Dissociative cooling - Effect on stagnation heat transfer of gas mixture injection

Abstract

By embedding a dissociating material into the porous outer structure of a projectile, stagnation-point heat transfer may be reduced by transpiration cooling resulting from the outflow of the dissociated gas products. The principal material considered is ammonium chloride, NHLjCl, which dissociates at temperatures over 613 K, hence providing additional heat-transfer reduction. Stagnation-point heat-transfer solutions for the injection of the dissociation products of NH^Cl into both equilibrium and frozen boundary layers are presented and compared with those for the injection of other gases such as helium. Results show that dissociative cooling has the potential to provide a significant reduction in stagnation-point heat transfer as temperatures rise, because the gas injection rate increases with the temperature of the NH4C1 interface. NomenclatureCi = mass fraction, p//p c p = specific heat per unit mass at constant pressure c p = mass-average specific heat of gas mixture D -multicomponent diffusion coefficient T) = binary-diffusion coefficient / = quantity defined by Eq. (8) h = enthalpy 7 = diffusive mass flux k = thermal conductivity I = pn/(pn)u, Le = Lewis number, D p c p /k M -molar mass ___ M = blowing parameter, p w v w /^/p e^e K N = molar flux Nu = Nusselt number, q" w x/(T e ~ T w )k P = pressure Pr -Prandtl number, H,C P /K q" = heat flux R -gas constant r = cylindrical radius Re = Reynolds number, u e x/v T = temperature u = x -component velocity v = ^-component velocity x = distance along body surface y = distance normal to body surface s = well depth in intermolecular potential r) = transformation defined by Eq. (6) 9 = T/T e K = velocity gradient, du e /8x IJL = viscosity v = kinematic viscosity £ = transformation defined by Eq. (7) p = density a = collision diameter $>ij = function of molar masses, ^Mj/M t 0 = porosity