Influence of Ablation on Radiative Heating for Earth Entry.

Using the coupled ablation and radiation capability recently included in the LAURA flowfield solver, this paper investigates the influence of ablation on the shock-layer radiative heating for Earth entry. The extension of the HARA radiation model, which provides the radiation predictions in LAURA, to treat a gas consisting of the elements carbon, hydrogen, oxygen, and nitrogen is discussed. It is shown that the absorption coefficient of air is increased with the introduction of the carbon and hydrogen elements. A simplified shock-layer model is studied to show the impact of temperature, as well as the abundance of carbon and hydrogen, on the net absorption or emission from an ablation contaminated boundary layer. It is found that the ablation species reduce the radiative flux in the vacuum ultraviolet, through increased absorption, for all temperatures. However, in the infrared region of the spectrum, the ablation species increase the radiative flux, through strong emission, for temperatures above 3000 K. Thus, depending on the temperature and abundance of ablation species, the contaminated boundary layer may either provide a net increase or decrease in the radiative flux reaching the wall. To assess the validity of the coupled ablation and radiation LAURA analysis, a previously analyzed Mars-return case (15.24 km/s), which contains significant ablation and radiation coupling, is studied. Exceptional agreement with previous viscous-shock-layer results is obtained. A 40% decrease in the radiative flux is predicted for ablation rates equal to 20% of the freestream mass flux. The Apollo 4 peak-heating case (10.24 km/s) is also studied. For ablation rates up to 3.4% of the freestream mass flux, the radiative heating is reduced by up to 19%, whereas the convective heating is reduced by up to 87 %. Good agreement with the Apollo 4 radiometer data is obtained by considering absorption in the radiometer cavity. For both the Mars-return and the Apollo 4 cases, coupled radiation alone is found to reduce the radiative heating by 30-60% and the convective heating by less than 5%. [ABSTRACT FROM AUTHOR]