DIESEL SPRAY DEVELOPMENT FROM VCO NOZZLES WITH COMMON-RAIL

Transcription

1 DIESEL SRAY DEVELOMENT FROM VCO NOZZLES WITH COMMON-RAIL CHOONGSIK BAE, JINSUK KANG AND HANG-KYUNG LEE Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology 373-1, Kusong-dong, Yusong-gu, Taejon , Korea ABSTRACT Spray characteristics of diesel fuel injection system is one of the most important factors in diesel combustion and pollutant emissions especially in HSDI (High Speed Direct Injection) diesel engines where the interval between the onset of combustion and the evaporation of atomized fuel is relatively short. An investigation into various spray characteristics from different holes of VCO(Valve Covered Orifice) nozzles was performed. The global characteristics of spray, including spray angle, spray tip penetration, and spray pattern were measured from the spray images, which were frozen by an instantaneous photography with a spark light source and ICCD. These spray images were acquired sequentially from the first injection to fifth injection to investigate injection-to-injection variation. For better understanding of spray development and their internal structures, a long-distance microscope was used to get magnified spray images at the vicinity of the nozzle hole with a laser sheet illumination. Also backward illuminated images with a spark light source were taken at various points of the spray field including vicinity of the nozzle hole to understand surface structures and breakup process of dense spray from VCO nozzle incorporated with common- injection system. As injection pressure increases interaction between spray and ambient air becomes important to liquid penetration and spray angle. Macroscopic spray angle increases due to air entrainment as injection pressure increases though spray angle near the hole seems independent from injection pressure. Liquid penetration is initially affected by injection rate increase as needle is moving upward and liquid penetration increase rate is in accordance with injection pressure. After this stage, air entrainment and high potential of evaporation makes the increase rate slower and this tendency is more obvious for higher injection pressure. Microscopic images taken at the vicinity of the nozzle hole exit reveal that central dense region consists of thick ligaments or membranes and most of the liquid droplets are formed at the tip of ligaments from spray surface due to the waves developed on it. Some smaller liquid droplets seem to be generated from the bubble or membrane breakup process. Droplet sizing was performed from the microscopic images, which were frozen by spark light source that has light duration of ns and high-resolution CCD camera equipped with long distance microscope whose magnification factor is more than six. Fuel particle sizes, described as SMD (Sauter Mean Diameter) in many points, decreased during injection durations and higher injection pressure induced smaller value. INTRODUCTION Direct injection (DI) diesel engine is one of the most promising economic engines being developed. However, combustion characteristics of DI diesel engine should be controlled carefully to meet the emission regulations, which are becoming more and more strict. The spray characteristics of DI diesel engine is crucial for combustion so that some advanced fuel injection systems, which have higher injection pressure and more controllability, have been developed recently. Common injection system is a newly developed fuel injection system of high injection pressure that is independent of engine speed and load. Moreover, injection timing and duration can be controlled regardless of engine operating conditions. Diesel injection nozzle can be classified into two groups: sac-type nozzle and VCO nozzle, according to the presence of sac volume in the nozzle. Sac type nozzle has been used widely and it gives relatively uniform flow field across the nozzle holes, however, the remaining fuel in the sac after injection period is a source of hydrocarbon emissions. VCO (Valve Covered Orifice) nozzle has no sac volume in principle.

