Effect of Mixing Ratio of Biodiesel on Breakup Mechanisms of

Energy & Fuels 2006, 20, 1709-1715

1709

Effect of Mixing Ratio of Biodiesel on Breakup Mechanisms of Monodispersed Droplets Sung Wook Park,† Sayop Kim,‡ and Chang Sik Lee*,§ Mechanical Engineering and Technology Research Institute, Hanyang UniVersity, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea, 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 10, 2005. ReVised Manuscript ReceiVed March 29, 2006

In this paper, the breakup characteristics of a monodispersed droplets were analyzed as a function of a mixing ratio of the biodiesel based on the images captured using a long-distance microscope and a spark lamp. To investigate the effects of physical properties of biodiesel such as high surface tension and viscosity on the breakup mechanism of a monodispersed droplet, the experiments were performed at various mixing ratios of biodiesel. In addition, the physical properties such as density, kinematic viscosity, and surface tension of the biodiesel blended fuels were measured to study the relations between Weber number and the transition of breakup regime. The experimental apparatus of this study consisted of a droplet generation system, an air flow nozzle, a light source, a long-distance microscope, and a CCD camera. In the first breakup stage, the droplet deformation rate was measured in order to reveal out the effect of high surface tension and viscosity on the deformation rate of a droplet of biodiesel blended fuels. The results of these experiments showed that breakup regime of droplet transited to deformation, bag breakup, stretching and thinning breakup, and catastrophic breakup regimes sequentially as the Weber number is increased. In addition, the higher surface tension and kinematic viscosity of biodiesel slowed the disintegration of a droplet and transition of breakup regime. The lower mixing ratio of biodiesel promotes the deformation of droplet in the first breakup stage, and the breakup mechanism transited to next regime at higher relative velocity between droplet and ambient gas as the mixing ratio is increased.

1. Introduction Nowadays the attentions on the alternative fuels such as biodiesel, CNG (compressed natural gas) and DME (dimethyl ether) have been increased because these alternative fuels can reduce the pollutant emissions dramatically.1-4 Among these alternative fuels, biodiesel is the most promising diesel alternative fuel because the biodiesel blended fuels can be used for compression ignition engine without engine modification.1,5 Owing to these advantages of the biodiesel, the biodiesel fuels are encouraged to use for diesel engine as blended forms with the conventional diesel in many countries. As a result, the production and use of biodiesel have been increased. To investigate the emission characteristics of compression ignition engine fueled with biodiesel or its blends, many studies have been performed. Sharp et al.6,7 measured the regulated and * Corresponding author. Tel: +82-2-2220-0427. Fax: +82-2-2281-5286. E-mail: [email protected]. † Mechanical Engineering and Technology Research Institute. ‡ Graduate School of Hanyang University. § Department of Mechanical Engineering. (1) Lee, C. S.; Park, S. W.; Kwon S. I. Energy Fuels 2005, 19, 22012208. (2) Song, J.; Huang, Z.; Qiao X.; Wang, W. Energy ConVers. Manage. 2004, 45, 2223-2232. (3) Yamada, H.; Yoshii M.; Tezaki A. Proc. Combust. Inst. 2005, 30, 2773-2780. (4) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C. Renewable Energy 2005, 30, 795-803. (5) Usta, N. Biomass Bioenergy 2005, 28, 77-86. (6) Sharp, C. A.; Howell, S. A.; Jobe, J. SAE Tech. Pap. Ser. 2000, No. 2000-01-1967.

unregulated emissions from diesel engines fueled with biodiesel blended fuels. They showed that CO and HC emissions were reduced by using biodiesel, whereas NOx emissions increased by 12% because the oxygen in the biodiesel increased the combustion temperature. To overcome the increase of NOx emissions, Kim et al.8 suggested a homogeneous charge compression ignition as a proper method to decrease the combustion temperature. Also Yoshimoto and Tamaki9 showed that the water emulsion and EGR (exhaust gas recirculation) can be used for the reduction of NOx emission in the compression ignition engine fueled with biodiesel blended fuels. Despite of advantages of biodiesel fuel, there still exist some problems and uncertainties that should be solved for the optimization of compression ignition engine fueled with biodiesel blended fuels. It is known that the surface tension and kinematic viscosity of biodiesel are higher than those of conventional diesel.1,10 The higher surface tension and kinematic viscosity cause higher SMD (Sauter mean diameter) of fuel spray, this makes the fuel evaporation difficult. Lee et al.1 studied the atomization and combustion characteristics of biodiesel blended fuels experimentally. In this study, they concluded that the atomization performance of biodiesel blended (7) Sharp, C. A.; Howell, S. A.; Jobe, J. SAE Tech. Pap. Ser. 2000, No. 2000-01-1968. (8) Kim, D. S.; Kim M. Y.; Lee, C. S. Energy Fuels 2004, 18, 12131219. (9) Yoshimoto, Y.; Tamaki, H. SAE Tech. Pap. Ser. 2001, No. 200101-0649. (10) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C. Renewable Energy 2004, 29, 727-742.

