were used to control the fuel injection timing and duration. (8) Park, J. H.; Suh, H. K.; Park, S. W.; Lee, C. S. Trans. KSAE 2005, 1, 369–374. (9) Suh, H. K.; Park, S. W.; Kwon, S. I.; Lee, C. S. Energy Fuels 2006, 20, 1471–1481.

Macroscopic and Microscopic Spray Characteristics of Fuels

Energy & Fuels, Vol. 22, No. 3, 2008 2093

Figure 2. Single hole injector and measured points of spray. Table 2. Experimental Conditions injection system

common rail

fuels hole diameter (mm) injection pressure (MPa) injection duration (ms) ambient pressure (MPa) ambient temperature (K)

biodiesel, BDE20, DME 0.3 60 0.8 0.1 293

Also, the injection timing was controlled by the synchronization of the injector driver and digital delay generator. To visualize the spray developing process, the light source from the Nd:YAG laser was used and the phase Doppler particle analyzer system was composed of the light source from the output of the Ar-ion laser which was determined to be 700 mW, a transmitter, a receiver, and signal analyzer for the measurement of the droplet size and velocity. 2.2. Experimental Procedures. The experiment was conducted with biodiesel fuel made from soybean oil and biodiesel-ethanol blended fuel which mixed biodiesel fuel with the volumetric ratio 20% of 99.9% degree of ethanol purity for the creation of a blended fuel with biodiesel and ethanol. In the case of DME fuel, it was pressurized to 1 MPa in a fuel tank using the pressure of nitrogen gas because it is gaseous at room temperature. Table 2 shows the used fuels and properties of the fuel in the study. For the investigation of atomization characteristics, the fuel was free injected at 60 MPa of the injection pressure. As can be seen in Figure 2, measured points were selected from 10 to 60 mm of the axial distance at 5 mm intervals for the study of droplet size and velocity. The detailed experimental conditions are shown in Table 1.

3. Numerical Approach 3.1. Applied Fuels. Teng et al.10 have conducted numerical investigations on the thermophysical characteristics of liquid DME such as density, vapor pressure, viscosity, and latent heat according to the temperature based on the existing molecular theory. For the fuel-system design and modeling, equations including the combustion characteristics and thermodynamic properties of DME were developed by the general thermodynamic theory.11,12 In the case of the biodiesel fuel, properties of biodiesel according to the temperature were based on the results of experiments which were conducted by Yoon et al.13 Also, the density and viscosity of biodiesel-ethanol blended fuel (BDE20) according to the temperature were measured by a density meter and viscometer. Figure 3 shows the viscosity (10) Teng, H.; McCandless, J. C.; Schneyer, J. B. SAE Tech. Pap. Ser. 2001, 200, 0101–0154. (11) Teng, H.; McCandless, J. C.; Schneyer, J. B. SAE Tech. Pap. Ser. 2003, 2003–01–0759. (12) Teng, H.; McCandless, J. C.; Schneyer, J. B. SAE Tech. Pap. Ser. 2004, 2004–01–0093. (13) Yoon, S. H.; Park, S. H.; Lee, C. S. Energy Fuels 2008, 22, 652– 656.

Figure 3. Viscosity and density of test fuels according to the temperature.

and density of each fuel according to variation in temperature. As can be seen in Figure 3a, the viscosity of fuels was decreased by the increase of temperature and the viscosity of the BDE20 has a smaller value than that of the biodiesel fuel. Also, it can be said that the decreasing pattern of the BDE20 is similar to that of the biodiesel fuel and the viscosity of the liquefied DME fuel has a very low value at less than 0.2 cP. In the case of density, the biodiesel and BDE20 show a similar trend of viscosity according to the increase of temperature and the density of DME fuel rapidly decreased more than that of biodiesel and BDE20 as shown in Figure 3b. 3.2. Hybrid Breakup Model and the Determination of Breakup Model Constant. In order to analyze the spray characteristics of fuels, a numerical investigation was conducted by the hybrid breakup model combined with the primary and secondary breakup. In the modeling of the breakup, the breakup constant (B1) of the Kelvin-Helmholtz (KH) model is an important factor in the primary breakup. In this analysis, the calculated results of various B1 from 20 to 60 were compared with the experimental results for the optimization of a breakup constant. From these results, it was known that the value of 40 for B1 agreed well with the experimental results and the optimum value of the breakup constant B1 was decided at 40 in this study. The Kelvin-Helmholtz instability is based on the result of aerodynamic interaction between the liquid jet and gas on the boundary layer which was proposed by Reitz,14 and the KH breakup model was used as the primary breakup model. The secondary breakup model was applied to the drop deformation and breakup (DDB) model that was suggested by Ibrahim et (14) Reitz, R. D. Atomisation Spray Technol. 1987, 3, 309–337.


