Characterization of high enthalpy flows in the IISc free piston driven shock tunnel using Two Colour Ratio Pyrometry

Abstract

High temperature and its associated effects set the hypersonic flow regime apart from other class of flows. Viscous dissipation raises the internal energy of the high-kinetic-energy gas as it slows down in the boundary layer. For slow heat conduction into the vehicle surface, the gas temperature increases drastically, leading to real gas effects, dissociation and ionization of molecules. Due to the thickened boundary layer, strong viscous interaction exists between the outer inviscid shock layer and the boundary layer, leading to a pressure rise compared to an inviscid case. This pressure in turn tends to make the boundary layer thinner than expected and increases the temperature gradient (for a cold wall case) and therefore the aerodynamic heating of the vehicle. High temperature near the vehicle airframe may cause surface ablation of the heat shield, releasing energy into the shock layer. Short duration flights, with a small time of flight may not experience ablation because the heat load on such vehicles may be too little, despite the high temperature in their vicinity. Energy release affects its aerothermodynamics and its performance. Effect of energy deposition (by means like plasma heat addition, electric arc discharge, chromium oxidation, etc.) on wave drag has been well documented in open literature. However, no concrete measurements exist on the aerothermodynamic effects of heat addition. Apart from ablation, flow separation may occur due to shock-boundary layer interaction (SBLI) towards the aft of a blunt nosed vehicle. Temperature in the dead air region (separated region with circulatory flow) inside the shear layer influences the performance of the vehicle and must be characterized. Apart from SBLI, another cause of intense heating is shock-shock interaction as classified by Edney. Although studies on wave drag galore, no experiments exist to characterize the temperature distribution in the separated shear layer. Characterization of these phenomena in an in-house free piston driven shock tunnel...