2 However, at low needle lift condition it is known to produce unsymmetric pressure field inside the nozzle and nozzle holes. There have been various efforts to understand the nature of diesel spray, and some remarkable researches are available on literatures. [1-3] Nevertheless, the details of transient diesel spray formation are not certain yet. This uncertainty mainly arises from the difficulty of optical access to the dense region of the spray near the hole and the lack of information about internal flow conditions. Lately, internal flow of real size sac-type nozzle was visualized followed by a series of researches with large-scale transparent VCO and sac nozzles. It is expected that in the future two-phase CFD models will become available which will incorporate sub-models for homogeneous and heterogeneous nucleation as well as for bubble dynamics, thus allowing the calculation of the two-phase flow at the nozzle exit and the subsequent spray development in diesel engine cylinders under the whole range of engine operating conditions. The temporally and spatially resolved characterization of spray, focusing on the structure in the vicinity of the nozzle exit is becoming more important. It can offer not only validation data for two-phase internal flow model of the nozzle, but also clues to understand the transient diesel spray formation mechanism, of which nature have been assumed from the indirect methods or extrapolative experimental results at dilute regions of the spray. In this study, non-evaporative transient diesel spray including dense region near the nozzle exit was visualized to understand atomization process and its effect on the diesel spray characteristics from VCO nozzle of common- injection system. 80mm diameter. Two visualization techniques were used for macroscopic observation. Spray images frozen by spark light source, which had light duration of shorter than 0ns were acquired with CCD camera and taken by a frame grabber. Shadowgraph technique was also applied to all sprays from the nozzle holes with Ar-ion laser and ICCD camera whose gating time was 70ns. Figure 1 shows schematic layout of experimental setup for macroscopic and microscopic visualizations. Microscopic visualization was performed in two ways. Thin laser sheet was incident through one spray and scattered light was acquired with ICCD camera, which has gating time of 70ns. Long distance microscope was used to get high magnification more than six times. Back illumination technique was utilized to investigate initially developed spray surface profile. Highresolution CCD camera equipped with long distance microscope was also used to freeze the image within ns. The VCO nozzles investigated in this study have five holes of an identical inclination angle and spray cone angle of 152. Each nozzle has double-guided needle. The sac nozzle compared with this nozzle has five holes of which diameter is 0.146mm and has spray cone angle of 152. Experimental conditions are summarized in Table 1. NOMENCLATURE C,C1,C2 : constant d 0 : nozzle hole diameter S : spray tip penetration SOI: start of injection t : time after start of injection : pressure drop across the nozzle hole : common- pressure ρ g : ambient gas density ρ 0 : reference gas density; ρ 0 =1kg/m 3 EXERIMENTAL SETU Spray characteristics from VCO nozzles equipped with common- system were investigated under atmospheric and pressurized conditions in a spray chamber through various optical observations. The BOSCH common- system is composed of C3 pump, which can raise the fuel pressure up to 1350bar and common-, which has internal volume of 18cm 3 and supplies fuel to four injectors. High-pressure chamber was pressurized by filling nitrogen up to 30bar and depressurized down to 6mmHg with vacuum pump. The pressure chamber allows optical accesses through three circular windows of Fig. 1 Experimental setup for macroscopic and microscopic visualization Table 1 Summary of experimental conditions. Nozzle types VCO 0.176mm,5holeX152, double-guided needle VCO 0.144mm,5holeX152, double-guided needle Chamber pressure 6mmHg Ambient 30bar (N 2 ) Common- pressure ( ) 250bar 1200bar Sac 0.146mm,5holeX152 * Engine speed: 1200 rpm * Injector solenoid energizing duration: 1.2ms@250bar, 0.41ms@1200bar

3 RESULTS AND DISCUSSIONS It was found that, injection-to-injection variability of spray patterns was negligible in a macroscopic point of view except that the penetration of initially injected one or two sprays were shorter than the consecutive sprays. This was observed in both VCO and sac nozzles. Hole-to-hole variations were only observed in sac nozzle in terms of spray angle. The penetration of spray from each hole of sac nozzle was almost identical and slight difference might be induced from the different momentum exchange rate with ambient gas. When chamber was vacuumed and the effect of ambient gas was negligible, spray penetration of each hole was identical though spray angle variation still existed. Spray characteristics such as spray tip penetration, spray angle, and its shape from spray images were observed as a function of injection pressure and ambient gas density. Figure 2 represents averaged penetrations of sprays from VCO nozzle injected into atmospheric ambient condition with various common- pressures. It shows that the spray penetration can be divided into three stages according to its increasing rate. At the first stage, penetration length is in accordance with the common- pressure. Details of this stage are discussed with Fig. 3. At the second stage, penetration increase rate with higher common- pressure is lower. Some correlations for non-evaporating diesel spray are available in the literature. These correlations show that after complete atomization the spray tip penetration S is as follows [1-2]: S=C ( /ρ g ) 1/4 (d 0 t) 1/2 (1) It means that spray tip penetration should be proportional to the square root of time regardless of injection pressure. However, Fig. 2 shows that the time dependency of penetration increase, represented by an exponent in power law, is a function of common- pressure. As pressure increases, the exponent decreases and seems to converge to the number 0.5, predicted by equation (1), which was obtained by analogy with an equivalent gas jet, with the same momentum flow rate as that of the liquid flow from the nozzle.