Effect of Cavitating Flow on the Flow and Fuel Atomization

Energy & Fuels 2008, 22, 605–613

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Effect of Cavitating Flow on the Flow and Fuel Atomization Characteristics of Biodiesel and Diesel Fuels Su Han Park,† Hyun Kyu Suh,† and Chang Sik Lee*,‡ Graduate School of Hanyang UniVersity, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea, and Department of Mechanical Engineering, Hanyang UniVeristy, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea ReceiVed June 12, 2007. ReVised Manuscript ReceiVed October 15, 2007

The aim of this work is to investigate the effect of cavitation on the fuel flow and atomization characteristics of the biodiesel fuel. To study these characteristics of biodiesel as an alternative fuel, two different nozzles which have different length-to-width ratios were utilized in this experiment. The visualization system visualized the internal and external flow for investigating the formation and development of cavitation inside the orifice, and the internal flow characteristics were analyzed quantitatively using dimensionless numbers such as the Reynolds number, Weber number, cavitation number, and discharge coefficient. Moreover, the droplet measuring system was installed to study the effect of the formed cavitation on the fuel atomization such as the mean droplet size and the axial and radial mean velocity. On the basis of the results of the cavitating experiment, it was revealed that the mean droplet size of biodiesel is larger than that of diesel fuel. The droplet size became small when it formed the cavitation inside the orifice, and in the case of the high L/W ratio nozzle, the droplet size is also smaller than that of the low L/W ratio nozzle. From these results, it can be concluded that cavitation promotes the atomization of fuels at the nozzle exit. Also, it can be concluded from the results of flow characteristics that the cavitation formed along the nozzle orifice wall because of the change in the flow direction and the flow velocity near the wall due to the geometry of the orifice inlet.

1. Introduction Nowadays, a common-rail injector is used for the injection strategy and the precise control of injection quantity in the high speed direct injection diesel engine. This diesel injection system realizes a highly pressurized and minimized nozzle hole size in order to improve the combustion and emission characteristics. These conditions, such as the high pressure and the reduction of nozzle hole, provide surroundings for the occurrence of cavitation. Therefore, from the viewpoint of the precise control of the injection and the promotion of the engine performance, it is necessary to study the formation and the development process of the cavitation. In the occurrence and growth of cavitation by flow characteristics, the nozzle geometry and fuel properties such as density, viscosity, and surface tension are the main factor of the liquid atomization. In general, cavitation is considered to be the transition of a fluid from liquid to vapor due to the low pressure, provoked at the inlet of the nozzle orifice and caused by strong direction changes in cross section.1 The cavitation generated inside the nozzle orifice promotes the atomization of the liquid and the breakup of the issuing jet at the nozzle exit, which can be used as a means for the reduction of energy for the disintegration of fuel droplets in diesel engines. Research about cavitation has progressed actively by many researchers. Desantes et al.2 and Payri et al.3 studied the effect of cavitation on the injection velocity in the nozzle exit vicinity, * Corresponding author. Phone: +82-2-2220-0427. Fax: +82-2-22815286. E-mail: [email protected]. † Graduate School of Hanyang University. ‡ Department of Mechanical Engineering, Hanyang Univeristy. (1) Lefebvre, A. H. Published by Taylor & Francis, 1989, ISBN 0-891116-603-3. (2) Desantes, J. M.; Arregle, J.; Lopez, J. J.; Hermens, S. SAE Tech. Pap. Ser. 2005, 2005-01-2120.

