Influence of shock structure on heat transfer characteristics in supersonic under-expanded impinging jets

Abstract

In this study, the relationship between the shock structure of impinging jet flow and target wall heat transfer characteristics was investigated. In the case of a high Reynolds number impinging jet that can enter the supersonic flow regime, the stagnation temperature of the impinged surface is affected by the jet structure as well as other factors, such as the nozzle to plate distance and radial distance. When the jet flow velocity becomes supersonic, shock structures form at the downstream of the nozzle exit. Complicated shock structures, such as the Mach shock disk and plate shock, are expected to affect the heat transfer characteristics of the impingement surface. In this study, the cooling performance of a supersonic nitrogen (N2) jet is investigated by measuring the impinged surface temperature, and the flow of the jet is visualized by schlieren imaging. The visualized image and surface temperature are compared to clarify the shock structure-related heat transfer characteristics. Under experimental conditions where the nozzle pressure ratio changes to a moderate and highly under-expanded regime, the surface temperature fluctuates according to the changes of jet structures in each condition. According to a position of Mach crossing point, which is an intersection of oblique shock structures, a behavior of jet fluid is decided to be trapped inside the recirculation zone or to be spread to wall jet region. In case that the jet fluid is trapped, the cooling performance of the supersonic jet is significantly reduced and vice versa. Therefore, the heat transfer characteristics can be explained by roles of the jet flow field structures including the recirculation flows and shock locations.