Near limit flame spread over thin solid fuels in a low convective microgravity environment

Abstract

A few microgravity experiments have reported formation of flamelets at near extinction of freely propagating opposed flow flame over solid fuels. Inspired by these observations, about which little is known so far, an established and elaborate numerical model in 3D is used explore opposed flow flame spread phenomena near oxygen extinction limit in quiescent and low convective environment (with flow velocity, U∞ = 5 cm/s). Simulations were carried out for various fuel widths show that 1 cm wide fuel has the least oxygen extinction limit among fuel widths varying between 0.5 cm and the 2D limit even after accounting for the presence of flamelets over wider fuels (except in 2D limit). Unlike quiescent environment where flamelets were found to be inherently unsteady and eventually extinguished, steadily propagating flamelets were also obtained in low convective space environment. The flamelet oscillations which arise due to thermo-diffusive instability were found to have time period close to sum of times scales in gas phase and solid phase. A scaling analysis showed that fuel area density, flow velocity and fuel Lewis number are key factors that influence size of a steadily propagating flamelets. Several of these features have been observed in experiments and predicted in the present simulations.