Schlieren measurements and analysis of concentration field in self-excited helium jets

Abstract

A low-density gas jet injected into a high-density ambient gas is known to exhibit self-excited global oscillations accompanied by large vortical structures interacting with the flow field. In this study, the formation and evolution of vortices and scalar structure of the flow field are investigated in buoyant helium jets discharged from a vertical tube into quiescent air. This is accomplished by applying the quantitative rainbow schlieren deflectometry technique to optically measure the local helium mole percentage across the whole field. Data were acquired over downstream locations extending from tube exit to about 3.0d (d=31.8 or 19.1 mm is the jet tube inside diameter) at spatial resolution of 0.14 mm and temporal resolution of 16.7 ms. Oscillations at identical frequency were observed throughout the flow field. The evolving flow structure is described by helium mole percentage contours during an oscillation cycle. Instantaneous, mean, and rms concentration profiles are presented to describe interactions of the vortex with the jet flow. Oscillations in a narrow wake region near the jet exit are shown to spread through the jet core near the downstream location of the vortex formation. The effects of jet Richardson number on characteristics of vortex and flow field are investigated and discussed.