10.1021/ef050372z CCC: $33.50 © 2006 American Chemical Society Published on Web 05/05/2006

1710 Energy & Fuels, Vol. 20, No. 4, 2006

fuels were inferior to the conventional diesel due to the higher surface tension and kinematic viscosity although the HC emissions were reduced by using biodiesel blended fuels up to 55% in comparison to the conventional diesel fuel based on the experimental results of fuel physical properties, atomization characteristics, injection rate, combustion performance, and emission characteristics. In addition, they suggested the further research on the atomization characteristics of biodiesel blended fuels were required for the improvement in engine performance. For investigating the effect of mixing ratio of biodiesel on the spray development, Grimaldi and Postrioti11 compared the processes of spray developments between conventional diesel and biodiesel blended fuels using a common-rail fuel injection system. Their results indicated that the spray tip penetration increased in accordance with the increase of mixing ratio of biodiesel because the higher surface tension and kinematic viscosity of biodiesel prevented the atomization of fuel spray. Analyzing the previous studies1,10,11 on the atomization characteristics of biodiesel fuels, it can be said that the breakup mechanism of biodiesel is much different with those of the conventional diesel; therefore, the researches focused on the breakup mechanism of biodiesel are required. In measuring the atomization characteristics of fuel spray, the PDPA (phase Doppler particle analyzer) system, which is widely used for obtaining droplet size and velocity, has been utilized to investigate the atomization characteristics of fuel spray.12-14 However, the results based on the PDPA system have some limitations in that the PDPA system only provides the size and velocity information. The experiments using the PDPA system are also possible in the limited measurement area and experimental conditions. To reveal the breakup mechanism, the analysis based on the droplet images during breakup can be the proper method using a long-distance microscope and a droplet generation system. By investigating the images of droplet breakup, the deformation of droplet and the distributions of breakup regime can be obtained. These results are useful in studying the breakup characteristics of droplets according to the physical properties of liquid. For investigating the droplet breakup mechanism, Lee and Reitz15 investigated the effect of liquid properties on the distortion and breakup mechanisms of liquid droplets using a monodisperse droplet generator and a long-distance microscope and showed that the breakup mechanisms of droplets with different liquids were similar at atmospheric and elevated ambient pressure conditions. Also Hwang et al.16 compared the experiments and calculations on the droplet deformation in the first breakup regime. They applied the TAB (Taylor analogy breakup) model and DDB (droplet deformation and breakup) model for calculating the droplet deformation and concluded that the time of flattening is well-predicted by the TAB model in all breakup regimes. On the basis of the previous studies on the atomization characteristics of biodiesel blended fuel and breakup mechanism of a droplet, it can be postulated that breakup mechanisms of biodiesel blended fuels are different with those of the conventional diesel fuel. In this point of a view, the research on breakup (11) Grimaldi, C.; Postrioti, L. SAE Tech. Pap. Ser. 2000, No. 200001-1252. (12) Park, S. W.; Lee, C. S. Exp. Fluids 2004, 37, 745-762. (13) Lee, C. S.; Lee, K. H.; Chon, M. S.; Kim, D. S. KSME Int. J. 2001, 11, 35-48. (14) Park, S. W.; Suh, H. K.; Lee, C. S. Int. J. Automot. Technol. 2005, 6, 315-322. (15) Lee, C. S.; Reitz, R. D. Atomization Sprays 2001, 11, 1-19. (16) Hwang, S. S.; Liu, Z.; Reitz, R. D. Atomization Sprays 1996, 6, 353-376.

