Low pressure flame blowoff of the stagnation region of cast PMMA cylinders in axial mixed convective flow

Abstract

Low-pressure blowoff experiments were conducted with a stagnation flame stabilized on the forward tip of cast PMMA rods in a vertical wind tunnel. Pressure, forced flow velocity, gravity, and ambient oxygen concentration were varied. Both normal gravity and microgravity tests were conducted to determine the influence of buoyant flow velocity on the blowoff limits. A linear shift of a buoyant flow of ~ 25 cm/s was adequate to match the microgravity data. The flame standoff distance before blowoff varies with the inverse square root of pressure and stretch rate via the gas phase length scale δ = (α/a)1/2, where the thermal diffusivity varies inversely with pressure. For a constant critical Damkohler number, the reaction rate must increase in tandem with the flow rate. The flame standoff distance responds to environment changes to maintain both the critical Damkohler number and the molar flux of oxygen along the blowoff boundary. Via Fick's law, there is a ubiquitous linear relationship between the partial pressure of oxygen at blowoff and the total pressure regardless of forced flow velocity and gravity level. At very low pressures, reactant leakage appears to cause a two-stage reaction zone. For very dilute oxidizer mixtures, an apparent high pressure blowoff limit is found, believed to be caused by chain-terminating three body reactions. The overall reaction order becomes negative along the high pressure limit. A normal gravity materials flammability screening methodology is suggested for spacecraft exploration atmospheres where the blowoff partial pressure of oxygen versus total pressure blowoff limits are measured for the expected exploration atmospheres (oxygen concentrations). As long as the blowoff boundary is above the Earth-normal partial pressure of oxygen (21 kPa) threshold, the material is safe for use in exploration spacecraft.