Numerical Analysis of Gas and Soot Radiation in Hybrid Rockets with Pyrolyzing Fuels

Computational fluid dynamics simulations are a valuable tool during the design phase of hybrid rockets. An accurate prediction of fuel regression rate for all mass fluxes and operating conditions requires, however, to estimate both convective and radiative contributions to the wall heat flux. While convection is usually the main contribution, radiation can significantly impact the regression rate at low mass fluxes. However, due to the large computational cost of radiative calculations, coming from the directional and spectral nature of radiation heat transfer, this effect is seldom taken into account. In this work, a numerical framework based on Reynolds-Averaged Navier-Stokes simulations, with submodels accounting for chemistry, turbulence, gas-surface interaction, and soot generation is coupled with a radiative heat transfer tool for the evaluation of spectrally resolved gas and soot radiation. Nine firing tests of a 1 kN class hybrid rocket burning gaseous oxygen and hydroxyl-terminated polibutadiene are analyzed, with oxygen mass fluxes ranging from 40 to 225 kg/(m2 ·s), revealing that gas radiation may increase the regression rate up to 20%. Soot calculations performed employing C2H2 as precursor show how, in fuel rich conditions, a significant share of the total heat flux may come from particulate radiation.