Paper ID ICLASS APPLICATION OF NEUTRON RADIOGRAPHY FOR VISUALIZATION OF CAVITATION PHENOMENA IN A FUEL INJECTION NOZZLE OF A DIESEL ENGINE

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1 ICLASS-2006 Aug.27-Sept.1, 2006, Kyoto, Japan Paper ID ICLASS APPLICATION OF NEUTRON RADIOGRAPHY FOR VISUALIZATION OF CAVITATION PHENOMENA IN A FUEL INJECTION NOZZLE OF A DIESEL ENGINE N.Takenaka 1, K.Kawabata 1, Y.Kawabata 2, I.C.Lim 3 and C.M.Sim 3 1 Department of Mechanical Engineering, Kobe University, takenaka@mech.kobe-u.ac.jp 2 Institute of Research Reactor, Kyoto University 3 Korean Atomic Energy Research Institute ABSTRACT Visualization of cavitation phenomena in a Diesel engine fuel injection nozzle was carried out by using neutron radiography system at KUR in Research Reactor Institute in Kyoto University and at HANARO in Korea Atomic Energy Research Institute. Light oil behavior in a metallic nozzle was visualized. A pulsed neutron beam was irradiated to the fuel injection nozzle by using a neutron chopper synchronized to the fuel pump rotation. A multi-exposure method was applied to obtain a clear image as an ensemble average of the synchronized images. Some constitutive images in the metallic real nozzle during the fuel spray were successfully obtained. The visualized images suggested that the cavitation occurred in a nozzle hole of a Diesel engine fuel injection nozzle. Keywords: Diesel engine, Fuel injection nozzle, Cavitation, Neutron radiography, Visualization 1. INTRODUCTION It is supposed that cavitation occurs inside a fuel injection nozzle of a Diesel engine. The cavitation affects much on the fuel injection and the performance of the engine. Visualization of cavitation phenomena in a simulated nozzle made of optically transparent material has been reported. [1,2]. However, nucleation process such as cavitation depends much on the liquid and the wall conditions in the nozzle. Therefore, visualization of the cavitation phenomena of a real fuel inside a real metallic nozzle of a Diesel engine has been required. Neutron radiography is suitable for visualizing the fuel behaviors inside the metallic nozzle. Visualization of cavitation phenomena in a Diesel engine fuel injection nozzle was carried out by using neutron radiography system at KUR in Research Reactor Institute in Kyoto University and at HANARO in Korea Atomic Energy Research Institute. A pulsed neutron beam was irradiated to the fuel injection nozzle by using a neutron chopper synchronized to the fuel pump rotation. The neutron flux was not enough for obtaining a clear image by one neutron pulse. Therefore, a multi-exposure method was applied to obtain a clear image by integrating the images by more than 1000 frames as an ensemble average of the synchronized images. Popular image intensifiers using Multi-Channel Plate (MCP) can intensify the neutron radiography images more than 1000 times but was not used in the present visualization because the spatial resolution with the intensifier was not enough for the visualization of the cavitation in the nozzle hole. Clear images without the ensemble averaging will be obtained if an intense pulsed neutron source by an accelerator system constructing in J-PARC project [3] is used or high spatial resolution image intensifier is developed. Some consecutive images in an interval of 3.6 degree in the cam angle of the fuel injection pump during the fuel spray were successfully obtained for the fuel pump revolutions from 360 to 600 rpm. 2. NEUTRON RADIOGRAPHY Radiography is a visualization technique to obtain a shadow image by the difference of the attenuation rate of radioactive rays for materials in the object. Roentgen is a typical X-rays radiography. The attenuation characteristics of X-rays and neutron rays in elements are shown in Fig.1. The mass attenuation coefficients of X-rays are shown by the solid line and those of neutron rays are plotted by the symbols against the atomic numbers. The mass attenuation coefficients of X-ray increase with increasing the atomic number numbers. X-ray radiography is suitable to visualize high atomic number elements in small atomic number elements. However, those of neutron rays depend much on the elements. They are high for the light elements like Hydrogen, Lithium and Boron and some special element like Cadmium, Gadolinium etc. and are low for most of the metals. Therefore, neutron radiography is suitable for the visualization of the light or the special elements in metals. It can be seen that neutron radiography is applicable to the visualization of water or oil behavior in metallic machines, i.e., neutron radiography is a Roentgen for the machines while X-rays radiography is a Roentgen of human bodies. Therefore, neutron radiography was applied to the visualization of the fuel behavior in a metallic nozzle in the present study. 3. EXPERIMENTAL APPARATUS AND PROCEDURE Neutron flux at the KUR and the HANARO systems is in the order of 10 7 and 10 8 neutrons/cm 2 s. Neutrons more than 10 8 neutrons/cm 2 are required to obtain a clear image, i.e., more than 1 or 10 seconds exposure time is required for the visualization by the present system without the image intensifier. Some methods were examined to obtain the images with the exposure time for the cavitation visualization from 0.2 ms to 2 ms by the practical neutron radiography system. The fuel injections are repeated periodically. Therefore,

