Gas temperature layer visualization in hypersonic shock tunnel using electric discharge

A novel technique for visualizing the gas temperature layer around bodies flying at hypersonic speeds is presented. The high temperature zone is visualized by photographing the light emitted from the electric discharge generated over a model exposed to hypersonic flow in a shock tunnel. The technique is based on electrical discharge phenomena, where the frequency of radiation emitted by the discharge path passing through the flow field varies with the temperature of the gas medium in the discharge path. The experiments are carried out in the hypersonic shock tunnel HST-1 at a nominal Mach number of 5.75 using helium as the driver gas, with free stream velocity of 1.38 km/s and free stream molecular density of $2.3396×10^{16} molecules/cm^3$. The electric discharge is generated across a line electrode embedded in the model surface and a point electrode suspended in the free stream. A high voltage discharge device (1.6 kV and 1 A) along with a micro-controller based pulse delay control module is integrated with the shock tunnel for generating and controlling electric discharge which lasts for ~2 μs. The gas temperature layer at zero angle of incidence around a flat plate and slightly blunted (5 mm bluntness radius) 20° apex angle slender cone model are visualized in this study. The visualized thickness of the high temperature layer around the flat plate is ~2 mm, which agrees well with numerical simulation, carried out using 2-D Navier-Stokes equation