Effect of High-Enthalpy Air Chemistry on Stagnation Point Heat Flux

Abstract

R EENTRY flows are characterized by high-temperature effects, like internal energy excitation, chemical reactions, and ionization. The thermochemical processes and their nonequilibrium effects determine the temperature of the gas that envelops the vehicle. They also alter the gas composition and associated properties, like conductivity and viscosity of the gas mixture. These have a dominant effect on the shock standoff distance, surface heat flux, and other aerothermodynamic characteristics of the vehicle. The accuracy of computational fluid dynamic simulations at reentry conditions depends on the fidelity of the different thermochemicalmodels used in the simulation. Several research efforts have been directed in the recent past toward quantifying the sensitivity of the aerothermal predictions to the uncertainty in the model parameters [1,2]. Flow simulations usually involve a large number of thermochemical parameters, like the rate constants of chemical reactions, vibrational-translational relaxation times, and species diffusion coefficients. The chemical rate constants form a significant portion of the parameter space for uncertainty analysis [3]. In addition, the typical uncertainty in the available reaction rate values ( 1 order of magnitude) is one of the highest among all the model parameters [4]. It is therefore important to study the effect of chemical reaction rates on aerothermodynamic predictions. Many different thermochemical reactions can occur in a given reentry flow and there are often competing effects between these processes. A detailed study of the underlying physics is required to identify the dominant mechanisms and their rate-limiting steps. Such insight can help explain flow phenomena observed at high-enthalpy conditions. It can also provide valuable information regarding the sensitivity of aerothermodynamic predictions to uncertainty in the thermochemical parameters. Within this scope, Reddy and Sinha [5] investigated the effect of chemical reaction rates on convective heat flux prediction at the 35 km altitude trajectory point (1652.75 s after launch) of the Fire II capsule. Numerical simulations were performed by varying the reaction rate constants as per their uncertainty limits. The flowfield solutions thus computed were analyzed in terms of the variations in local chemical reaction rates and the species source terms. The resultant effect on the gas composition and temperature in critical regions of the flowfield, like the thermal boundary layer on the surface, was studied. It was found that the chemical state of the gas, i.e., equilibrium, nonequilibrium, or frozen, and the extent of recombination reactions in the boundary layer are the critical factors that determine the sensitivity of the heating rate to variations in the reaction rate coefficients. In the current Note, we apply a similar approach to a high-enthalpy AS-202 reentry condition, which has been numerically simulated by Wright et al. [6]. The Mach number is 26.2 and the total enthalpy is 31.53 MJ∕kg. The dominant chemical activity observed at the 35 km altitude condition (total enthalpy of 12 MJ∕kg) is the dissociation and recombination of oxygen. The surface heating rate is therefore sensitive to only these reaction rates. A higher total enthalpy at the 70 kmcondition can cause dissociation and recombination of bothO2 and N2 and multiple reactions are expected to proceed simultaneously.We aim to investigate the sensitivity of the flowfield solution to variation in these reaction rates, and to identify the critical parameters that influence the accuracy of the surface heat flux predictions. The focus of the current work is on gas-phase chemistry and therefore, wall catalysis effects are not included.