Radiative and Total Heat Transfer Measurements to a Titan Explorer Model

Abstract

This paper reports on the design and testing of a heat transfer gauge suitable for the spectrum of radiation encountered during entry to the atmosphere of Titan. Experiments were performed in the X3 expansion tube at a velocity of 6 km/s, and conditions corresponding to aerocapture with a 70 degree sphere-cone at 16 degrees angle of attack. The radiation gauges used thin film sensing elements mounted behind a borosilicate window on the windward surface, and were therefore shielded from the convective heat transfer. The sensors are uniformly sensitive for radiation in the wavelength range from 0.2 to 2μm, and give an integrated measurement of the total radiant flux in this band. Externally mounted thermocouple gauges were used to measure the total heat transfer, and hence the ratio of radiant to convective heat flux could be found. The Titan atmosphere was simulated by a mixture of 5% methane (by volume) in nitrogen. Comparative tests using pure nitrogen confirmed that without the methane content, negligible radiation was produced, consistent with CN being the primary radiator. The issues of scaling radiation from flight to laboratory model are discussed, and it is shown that for weakly coupled flows, the absolute level of surface radiant heat flux stays the same to a first order if binary scaling is used. The gauges, and the super-orbital expansion tube facility are shown to be useful resources for the study of nonequilibrium radiating flows.