Nonequilibrium Radiation Intensity Measurements in Simulated Titan Atmospheres

This paper details the experimental work conducted at the University of Queensland to measure the nonequilibrium radiation intensity behind a shock in simulated Titan atmospheres, as would be seen during planetary entry. Radiation during Titan entry is more important at lower speeds (about 5-6 km/s) than other planetary entries due to the formation of cyanogen in above equilibrium concentrations in the shock layer, which is a highly radiative species. The experiments were focused on measuring the nonequilibrium radiation emitted from cyanogen between the wavelength range of 310-450 nm. This paper includes experimental results for radiation and spectra found in the postshock region of the flow. Experiments have been conducted at various ambient pressures, shock speeds, and chemical compositions. This leads to a comprehensive benchmark data set for Titan entry, which will be useful for validation of theoretical models. Spectra were recorded at various axial locations behind the shock, enabling the construction of radiation profiles for Titan entry. Furthermore, wavelength profiles can also be constructed to identify various radiating species, in this case, predominately cyanogen violet. Furthermore, this paper includes comparisons with experiments performed at NASA Ames Research Center on their electric arc-driven shock tube in Titan compositions. Excellent quantitative agreement has been obtained between the two facilities.