2 many images in the same injection condition can be obtained if the exposure is synchronized to the engine rotation. A method is integration of the many images with a short time exposure synchronized by a CCD camera to the engine rotation. However, the read out noises are also integrated if many images are integrated. Therefore, a multiplex exposure method with opening the electrical shutter of a CCD camera by using a neutron chopper was employed in the present study. It is known that the cavitation is chaotic phenomena depending much on the small difference of the initial conditions and the obtained image is an ensemble-averaged image. In future, a method will be possible by synchronizing the fuel pump rotation to a high intensity pulsed neutron source generated by an accelerator [3]. Fig.2 shows the experimental apparatus. A pulsed neutron beam was obtained by the neutron chopper and was irradiated to the nozzle. The neutron image was converted to the optical image by the scintillation converter and was accumulated by the CCD camera. The slit angle was 3.6 degrees for taking several image frames for the fuel injection. Since the cam and the chopper were rotated by the same shaft, they were well synchronized. The exposure time was determined by the slit angle. The timing of the fuel injection visualization was determined by the delay angle between the chopper and the cam. The short exposure time images in one rotation were accumulated in the CCD camera for many rotation numbers to obtain the clear images. Mass Attenuation coefficient : µ m cm 2 /g H B X-rays (100 kev) Cd Sm Gd Eu Li H 2 O Pu Ir Ac Rh In Sc Tm Be N Er Hf Hg Co Xe Dy Pa C O Na Ni Kr Nd Mn Se Ag Lu Re Au P Ti Zr Pm F Ne Mg Cl VCr Fe Cu Br Cs SrMoPd La Tb He Ra K Ga As Sb Ho Yb Os Pr Ta W Al Ca Zn Pb Si S Ar GeRb Y Nb Ru Te Pt Tl Th Ba U Sn Ce Bi Atomic Number I Fig. 1 Mass attenuation coefficients of X-rays and neutron rays CCD Camera Camera Box Coupling Pressure Sensor Pump Cam Nozzle Motor CVT Gear(100 teeth) Converter Mirror Chopper Slit(3.6degree) Neutron Fig. 2 Schematic illustration of experimental apparatus

3 The fuel supply was controlled by a governor and the fuel supply rate was measured by a sampling as shown in Fig.3. Light oil at room temperature was supplied at the maximum pressure around 20 MPa. The pressure fluctuations were measured during the visualization. The rotation of the motor was changed from 360 rpm to 600 rpm. Twelve images for the exposure time of 20 seconds, i.e., 240 seconds in total, were obtained for one experimental condition. Each obtained image was filtered by a mathematical Morphology filter to reduce the star like noises due to direct irradiation of radioactive rays to the CCD elements. The twelve filtered images were added and the image ensemble-averaged from 1440 to 2000 times was obtained. Fig.4 shows the tested one-hole nozzle made by iron, 8 mm in the outer diameter and 6 mm in the needle diameter. The diameter of the nozzle hole was 0.38 mm and the lift was 0.2 mm. Preliminary tests were made at KUR and the main experiments were conducted at HANARO. Light oil commercially sold in Korea was used for the experiments. close min] Fuel supply rate [cc/ open minimum maximum rotation [rpm] Fig. 3 Fuel supply rate Body Needle Nozzle chamber Seat Sac chamber Nozzle hole Fuel spray Fig. 4 Tested one-hole nozzle 4. VISUALIZED RESULTS AND DISCUSSION The optical spray images were obtained using the same experimental apparatus as shown in Fig.2 using optical rays instead of neutron rays and the converter. The synchronize timing was change by changing the position the gears to obtain each image. The phase 0 was determined when the needle lift was the highest. Six images from phase -3 to phase +2 are shown in Fig.5 for 360 rpm and the minimum fuel supply condition. One phase was equivalent to 3.6 degree. It can be seen that the fuel was injected around phase 0 from phase -2 to phase +2, i.e., for about 15 degree of the cam angle. Neutron radiography images were obtained in the same conditions as shown in Fig.5. Fig.6 shows an example of the neutron radiography images. The optical image of the spray, neutron radiography image and the image enlarged near the nozzle hole and pseudo-colored are shown. One image element was about 0.3 mm. The spatial resolution of neutron radiography is limited by the converter resolution of around 0.1 mm. It means that the image has blur of about 0.1 mm, i.e., a few image elements, in this image. It is difficult to visualize the details of the fuel behavior in the nozzle hole of 0.38 mm in diameter but it may be seen that the hole is filled by the fuel or not. An image processing method was applied to obtain the fuel image in the nozzle hole by vanishing the nozzle body in the image as shown in Fig.7. The image with the fuel was divided by the image without the fuel to vanish the nozzle body. The image processing for the nozzle filled with the fuel was shown in Fig.7 as an example. It can be seen that the fuel filled in the nozzle hole was extracted with some blur.

