Scaling Sub-Surface Layer Circulation Induced by Pulsating Flame Spread over Liquid Fuels

Abstract

The pulsating flame spread over liquid fuels consists of main-pulsation with 0.5 to 2 Hz frequency and sub-pulsation with 5 to 10 Hz frequency. The former originates in existence of a cold temperature valley at the liquid surface ahead of the spreading flame. The cold temperature valley is formed by the surface wave in connection with a sub-surface layer circulation. In this study, the instability analysis for the liquid surface ahead of a flame leading edge was performed to clarify the onset of surface wave. Moreover the effect of gravity on surface wave generation was examined. The theoretical result shows that the onset of surface wave is firstly controlled by Marangoni force and secondary by buoyancy force. The critical condition for onset of surface wave was expressed to the non-dimensional form. Three dimensionless parameters, the Marangoni number, the Weber number and the Froude number include the ratio, ħ/L, of the characteristic length, L, and the depth, ħ, of sub-surface layer circulation (shown in Fig. 1). F1 The circulation scale for seven different thickness of liquid fuel from 2 to 20 mm was measured using a schlieren photograph and thermography. The ratio, ħ/L, decreases with decreasing fuel layer thickness less than 5 mm and consists to μg condition at the fuel layer thickness of 3 mm.