the measurement of the injection rate, and the momentum flux using three nozzles with different geometry, both experimentally and numerically. They reported that the increase in exit velocity with the appearance of cavitation seems to be caused by the variation of the characteristic of fuel density at the exit of the nozzle due to this cavitation. They also studied the relationship among the formation of the cavitation, the spray penetration, and the flame shape through the spray and flame visualization. In their study, the generation of the cavitation was affected by the shape and dimension of the nozzle orifice. Gavaises et al.4 explained that cavitation is formed not only at the hole entrance due to the local pressure drop caused by nozzle inlet geometry but also at the sac volume of the nozzle tip inside the multihole injector for large diesel engines. In addition, through the CFD calculation, they revealed that the needle lift, cavitation number, and Reynolds number affected the formation and growth of the cavitation. Arcoumanis et al.5,6 introduced the breakup model as a similar approach based on the assumption that the breakup is dominated by the bubble behavior at the outside of the nozzle. In their study, a diesel injector was compared with a scaled-up diesel injector and revealed that the Reynolds number (Re) and cavitation number (K) are the dominant factors influencing the pattern of cavitating flow. Sou et al.7 visualized the generation and growth progress of the cavitation in the transparent acrylic (3) Payri, F.; Arregle, J.; Hermens, S. SAE Tech. Pap. Ser. 2006, 200601-1391. (4) Gavaises, M.; Andriotis, A. SAE Tech. Pap. Ser. 2006, 2006-011114. (5) Arcoumanis, C.; Gavaises, M. Atomization Sprays 1998, 8, 3. (6) Arcoumanis, C.; Badami, M.; Flora, H.; Gavaise, M. SAE Tech. Pap. Ser. 2000, 2000-01-1249. (7) Sou, A.; Lihan, M. M.; Hosokawa, S.; Tomiyama, A. Proc. 10th ICLASS 2006, ICLASS 06-043.

10.1021/ef7003305 CCC: $40.75  2008 American Chemical Society Published on Web 12/07/2007

606 Energy & Fuels, Vol. 22, No. 1, 2008

resin 2D nozzle. They studied the effect of cavitation number (K) and Reynolds number (Re) on the cavitating flow under various flow rate conditions by applying the laser doppler velocimetry (LDV) system. From their study, the pattern of cavitating flow was divided into four steps: no cavitation and wavy jet, developing cavitation and wavy jet, super cavitation and spray, and hydraulic flip and flipping jet. Roth et al.8 carried out the CFD calculation of various nozzle shapes in order to investigate the effect of the nozzle shape on the internal flow characteristics of a diesel injector nozzle. They reported that increasing the orifice inlet radius leads to increasing the mean exit velocity and discharge coefficient near the orifice wall, while the region of the cavitation decreases. Soteriou et al.9 applied a LDV system inside an enlarged plain orifice nozzle under noncavitating conditions. They also observed small bubbles in the downstream direction at the inlet rim and concluded that turbulence within the cavitating flow is a major factor for promoting atomization. Daikoku et al.10 also investigated the effect of the nozzle length-to-diameter ratio or width on the liquid breakup in the 2D nozzle. They reported that when the length-to-width (L/W) ratio is low, the atomization process is affected by the generation and disappearance of cavitation. Further, the liquid is ejected as a sufficiently turbulent form, which promotes the atomization. Extensive experimental and numerical studies on nozzle cavitation have been carried out by Lee et al.11 and Sarre et al.12 However, the majority of previous studies provide an understanding of the formation and influence of the cavitation. In addition, most of these studies used water for the working fluid, which does not reflect the properties of fuel. In the viewpoint of the environment and the energy situation, the use of biodiesel fuel is under consideration because it can be used without modification of the fuel supply system in a diesel engine, and it is already using in many countries. Further, it can be expected to improve the emission characteristics and to increase the thermal efficiency by the entire combustion because the cetane number of biodiesel fuel is higher than that of diesel fuel.13–16 In a diesel engine, the fuel properties of biodiesel influence the spray characteristics and combustion performance. In this point of view, the effect of fuel properties such as viscosity and surface tension on the biodiesel fuel atomization was conducted by Ejim et al.17 Also, the investigation on the formation of cavitation and atomization of biodiesel fuel is a necessity because spray characteristics and structure were affected by different properties of biodiesel fuel compared to diesel fuel. The aim of this paper is to investigate the effect of cavitation on the flow and atomization characteristics of biodiesel fuel in visualized and enlarged nozzles. Moreover, the experiment was performed to analyze the influence of the different nozzle length(8) Roth, H.; Gavaises, M.; Arcoumanis, C. SAE Tech. Pap. Ser. 2002, 2002-01-0214. (9) Soteriou, C.; Andrews, R.; Simth, M. SAE Tech. Pap. Ser. 1999, 1999-01-1486. (10) Daikoku, M.; Furudate, H.; Inamura, T. Proc. 9th ICLASS 2003, Paper No. ICLASS 12-7. (11) Lee, J. W.; Min, K. D. Trans. KSME 2006, 30–6, 553–559. (12) Sarre, C. K.; Kong, S. C.; Reitz, R. D. SAE Tech. Pap. Ser. 1999, 1999-01-0912. (13) Yoon, S. H.; Park, S. W.; Kim, D. S.; Kwon, S. I.; Lee, C. S. Proc. ICEF 2005, 2005–1258. (14) Lee, C. S.; Park, S. W.; Kwon, S. I. Energy Fuels 2005, 2201– 2208. (15) Suh, H. K.; Park, S. W.; Kwon, S. I.; Lee, C. S. Trans. KSAE 2004, 12–6, 23–29. (16) Zhang, Yu.; Boehman, A. L. Energy Fuels 2007, 21, 2003–2012. (17) Ejim, C. E.; Fleck, B. A.; Amirfazli, A. Fuel 2007, 86, 1534–1544.