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Figure 1. Schematic diagram of the experimental apparatus.

mechanisms of droplets is required for finding the optimal injection conditions of biodiesel blended fuels. The breakup mechanisms according to the mixing ratio can be especially important results to study the effect of mixing ratio on the atomization performance of fuel sprays. The aim of this study is to investigate the breakup mechanism of biodiesel according to the mixing ratio of biodiesel and conventional diesel. In this experiment, the breakup mechanisms are analyzed as a function of relative velocity between the droplet and the ambient gas and Weber number. In addition, to reveal the effect of high surface tension and kinematic viscosity of biodiesel on the droplet deformation, the deformation of a droplet is measured as a function of time after the droplet is exposed to the steady gas flow in the first breakup stage. In the first breakup stage, the breakup characteristics of biodiesel blended fuels are studied according to the breakup regimes such as bag breakup, shear breakup, and catastrophic breakup. 2. Experimental Apparatus and Procedure 2.1. Experimental Apparatus. To analyze the effect of mixing ratio of biodiesel on the breakup mechanism of monodispersed droplets, an experimental apparatus composed of a droplet generator, a gas flow nozzle, a spark lamp, and a long-distance microscope with a CCD camera was utilized as shown in Figure 1. In this system, the monodispersed droplets were generated using a vibrating orifice droplet generator, and then the generated droplets were disintegrated after they were exposed to the flow stream made by gas jet injected through the nozzle. The injection pressure of the gas was changed according to the relative velocity between droplet and ambient gas, and the pressure difference between droplet generator and ambient gas was set constant to 0.1 MPa in gauge pressure. Diameter of the nozzle was 2 mm, and the distance between the nozzle exit and the passage of the monodispersed droplet was set constant to 1.5 mm. In this experiment, the droplet generator has a 100 µm orifice nozzle, and the optimal frequency (fopt) applied to the vibrating piezo stack of the droplet generator is determined from fopt ) 0.282 Q/Dn3, where Q is the volumetric flow rate and Dn is the orifice diameter of the nozzle. The amplitude applied to the piezo stack was 30 V, and the signal was generated using the function generator and the voltage amplifier. The system for capturing the images of droplet breakup consists of a spark lamp, a long-distance microscope (QM-100, Questar), a CCD camera, and a digital delay generator (Berkeley Nucleonics Corp., model 555) for synchronizing the spark lamp and the exposure time of the CCD camera. 2.2. Experimental Procedure. The experiments were performed by changing the mixing ratio of biodiesel and the injection velocity of gas jet. For each experimental case, 100 images were captured and analyzed in terms of breakup regime, rate of droplet deforma-

Mixing Ratio of Biodiesel

Energy & Fuels, Vol. 20, No. 4, 2006 1711

Table 1. Test Fuels fuel composition

ref

conventional diesel soybean oil (20%) + diesel (80%) soybean oil (40%) + diesel (60%) soybean oil (60%) + diesel (40%) soybean oil (80%) + diesel (20%) soybean oil

D100 BD20 BD40 BD60 BD80 BD100

tion, droplet trajectory, and wavelength of Rayleigh-Taylor instability on the droplet surface. The size of droplet generated by the piezo stack droplet generator is determined by D)

( ) 6Q π fopt

1/3

(1)

where D is the droplet diameter generated by the droplet generator of Figure 1, and fopt is the optimal frequency. In this study, the diameter of orifice nozzle of the droplet generator (Dn) is 100 µm, and the calculated droplet size by eq 1 yields 189 µm. The liquids used in this experiment are neat biodiesel, and its blends with the conventional diesel and the fuel compositions are listed in Table 1. As listed in Table 1, the experiments were performed using various kinds of fuels with different mixing ratios in order to investigate the effect of mixing ratio of biodiesel on the breakup mechanism of a droplet. In this experiment, the biodiesel was made of soybean oil, and the biodiesels are mixing with the conventional diesel in mixing ratios from 0% to 100% in steps of 20 vol %. 2.3. Influencing Parameters of Droplet Breakup. 2.3.1. Nondimensional Numbers and Coordinates. Nondimensional numbers such as Weber and Reynolds numbers are important in investigating the breakup characteristics of droplets because they indicate the effect of surface tension and viscosity properly. To investigate the correlations between the nondimensional numbers and breakup characteristics, Weber (We) and Reynolds (Re) numbers are defined as follows: We )

Re )

(2)

