Influence of gap height and flow field on global stoichiometry and heat losses during opposed flow flame spread over thin fuels in simulated microgravity

This study characterizes thin fuel opposed flow flame spread in simulated microgravity for a range of gap heights and airflow velocities in a Narrow Channel Apparatus (NCA). One objective was to estimate gap heights that suppress buoyancy without promoting excessive heat losses to the channel walls. A corollary of this objective was to assess the dependence of heat losses on the channel height. A second objective was to determine the influence of global combustion stoichiometry on simulated microgravity flame spread in the NCA. Whatman 44 filter paper was used for NCA gap heights ranging from 6-20 mm (half-gap below and above sample) and average opposed flow velocities 1-40 cm/s. Flames at low flows were fuel rich when the forced flows were of the same magnitude as the diffusive flow. For thin fuels, a full gap of 10 mm (5 mm half-gap) provided a compromise between buoyancy suppression and heat loss. Calculations were made of flame stoichiometry and of the influence of the velocity profile on flame spread rates (comparing it with previous theory). This part of the analysis provided support for the velocity gradient theory of flame spread. The information provided in this work on the theoretical nature of opposed flow flame spread over thin fuels is based on experimental measurements in simulated microgravity conditions.