14th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2008 - 1 - Microscopic Visualization of Liquid Column Break-up Process in Gasoline PFI Injector Nobuyuki KAWAHARA *1 , Eiji TOMITA *1 , Mamoru SUMIDA *2 1: Department of Mechanical Engineering, Okayama University, Okayama, JAPAN, kawahara@mech.okayama-u.ac.jp 2: Mitsubishi Electric Corp., Himeji, JAPAN Abstract The purpose of this study is to observe the fuel break-up process very close to an injection hole of a practical PFI injector and to examine the effect of internal structure on characteristics of spray behavior. In order to investigate the atomization process at the nozzle exit, visualization of spray structure was performed using an ultra high-speed camera (max. camera speed 1Mfps) with a long-distance microscope. The visualized experiments were carried out using PFI injector in a closed chamber at the atmospheric pressure. Three main conclusions were obtained from this study. It has been shown that the liquid fuel column was injected. During the injection period, the spray indicates the quasi-steady state mode. Surface waves of liquid column can be recognized. Liquid column very close to the nozzle exit were broken up to liquid ligament structure. These surface waves cause the break-up of liquid column to liquid ligaments. Break-up process of liquid ligaments to droplets can be investigated using high-speed magnified images. Moreover, interactions between droplets, such as penetration or coalescence, can be visualized. 1. Introduction It is necessary for internal combustion engines to improve thermal efficiency and to reduce exhaust emissions because of saving energy resource, problems of exhausting nitrogen oxide, unburned hydrocarbons, carbon monoxide, particulate matters, and carbon dioxide. The fuel Injection systems for usual spark-ignition engines inject the fuel into the intake port. The advantages of gasoline Port Fuel Injection (PFI) system are increased power and torque, more uniform fuel injection, more rapid engine response, more precise control of the air-fuel ratio during cold-start and engine warm-up. Current subject of PFI injector is decrease of unburned hydrocarbon during cold-start. To solve this subject, atomization of PFI injector should be improved (Heywood 1988, Zimmerman et al. 1999, Liu et al. 2006). In order to understand the spray characteristics formed by multi-hole injector, investigations of gasoline injection sprays using several measurement techniques like laser sheet method with laser- induced (exciplex) fluorescence (LIF), phase Doppler anemometor (PDA) (Ahao et al. 1995), particle image velocimetry (PIV) have been carried out for better control of spray and combustion characteristics. TAB (Taylor Analogy Break-up) model (O’Rourke and Amsden 1987) and DDM (Discrete Droplet model) are widely used for numerical simulation (Dukowicz 1980, Glodowski et al. 1996). These results provided the detailed information about spray tip penetration, spray cone angle, distribution of the liquid/ vapor phase, or droplet velocity and diameter due to secondary break-up process. However, one of the key processes affecting spray behavior is the primary spray break-up, because it defines the starting conditions for the spray distribution, the evaporation process, and mixture formation. Experimental investigations of atomization process were restricted due to very high-speed and very small region phenomena. Although scale-up models have been used to study the primary spray structure, it is impossible to match Reynolds, Weber, and cavitation numbers and time scales simultaneously in practical high-pressure multi-hole injector. Microscopic and very fast investigation of primary spray structure of practical multi-hole injector is strongly needed.