Kim et al.

and the radius of the droplet after breakup rn can be found by the following equation. rc ) 0.61ΛKH,

τKH )

3.726B1r ΩKHΛKH

r - rn r - rc ) dt τKH

(3) (4)

The DDB model for the secondary breakup is expressed in terms of the internal energy of the half-droplet and the worked energy from the outside, and the equation of the DDB model is given by the following equation15 d2y 4N 1 dy 27π2 [ 3 + y 1 - 2(cy)-6] ) (5) + 2 2 dt Re 16We 8 dt y The solution of this equation was calculated by a fourth-order Runge–Kutta method and the critical condition of the droplet breakup is expressed in the following equation on the supposition that the viscous dissipation and kinetic energy of the droplet are negligible. K

a We ) (6) r 6π where a is the ratio of the major semiaxis of the ellipsoidal cross section of the oblate spheroid and r is the initial droplet spheroid radius. In the DDB model, the drag coefficient of the sphere CDS is related to the Reynolds number of the droplet and the drag coefficient CD was calculated by the consideration of the droplet deformation as given by17 CD ) CDS(1 + 2.632y)

(7)

where y is the magnitude of the droplet deformation and it is calculated through the following equation:

{ ( ar - 1)}

y ) min 1, Figure 4. Comparison of experimental and calculated spray development processes according to fuels.

al.15 It is assumed that a droplet is warped by pure extension flow and it is based on the conservation equation of energy at a deformed droplet. Therefore, the calculation for the comparison of experimental results was conducted by the KelvinHelmholtz and drop deformation breakup (KH-DDB) model combining two breakup models in this study. Also, the breakup boundary condition of the two breakup models assumed that a conversion between primary and secondary breakup occurred when the droplet size has less than maximum 95% of initial droplet size as proposed by Beatrice et al.16 In the breakup characteristics of the KH breakup model, the maximum growth rate (ΩKH) and the corresponding wavelength (ΛKH) which are derived from the calculated solution of the dispersion relation equation are given by14 ΛKH (1 + 0.45Z1/2)(1 + 0.4T0.7) ) 9.02 r (1 + 0.87We1.67)0.6 ΩKH

[ ] Ffr3 σ

(1)

g

1/2

)

(0.34 + 0.38We3/2 g ) (1 + Z)(1 + 1.4T0.6)

(2)

Also, under an assumption that the radius of a droplet decreases to the critical radius rc during the breakup time, τKH (15) Ibrahim, E. A.; Yang, H. Q.; Prezkwas, A. J. AIAA J. Propulsion Power 1993, 9, 652–654. (16) Beatrice, C.; Belardini, P.; Bertoli, C.; Cameretti, M. C.; Cirillo, N. C. SAE Tech. Pap. Ser. 1995, 950086.

(8)

4. Results and Discussion 4.1. Macroscopic Characteristics of Spray Development. Figure 4 shows the comparison between the experimental images that were obtained through the visualization system and numerical spray developing processes using the hybrid breakup model of the three fuels according to the time after the start of the injection. The numerical results that are indicated on the left side of the spray axis show a good agreement with the experimental images as compared in Figure 4. The comparisons with both results between predicted and experimental images of the three fuels are similar evolution progress for the same elapsed time after the start of the injection. The phenomenon of a spray vortex obviously appeared by the drag force in the exterior of the spray. In the case of the experimental results, it was confirmed that the spray width of the DME fuel was remarkably narrower than those of the two other fuels, and DME fuel at the downstream blurred the spray images because droplets from the outside rapidly evaporated by the vaporizing property of DME fuel at room temperature. Also, the spray developing processes of the biodiesel and BDE20 were almost equal in spray shapes but the vortex shape of BDE20 at the outer spray shows more clearly than that of biodiesel fuel because the fuel atomization is activated by the effect of the low viscosity of BDE20. In the calculated spray developing process, it was (17) Hwang, S. S.; Liu, Z.; Reitz, R. D. Atomization Sprays 1996, 6, 353–376.