4 phase-3 phase-2 phase-1 phase 0 phase+1 phase+2 Fig. 5 Optical images y of the spray, 360 rpm, fuel supply minimum 1mm 0.38mm Optical spray image Neutron radiography image Expanded and pseudo-colored image near nozzle hole Fig. 6 Neutron radiography image

5 Nozzle with fuel Nozzle without fuel division = Enlarged and pseudo-colored Fuel image without nozzle body Fig. 7 Image processing method to obtain the fuel image Consecutive images near the nozzle hole by neutron radiography are shown in Fig.8 in the same condition of the optical visualization of the fuel spray in Fig.5 for the rotation of 360 rpm with the minimum fuel supply. The image-processed results of Fig.8 in the same image processing method in Fig.7 are also shown in Fig.9. It can be seen that the fuel is scarcely found in the nozzle hole though the fuel spray is visualized out of the nozzle hole in the optical observation in Fig.5. The fuel can be observed near the inlet of the nozzle hole from phase -1 to phase +1 in Fig.9 but no clear fuel can not be found after the inlet. The fraction of the fuel in the nozzle hole was very small comparing the image filled by the fuel shown in Fig.9. Similar results were obtained for the rotation of 480 and 600 rpm. It can be estimated that the cavitation occurred near the inlet of the nozzle hole and the fraction of the liquid fuel in the nozzle hole was very small due to the rapid increase of the gas phase. It is also supposed that the two-phase flow pattern in the nozzle hole was a mist flow since no liquid core was found in the nozzle hole. These images were ensemble-averaged images over 1000 times of the fuel injections as described above. Various fuel behavior of each injection may be integrated in the images since the cavitation phenomena were chaotic. However, the results indicated that the liquid fuel fraction in the nozzle hole was very small and the cavitation occurred in the nozzle hole for most of the injections. 5. CONCLUSIONS The visualization of the cavitation phenomena in a nozzle of a Diesel engine was performed by neutron radiography. 1. A multiplex exposure method by using a neutron chopper with opening an electrical shutter of a CCD camera was proposed for visualization of cavitation in a fuel nozzle of a Diesel Engine. 2. The cavitation phenomena in a real metallic fuel nozzle were successfully visualized. 3. It is supposed that the flow pattern in the nozzle hole is a mist flow. REFERENCES 1. Baz,I., Lance, M., Champoussin,J., Marie,J., "Investigation of two-phase flows generated by cavitation inside high-pressure injection nozzles", 5th Int. Conf. Multiphase Flow, ICMF'04, Yokohama, Japan, May 30-June 4, 2004, Paper No Sou,A., Tomiyama,A., Hosokawa,S., Nigorikawa,S., Matsumoto,Y., "Visualization of cavitation in a two-dimensional nozzle and liquid jet", 5th Int. Conf. Multiphase Flow, ICMF'04, Yokohama, Japan, May 30-June 4, 2004, Paper No J-PARC, Japan Proton Accelerator Complex project,

6 Rotation 360 rpm Fuel supply Minimum 1.67msec phase mse c phase -2 phase -1 phase 0 phase +1 phase +2 Fig. 8 Consecutive neutron radiography images Divided images Rotation 360 rpm Fuel supply Minimum Filled phase -3 phase -2 phase -1 phase 0 phase +1 phase +2 Fig. 9 Consecutive image-precessed neutron radiography images