Blow-Off Velocities of Jet Flames

There have been significant successes in the mathematical modelling of lift-off distances and plume heights of fuel jet flames, and in correlating these parameters experimentally. A dimensionless flow number, or jet velocity, has been developed, with which experimental dimensionless plume heights and flame lift-off distances have been successfully correlated. However, the prediction of blow-off heights presents significantly more difficulty. This is because of the strong non-linearities at blow-off, involving complexities of mixing, strain rates, flamelet burning velocities and curvatures, with localised flame extinctions. Because of these, it is difficult to obtain consistency in the measurement of blow-off heights. Consequently, the practice adopted by Kalghatgi of measuring dimensionless fuel jet velocities at blow-off was followed in the present study. Just before blow-off, strong instabilities and flame oscillations develop, and a correlation of blow-off is proposed in terms of the the last stable dimensionless flow number, before blow-off occurs. The study covers acetylene, commercial butanes, ethylene, hydrogen, methane and propane. Blow-off is characterised from plots of the dimensionless flame lift-off distance against the flow number for each fuel, with identification of the last stable dimensionless flow number prior to blow-off. This enables the dimensionless flow numbers for blow-off to be identified for the different fuels, along with the associated dimensionless lift-off distance.