Characterization of Low-Density Gaseous Jet Structure and Vorticity Using PIV Technique

In view of the increasing interest towards hydrogen, such as its utilization for road transport sector decarbonization, the present study proposes the use of helium as substitute to characterize the jet structure through the use of the particle image velocimetry (PIV) technique. The experimental test campaign involved the use of a gaseous injector capable of delivering helium up to 50 bar in a constant volume chamber (CVC), 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 jet, the second including a metal plate positioned perpendicularly 50 mm from the injector tip, thus making it possible to observe the jet – 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 7 to 34. The use of the streamlines made it possible to determine the presence of the vortex structures, further highlighting the capacity of the jet to promote or not the mixing phenomena. In the absence of obstacles in the path, as the pressure inside the chamber increased, a narrowing of the jet was observed and a tendency for the vortexes to dissipate their intensity more quickly, only to be replaced by randomly distributed small eddies. The main ring structures extended to the intermediate region (≈40 - 50 mm) as CVC pressure increased, without ever reaching the head of the jet. Instead, the interaction with the wall contributed to the formation of vortexes that were able to pick up the mixing phenomena with reduced intensity values (≈ 30 - 40 %) compared to the maximum measured peaks.