Laminar jet methane/air diffusion flame shapes and radiation of low air velocity coflow in microgravity

摘要

Observations based on short-duration experiments under microgravity of the characteristics of laminar jet diffusion flames burning in coflowing air conditions are described. Experimental conditions were such to establish a flow with Reynolds number of 140 and low air flow velocities of 0-0.5 m/s to produce steady laminar flames. Previous studies ignored effects of air flow velocity with small air stream Froude number, indicating that flame length and diameter were independent of air flow velocity. Distinct from previous studies, we introduced integrated flame shape (maximum flame diameter and ratio of flame length to maximum flame diameter) and residence time formulas, the coflowing air effects on the flame length and maximum diameter together with the flame oscillation, flame color and radiation were investigated both in microgravity and normal gravity. The experimental results obtained showed that the characteristics of microgravity laminar jet diffusion flames were significantly affected by air-flow velocities for the absence of buoyancy. Under microgravity, with increasing coflowing air velocity, the mixing rate of fuel and air increased and the flame sheet moved closer towards the nozzle, maximum flame diameter decreased with air flow velocity increasing; in contrast, the air velocity had little effect on flame shapes in normal gravity. The microgravity flame exhibited no oscillations with the absence of buoyancy, whereas in normal gravity, flame oscillation frequency increased accompanied by a smaller amplitude with air velocity increasing. In microgravity, the residence time for soot converted by the local flow also decreased with air velocity increasing; the radiative heat loss flux of the flame then decreased, but changed little if residence time was sufficiently large. Compared with microgravity flame, the flame radiation heat loss flux of normal gravity flame was smaller with a much shorter residence time.