Quantitative Measurements of Liquid and Vapor Distributions in Flash Boiling Fuel Sprays using Planar Laser Induced Exciplex Technique

The flash boiling phenomenon occurs at some operating conditions when fuel is directly injected into the cylinder of a homogeneous charge spark ignition direct injection (SIDI) engine due to the higher temperature of the injected fuel and lower back pressure. A flash boiling spray has significantly different characteristics from a conventional DI gasoline spray. In this paper, the planar laser-induced exciplex fluorescence (PLIEF) technique with two specially designed dopants of the fluorobenzene (FB) and the diethyl-methyl-amine (DEMA) in n-hexane was implemented to investigate the liquid and vapor phases of sprays from a multi-hole injector. A vapor phase calibration was carried out to quantitatively correlate the fluorescence signal with vapor concentration. The quantitative vapor concentration distribution is then obtained by applying the calibration. Vapor phase calibration shows that at the temperature range of 30°C ~ 75°C, the vapor phase fluorescence is directly proportional to the vapor concentration. The concerned effect of vapor temperature is negligible in this particular LIEF system. For fuel sprays, the fuel temperature and back pressure play a dominant role to the spray structure under superheated conditions. As the superheat degree increases, spray plumes (both liquid and vapor phases) from different holes of the nozzle tend to merge into each other and finally form a single plume spray near the centerline. The vapor phase collapses more severally than the liquid phase. At high superheat degrees, all vapor phases collapse into the centerline of the injector to form a highly concentrated "gas jet" structure. The total vapor mass of the spray increases significantly after the collapsing, indicating dramatically increased vaporization rate due to the flash boiling.