Experimental Studies on Gaseous Mixing in a Low Area Ratio Rectangular Supersonic Confined Jet

Abstract

When a supersonic jet passes through a confined passage, it is called as supersonic confined jet. They are encountered in many aerospace applications such as supersonic ejectors, supersonic mixers/purgers, and thrust augmenters. When the supersonic jet goes through a very narrow confined passage, the flow physics of the jet drastically varies in comparison with the supersonic jet exhausting into the ambient. The growing mixing layer along the jet periphery disintegrates quickly as it interacts with the bounding wall in the close proximity. Hence studying the aspects of fluid mixing in the confined passage is not only important but also difficult. In such cases, identifying the non-mixed and the mixed portion is found to be helpful. Parameters like non-mixed length (LNM) and potential core length (LPC) of the primary flow are found to be useful in demarcating the non-mixed region, whereas mixed length (LMIX) is used to identify the mixed region in the confined passage. In the present research, an existing supersonic jet test facility in LHSR-IISc is used as a platform to study the mixing progression observed in a typical supersonic confined jet. Experiments are carried out in a low area ratio rectangular supersonic confined jet having a constant area mixing duct with air as the working fluid in both primary and secondary flow. The design Mach number of the nozzle (MPD=1.5 – 3.0) and the primary flow stagnation pressure (POP=4.89 – 9.89 bar) are the parameters that are varied during the experiments. Values of LNM and LPC are calculated using the planar laser Mie scattering experiments, whereas values of LMIX are calculated using the acetone planar laser-induced fluorescence experiments. Supersonic confined jet characteristics like the potential core length and the shock cell spacing of the supersonic jet are also obtained using the similar technique. Thermodynamic parameters of the confined jet like the stagnation pressure ratio, compression ratio, and the entrainment ratio are calculated using pressure measurements. They are found to be useful in understanding the mixing process. Dominant spatial and temporal modes in the supersonic confined jet are computed from the high-speed Schlieren imaging at a higher repetition rate using proper orthogonal decomposition technique to study the mixing process. Two-dimensional planar image velocimetry experiments are carried out to understand the flow kinematics of the supersonic confined jet.