Figure 6. Comparison of the spray tip penetration of fuels according to the time after the start of the injection.

Figure 5. Comparison between the experimental and calculated outer line of the spray images according to fuels (tinj ) 1.0 ms).

shown that droplets of a large size were injected from the nozzle at the 0.4 ms after the start of the injection and the breakup of droplets remarkably occurred within 30 mm of the axial distance from the nozzle tip. Also, it was considered that droplets of DME spray have a small size compared with the biodiesel and BDE20 sprays because the atomization rapidly progressed from the early stage of injection by the effect of the low viscosity and surface tension of the DME fuel. In the case of 1.0 ms after the start of the injection, the visible shape of the vortex can be seen from the exterior of the biodiesel and BDE20 sprays but the DME spray shows a faint vortex shape as it may be inferred that the Weber number influenced the drag coefficient and was increased by low surface tension of the DME fuel and the droplets after the breakup disappeared by evaporation at the outside of the spray. Figure 5 shows the comparison between the experimental and calculated results of the outer line of the spray images according to fuels at 1.0 ms after the start of the injection. It was known that the outer line of the biodiesel spray has a maximum length and that of the DME spray has a minimum length. Also, the shape of the biodiesel and BDE20 sprays show a similar tendency but the DME fuel showed a spray width of 5 mm and below under the influence of viscosity and surface tension. The calculated results show good agreement with the experimental results and it may be considered that the tendency according to characteristics of fuel has well predicted accuracy. However, it was estimated that the calculated outer line of DME fuel shows a different result compared to experimental results by reason of the nonevaporating condition on the calculated process. The spray tip penetration which was measured by spray images through the visualization system was compared with calculated results as illustrated in Figure 6. As for results of the spray developing processes, the spray tip penetration of the biodiesel and BDE20 fuels shows longer penetration than that of the DME fuel because biodiesel and BDE20 have similar properties of fuel but those of DME fuel are different from the other fuels. It was considered that a low surface tension of DME fuel promoted the atomization of droplets at the early stage of injection and there was a wide difference from 1.0 ms after the start of the injection by effect of the evaporation. Also, the calculated results well predicted the tendency of spray tip penetration according to fuels and it was shown that the

increasing rate of spray tip penetration slowed from 0.5 ms after the start of the injection due to the effect of the drag force and secondary breakup. After 0.8 ms of injection time, it was judged that calculated results show longer penetration than the experimental results because the kinetic energy in the droplets was increased by the coalescence and collision of droplets at the lower part of spray. 4.2. Atomization Characteristics of the Spray. In order to analyze the atomization characteristics, the microscopic characteristics of fuels such as droplet mean diameter and its distribution according to the axial distance and time were measured by a phase Doppler particle analyzer and they were compared with calculated results. Figure 7 shows the comparison between the experimental and calculated frequency distribution of droplet number of the three fuels according to droplet size at the 20 and 40 mm axial distance from the nozzle tip. The droplet size was plotted in the droplet diameter axis with 2 µm intervals and the frequency distribution was defined as the number of droplets according to droplet size to the number of total measured droplets. In the case of experimental results, the frequency distribution of all the fuels shows that the distribution of droplets with 5 µm or larger droplet size at 40 mm of axial distance was greater than that at 20 mm. It means that the concentrated droplets at the center of the spray collided and coalesced with floating droplets which vanished the kinetic energy as the spray was going downstream. The experimental and calculated results have a distribution ratio of a similar trend, but the calculated results of the frequency distribution over 15 µm predicted larger droplet size than that of the experimental results. The frequency of the droplet diameter of the biodiesel fuel counts for about 70% under 5 µm of the droplet size, and it indicates that the breakup actively occurred at 20 mm of axial distance. Therefore, the increase of the droplets of 5-10µm at 40 mm of the axial distance from the nozzle tip brought about the worse of the atomization of droplets compared to the case of the axial distance of 20 mm. The frequency distribution of the BDE20 fuel shows a resembling tendency of the biodiesel fuel but it can be seen that the frequency distribution of the BDE20 at 20 mm of axial distance has a similar trend of the biodiesel fuel at 40 mm of axial distance because the breakup of droplets happened before the 20 mm of axial distance. Also, it was confirmed that the atomization of the DME fuel was almost completed before the 20 mm of axial distance as shown in Figure 7. The frequency distribution of the DME fuel at 20 mm of the axial distance shows a similar distribution of the other fuels at 40 mm of axial distance.


