Characterization of Radiative Heating Anomaly in High Enthalpy Shock Tunnels

Surface heating measurements in shock tunnels operating at high enthalpy have previously showed anomalously high heating, the mechanisms for which have been the subject of much speculation. However, recent tests in JAXA’s High Enthalpy Shock Tunnel (HIEST) have presented compelling evidence that the heating anomaly is due to radiation from the shock layer, produced by impurities within the shock. The present paper describes test series performed in HIEST with a flat-faced cylinder model and an Apollo capsule model at angle of attack. The flat-faced cylinder is instrumented with three types of thermocouple heat flux gauges to separate contributions from radiative and convective heating. Most of the augmentation is shown to be due to radiation in this configuration. The Apollo capsule model is instrumented with a trip element and thermocouple gauges to study transition at high enthalpy, but transition data are obscured by the presence of radiation. Both test articles are embedded with fiber optics to measure the magnitude and characteristics of the radiative heating. Analyses of the radiation spectra, estimates of radiative heating corrections and comparison to computational fluid dynamic simulation of the shock tunnel are presented. The data show the radiative correction to be meaningful at high pressure or high enthalpy conditions; however, additional discrepancies remain.