Spray Characteristics with High-Speed Tomographic Particle Image Velocimetry Under Non-Flash and Flash Boiling Conditions

Compared with port fuel injection engines, direct injection(DI) gasoline engine is becoming the mainstream of gasoline engines because of its higher fuel economy and excellent transient response. It has been proven that fuel spray characteristics in DI engines are crucial to the performance and emission quality of the engine. Flash boiling spray has great potential to achieve high fuel economy and low emission by dramatically improving the fuel atomization and vaporization and it has different spray-air interaction behavior as compared with non-flash boiling one, while its mechanism is more complex as compared with subcooled spray. We investigate the time-resolved spatial velocity field of the spray using 2-camera high-speed 3 D3 C(3-dimension 3-component)tomographic particle image velocimetry(PIV) diagnostic technique. A 10 mm thick laser sheet is used to illuminate the fuel spray. Characteristics of both non-flash and flash boiling sprays are studied. A single-hole injector is mounted within a heat exchanger so that different fuel temperature can be accessed. In the experiment, n-pentane is used as the fuel. For the non-flash boiling spray, the velocity field of the liquid spray is mostly consistent to the injection direction. With the increase of the degree of superheat(Do S), the overall velocity scale decreases especially at the spray tip. Meanwhile, larger swirls occur at the lower part of the flash boiling spray, which means stronger spray-air interaction occurs at a higher Do S.

Spray Characteristics of Air-Assisted Injector Under Different Ambient Pressures

Spray atomization of liquid fuel plays an important role in droplet evaporation,combustible mixture formation and subsequent combustion process.Well-atomized liquid spray contributes to high fuel efficiency and low pollutant emissions.Gasoline direct injection(GDI)has been recognized as one of the most effective ways to improve fuel atomization.As a special direct injection method,the air-assisted direct injection utilizes high-speed flow of high-pressure air at the injector exit to assist liquid fuel injection and promote spray atomization at a low injection pressure.This injection method has excellent application potential and advantages for high performance and lightweight engines.In this study,the hollow cone spray emerging from an air-assisted injector was studied in a constant volume chamber with the ambient pressures ranging from 5 kPa to 300 kPa.External macro characteristics of spray were obtained using high speed backlit imaging.Phase Doppler particle analyzer(PDPA)was utilized to study the microcosmic spray characteristics.The results show that under the flash boiling condition,the spray will generate a strong flash boiling point which causes the cone shape spray to expand both inwards and outwards.The axisymmetric inward expansion would converge together and form a lathy aggregation area below the nozzle and the axisymmetric outward expansion greatly increases the spray width.The sauter mean diameter(SMD)of flash boiling condition can be reduced to 5μm compared to the level close to 10μm in the non-flash boiling condition.