Air-Carbon Ablation for Hypersonic Flow Environments

Abstract

Recent molecular beam experiments of high-velocity O, N, and O2 impacting carbon material at high temperatures produced detailed surface chemistry data relevant for carbon ablation processes. New data on O and N reactions with carbon has been published using a continuous molecular beam with lower velocity (2000 m/s) and approximately 500 times higher beam flux than previous pulsed-beam experiments. This data is interpreted to construct a new air-carbon ablation (ACA) model for use in modeling carbon heat shield ablation. The new model comprises 20 reaction mechanisms describing reactions between impinging O, N, and O2 species with carbon and producing scattered products including desorbed O and N, O2, and N2 formed by surface-catalyzed recombination, as well as CO, CO2, and CN. The new model includes surface-coverage-dependent reactions and exhibits non-Arrhenius reaction probability in agreement with experimental observations. All reaction mechanisms and rate coefficients are described in detail and each is supported by experimental evidence or theory. The model predicts pressure effects and is tested for a wide range of temperatures and pressures relevant to hypersonic flight. Model results are shown to agree well with available data and are shown to have significant differences compared to other models from the literature. A preliminary step towards validating the ACA model required simulating the plasma flows in the plasma chamber of von Karman Institute's (VKI's) Inductively Coupled Plasma (ICP) facility called Plasmatron using US3D, a 3D unstructured Navier-Stokes equations solver developed at the University of Minnesota. First, a parameter study of transport properties and the wall-catalycity of a catalytic probe used to characterize the plasma flow was conducted. It was found that the Gupta-Yos mixing rule with collision cross-section data performed better than Wilke's mixing rule with Blottner curve fits and Eucken relation to compute mixture viscosity and thermal conductivity. Also, the wall-catalycity had a strong effect on the boundary layer edge properties along the stagnation line for lower pressure flows in the Plasmatron. It was also found that the Self Consistent Effective Binary Diffusion (SCEBD) model predicted higher stagnation line enthalpy at the boundary layer edge for a flow over a non-catalytic wall when compared to the Fickian diffusion model that attributes a single diffusion coefficient to all the species in the mixture. Then, a series of iterative US3D simulations were performed to characterize the plasma flows over a fully-catalytic wall for seven air-plasma experiments in the Plasmatron. The simulations matched the experimentally measured cold wall heat flux and agreed relatively well with the boundary layer edge properties predicted by VKI's own analysis, giving confidence that the plasma freestream was well characterized in the seven experiments. Then, preliminary simulations of carbon ablation using the seven plasma flows were performed. Two ablation models called the ZA model and the MURI model gave comparable carbon mass loss rates with the experiments. However, the ZA model predicted lower surface heat flux than the MURI model due to the presence of spurious gas-surface reactions. Further experiments of carbon ablation measuring the surface heat flux are suggested in addition to mass-loss measurements. Finally, an analytical framework was developed that characterizes a flight mission or an experimental condition as reaction-limited or diffusion-limited with respect to carbon ablation. The framework uses the flight conditions such as the velocity, altitude, nose radius of the vehicle, and the surface temperature of the ablating heat shield to calculate time scales for diffusion and gas-surface chemical reactions. A new Damkohler number for ablation, defined as the ratio of diffusion time scale to the time scale for gas-surface chemical reactions was proposed. The framework was applied to several flight conditions in the existing literature. It was found that ablation of larger heat-shields like the Stardust re-entry capsule and Orion space-crew vehicle falls under a diffusion-limited regime, while the ablation of smaller objects like the nose tip of the Re-entry F vehicle falls under a reaction-limited regime. Future CFD simulations of ablation on various heat shields using the existing ablation models in the literature are recommended to establish a reference Damkohler number for the classification of the ablation regimes.