Study of Air Entrainment of Multi-hole Diesel Injection by Particle Image Velocimetry - Effect of Neighboring Jets Interaction and Transient Behavior After End of Injection. 2010-01-0342

The air entrainment of multi-hole diesel injection is investigated by high speed Particle Image Velocimetry (PIV) using a multi-hole common rail injector with an injection pressure of 100 MPa. The sprays are observed in a high pressure, high temperature cell that reproduces the thermodynamic conditions which exist in the combustion chamber of a diesel engine during injection. Typical ambient temperature of 800K and ambient density of 25 kg/m3 are chosen. The air entrainment is studied with the PIV technique, giving access to the velocity fields in the surrounding air and/or in the interior of two neighboring jets. High acquisition rate of 5000 Hz, corresponding to 200 μs between two consecutive image pairs is obtained by a high-speed camera coupled with a high-speed Nd:YLF laser. The effect of neighboring jets interaction is studied by comparing four injectors with different numbers of holes (4, 6, 8 and 12) with similar static mass flow rate per hole. The results show that both the maximum air entrainment level and the total mass of entrained air are similar for all the injectors, and therefore are not affected by neighboring jets in the conditions studied. However the transient behavior of the air entrainment process is affected when the number of holes is high, hence when two neighboring jets are near (12 holes nozzle): the air entrainment reaches a maximum at later timings compared to the other nozzles. An analysis of the velocity fields between the two jets shows that this result is due to the air flow inertia generated when the two jets are near. The transient behavior after the End Of Injection (EOI) is also studied using the 4-holes injector. The results show a very rapid decrease of the mean axial velocity near the nozzle after the EOI. The air entrainment becomes maximum at a given position and this position propagates downstream towards the jet tip.