Radiation-kinetics interactions: A comparison of opposed-flow flame spread in a low-velocity microgravity and low-pressure downward environments

Abstract

Flame spread over solid fuels in an opposed-flow environment has been studied for decades. While the thermal regime, where finite-rate kinetics and radiation are ignored, has a closed-form theoretical solution for the spread rate, only experimental and numerical studies have been used to correlate spread rate data in regimes where kinetics and radiation play important roles. In a low-opposing flow environment such as in microgravity, radiation has been shown to play a dominant role due to the increased residence time leading to radiative flame extinguishment. Experiments with downward flame spread in low pressure environments show remarkable similarities with microgravity flames, and this analogy has been used to replicate and expand the microgravity data. In this study, a scale analysis is used to identify non-dimensional numbers - the Damköhler number and the radiation number controlling the finite-rate kinetic and radiative losses - to explore the similarities and differences between a low-pressure downward configuration and a quiescent low-gravity environment. A comprehensive numerical model with finite-rate kinetics and gas and solid phase radiation is used to explore flame extinguishment by reducing ambient pressure and gravity level independently. It is shown that a reduction of pressure in a downward configuration accompanies enhanced radiative loss and finite-rate chemical effects, a combination that brings about flame weakening leading to extinction. When the gravity level is reduced, the radiation and Damköhler numbers increase. While the enhanced radiative losses cool the flame significantly, the increased residence time allows sufficient combustion for a cooler flame to sustain steady spread until radiative extinction occurs at a very low gravity level. Despite the radiative similarities, the role of finite-rate kinetics is shown to be completely opposite in the two configurations.