Kim et al.

Figure 7. Comparison of the frequency distribution of droplet size according to fuels at 20 and 40 mm of the axial distance.

Figure 8 shows the comparison between the experimental and calculated results of the mean droplet size of the three fuels according to the axial distance. The mean droplet sizes of the biodiesel and BDE20 fuels have the range of 15-30 µm and similar values of mean droplet size. But the mean droplet size of the DME fuel was shown to have the value from 5 to 15 µm by the effect of the lower surface tension and viscosity of biodiesel and BDE20 fuels. Also, there is the difference about 5-10 µm of mean diameter between the calculated and experimental results from 15 to 30 mm of the axial distance and it can be known that the breakup of the droplets rapidly occurred between 10 and 20 mm of the axial distance. In the case of DME fuel, it was guessed that the atomization nearly finished before the 10 mm of the axial distance because the mean droplet size was smaller than that of the other fuels at 10

mm of the axial distance. As the axial distance proceeded downward, it can be seen that the mean droplet size of the biodiesel and BDE20 sprays minutely increased from 25 mm of the axial distance, but it may be conjectured that the mean droplet size of the DME spray did not increase according to the axial distance because the collisions of the droplets at the lower part of the spray occurred less than that for the two other fuels due to the effect of evaporation. The comparison between the experimental and calculated results of the Sauter mean diameter (SMD) of the fuels according to the axial distance is illustrated in Figure 9. The calculated results show similar patterns with the experimental results. Also, it can be shown that the experimental and calculated results obviously have different patterns of SMD according to the fuels and the SMD of the biodiesel spray slowly decreased as the


Figure 8. Comparison of the mean droplet size of fuels according to the axial distance.


injection. So, it was shown that the overall SMD was slightly increased by effect of the collision of droplets or floating particles. Also, the overall SMD of the biodiesel and BDE20 sprays has a stable value at about 30 µm because the BDE20 fuel shows an overall SMD somewhat below that of the biodiesel fuel by the minor difference of fuel property. On the other hand, the DME spray has the value at about 20 µm of the overall SMD because DME fuel makes a great difference of fuel properties such as the density, viscosity, and surface tension. As can be seen in Figure 10, the calculated results show a similar tendency with the overall SMD according to properties of a fuel. Figure 11 shows the mean droplet size and Weber number of the experimental results according to the time after the start of the injection. The comparisons between the experimental and numerical results of the Weber number were based on factors such as the mean velocity of droplets obtained through the droplet measurement system, nozzle hole size of the injector, and surface tension of fuels and show similar patterns for the elapsed time. In the figure, the Weber number according to mean velocity of droplets was defined as Wedv )

FairUd2dn σf

(9)

where the mean velocity of droplets was calculated by Ud ) √Udx2 + Udy2

Figure 9. Comparison of the SMD of fuels according to the axial distance.

(10)

and σf is the surface tension of the fuels. In the case of the mean droplet size, it was shown that the mean diameter of droplets in the biodiesel and BDE20 sprays rapidly decreased but the mean diameter of droplets in the DME spray has an irregular tendency in the region from 1 to 2 ms. It can be said that small droplets after breakup evaporated quickly and the mean droplet size of the DME spray irregularly shows larger than that of other fuels at the range from 1 to 2 ms after the start of the injection. As shown in Figure 11, the predicted Weber number shows a similar pattern as measured results. Also, it was shown that the experimental Wedv of the biodiesel and BDE20 were close to zero at 1 ms after the start of the injection. In the case of the DME spray, it was shown that Wedv has by far the largest value of about 150 at the initial injection due to the lower surface tension of about 40% compared with the two other fuels and Wedv of the DME spray was continuously decreased by the diminishment in mean velocity of the droplets until 3 ms after the start of the injection. 5. Conclusions

Figure 10. Comparison of the overall SMD of fuels according to the time after the start of the injection.

