Simultaneous PIV/LIF-PIV measurements and numerical simulation of liquid flow and ambient gas flow for transient diesel spray

The characteristics of spray/ambient gas flow and the turbulent mixing process significantly influence the subsequent combustion feature. There are some related studies on the simultaneous measurement of spray liquid/gas phase flow using optical diagnosis techniques. However, the two-phase flow characteristics of the transient Diesel spray have not been discussed in detail, few works have verified the measuring accuracy of the methods. This study proposes a PIV/LIF-PIV measuring method for simultaneously investigating the liquid/gas flow of transient Diesel spray. The experimental process and effects of imaging parameters/digital image processing method on the measuring accuracy are discussed based on the detectability of cross-correlation peak. The characteristics of the local turbulent mixing of liquid/gas flow for Diesel spray are presented. Numerical simulation using the Large-Eddy Simulation (LES) Eulerian-Lagrangian method is also carried out for extending the understanding regarding the liquid/gas phase flow process. The results indicate that the upstream vortex structures of spray flow lead to the random direction of velocity vectors at the tip, which is related to the shot-by-shot variation of spray structure at early timing after SOI. A similar streamline of spray flow and ambient gas flow could be observed in the spray region especially at 2.0 ms ASOI, which implies the gaseous phase is accelerated rapidly by the internal spray droplets as soon as entering the spray boundary, and both phases reach a “balanced state” at the end of the injection period. Similar results could be derived from the simulation results, the position where the maximum spray velocity exists is quite different because of the randomness of the instantaneous flow. An improved quasi-dimensional velocity predicting model that accounts for the spatial variation of velocity distribution is also developed.