[3] It will be discussed later in this paper with Figs. 4 and 5 that higher injection pressure results in smaller liquid droplets and increase of air entrainment into the spray. Consequently, momentum exchange with ambient gas is more vigorous for the spray with higher injection pressure. In case of common- pressure 600bar and 00bar, the spray tip penetration seems to be saturated about 1ms after start of injection. This phenomenon can be explained by considering that the spray penetration is acquired from the macroscopic mie scattering images, in which the intensity of the scattered light is directly proportional to particle number density multiplied by square of liquid droplet diameter. However, the decrease rate of droplet size is inversely proportional to the droplet diameter and air entrainment diminishes the number density of liquid droplet. The maximum liquid penetration length is possibly affected by the atomization quality of the nozzle and thermodynamic conditions of the ambient air. Spray tip penetration(mm) 0 1 VCO nozzle (0.176mm x 5hole) Atmospheric ambient condition =250 bar =300 bar =600 bar =00 bar Time after start of injection(ms) Fig. 2 Spray penetration from VCO nozzle under atmospheric ambient condition Figure 3 represents the early spray penetrations of VCO nozzles. As mentioned above, this figure shows the result of the first stage of spray tip penetration. The penetrations seem to increase almost linearly according to the time as assumed by Arai et al. [2]. They assumed that jet velocity within the intact length is equal to the initial jet velocity, while Yule et al. [3] pointed out that in Arai s equation initial jet velocity was low and suggested to use hyperbolic tangent function to match gradual transition from the nozzle to the downstream spray. Microscopic visualization shows that in the first stage the spray tip consisted of liquid blobs connected by thick ligaments. From the start of injection, momentum is lost by breakup, so the initial jet velocity is naturally different form the injection velocity. The early penetration data of Fig. 3 have the tendency as follows: S=C1(2 /ρ l ) 1/2 (ρ 0 /ρ g ) C2 t (2) From the findings of this study, C1 has the value between 0.3 and 0.4. This range is about the half of typical discharge coefficient of diesel nozzle. Thus, spray tip penetration velocity at the first stage is about the half of injection velocity. The effect of ambient gas density on penetration is rather weak. C2 has the value between 0.1 and 0.3.

4 30 VCO nozzle(0.144mm x 5hole) Spray tip penetration (mm) vco, =250 bar, ρ gas =1.23 kg/m 3 vco, =1200 bar, ρ gas =1.23 kg/m 3 vco, =1200 bar, ρ gas =33.8 kg/m 3 Y Time after start of injection (µs) Fig. 3 Spray penetrations of VCO nozzle at early stage Spray angle ( o ) VCO nozzle(0.176mm x 5hole) Atmospheric ambient condition 1ms after SOI Common pressure (bar) Fig. 4 Effect of injection pressure on the spray angle from VCO nozzle Figure 4 represents averaged spray angles of VCO nozzle at 1ms after SOI as a function of common- pressure. It is derived from macroscopic images and it shows that higher injection pressure induce larger spray angle. This result is corresponding to the general trend. [4] Microscopic images taken at the nozzle exit show that the spray angle at this stage is almost independent from injection pressure, so that spray angle increase according to common- pressure is thought to be a result of ambient air entrainment into the spray. The effect of injection pressure on SMD is represented in Fig. 5. It shows that SMD value continuously decreases during injection duration and higher injection pressure produces smaller value. Figures 4 and 5 give a qualitative explanation about the maximum liquid penetration length difference in Fig. 2. SMD(µm) Injection duration at =1200bar Injection duration at =250bar Atmospheric ambient condition =250bar,Y=23.5mm,R=2.2mm =250bar,Y=23.5mm,Center =250bar,Y=37.6mm,R=3.5mm =250bar,Y=37.6mm,Center =1200bar,Y=23.5mm,R=2.2mm =1200bar,Y=23.5mm,Center =1200bar,Y=37.6mm,R=3.5mm =1200bar,Y=37.6mm,Center Time after SOI(µs) Fig. 5 Effects of injection pressure and time on SMD of the spray from VCO nozzle (0.144mm x 5hole) Figure 6 shows that VCO spray structures vary with the ambient gas density. When chamber was vacuumed, conical spray shape maintained during injection period and periphery of the spray was smooth, because aerodynamic interaction between spray and ambient gas was negligible. The shape of the spray tip at high ambient gas density is sharper and when ρ g was 33.8kg/m 3, a feather like structure with puffy edge appeared. Lai et al.[5] also observed puffy structure along the edge of the spray from VCO nozzle, injected into the chamber which was pressurized to 2.8Mpa with nitrogen. Injection pressure was 1350bar. Recently, Interesting result was presented by Di Stasio and Allocca [6] showing the effect of ambient gas. ressure chamber was filled with some different gases to maintain the same ambient gas density. Although, spray tip penetrations were almost the same at a certain time, spray shapes were different according to the choice of ambient gas. The authors discussed that it was induced from different acoustic velocities of ambient gases.

