Thermo-chemical ablation of heat shields under earth re-entry conditions.

摘要

The process of ablation for Earth atmospheric entry is modeled. The flowfield surrounding the ablator is modeled by an extended set of Navier-Stokes equations that include the effects of thermo-chemical nonequilibrium. This set of equations encompasses the conservation of mass for each chemical species, conservation of momentum, the conservation of vibrational energy, and the conservation of total energy. The heat conduction into the ablator material is modeled by using Fourier's Law of heat conduction and the heat equation. The flowfield and ablator are coupled by a thermo-chemical ablation model that includes a surface mass balance and a surface energy balance. The ablation model takes into account chemical reactions of the flowfield species with the surface material, surface material acting as a catalytic surface, and sublimation of the surface material.; To solve the governing equations for the model, a computational fluid dynamics approach is used where the flowfield is solved using a Modified Steger-Warming flux vector splitting scheme and the solid is solved using a centrally differenced scheme. A Gauss-Seidel Line Relaxation technique is implemented to speed numerical convergence.; The flowfield model is verified by comparing to flowfield computations of other researchers and to experimental data. The ablator/heat shield model is validated by a direct comparison between an exact analytical solution and a numerical solution. The thermo-chemical ablation model is verified by comparing to the experimental results of the PAssive Nosetip Technology (PANT) program.; The model is used to calculate steady-state ablation data for sphere-cone re-entry bodies. Two bodies with nose radii of 0.0127 m and 0.1270 m are tested at a velocity of 8 km/s. The ablator material is selected to be a commercial grade graphite. Due to the ablator selection, the flowfield is limited to 11 chemical species and two internal energy modes. A standard Earth atmosphere is selected at altitudes ranging from 40 km to 80 km.; The major results concern the thermo-chemical ablation model. Both oxidation and sublimation mechanisms are evident and are dependent on freestream conditions and the re-entry body shape. At low heat flux levels, the primary ablation mechanism is due to oxidation. However, as heat flux increases, sublimation of the graphitic material is the dominant ablation mechanism. Sublimation effects can become strong enough to force-off or blow all flow species away from the body's surface. Several ablation species are present within the surrounding flowfield, but the primary species are CO, C, and CN. The cyano radical, CN, is of most interest since it is often omitted from the species studied in ablation models. Carbon monoxide, CO, is present due to surface oxidation and to flowfield oxygen reacting with sublimation species.