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Figure 1. Schematic of the flow visualization system.

to-width (L/W) ratios and the formation process of the cavitation inside the nozzle orifice and its effects on the external flow pattern of biodiesel and diesel in the vicinity of the 2D nozzle exit by using dimensionless numbers such as the Reynolds number (Re), Weber number (We), cavitation number (K), and discharge coefficient (Cd). To investigate the effect of cavitation on the fuel atomization, the droplet measuring system was used to study the mean droplet size, the axial mean velocity, and the radial mean velocity. 2. Experimental Apparatus and Procedures 2.1. Flow Visualization and Spray Measuring System. In this study, the experimental apparatus, as shown in Figure 1, was designed to investigate the flow characteristics and visualized the formation of cavitation inside the nozzle orifice and its effects on the external flow characteristics of diesel and biodiesel fuel. The experimental setup consisted of the fuel supply system and the flow visualization system. Test fuels were filtered in order to remove the impurities and were pressurized by nitrogen gas. At the same time, the instant flow rate and pressure were measured at various injection pressures by a flow meter (A109LMA, GPI) and pressure gauge. When diesel and biodiesel fuel passed through the nozzle orifice, the internal and external flow images were visualized with the high resolution ICCD (intensified charge couple device) camera (Dicam PRO, The Cooke Corp.) with a spot lamp as a light source. The injected test fuel was recirculated through the circulation pump (PW-200M, WILO) to the fuel tank. Two different types of nozzles were used to investigate the effect of the L/W ratio on the internal flow characteristics and the formation of cavitation at the orifice, as shown in Figure 2. Detailed specification and reference about nozzles was shown in Table 1. The fuel droplet measuring system (PDPA, phase Doppler particle analyzer) was installed for the measurement of the droplet mean diameter (SMD, Sauter mean diameter), the axial mean velocity, and the radial mean velocity of injected fuels in the nozzle. Considering the measuring accuracy and the signal intensity of the signal analyzer, the power of the Ar-ion laser was set at 0.7 W as a light source of the PDPA system. Moreover, the droplet measuring system consisted of a transmitter, a receiver, and a signal analyzer. 2.2. Experimental Procedure. In order to examine the internal and external flow characteristics and the effect of cavitation on the atomization characteristics of biodiesel and diesel fuel in the nozzle, the experiment was performed by using two nozzles, a flow visualization system, and a fuel droplet measuring system, as illustrated in Figures 1 and 2. Test transparent nozzles were made from the acrylic acid resin. The main raw material of it is a “methyl methacrylate”, and it has a transmissivity of 98% and a reflexibility of 1.49. Diesel and biodiesel fuel derived from soy bean oil was used for the test fuel in this study. The fuel properties of the test fuel are listed in Table 2.

Biodiesel Flow and Fuel Atomization Characteristics

Energy & Fuels, Vol. 22, No. 1, 2008 607

Figure 2. Test nozzles and visualization region.

Figure 4. Measurement points of the droplet measuring experiment. Figure 3. Schematic of the droplet measuring system.

