Abstract
Experimental investigations of fuel breakup very close to nozzle of practical high-pressure swirl injector, which is used in direct injection spark ignition (DISI) engine, were carried out. In DISI engines, fuel is directly injected into cylinder therefore the spray characteristics and mixture formation are of primary importance. Many experimental investigations using several measurement techniques like laser sheet method with high-speed camera, LIF or PDA have been carried out for better understandings of spray and combustion characteristics. However, 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 in practical high-pressure swirl injector. Microscopic investigation of primary spray structure of practical swirl injector is needed. On the other hand, numerical simulations have been conducted for better understanding of spray formation process. These researches indicated qualitatively good agreement, but the initial conditions, such as liquid sheet thickness and break-up length, were not accurately since break-up process of the liquid sheet from swirl injector has not been examined. In numerical simulation of spray behavior, a sub-model for atomization phenomena is very important. Therefore the primary atomization process and the break-up process of liquid sheet play an important role. In this research, visualizations of primary spray formation process were demonstrated using a high-speed video camera (maximum speed: Imfps) with a long-distance microscope. Initial state and development of the spray were discussed under the different ambient (back) pressure condition. During the injection period, the length and thickness of the liquid sheet, which is produced from the nozzle exit, were measured using Ar-ion laser sheet and high-speed camera. Moreover, fluctuations of the length and thickness of liquid sheet were discussed. Three main conclusions were drawn from this study. (1) It has been shown that the liquid fuel column without swirl motion was injected as a compact jet at the beginning of the injection. During the injection period, the spray indicates the quasi-steady state mode. (2) Liquid film sheet has a ligament structure. Using Ar-ion laser sheet and high-speed camera, length and thickness of the liquid sheet can be measured. (3) Surface waves of liquid sheet can be recognized. Higher ambient (back) pressure makes shorter wavelength of surface waves of liquid sheet of swirl injector.