5 Fig.6 Mie scattering and shadowgraph images of sprays injected from VCO nozzle with injection pressure 1200bar at 0.25ms after SOI (from top : first row ρ gas =33.8kg/m 3 second row ρ gas =1.23kg/m 3 third row p gas =6mmHg) Fig.7 Mie scattering and shadowgraph images of sprays injected from sac nozzle with injection pressure 1200bar at 0.16ms after SOI (from top : first row ρ gas =33.8kg/m 3 second row ρ gas =1.23kg/m 3 third row p gas =6mmHg) Figure 7 represents the sac spray structures at different gas densities. Like VCO nozzle, the feather like structure appeared at high ambient gas density condition. It is noticeable that breakup takes place inside of the spray at vacuum condition. Because aerodynamic breakup is hard to occur, hydraulic breakup or flash atomization could be a mechanism of this phenomenon. However, considering above images acquired at ambient pressure condition (ρ g =1.23kg/m 3 ), velocity rearrangement or turbulence effect inside the spray is not appropriate. Though it is not certain yet, cavitation bubble expansion might be a mechanism of this spray breakup. Fath et al. [7] proposed that the cavitation bubbles produced inside the nozzle would enhance the disintegration of liquid jet from sac nozzle. Based on their microscopic visualization Lai et al.[5] discussed the comparison of the mini-sac nozzle spray with those of the VCO s showing that sac-type nozzle had less turbulent primary breakup and different cavitation breakup features, which was consistent with its internal flow characteristics. They also proposed that though some optical artifacts could not be completely ruled out, close examination of the fussy interface suggested formation of micro explosion or bursting bubbles. There are series of researches about nozzle internal flow and the effects of cavitation on atomization, which could give some tips on the atomization mechanisms. [7-9] Figure 8 shows microscopic images of cross section of the 5-O clock VCO spray shown in Fig. 6 when injection pressure was 250bar and injection duration was 1.2ms. Figure 9 shows microscopic images of the same spray in Fig. 8, which was acquired with back illumination to investigate surface profile. At 3µs from SOI, transparent bubble appears. This bubble seems to be the membrane that has been attached inside the hole with surface tension. As the liquid column exits from the hole, this membrane deformed to a bubble that is followed by arbitrary shapes at the tip of liquid column. After 30µs from SOI, asymmetric waves developed on the liquid column and about 0.1ms from SOI, a typical spray pattern of atomization regime appears. Because of large relative velocity, unstable ligaments are forming around the spray surface. In the previous study [], it was observed that droplets are forming from the ligaments. Figure is microscopic images of cross section of the 5- O clock VCO spray depicted in Fig. 6 when injection pressure was 1200bar and injection duration was 0.4ms. Compare to Fig. 8, the more homogeneous scattered light intensity is observed inside and at the edge of the spray. This might be an evidence of better atomization and relatively small eddy structure. Figure 11 shows microscopic images of the same spray in Fig., which was acquired with back illumination to investigate surface profile. Unlike the spray of injection pressure 250bar, typical pattern of atomization regime appears without transition period. It was observed that about 20µs from SOI, droplets are forming from ligaments.

