Contrary Effects of Nozzle Length on Spray Primary Breakup under Subcooled and Superheated Conditions 2018-01-0302
Nozzle length has been proven influencing fuel spray characteristics, and subsequently fuel-air mixing and combustion processes. However, almost all existing related studies are conducted when fuel is subcooled, of which fuel evaporation is extremely weak, especially at the near nozzle region. In addition, injector tip can be heated to very high temperature in SIDI engines, which would trigger flash boiling fuel spray. Therefore, in this study, effect of nozzle length on spray characteristics is investigated under superheated conditions. Three single-hole injectors with different nozzle length were studied. High speed backlit imaging technique was applied to acquire magnified near nozzle spray images based on an optical accessible constant volume chamber. Fuel pressure was maintained at 15 MPa, and n-hexane was chosen as test fuel. Fuel temperature ranged from 25 °C to 85 °C, and ambient pressure ranged from 20 kPa to 200 kPa, which provided a wide range of subcooled and superheated conditions. Results show that nozzle length has contrary effect on spray primary breakup process under subcooled and superheated conditions. Longer nozzle led to narrower near nozzle spray under subcooled conditions due to stronger restriction effect of nozzle wall. However, longer nozzle could result in stronger in-nozzle fuel evaporation and more in-nozzle bubbles near the nozzle exit under superheated conditions, which led to faster and stronger fuel atomization and evaporation processes, and thus wider fuel spray.