Multidimensional Modeling of Pyrolysis Gas Transport Inside Charring Ablative Materials

Abstract

The behavior of pyrolysis gas transport in arcjet test samples is numerically studied. The simulation of the pyrolysis gas flow inside a porous material is presented, using two different geometries. The effects of allowing the gas to flow out of the sidewall are especially highlighted. Results show that the flow inside the test article is complex, and that the zero-dimensional or one-dimensional assumption made in most material response codes are not necessarily valid for certain geometries. The importance of three-dimensionality for modeling an ablative test article is addressed. Nomenclature A= face area of a control volume, m 2 A i = preexponential factor of solid component i, s −1 E g = overall gas energy per unit volume, J∕m 3 or kg∕ms 2 E i ∕R = activation energy of solid component i over universal gas constant, K E s = overall solid energy per unit volume, J∕m 3 or kg∕ms 2 F = matrix of convective fluxes through the face of a control volume F d = matrix of diffusive fluxes through the face of a control volume G = G x ; G y ; G z inviscid flux of momentum equation; e.g., G x is equal to ∂ϕρu 2 ∕∂x ∂ϕρuv∕∂y ∂ϕρuw∕∂z, Pa∕m H = gas enthalpy, J∕kg K = material sample permeability, m 2 k = solid material thermal conductivity, W∕m · K L = arc length; distance to the origin of Cartesian coordinate system;x 2 y 2 z 2 p , m _ m 0 0 = mass flow rate per unit area; ϕρ g u · n, kg∕m 2 s j _ m 0 0 j = gas momentum; ϕρ g u 2 v 2 w 2 p , kg∕m 2 s n = face normal direction P = vector of primitive variables p = static pressure, Pa p w = sample surface pressure, Pa Q = vector of conservative variables q w = sample surface heat flux, W∕m 2 R = R x ; R y ; R z ; residual of Darcy's law; e.g., R x is equal to −∂p∕∂x − ϕμ∕Ku, Pa∕m R HS = right-hand side of the linear system to be solved S = vector of source terms in a control volume S D = diffusive source term for flow through porous media; −ϕμ∕Ku 2 v 2 w 2 , kg∕ms 3 T = temperature, K T react = reaction initiate temperature, K t = time, s u = u; v; w; velocity, m∕s V = volume of a control volume, m 3 Γ i = volume fraction of virgin solid composite i ρ g = overall density of pyrolysis gases, kg∕m 3 ρ s = overall solid density, kg∕m 3 ρ s i = density of solid component i, kg∕m 3 ϕ = porosity of material ψ i = phenomenological parameter for component i ω g = pyrolysis gas generation rate, kg∕m 3 s ω s i = decomposition rate of solid component i, kg∕m 3 s Subscripts c = char state of the material g = overall pyrolysis gas i = solid species i s = overall solid v = virgin state of the material w = sample surface wall x = first component of Cartesian coordinate y = second component of Cartesian coordinate z = third component of Cartesian coordinate

Numerical Investigation of Thermal Response Using Orthotropic Charring Ablative Material

“…Recently, Weng and Martin [4] and Weng et al [5] showed that, due to the high enthalpy carried by the pyrolysis gas, the gas flow behavior within a charring ablator is crucial to the inner thermal response of the material. In addition, since the gas is eventually blown into the chemical reacting boundary layer [6], correct modeling of the pyrolysis gas is also important to help determine the surface boundary conditions.…”

Three dimensional radiative heat transfer model for the evaluation of the anisotropic effective conductivity of fibrous materials

“…The methodology developed and presented here could be use to obtain accurate values of the effective conductivity of realistic three-dimensional material geometry, such as the ones obtained from computed X-ray microtomography [20]. These values could then be used in volume-averaged material response code [31,32] to increase the fidelity in heat transfer analysis.Moreover, performing such an analysis on a real material would allow to compare to experimental analysis.…”

Protection of pyrolysis gases combustion against charring materials’ surface ablation

“…Table 4 gives the critical velocity result for each Ma. 4 , CO, CO 2 and C 2 H 2 for Ma=8, 9 and 10. It is visible that with an increasing Ma, the types of combustion components increase, the consumption rate of H 2 increases, the production amounts of CH 3 increases at first then decreases, the production amount of CH 4 decreases, the production amount of CO has little difference and the production amounts of H, CO 2 , C 2 H 2 and H 2 O increase.…”

“…The pyrolysis of charring ablation materials and the flow of pyrolysis gases in the material can bring off amounts of heat [4][5][6][7]. Meanwhile, the char on the material surface usually ablates because it reacts with the oxygen in the boundary layer behind the shock wave.…”