axial distance. However, the decrease on the SMD of the BDE20 and DME sprays in the range from 15 to 20 mm shows steep decrease as indicated in the SMD distribution. It was supposed that the viscosity of the BDE20 fuel was diminished by a mixing of ethanol fuel, and the SMD of the DME spray was affected by its low surface tension and viscosity. Figure 10 indicates the comparison of the experimental and calculated results of the overall SMD according to the time after the start of the injection. Results of the numerical analysis were in good accordance with the experimental results and it was known that the breakup of droplets occurred suddenly at 0.2 ms after the start of the injection and the atomization of fuels was almost completed before 0.5 ms after the start of the

In order to analyze the effects of biodiesel, DME, and biodiesel blended fuel on the atomization characteristics, an experimental and computational study was conducted for the investigation of the macroscopic and microscopic characteristics such as the development process of spray, the frequency distribution of droplets, the mean droplet size, and overall SMD. Also, a numerical study was performed by KIVA code and the calculated results compared with the experimental results such as the development process of spray, SMD distribution of droplets, and overall SMD according to the time after the start of the injection. The conclusions of the comparison between the experimental and calculated studies are summarized as follows. 1. In the spray development process, the numerical results show good agreement with the experimental results and the


Kim et al.

tip penetration of the biodiesel and BDE20 fuels showed longer spray than that of the DME fuel. 2. The frequency distribution of the droplet size for the three fuels shows that the distribution of the droplets with 5 µm or larger droplets at 40 mm of the axial distance was greater than that at 20 mm. The mean droplet sizes of the biodiesel and BDE20 fuels have the range of 15-30 µm but the mean droplet size of the DME fuel has the value from 5 to 15 µm by the effect of lower surface tension and viscosity than the biodiesel and BDE20 fuels. Comparing the SMD results according to the axial distance, the SMD of biodiesel spray slowly decreased as the axial distance increased but the rapidly decreasing SMD of the BDE20 and DME sprays appeared in the range between 15 and 20 mm. 3. In the case of the overall SMD, the breakup of droplets occurred suddenly at 0.2 ms after the start of the injection and the atomization of fuels was almost completed before 0.5 ms after the start of the injection. The overall SMD of the biodiesel and BDE20 sprays has a stable value of about 30 µm and the DME spray has the value at about 20 µm of the overall SMD. 4. It was shown that the mean diameter of droplets in the biodiesel and BDE20 sprays rapidly decreased but the mean diameter of the droplets in the DME spray has the irregular range between 1 and 2 ms of elapsed time. Also, the mean diameter of the droplets was closely related to the Weber number according to the mean velocity of the droplets and the calculated Wedv had a similar pattern of the experimental results. Acknowledgment. This study was supported by the CEFV (Center for Environmentally Friendly Vehicles) of the Eco-STAR project from the MOE (Ministry of Environment, Republic of Korea). Also, this work was 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. This work was also supported by the Second Brain Korea 21 Project in 2007.

Nomenclature

Figure 11. Distribution of droplet diameter and Weber number according to the time after the start of the injection.

phenomenon of the vortex obviously appeared due to the drag force in the exterior of the spray. The spray width of the DME spray was remarkably narrower than those of the biodiesel and BDE20 fuels and the DME fuel at the downstream blurred the spray images. So, the spray developing processes of the biodiesel and BDE20 were almost equal in spray shapes. Also, the spray

a ) major semi-axis of the ellipsoidal cross section of the oblate spheroid B1 ) constant of the KH breakup CD ) drag coefficient CDS ) drag coefficient of the sphere dn ) diameter of nozzle hole K ) liquid to gas density ratio N ) liquid to gas viscosity ratio r ) droplet radius Re ) Reynolds number T ) Taylor number Ud ) mean velocity of droplets We ) Weber number Wedv ) Weber number according to mean velocity of droplets Z ) Ohnesorge number Λ ) corresponding wavelength Fair ) density of air τ ) breakup time Ω ) maximum growth rate mean diameter ) average diameter of all the droplets (D10), ∑NiDi/ ∑Ni local SMD ) Sauter mean diameter (D32) at the measuring point, ∑NiDi3/∑NiDi2 overall SMD ) Sauter mean diameter (D32) of total droplets KH ) KH breakup EF700692W

An Experimental and Numerical Investigation of Atomization

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