Diffusion flames and their flickering motions related with Froude numbers under various gravity levels

Abstract

Combustion phenomena related to Froude number and Strouhal number were investigated in diffusion flames under various gravity levels. The Froude number of the fuel jet was controlled by gravity level, diameter of the nozzle, fuel properties, and fuel flow rate. Artificial gravity was created by a centrifuge and its level was varied from 1 G to 14 G. On the other hand, microgravity condition was formed by a drop tower. CH 4, C 3H 8 and CH 4 + C 3H 8 (54:46 mol %) were used as the test fuels. Many kinds of diffusion flames were observed on nozzles ranging in diameter from 0.5 to 10.0 mm. Flame lengths were measured and correlated by a dimensionless parameter which was proposed by Altenkirch et al. Using their parameter, an empirical equation was obtained as follows: L f / d = 0.58 Re 0.667 Fr 0.089, where L f was the average flame length and d was nozzle diameter, where Re is the Reynolds number (5 ≦ Re ≦ 570) and Fr is the Froude number (4.2 × 10 −4 ≦ Fr ≦ 5.3 × 10 3). Since gravity levels of experiments were accounted for via the Froude number, this equation could express the flame lengths at various gravity levels (low- and high-gravity conditions). Also, flickering frequencies under various gravity levels (1.0–5.0 G) were measured. Over a wide range of fuel jet velocity (from 0.03 m/s to 36.2 m/s), flickering frequencies were correlated by two relations coupled through the Froude number. One relation was St ∝ Fr −0.50 for low Froude number conditions, and the other was St ∝ Fr −0.41 for high Froude number conditions. It meant that the Froude numbers of the low and high levels were dividing criteria for the flickering frequencies, and that flickering motions expressed by these two equations were affected by buoyancy in different ways.