An experimental study on the lift-off characteristics of triple flames in a diverging channel with fuel dilution

Abstract

When triple flames propagate in a diverging channel, the effects of fuel dilution on the lift-off characteristics of triple flames are investigated. A multi-slot burner with a diverging channel is used to stabilize the lift-off flame with small fuel concentration gradient. It has been reported that there is a maximum propagation velocity at a critical concentration gradient, the inlet concentration gradient of the minimum liftoff height, in an open jet. The enhancement of a diffusion branch is used to increase the propagation velocity of triple flames near the critical concentration gradient. However, the influence of the confined channel on the structure and propagation velocity of triple flames according to the fuel dilution rate needs to be investigated. The lift-off height and propagation velocity of triple flames are examined by employing three types of fuel compositions diluted with nitrogen (N2 0%, 25%, and 50% diluted methane). The fuel dilution reduced the propagation velocity of triple flames in the confined channel primarily due to the decrease in the flame temperature in the premixed branches. Regardless of the different fuel dilution rates, the propagation velocity of triple flames has a maximum value at the critical concentration gradient even though the critical concentration gradient increases with the fuel dilution rate. Furthermore, the critical concentration gradient in the confined channel is larger than that in an open jet due to the enhancement of convective diffusion. © 2007 Combustion Institute. All Rights Reserved.