The Influence of Ablation on Radiative Heating for Earth Entry

Abstract

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 C, H, O, and N is discussed. It is shown that the absorption coefficient of air is increased with the introduction of the C and H elements. A simplified shock layer model is studied to show the impact of temperature, as well as the abundance of C and H, 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 3,000 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 free-stream mass flux. The Apollo 4 peak-heating case (10.24 km/s) is also studied. For ablation rates up to 3.4% of the free-stream mass flux, the radiative heating is reduced by up to 19%, while 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%. = thickness of the constant-property layers defined in Fig. 3, with i = 1 or 2 (cm) h = absorption coefficient (cm -1 ) = blowing reduction parameter = trasmissivity defined in Eq. (2) Subscripts h = indicates a spectral dependence in terms of eV Superscripts abl = refers to ablation species, meaning species containing any C or H atoms air = refers to air species, meaning species containing no C or H atoms Abbreviations eV = electron volts; the frequency in eV, labeled h , is equal to 1.24x10 -4 / c IR = infrared; refers here to the spectral region below 6 eV NIST = National Institute of Standards and Technology OP = Opacity Project VUV = vacuum ultraviolet; refers to the spectral region above 6 eV

Assessment of Radiative Heating Uncertainty for Hyperbolic Earth Entry

“…This study showed that the convective heating during the peak heating pulse is reduced to essentially zero. Navier-Stokes simulations with coupled radiation 16,17 and coupled radiation and ablation 18 have been performed recently and shown to agree well with past viscous shock layer results.…”

“…The ablation rates for this case were an orderof-magnitude lower than for the 15 km/s Mars-return case. The study by Johnston et al 18 applied the same Laura/Hara analysis considered in this work to the Apollo 4 peak heating condition at 10.2 km/s. The influence of ablation was shown to be less than 3% on the radiative intensity below 6 eV.…”

“…The Hara code applies up-to-date spectral and excitation rate data to an efficient computational algorithm to allow fast and accurate radiation predictions. The baseline radiation properties for air species are discussed by Johnston et al, 46 while the radiation properties for ablation properties are discussed by Johnston et al 18 The baseline Hara predictions apply a smeared rotational band (SRB) approach for modeling molecular band systems. The accuracy of the SRB model for the present cases is examined in Section VI-E.…”

Implementation Of Radiation, Ablation, And Free Energy Minimization Modules For Coupled Simulations of Hypersonic Flow

“…al. 5,6 Whereas these companion papers focus on specific applications, the present paper focuses on the modeling and the numerical details of model implementation. The baseline flow solver is a newly refactored version of the LAURA code 7 that utilizes Fortran 95 modules.…”

Radiation Coupling with the FUN3D Unstructured-Grid CFD Code

“…HARA and LAURA have been coupled for radiation with ablation products using a structured-grid flow solver. 2,16 HARA evaluations take about 10 seconds per line of sight on a current-generation central processing unit. In the LAURA/HARA best practices, radiation evaluations are called from every few hundreds to few thousands of LAURA iterations.…”