Effect of tab length on supersonic jet mixing

Abstract

An experimental investigation has been carried out to assess the efficiency of rectangular tabs of 5% blockage with three different aspect ratios of 1, 1.5, and 2, in promoting the mixing of a Mach 1.73 axisymmetric free jet, in the presence of adverse and favorable pressure gradient, by varying the operating nozzle pressure ratio (NPR) from 4 to 8, in steps of 1, covering all the three levels of expansion, namely, the over-, correct-, and underexpaned states at the nozzle exit. For NPR 4, the Mach 1.73 jet is overexpanded with an adverse pressure gradient of about 23%. At NPR 5, the nozzle is almost correctly expanded with an adverse pressure gradient of only about 3%. At NPRs 6, 7, and 8, the nozzle is underexpanded with favorable pressure gradients of 16%, 35%, and 55%, respectively. It is found that as high as about 88% reduction in the core length is achieved by the tab with an aspect ratio of 1.5 at NPR 7. However, the reduction caused by tabs with aspect ratios of 1 and 2 is only 73.3% and 42.3%, respectively. It is to be noted that though the mixing promotion caused by the tab with an aspect ratio of 1.5, at NPR 7, leads to the core length reduction to the tune of about 88%, it is at the cost of momentum thrust loss of about 5% and the drag caused by the tabs. It is also found that except at NPR 4, the mixing caused by the tab with an aspect ratio of 1.5 is better than tabs with aspect ratios of 1 and 2. The shadowgraph pictures for the uncontrolled and controlled jets demonstrate the effectiveness of the tabs in weakening the waves present in the jet core.