Characterization of low-density gas spray structure and vorticity using PIV technique

In view of the increasing interest towards hydrogen, such as its utilization for road transport sector decarbonisation, in the current study helium has been employed as substitute to characterize the spray structure through the use of the particle image velocimetry (PIV) technique. The experimental test campaign implied the use of a gaseous injector capable of delivering the substitute fuel up to 50 bar in a constant volume spray – chamber, which pressure has been varied in order to scrutinize the influence of environment density on jet structure. Two configurations were employed: one consisting of a free path of distribution of the spray, the second including a metal plate positioned perpendicularly 50 mm from the injector tip, thus making it possible to observe the spray – wall interactions under several conditions. The illumination was provided by a dual cavity Nd:YAG laser and a 4-megapixel camera used for image capture. The influence of pressure ratio (PR) was evaluated over a wide range, from 3 to 34, relating it to the morphological outcomes. Then, the use of the streamlines made it possible to trace the vortex structures, further highlighting the capacity of the spray to promote or not the mixing phenomena. In the absence of obstacles in the path, as the pressure inside the chamber increased, the jet was observed to narrow and the vortex rings first dissipated their intensity, only to be replaced by randomly distributed small eddies. On the contrary, the interaction with the wall contributed to the formation of vortices that were able to pick up the mixing phenomena with surrounding environment from the rear areas.