Passive scalar mixing studies to identify the mixing length in a supersonic confined jet

Supersonic jet with a co-flow, closely bounded by walls is known as supersonic confined jet. Supersonic confined jet is encountered in practical devices like the supersonic ejector. Mixing of the primary and the secondary fluid inside the confined passage is complex. From a design perspective, it is necessary to have an accurate knowledge of the mixing length (L _MIX). Tracers that do not actively participate in the flow behavior but rather mark the fluids such that they faithfully follow the fluid motion are known as passive scalars. Passive scalars help in the understanding the progression of mixing amidst interacting flows. In this work, we have performed passive scalar mixing studies in a supersonic confined jet for different operating conditions using an existing low area ratio (AR = 3.7) rectangular supersonic gaseous ejector. Air is used as the working fluid in both the primary and the secondary flow. The design Mach number of the primary flow nozzle (M _PD = 1.5–3.0) and the total pressure of the primary flow (P _OP = 4.89–9.89 bar) are varied during the experiments. Using the planar laser-induced fluorescence (PLIF) technique and acetone as the passive scalar, L _MIX is determined. A 266 nm Nd-YAG laser with a repetition rate of 8 Hz is used to excite the acetone molecules in the flow field, and the emitted fluorescence is captured by an ICCD camera. A new method is proposed to study the passive scalar distribution from the acetone PLIF images through digital image processing. Spatial Scalar Fluctuations Intensity (SSFI or ψ) is a parameter defined at every transverse section along the flow direction. Based on the variation of ψ along the jet, the location of L _MIX can be identified. L _MIX is defined as the length from the supersonic nozzle exit where ψ first attains a value of 0.05. For the first time, L _MIX is quantified in a supersonic confined jet. L _MIX values are observed to be in the range of 3H to 6H for the cases under study, where H is the height of the confined passage. The behavior of L _MIX is closely dependent on the nozzle operating conditions. The values of L _MIX are found to be reduced by 17.67% for the over-expanded flows and increased by 15.76% for the under-expanded flows from the perfectly expanded condition. This study also provides other supersonic confined jet characteristics like the potential core length (L _PC) and the shock cell spacing (S _x) of the primary supersonic jet. Compared to the supersonic free jet, values of L _PC and S _x are found to be different in the supersonic confined jet.