Radiation properties modeling for plasma-sprayed-alumina-coated rough surfaces for spacecrafts

Abstract

Spacecraft thermal control materials (TCMs) play a vital role in the entire service life of a spacecraft [NASA Preferred Reliability Practices: Spacecraft Thermal Control Coatings Design and Application (Practice No. PD-ED-1239), 1995, pp. 1–6]. Most of the conventional TCMs degrade in the harmful space environment [R.D. Karam, Satellite Thermal Control for Systems Engineers, AIAA, Virginia, USA, 1998, pp. 147–163]. In the previous study, plasma sprayed alumina (PSA) coating was established as a new and better TCM for spacecrafts, in view of its stability and reliability compared to the traditional TCMs [R.M. Li, S.C. Joshi, H.W. Ng, Presented in 2nd International Conference on Materials for Advanced Technologies & IUMRS, Singapore, 2003, pp. 1–4]. During the investigation, the surface roughness of PSA was found important, because the roughness affects the radiative heat exchange between the surface and its surroundings. Parameters such as root-mean-square roughness cannot properly evaluate surface roughness effects on radiative properties of opaque surfaces [M.F. Modest, Radiative Heat Transfer, 2nd ed., Academic Press/Elsevier Science, USA, 2003, pp. 90–92]. Some models have been developed earlier to predict the effects, such as Davies’ model [H. Davies, Proceedings of IEEE vol. 101, part IV, 1954, pp. 209–214], Tang and Buckius's statistical geometric optics model [K. Tang, R.O. Buckius, Int. J. Heat Mass Transfer 44 (2001) 4059–4073]. However, they are valid only in their own specific situations. In this paper, an energy absorption geometry model was developed and applied to investigate the roughness effects with the help of 2D surface profile of PSA coated substrate scanned at micron level. This model predicts effective normal solar absorptance ( α ne) and effective hemispherical infrared emittance ( ɛ he) of a rough PSA surface. These values, if used in the heat transfer analysis of an equivalent, smooth and optically flat surface, lead to the prediction of the same rate of heat exchange and temperature as that of for the rough PSA surface. The model was validated through comparison between a smooth and a rough PSA coated surfaces. Even though not tested for other types of materials, the model formulation is generic and can be used to incorporate the rough surface effects for other types of thermal coatings, provided the baseline values of normal solar absorptance ( α n) and hemispherical infrared emittance ( ɛ h) are available for a generic surface of the same material.