Phenolic Impregnated Carbon Ablator was the heatshield material for the Stardust probe and is also a candidate heatshield material for the Orion Crew Module. As part of the heatshield qualification for Orion, physical and thermal properties were measured for newly manufactured material, included emissivity, heat capacity, thermal conductivity, elemental composition, and thermal decomposition rates. Based on these properties, an ablation and thermal-response model was developed for temperatures up to 3500 K and pressures up to 100 kPa. The model includes orthotropic and pressure-dependent thermal conductivity. In this work, model validation is accomplished by comparison of predictions with data from many arcjet tests conducted over a range of stagnation heat flux and pressure from 107 W/cm2 at 2.3 kPa to 1100 W/cm2 at 84 kPa. Over the entire range of test conditions, model predictions compare well with measured recession, maximum surface temperatures, and indepth temperatures. Nomenclature A, B, C, D = four thermocouple placement options E = fractional error in recession I, II, III = three model geometry options S = centerline recession, mm X, Y = Cartesian coordinates perpendicular to Z, cm Z = Cartesian coordinate parallel to the axis of the geometry, cm
A formulation of finite rate ablation surface boundary conditions, including oxidation, nitridation, and sublimation of carbonaceous material with pyrolysis gas injection, based on surface species mass conservation, has been developed. These surface boundary conditions are discretized and integrated with a Navier-Stokes solver. This numerical procedure can predict aerothermal heating, chemical species concentration, and carbonaceous material ablation rates over the heat-shield surface of reentry space vehicles. Two finite rate gas-surface interaction models, based on the work of Park and of Zhluktov and Abe, are considered. Three test cases are studied. The stream conditions of these test cases are typical for Earth reentry from a planetary mission with both oxygen and nitrogen fully or partially dissociated inside the shock layer. Predictions from both gas-surface interaction models are compared with those obtained by using chemical equilibrium ablation tables. Stagnation point convective heat fluxes predicted by using Park's finite rate model are usually below those obtained from chemical equilibrium tables and Zhluktov and Abe's model. Recession predictions from Zhluktov and Abe's model are usually lower than those obtained from Park's model and from chemical equilibrium tables. The effect of species mass diffusion on the predicted ablation rate is also examined. Nomenclature B= dimensionless mass blowing rate,ṁ/ρ e u e C m C i = mass fraction for species i C m = Stanton number for mass transferm 2 /s D = bifurcation diffusion coefficient, m 2 /s E = total energy per unit volume, J/m 3 F = nonlinear equation, Eq. (24), or P 0 / √ (2πm i kT ) f i = diffusion factor of species i h = Planck's constant, J · s, or enthalpy, J/kg J = mass diffusion flux, kg/m 2 · s K i = equilibrium constant K t = thermal conductivity of translation temperature, W/m · K K v = thermal conductivity of vibration temperature, W/m · K k = Boltzmann constant, J/K k f = forward reaction rate, Eq. (18) k r = backward reaction rate, Eq. (18) M = molecular weight, kg/mole m i = mass of species i, kġ m = mass flux, kg/m 2 · ŝ N i = Eq. (12) p = pressure, N/m 2 p E = saturated vapor pressure, N/m 2 Q T − v = rate of translation and vibration energy exchange, W/m 3 q conv = convective heat flux, W/m 2 q v = heat flux due to species diffusion, W/m 2 R = universal gas constant, J/kmol · K R b = base radius, m R c = corner radius, m R n = nose radius, m r i = reaction rate; Eq. (14) S = recession rate, m/s T = temperature, K t = time, s u = fluid velocity, m/s v s = species diffusion velocity, m/s v w = mass injection velocity, m/s w = species source term in Eq. (1), kg/m 3 · s x = Cartesian coordinate system, m Z i = bifurcation diffusion quantity of species i; Eq. (4) α = surface absorptance β = efficiency of gas-surface interaction ε = surface emissivity ε i = factor in ith heterogeneous reaction η = general body-fitted coordinate system normal to surface, m i = surface coverage concentration of species i 0 = free surface concentration λ = blowi...
A formulation of finite-rate ablation surface boundary conditions, including oxidation, nitridation, and sublimation of carbonaceous material with pyrolysis gas injection, has been developed based on surface species mass conservation. These surface boundary conditions are discretized and integrated with a Navier-Stokes solver. This numerical procedure can predict aerothermal heating, chemical species concentration, and carbonaceous material ablation rate over the heatshieid surface of re-entry space vehicles. In this study, the gas-gas and gas-surface interactions are established for air flow over a carbon-phenolic heatshield. Two finite-rate gas-surface interaction models are considered in the present study. The first model is based on the work of Park, and the second model includes the kinetics suggested by Zhluktov and Abe. Nineteen gas phase chemical reactions and four gas-surface interactions are considered in the present model. There is a total of fourteen gas phase chemical species, including five species for air and nine species for ablation products. Three test cases are studied in this paper. The first case is a graphite test model in the arc-jet stream; the second is a light weight Phenolic Impregnated Carbon Ablator at the Stardust re-entry peak heating conditions, and the third is a fully dense carbon-phenolic heatshield at the peak heating point of a proposed Mars Sample Return Earth Entry Vehicle. Predictions based on both finite-rate gassurface interaction models are compared with those obtained using B' tables, which were created based on the chemical equilibrium assumption. Stagnation point convective heat fluxes predicted using Park's finite-rate model are far below those obtained from chemical equilibrium B' tables and Zhluktov's model. Recession predictions from Zhluktov's model are generally lower than those obtained from Park's model and chemical equilibrium B' tables. The effect of species mass difhsion on predicted ablation rate is also examined. NomenclatureB' = riz / peueC,, , dimensionless mass blowing rate Ci = mass fraction for species i (C-i) = adatom i, 0 or N C, = Stanton number for mass transfer D = bifurcation diffusion coefficient, m2/s Di = diffision coefficient for species i, m2/s E = total energy per unit volume, J/m3 F = nonlinear equation defined in Eq (24), or Po / fi = diffision factor of species i 1 https://ntrs.nasa.gov/search.jsp?R=20040045247 2018-05-12T22:13:36+00:00Z
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