Table 3. Experimental Conditions

Table 1. Specifications of the Droplet Measuring System light source

Ar-ion laser

wavelength laser beam diameter beam expander ratio focal length

514.5 nm, 488 nm 1.4 mm 0.5 250 mm for transmitter 250 mm for receiver 30°

collection angle

Table 2. Test Fuel Properties fuel

diesel

biodiesel (soy bean oil)

density (kg/m3) surface tension (N/m) dynamic viscosity (Ns/m2) vapor pressure (MPa)

830 0.026 0.00223 ,0.1

880 0.028 0.00389 ,0.1

This work was carried out to investigate the effect of the nozzle L/W ratio on the internal and external cavitating flow characteristics. It was carried out using various enlarged nozzles, as classified in Figure 2. Figure 2 shows nozzles of the rectangular orifice inlet shape with 1.5 and 3.0 of the L/W ratio. In the figure, a dotted line indicates the interested visualization region. To investigate the atomization characteristics of biodiesel fuel under various injection conditions, the measuring points were selected at 10 mm intervals from 40 to 150 mm according to the axial direction and at 2 mm intervals to radial direction, as shown in Figure 4, assuming that the spray was axial symmetric. At each point, the approximately 20 000 droplets were captured, and droplets in the range from 2 to 80 µm were averaged and analyzed. The measurement of the SMD and axial velocity of biodiesel and diesel fuel droplets was conducted at the representative injection pressure for turbulent flow, growth of cavitation, and hydraulic flip by using nozzles R and L. The flow visualization and droplet measuring experimental conditions are listed in Table 3. In this investigation, the flow characteristics of cavitating flow were analyzed in terms of following a nondimensional number such as the Reynolds number, Re ) FVW/µ, and the cavitation number,

(a) Flow Visualization fuel injection pressure (MPa) test nozzles ambient temperature (K) ambient pressure (MPa)

0.13-0.45 nozzle R, nozzle L 293 0.1

(b) The Droplet Measuring System nozzle R (L/W ) 1.5)

nozzle L (L/W ) 3.0)

fuel

diesel

biodiesel

diesel

biodiesel

turbulent flow growth of cavitation hydraulic flip

0.16 MPa 0.30 MPa 0.42 MPa

0.16 MPa 0.30 MPa 0.42 MPa

0.20 MPa 0.35 MPa 0.42 MPa

0.20 MPa 0.35 MPa 0.45 MPa

K ) 2(Pb - Pv)/FV2 . In Re and K, F is the fuel density, V is the injected velocity of the fuel droplets, and Pb and Pv indicate the ambient pressure and vapor pressure, respectively. Additionally, W means the representative length of a nozzle orifice width. The injection flow rate, the injection pressure, and the fuel properties were analyzed and compared according to the nozzle length-to-width ratio. Moreover, the discharge coefficient (Cd) expresses all of the losses in the nozzle, and it should be considered because it is an important factor for analyzing the cavitating flow of the injector nozzle and for designing the nozzle. This factor is the ratio of the ideal flow rate to the actual flow rate. The ideal flow rate was derived by the continuous equation and Bernoulli’s equation, and the discharge coefficient can be expressed as the following equation. Cd )

Qact ) Qideal

Qact√1 - β2

F2 ∆P + 2g∆Z

A2

where the subscript 2 is for downstream and ∆Z is the position difference between upstream and downstream, β is the contraction ratio of the nozzle cross section (Anozzle) and the orifice cross section


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Figure 5. Comparison of the visualization images at various nozzle types.

(Aorifice), and ∆P is the difference of the ambient pressure and the vapor pressure.

3. Results and Discussion 3.1. Flow Characteristics of Diesel and Biodiesel Fuels. In the present study, the experiment was conducted to investigate the internal and external flow characteristics and to visualize the formation and development of cavitation of biodiesel and diesel fuel using nozzle R. Also, an experiment using nozzles

R and L was carried out to analyze the flow characteristics and the formation of cavitation of biodiesel on the effect of the nozzle L/W ratio. Figure 5 shows the cavitating flow patterns of diesel in nozzle R and biodiesel in nozzle L and nozzle R at various injection pressure and flow rates. The patterns can be divided into four regions: turbulent flow, beginning point of cavitation, growth of cavitation, and hydraulic flip.12 The beginning point of cavitation is the point where the cavitation bubble occurs


Figure 6. Comparison of the injection pressure and the flow rate in nozzle R and nozzle L.


