On Catalytic Recombination Rates in Hypersonic Stagnation Heat Transfer

Abstract

Stagnation laminar heat transfer at hypersonic speeds depends on the rate of recombination of the dissociated air behind the detached shock wave. This paper is concerned with the case of a large recombination t ime compared to the t ime of diffusion across the boundary layer. The conditions of existence of such a "frozen flow" and i ts coupling with the dissociation lag behind the shock are discussed. In order to account for finite catalytic recombination rates at the wall, a nonsimilar boundary condition is introduced which can be reduced to similarity for stagnation flow only. In this latter case, Lees' ( l ) 3 and Fay and Ridell's (2) heat transfer solutions are shown to correspond to the l imit ing case of an infinitely fast catalyst. The validity of their solutions is extended to the general case of a wall of finite catalytic efficiency, by introducing a correction factor (p. This factor is a s imple function of the flight condit ion, nose geometry and the wall catalytic recombinat ion rate constant . For a given nose material , the percentage of the heat transfer by catalysis is found to in crease wi th the velocity, the nose diameter and the wall temperature and to decrease with alt itude. Finally, the experimental values obtained for the catalytic recombination rates of oxygen and nitrogen atoms on various surfaces i l lustrate numerically the importance of the nature of the wall on the catalytic heat transfer to a missi le nose . In particular, the superiority of pyrex over metal l ic surfaces stresses the need for more experimental values for glassy and ceramic coatings.