Modelling Gas-Phase Radiation for Creeping Flame Spread at Microgravity
by
Robert A. Altenkirch
Mississippi State University
Coauthors: Matthew F. Bundy, Lin Tang, Subrata Bhattacharjee

For creeping flame spread over solid fuels in an opposing flow configuration at microgravity, gas-phase radiation is a controlling forward (propagating) heat transfer mechanism. Here two relatively computationally sophisticated methods of accounting for gas-phase radiation are compared against each other. Radiation source terms for the Navier-Stokes equations were calculated with both methods for a computational model for flame spreading over a cylindrical, polymethylmethacrylate solid.

The first gas-phase radiation method is the global balance radiation method (GBRM). Gas radiation is treated as a pure loss in the energy equation with a constant absorption coefficient that is calibrated by demanding that the energy loss computed with this absorption coefficient is identical to that determined from solving the equation of radiative transfer for an optically thin gas with a distribution of species and temperature throughout the flame.

The second gas-phase radiation method employs a discrete transfer method (DTM) and is developed in order to understand better how radiation influences flame structure and flame spread behavior. In the DTM, energy entering and leaving discrete volumes within the flame is accounted for so that the source terms in the Navier Stokes equations are local ones. GBRM in radiation modeling is accurate in predicting the total radiative heat loss from the flame, but it is not capable of accounting for variations in local emission within the flame. The results of the modeling effort show that the GBRM tends to over-predict local radiative heat losses from the flame compared to the DTM, and flame structure is affected.

Date received: April 22, 1999