Figure 8. Injection flow rate of diesel and biodiesel fuel (nozzle R).

Figure 7. Injection flow rate at various nozzle types and fuels.

initially, and the hydraulic flip is the phenomenon occurring when the fuel passes through the nozzle orifice without cavitation reattachment to the wall of the formed cavitation. The region before the beginning point of cavitation is called turbulent flow, and the interval from the beginning point of cavitation to hydraulic flip is called growth of cavitation. As shown in parts a and b of Figure 5, the cavitation was formed at an injection pressure of 0.20 MPa and an injection flow rate of 9.7 L/min in the case of diesel fuel, and it formed at an injection pressure of 0.20 MPa and an injection flow rate of 9.35 L/min in the case of biodiesel fuel. When the injection pressure increased, the cavitating flows of diesel and biodiesel fuels had a similar pattern. However, the injection flow rate of biodiesel fuel was a little lower than that of diesel fuel. Figure 5c illustrates the flow characteristics of biodiesel fuel at the nozzle with a L/W ratio of 3.0. The cavitation formed at an injection pressure of 0.25 MPa and an injection flow rate of 11.3 L/min, and the hydraulic flip began at an injection pressure of 0.45 MPa and an injection flow rate of 16.7 L/min. For nozzle L (L/W ) 3.0), the injection pressure and flow rate for the formation of cavitation increased in contrast to nozzle R (L/W ) 1.5). It can be conjectured that the friction loss between the wall and the fuel increased with the increase of pressure and flow rate. Figure 6 shows the comparison of the injection pressure and flow rate in two nozzles. On the basis of the results of Figure 5, Figure 7 shows the injection flow rate as the unit of the volume per minutes when

Figure 9. Cavitation number and Reynolds number along the injection pressure.

the injection pressure increases. In this figure, biodiesel fuel has a slightly lower injection flow rate than diesel fuel at all over the injection pressure. However, as the unit of the mass, biodiesel fuel has a little higher injection flow rate than diesel fuel because the liquid with a higher density has a lower volume at the same mass quantity, as shown in Figure 8. Figure 9a shows the relationship between the injection pressure and cavitation number (K) for biodiesel fuel at two nozzle types. Cavitation number (K) means the ratio of the


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Figure 11. Classification of cavitating flow patterns for the Weber number and Reynolds number.

Figure 10. Discharge coefficient for cavitation number.

dynamic pressure and the static pressure. Theoretically, a cavitation occurs when the cavitation number is below 1.0 in this investigation. As the cavitation number decreased, the cavitation intensity was stronger. Also, the cavitation number was in inverse proportion to the square of injection velocity. As shown in Figure 9a, the injection pressure for the formation of cavitation increases, the hydraulic flip was also occurred at the higher injection pressure as the change from nozzle R to nozzle L. In these results, the condition of the cavitation inception was much affected by the nozzle length-to-width ratio. Figure 9b shows the change of the Reynolds number according to the injection pressure. Biodiesel with a high viscosity has a lower range of values and a lower value than that of diesel fuel as the Reynolds number is the ratio between the inertia force and the viscous force. The Reynolds number was used as the measure of the turbulent flow; accordingly, it is conjectured that the flow irregularity of biodiesel is lower than that of diesel fuel due to the high viscosity. Figure 10 shows the change of the discharge coefficient when the cavitation number increases. As shown in Figure 10a, the discharge coefficient of both fuels increases a little after the occurrence of the cavitation. However, it immediately decreases after the transition to the hydraulic flip for both fuels. The discharge coefficient of diesel fuel is a little higher than that of biodiesel fuel. It was also affected by the density and viscosity of fuels. Figure 10b shows a comparison of the discharge

Figure 12. Mean droplet size distribution along the axial distance.

coefficient between two types of nozzles using biodiesel fuel. The Cd value of nozzle L is lower than that of nozzle R in most of the test range due to the long flow friction region. However, after the formation of cavitation, the Cd value of nozzle L is higher than that of nozzle R. It is explained that the ruptured energy of cavitation and the momentum were much stored through the long flow region compared with nozzle R. Figure 11 shows the classification of cavitating flow patterns for the Weber number between diesel and biodiesel fuels in nozzle R. Classification of the cavitating flow pattern was