FGUD µG

(3)

where U is the relative velocity between droplet and ambient gas and D is the diameter of the droplet. The subscripts L and G also mean the properties of fuel liquid and ambient gas, respectively. For the analysis of droplet trajectory and deformation according to the time after droplet entered the gas flow field, the coordinate and dimensionless time (t*) are defined as illustrated in Figure 2. In this figure, t is the elapsed time after droplet entered the flow field. Also VD and LD indicate the droplet velocity and distance between neighboring two droplets. Therefore, the dimensionless time after the droplet exposed to the ambient gas flow field (t*) becomes the count of droplets from the entrance of gas flow field. 2.3.2. Droplet Deformation Rate. In the conditions of low relative velocity, the droplet is deformed due to the external force without disintegration. In this regime, the deformation of droplet is an important factor in analyzing the breakup mechanism. In this study, the deformation rate of droplet is defined quantitatively as A D

Figure 3. Mean size distributions of biodiesel blended fuels.1 Table 2. Physical Properties of Test Fuels

FGU2D σL

)

Figure 2. Definitions of coordinate and t*.

(4)

where A and D represent the major axis of the deformed droplet and the droplet diameter of an intact droplet, respectively.

3. Results and Discussions As indicated in the previous research,1 the atomization performance of the biodiesel blended fuels is inferior to that of

fuel

density (kg/m3)

surface tension (kg/s2)

viscosity (kg/ms)

D100 BD20 BD40 BD60 BD80 BD100

831 840 849 860 871 880

0.026 0.0265 0.0272 0.0276 0.02795 0.02833

0.00223 0.0024 0.0028 0.00316 0.00352 0.00389

the conventional diesel because the surface tension and kinematic viscosity of biodiesel is higher than those of conventional diesel fuel. The measurement of SMD also shows that the biodiesel has higher SMD due to its physical properties as shown in Figure 3. In this study, the effect of physical properties on the breakup mechanism of biodiesel is studied in the microscopic viewpoint based on the image analysis captured using a long-distance microscope. The physical properties of the fuel according to the mixing ratio of biodiesel are investigated first of all. Then the breakup mechanisms are discussed in the first and second breakup stages. 3.1. Effect of Mixing Ratio of Biodiesel on the Physical Properties of Fuel. The breakup characteristics of fuels are dependent on the physical properties such as surface tension and viscosity. Considering that the physical properties of biodiesel are different with those of the conventional diesel, it is important to investigate the effect of mixing ratio of biodiesel on the physical properties of fuel. Table 2 lists the physical properties of biodiesel blended fuels according to the mixing ratio. As can be seen in this table, density, surface tension, and


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Figure 4. Deformation of droplet in the first breakup stage.

Figure 6. Effect of mixing ratio on the droplet trajectory in the first breakup stage.

Figure 5. Effect of mixing ratio on droplet deformation rate.

viscosity are increased with the increase of mixing ratio. On the basis of this result, it can be guessed that the droplets are hardly atomized with the increase of mixing ratio of biodiesel. 3.2. First Break Stage. 3.2.1. Deformation Characteristics. The breakup processes of a liquid droplet are classified into two stages, first and second breakup stages.15-18 During first breakup stage of droplet breakup, the droplets experience deformation of the droplet shape due to the external force without disintegration of droplets. After the droplet enters the steady gas flow field, the droplet is influenced by the distribution of aerodynamic pressure around the droplet. As a result, the droplet is deformed from its intact spherical shape. Since the steady gas stream flows around the droplet, the gas velocity distribution and the aerodynamic pressure distribution at any point on the droplet surface are not uniform. The gas velocity has a maximum value at the equator of the droplet and equals zero at the droplet pole, which is called a stagnation point. In accordance with Bernoulli’s equation, the aerodynamic pressure becomes higher at the pole and lower at the equator. Due to this phenomenon, the external aerodynamic pressure causes the spherical droplet to distort and becomes flattened to form an oblate ellipsoidal as shown in Figure 4. On the basis of the experimental results shown in the images of Figure 4, it can be seen that flattening occurs rapidly with the increase of Weber number. To investigate the effect of mixing ratio on the fuel deformation quantitatively, the deformation of droplet was measured as a function of dimensionless time after the droplet was exposed to the gas flow field. Figure 5 shows the droplet deformation rate according to the mixing ratio of biodiesel in the case that the relative velocity between the droplet and ambient gas is constant to 42 m/s. In this figure, it can be seen that the droplets are hardly deformed as the mixing ratio of biodiesel increases. (17) Liu, A. B.; Reitz, R. D. Atomization Sprays 1993, 3, 55-75. (18) Wu, P. K.; Faeth, G. M. Atomization Sprays 1993, 3, 265-289.

