Effects of oxygen depletion on soot production, emission and radiative heat transfer in opposed-flow flame spreading over insulated wire in microgravity

Abstract

The main objective of this article is to investigate experimentally and numerically the effects of reduced-oxygen contents on the soot production and emission from solid fuels in microgravity, which constitute an important issue in terms of fire safety for manned space missions. Due to its convenience for the implementation of soot-related optical diagnostics, the configuration of a flame spreading in an opposed flow over thin nickel chromium (NiCr) wires coated by low density PolyEthylene (LDPE) is considered. Experiments are conducted at a pressure of 101.3 kPa and an oxidizer velocity of 150 mm/s. The oxygen mole fraction in the oxidizer, XO2, is varied from 18% to 21% by nitrogen dilution of air. The modeling strategy lies on a surrogate fuel to mimic the combustion of LDPE by preserving its stoichiometry and the laminar smoke-point (LSP) flame height. The numerical model considers a detailed chemistry, a two-equation soot production model involving laminar smoke point (LSP)-based soot formation rates and oxidation by OH and O2, a radiation model coupling the Full-Spectrum correlated-k method with the finite volume method, and a simple degradation model for LDPE. Based on experimental evidence, the soot formation rate is scaled by the adiabatic flame temperature to account for thermal effects due to variation in XO2. The model reproduces quantitatively the increase in flame size, residence time, and soot volume fraction observed experimentally as XO2 is enhanced as well as the transition from a non-smoking to a smoking flame, which occurs for XO2 between 19% and 20%. The increase in soot volume fraction results of a combined enhancement in both residence time, owing to an increase in the fuel mass flow rate, and soot formation rate due to higher temperature in the soot formation region. The radiant fraction increases significantly with XO2 from about 17% for XO2=18% to about 36% for XO2= 21%. This increase in radiative losses is accompanied by a reduction of the temperature in the soot oxidation region. Therefore, for increasing XO2, the soot oxidation process is governed by a competition between oxygen-enhanced conditions that promote the formation of soot oxidizing species and the increase in radiative losses that dampens their formation. For the present flames, the first mechanism prevails as XO2 increases from 18% to 19% whereas the second dominates as XO2 is further increased, leading to smoking flames as actually observed for XO2=20 and 21%. The radiant fraction at the smoke-point transition and the soot oxidation freezing temperature are in line with those reported at normal gravity. Finally, model results show that, whatever XO2, the contribution of radiation to the heating process is negligible ahead of the pyrolysis front and is largely overcome by surface radiative losses along the pyrolysis region.