6 Fig.8 Microscopic images of early spray development of VCO nozzle, injection pressure 250bar, ρ gas =1.23kg/m 3 (from left above : 3, 5,, 15, 20, 25, 30, 40, 50, 0, 150µs after SOI) Fig. Microscopic images of early spray development of VCO nozzle, injection pressure 1200bar, ρ gas =1.23kg/m 3 (from left : 4, 6, 14, 19, 24µs after SOI) Fig.9 Microscopic images of early spray development of VCO nozzle, injection pressure 250bar, ρ gas =1.23kg/m 3 (from above : 3, 50, 150µs after SOI) Fig.11 Microscopic images of early spray development of VCO nozzle, injection pressure 1200bar, ρ gas =1.23kg/m 3 (from above : 1,, 20µs after SOI)

7 CONCLUSIONS Spray characteristics of VCO nozzle for common injection system were investigated through macroscopic and microscopic images with various optical techniques. The findings from this study are summarized as follows. 1. Injection-to-injection variability of spray was observed for first injected one or two sprays. 2. Hole-to-hole variation of spray from VCO nozzle with double-guided needle was negligible. However, hole-tohole variation was observed in sac nozzle in terms of spray angle. 3. Spray angle measured from the macroscopic images increase with injection pressure because of air entrainment. While, the spray angle measured from the microscopic images near the nozzle is independent of injection pressure. 4. Spray penetration can be divided into three stages according to their increasing rate. At the first stage, penetration increase rate is proportional to the common- pressure. At the second stage, penetration increase rate is decreasing with common- pressure. At the third stage, spray penetration is saturated to a certain value. 5. Spray shape is closely related to ambient gas density. When ambient gas density was high, a feather like spray structure appeared. 6. For spray of injection pressure 250bar, it takes about 0.1ms from SOI to transfer into atomization regime, however for spray of injection pressure 1200bar, typical pattern of atomization regime appears without transition period. 7. Higher injection pressure produce smaller SMD value and homogeneous internal structure of spray. 8. Droplets are formed at the end of ligaments around the spray surface. 5. Lai, M. C., Wang, T. C. T., Xie, X., Han, J. S., Henein, N., Schwarz, E., Bryzik, W., Microscopic Characterization of Diesel Sprays at VCO Nozzle Exit. SAE (1998). 6. Di Stasio, S., Allocca, L., Influence of the Gas Ambient Nature on Diesel Spray roperties as High Injection ressure: Experimental Results. Conference on Thermofluidynamic process in Diesel Engine, Valencia, September 2000, pp Fath, A., Münch, K. U., Leipertz, A., Spray Break-up process of Diesel Fuel Investigated Close to the Nozzle. ICLASS 97 (1997). 8. Soteriou, C., Andrews, R., Smith, M., Direct Injection Diesel Sprays and the Effect of Cavitation and Hydraulic Flip on Atomization. SAE (1995). 9. Arcoumanis, C., Flora, H., Gavaises, M., Kampanis, N., Horrocks, R., Investigation of Cavitation in a Vertical Multi-Hole Injector. SAE (1999). Bae, C. S., KANG, J. S., Diesel Spray Development of VCO Nozzles for High ressure Direct-Injection. SAE (2000). ACKNOWLEDGMENTS The authors would like to acknowledge the support of National Research Laboratory scheme, Korean government REFERENCES 1. Dent, J.C., A Basis for the Comparison of Various Experimental Methods for Studying Spray enetration. SAE7571 (1971). 2. Arai, M., Tabata, M., Hiroyasu, H., and Shimizu, M., Disintegrating rocess and Spray Characterization of Fuel Jet Injected by a Diesel Nozzle. SAE (1984). 3. Yule, A. J., Filipovic, I., On the Break-up Time and Lengths of Diesel Sprays. Int. J. Heat and Fluid Flow, Vol. 13, No. 2 (1992) 4. Bayvel, L., Orzechowski, Z., Liquid Atomization. Taylor and Francis, 1993, p 309.