It means that the higher surface tension and viscosity of biodiesel slow down the deformation of droplet. Especially, in the case of deformation, the viscosity of fuel can be an dominant factor of the droplet deformation because the deformation is closely related to the viscosity resistance of the internal energy of the droplet. As a result, the droplet of conventional diesel fuel is deformed rapidly in comparison to the biodiesel blended fuels. 3.2.2. Droplet Trajectory. Accompanying with the droplet deformation rate, the droplet trajectory is another important parameter in analyzing the breakup characteristics in the first breakup regime because droplet trajectory is closely related to the droplet deformation rate. It can be said that the projected droplet area in the gas flow direction is mainly determined by the surface tension and viscosity of fuel when the relative velocity between droplet and ambient gas is constant. Therefore as the mixing ratio is increased, the projected area of droplet becomes smaller, and the effects of drag force and the external force acting on the droplet are reduced. As a result, the droplet path traveled is decreased with the increase of mixing ratio of biodiesel as shown in Figure 6. Summarizing the results of Figures 4-6, it can be said that the higher surface tension and viscosity of biodiesel reduce the effect of the ambient gas flow on the deformation and trajectory of monodispersed droplets. 3.3. Second Breakup Stage. As the Weber number is increased, liquid droplets experience disintegration in the second breakup stage. In this stage, droplets encounter three breakup regimes, that is, bag breakup, stretching and thinning breakup, and catastrophic breakup. 3.3.1. Bag Breakup. If the Weber number becomes higher than the critical value of the first breakup stage, a thin hollow bag is formed in the center of flattened droplet. As time elapses in the gas flow field, the bags are burst, forming a number of small fragments. Eventually, the rims are disintegrated into small droplets as shown in Figure 7. In this figure, the Weber number is constant to 60 at various mixing ratio of biodiesel. In this figure, the photographs for the breakup phenomena of biodiesel blended fuels show a similar breakup pattern. However, at the same Weber number, the gas velocities are increased with an increase of mixing ratio of biodiesel fuel as shown in Figure 7. 3.3.2. Stretching and Thinning Breakup. As the Weber number is further increased from the bag breakup regime, droplets are disintegrated at the droplet edge because a suction stress toward flow direction is generated at the edge of the droplet. This type of breakup is called as stretching and thinning breakup. In this regime, shearing action due to the high-speed gas flow on the droplet causes the deformation of a boundary


Figure 7. Effect of mixing ratio on breakup mechanism in bag breakup regime (We ) 60).


Figure 9. Effect of mixing ratio on breakup mechanism in catastrophic breakup regime (We ) 380).

Figure 10. RT wavelength at catastrophic breakup regime (U ) 212.2 m/s).

and kinematic viscosity of the conventional diesel drops bring about slightly fast disintegration of the flattened drops as shown in Figure 8.

Figure 8. Effect of mixing ratio on breakup mechanism in stretching and thinning breakup regime (We ) 105).

layer at surface of the droplet. Then the boundary layer is separated from the droplet. Figure 8 shows the effect of mixing ratio on the pattern of droplet breakup in the stretching and thinning breakup regime. Comparing the breakup patterns of diesel drops with the biodiesel drops, it can be shown that the lower surface tension

3.3.3. Catastrophic Breakup. When the Weber number is higher than that of stretching and thinning breakup regime, the breakup mechanism transits to the catastrophic breakup regime. In this regime, the droplets are disintegrated into small droplets suddenly due to the higher relative velocity between drops and ambient gas as shown in Figure 9. As shown in the photographs of the catastrophic breakup regime in this figure, the drops of the neat biodiesel (BD100) has larger ligaments in comparison to those of the conventional diesel (D100) because of the higher surface tension and viscosity. In Figure 9, the wave instabilities on the droplet surface can be observed. There are two kinds of wave instabilities that can be observed in the catastrophic breakup regime. The first instability is KH (Kelvin-Helmholtz) wave instability, which is generated because of the shear stress on the surface. On the other hand, acceleration on the droplet surface generates the RT (Rayleigh-Taylor) wave instability. Also the previous


Park et al.

instability theory.19 The RT wavelength, λRT can be estimated as follows:

λRT ) 2π

x

3σL

a(FL - FG)

(5)

where the acceleration of droplet (a) is derived by 2 3 F GU a ) CD 4 F LD

Figure 11. Effect of mixing ratio on RT wavelength (U ) 212.2 m/s).

Figure 12. Transition of breakup regimes according to (a) relative velocity and (b) Weber number.

