Experimental study of temporal evolution of initial stage diesel spray under varied conditions

•Ultra-high speed imaging techniques were used to study the near-field diesel spray.•Timing of the maximum spray tip velocity was investigated.•A new way to discriminate between the two stages of fuel spray penetration was proposed.•Equations to predict the transition timing and near-field penetration was proposed.

Understanding of the initial stage of spray development is important in modelling of the injection process for optimisation of diesel engine design. The near-field spray characteristics of a single-hole diesel injector were studied using an ultra-high speed CCD digital video camera with the speed of up to 1 M fps. The effects of injection and ambient pressure on the spray behaviours have been examined in terms of initial spray penetration and tip velocity. Acceleration was observed during the initial stage of the spray process and the time corresponding to the spray tip peak velocity (tp) was extracted and compared to the breakup time (tb) in Hiroyasu correlation. It was found that tp is a natural and effective way to discriminate between the two stages during the spray process. The ambient pressure increase reduces the transition length considerably before the value reaches 1.5 MPa and after that, the effect becomes much smaller. Finally, empirical correlations for the prediction of spray penetration development were proposed.