Figure 13. Mean droplet size distribution along the radial of diesel and biodiesel fuel.

divided by the Weber number, such as turbulent flow (We < 34 000), growth of cavitation (34 000 < We < 75000), and hydraulic flip (We > 75 000). As shown in Figure 11, the Weber number of biodiesel fuel is larger than that of diesel fuel by about 2.6 times at the same Reynolds number. This is the reason why the ratios between the surface tension and the density of diesel and biodiesel fuels are almost the same [(F/σ)diesel ) 1.03(F/σ)biodiesel]; the velocity of biodiesel fuel is faster than that of diesel fuel by about 1.6 times. On the other hand, the Reynolds number of diesel fuel is larger than that of biodiesel fuel by about 1.6 times at the same Weber number because the ratio between the viscosity coefficient and the density of diesel is larger than that of biodiesel. 3.2. Atomization Characteristics of Diesel and Biodiesel Fuel. Fuel atomization as the concept of the extension of the surface area is an important factor in the design of diesel engines in terms of thermal efficiency and emission performance. In the present work, the experiment using the droplet measuring system was carried out to investigate the atomization characteristics of biodiesel fuel in nozzle R and L, such as the mean droplet size distribution, the axial mean velocity, and the radial mean velocity. In order to enhance the accuracy of the investigation, the experiment was conducted under a data rate over 150 Hz in all of the experimental conditions. Also, the data rate gradually increases along the axial distance. In the near region of the nozzle exit, the data rate is so low because the liquid jet was dense. On the other hand, the valid percent is more than 99.0% in all of the measuring

Figure 14. Comparison of the nozzle exit velocity for the injection pressure.

points. On the basis of these results, the results of the droplet measuring system can be trusted. Values of the data rate and valid percent were averaged at the same axial distance. Figure 12 shows the mean droplet size distribution when the axial distance increases from 40 to 150 mm. As shown in Figure 12a, the mean droplet size of biodiesel is larger than that of diesel fuel in the turbulent flow. In the region of growth of cavitation, the droplet size became small compared with the region of turbulent flow in both fuels. This is explained by homogeneous nucleation, one of the cavitation formation


Park et al

Figure 15. Axial velocity along the axial distance of fuels (nozzle R).

Figure 16. Droplet distribution along the axial velocity (LZ ) 100 mm, LR ) 0 mm).

theories.18

The cavitation grew from the energy difference between the stored energy at the surface of the bubbles by the surface tension and the work energy by the growth of bubbles. In the growth process, the maximum growth of cavitation was ruptured and its energy was diffused.18 It can be concluded that cavitation plays a key role in the liquid atomization because its breakup energy of cavitation affects the fuel atomization. Figure (18) Brennen, C. E. Published by Oxford university press, 1995, ISBN 0-19-509409-3.

Figure 17. Comparison of the radial and axial mean velocity distributions of fuel and nozzle type according to the radial distance.

12b represents the effect of the length-to-width ratio of the nozzle on the atomization characteristics of biodiesel fuel. After the occurrence of cavitation, the SMD of both nozzles became small. As the L/W ratio increases, the SMD became small after the formation of cavitation, because the ruptured energy considered the homogeneous nucleation theory was much stored on the surface of the cavitation bubbles in nozzle L. Figure 13 shows the size distribution of droplet mean diameter along the radial distance of diesel and biodiesel fuel. As shown in Figure 13, the droplet mean diameter at the region of turbulent flow is larger than that of the region of growth of cavitation in


