Experimental study of methane-air premixed flame extinction at small stretch rates in microgravity

Abstract

Extinction experiments on counterflow methane-air premixed flame at small stretch rates were conducted under microgravity conditions to measure the fuel concentration of the mixture at extinction over a range of very low stretch rates and to observe extinction characteristics of small stretch-rate flames in which radiative heat loss may be dominant. By employing a low-speed counterflow system and microgravity, it is possible to remove the effects of buoyancy, conductive heat loss to the burner, and flame curvature on the flame extinction simultaneously. Consequently, stable counterflow twin flames at low stretch rates can be established, and hence excellent conditions for extinction measurements and observations can be realized. Stationary counterflow premixed flames at stagnation velocity gradients ranging from 4 to 20 s ‫1מ‬ were successfully established and the influence of the stagnation velocity gradient on extinction was examined under conditions of a microgravity field of 10 ‫4מ‬ to 10 ‫5מ‬ g with a duration of 10 s. This field is generated by the 490-m drop shaft of the Japan Microgravity Center (JAMIC) in Hokkaido, Japan. Results show that extinction limits strongly depend on the stagnation velocity gradient even in the range of low stretch rates, and there is a turning point on the left portion of the extinction limit curve. A recent numerical calculation involving radiative heat loss indicated the existence of a radiation extinction limit at a certain low stretch rate, in addition to the stretch extinction limit at a large stretch rate. This suggests that the existence of the turning point observed in the present experiment may be the result of radiative heat loss