Stagnation-point heat transfer for a new binary air model including dissociation and ionization.

Abstract

Laminar stagnation-point heat transfer in dissociated and ionized air is considered. A simplified binary diffusion model is proposed and incorporated with the frozen thermal conductivity concept. This model takes advantage of the equilibrium nature of air in the sense that almost all molecules are dissociated before any significant amount of ions appear. Hence, the air may be considered to be a binary mixture of either air molecules and air atoms, or air atoms and air ion-electron pairs (assuming ambipolar diffusion of ions and electrons). The model is only binary with respect to diffusion, since the thermodynamic and transport properties are evaluated using molecular, atomic, ionic, and electronic species of air. With this model, calculations are made for both frozen and equilibrium boundary layers in air at velocities up to 50,000 fps and altitudes of 250 kft. The results show that approximations of the atom mass fraction can affect the boundary-layer chemistry incorrectly. Equilibrium heattransfer rates are progressively lower than the frozen rate through the dissociation regime as well as the ionization regime. Nitrogen does not adequately represent air either in terms of chemistry or heat transfer. Comparison with experimental results shows agreement with the calculations.