both fuels. From these results, it was concluded that the occurrence and collapse of cavitation affected the fuel atomization, like the preceding conclusions about the axial direction. In addition, the SMD and AMD increase with increasing radial distance. It was concluded that the larger droplets move to the outer side of the spray due to its momentum. When the injection pressure increased, the nozzle exit velocity increased, as shown in Figure 14. In this figure, the theoretical velocity was obtained by the Bernoulli equation. The experimental value was calculated from the flow rate and the crosssectional area of the nozzle orifice at each injection pressure. The mean droplet velocity from the PDPA system was calculated by the means of four points at 40 mm from the nozzle exit. In the case of diesel fuel, the difference between the theoretical and experimental value was 15.01% of the minimum value at 0.22 MPa in the beginning stage of cavitation and 27.5% of the maximum value of 0.43 MPa in the hydraulic flip region. In the case of biodiesel fuel, the difference between the theoretical and measured value was 18.7% of the minimum value at 0.20 MPa and 20.88% of the maximum value at 0.43 MPa. From these results, it can be said that internal and external flows were affected by the cavitation. The flow velocity of biodiesel fuel is lower than that of diesel fuel due to the larger flow resistance from higher fuel viscosity, density, and the friction between the orifice wall and the fluid. The flow velocity measured by the droplet measuring system is in agreement with the experimental value obtained from the flow meter except the point of hydraulic flip. Parts b and c of Figure 14 show the effects of different L/W ratios on the change of the axial velocity at various injection pressures. After 0.25 MPa of injection pressure to start the cavitation, the flow rate of nozzle L was larger than that of nozzle R at the same injection pressure due to the orifice length. This is why the energy for the fluid flow surpassed the consuming energy for overcoming the friction of the orifice wall and the fluid flow after an injection pressure of 0.25 MPa. The measurement error in Figure 14 was calculated as the difference of the theoretical and experimental values. This error is the energy loss by the friction of the orifice wall as well as the internal nozzle generated turbulence, cavitation effects of fluid density, and exit velocity profile. Figure 15 shows the distribution of axial mean velocity for nozzle R at the regions of turbulent flow and growth of cavitation. As shown in Figure 15, the axial mean velocity increases after the formation of cavitation. This is confirmed in Figure 16. Figure 16 represents the number of droplets related to the axial mean velocity. From turbulent flow to hydraulic flip, the axial velocity of the peak droplets number increases to the high velocity. Figure 17 shows a comparison of the radial and axial mean velocity distributions of fuel and nozzle type according to the radial distance. As shown in Figure 17, the formation of cavitation affects the increase of the axial and radial mean velocities of diesel and biodiesel fuels. Figure 17a illustrates a


comparison of the radial mean velocity of diesel and biodiesel fuels. Both fuels have nearly the same value and the increase pattern after the occurrence of cavitation. In Figure 17a and b, the velocity increases with increasing radial distance, due to the increase of the droplet momentum by the increase of the droplet size, as shown in Figure 12. On the other hand, the axial velocity decreases when the radial distance increases, as illustrated in Figure 17c. 4. Conclusions This work was carried out to examine the flow and fuel atomization characteristics of biodiesel and diesel fuel through an investigation on the effect of the length-to-width ratio on the formation of cavitation and the effect of cavitation on the external flow pattern. The conclusions of this study are summarized as follows: The cavitating flow rate of biodiesel fuel was slightly lower than that of diesel fuel, while the cavitating flow patterns of biodiesel and diesel fuel which the cavitation formed along the orifice wall were similar. When the length-to-width ratio of the nozzle increased from 1.5 to 3.0, a higher pressure by about 25% was needed to obtain the cavitation. The consuming energy of the higher L/W ratio nozzle increases for the occurrence of the cavitation in the nozzle. The discharge coefficient increases a little after the occurrence of the cavitation. However, it immediately decreased after the transition to the hydraulic flip. In the region of cavitation growth, the droplet size of biodiesel and diesel fuels became small compared with the region of turbulent flow in biodiesel and diesel fuel along the axial and radial directions. The axial mean velocity increases after the formation of cavitation. It is proved that the axial velocity at the peak droplets number increases to the high velocity from turbulent flow to hydraulic flip. On the basis of the SMD and velocity measurements, the cavitation in the nozzle orifice promoted the atomization of fuels at the nozzle exit. It can be concluded that the energy generated during the formation, growth, and rupture of cavitation enhances the energy for the atomization of fuels. Acknowledgment. This work was supported in part by the CEFV (Center for Environmentally Friendly Vehicle) of the Eco-STAR project of the MOE (Ministry of the Environment in Seoul, 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 Laboratory of Excellence. In addition, this study was supported by the Second Brain Korea 21 Project in 2006. EF7003305

Effect of Cavitating Flow on the Flow and Fuel Atomization

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