(6)

In Figure 11, the effect of mixing ratio of biodiesel on the RT wavelength is illustrated quantitatively with the estimated results using eqs 5 and 6 when the relative velocity between the droplet and ambient gas is constant to 212.2 m/s. In this figure, the wide range of experimental values in the RT wavelength indicates that the RT wave is very unstable. In addition, both experimental and calculated results show that the RT wavelength is increased with an increase of mixing ratio although the experimental results of RT wavelength is longer than the estimated values. It can be seen that the RT wavelength of biodiesel is longer than the conventional diesel because the surface tension of biodiesel is higher as shown in Table 2. The correlation between the RT wavelength and the surface tension can also be shown in eq 5, which shows that the RT wavelength is proportional to the root of surface tension. Also, based on the shorter predicted RT wavelength than the experiments shown in Figure 11, it can be said that eq 5 underestimates the effect of surface tension on the RT wavelength. 3.3.4. Effect of Mixing Ratio on Transition of Breakup Regimes. In a steady gas flow field, the breakup regime is dominated by the physical properties of droplet and relative velocity. As the relative velocity is increased, the liquid droplet experiences the deformation, bag breakup, stretching and thinning breakup, and catastrophic breakup regimes sequentially. The transition characteristics are important because it shows the atomization characteristics of the fuel. Figure 12 shows the transitions of breakup regimes according to the relative velocity between the droplet and the ambient gas and Weber number at various mixing ratios of the biodiesel. In Figure 12a, it can be observed that the breakup mechanism is transited at slightly higher relative velocity as the mixing ratio is increased. This result indicates that the droplet is disintegrated easily at the lower surface tension. On the other hand, the transition Weber number is almost constant regardless of mixing ratio as shown in Figure 12b. On the basis of the results of Figure 12, it can be said that the Weber number is the most dominant parameter in determining the breakup regime regardless of the physical properties of liquid droplet. 4. Conclusions

mechanism15,16

researches on the droplet breakup indicated that the wavelength of RT instability is longer than that of KH instability. On the basis of the theory of wave instability, it can be said that the dominant wave instability of this experiment is RT instability because the wave is generated in the orthogonal direction to the gas jet stream due to the external force exerted on the droplet surface as shown in Figure 10. Therefore, in this study, the RT wavelength was measured for the analysis of the effect of mixing ratio on the RT wave instability. Along with the measurement of RT wavelength, the estimation on the RT wavelength was performed based on the wave

In this study, the effect of mixing ratio of biodiesel on the breakup mechanisms of monodispersed droplets was investigated using the droplet generator, the spark lamp, and the long-distance microscope. For the quantitative analysis, the droplet deformation, droplet trajectory, and RT wavelength were measured. On the basis of the results, the conclusions of this study can be summarized as follows; (1) As the We number is increased, the droplet experiences deformation, bag breakup, stretching and thinning breakup, and catastrophic breakup sequentially. (19) Bellman, R.; Pennington, R. H. Q. Appl. Mech. 1954, 12, 151162.


(2) In the first breakup stage, as the mixing ratio of biodiesel is increased, the deformation of droplet and droplet path traveled is decreased due to higher surface tension and viscosity of biodiesel in comparison to the conventional diesel fuel. (3) The RT wavelength becomes longer as the mixing ratio is increased at the same relative velocity between droplet and ambient gas. However, the measured wavelength in the experiment is longer than the estimated wavelength based on the theory of RT wave instability. (4) The breakup mechanism is transited to next regime at the higher relative velocity as the mixing ratio is increased. On the other hand, the transition Weber number is almost constant regardless of the mixing ratio. (5) The atomization performance of biodiesel is inferior to the conventional diesel. Therefore, the atomization strategy should be considered for the higher engine performance in a compression ignition engine fueled with a biodiesel or its blends. Acknowledgment. This work is financially supported by the Ministry of Education and Human Resources Development (DOE), the Ministry of Commerce, Industry and Energy (MOCIE), and the Ministry of Labor(MOLAB) through the fostering project of Lab of Excellency.


Nomenclature A ) major axis of the deformed droplet a ) acceleration of droplet CD ) drag coefficient D ) droplet diameter Re ) Reynolds number U ) relative velocity between droplet and ambient gas We ) Weber number Greek Symbols ) deformation rate of a droplet µ ) viscosity F ) density σ ) surface tension Subscripts G ) gas properties L ) liquid properties EF050372Z

Effect of Mixing Ratio of Biodiesel on Breakup Mechanisms of

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