DEVELOPMENT OF FLOW AND HEAT TRANSFER MODELS IN HYPERSONIC RAREFIED GAS DYNAMICS

Abstract

The hypersonic re-entry of a space vehicle into any planetary environment is a challenging process. Space vehicles designed for hypersonic entry to the Martian atmosphere may achieve a speed of 5-9 km/s. At these speeds, a shock layer of high-temperature gases is generated in front of the re-entry vehicle; and the gas species tend to undergo dissociation, ionization, and chemical exchange reactions due to which chemical non-equilibrium occurs. Simultaneously, due to low collision rates among molecules at high altitudes and high velocity of gases in the shock layer, the gas state is far from thermodynamic equilibrium. At this extreme condition, the radiative heat transfer to the surface of the vehicle becomes an essential part of the study. A molecular-level chemistry model is desired to simulate the flow with non equilibrium chemical kinetic effects. Such a chemistry model should be able to estimate reaction rates using the kinetic theory and fundamental molecular properties only. In this work, a chemistry model based on Quantum-Kinetics (Q-K) is presented for the Direct Simulation Monte Carlo (DSMC) method in a rarefied gas environment. The dsmcFoam solver in OpenFOAM CFD software is modified to account for high-temperature non thermodynamic equilibrium effects and to include vibrational relaxation and chemical reactions. An eight-species (CO2, N2, CO, O2, NO, C, N, and O) chemistry model is used to simulate the chemical reactions in the Martian environment. The chief constituent of the Martian atmosphere is CO2, which is a polyatomic molecule having four vibrational modes. The reaction rates are validated with previously published data based on the total collision energy model and Arrhenius rates, as well as with the experimental data. The DSMC method, along with the newly added vibrational relaxation model for polyatomic molecules and the Quantum-Kinetics chemical reaction model, is applied to the hypersonic flow over a cylinder in the Earth's atmosphere for the verification of the code. The model is then applied to study the evolution of a gas mixture in the Martian atmosphere at high temperature with the effects of dissociation and exchange reactions included in the simulation. Finally, the developed solver was applied to simulate the flow around the Pathfinder vehicle and a larger size spacecraft in the Martian atmosphere at 74.28 km altitude and 7.661 km/s free stream velocity. The stagnation point heat flux and drag coefficient for the Pathfinder vehicle was comparable with previously published studies and data. vii Next, we estimate the effect of non-gray gas radiation on surface heat flux at the stagnation point. We employ the P1 model and the exact solution to the parallel slab problem to calculate the radiative heat flux at the stagnation point of the spacecraft. The simulation is done with a non-homogeneous temperature field with data taken from the actual DSMC simulation and found that P1 method performs poorly under optically thin conditions. We developed a code to model non-gray radiation from significant bands of CO using the line by-line spectral model. It was found that, at 74.28 km altitude, radiative heat flux dominates and can reach up to 2-3 times the convective heat flux for both Pathfinder and the spacecraft. However, at lower altitudes, the radiative heat flux was around one-fifth of the convective heat flux. The radiative heat flux at the stagnation point for 5 km/s entry speed was negligible.