Premixed turbulent burning during explosions

Abstract

Experimental measurements during the early stages of spark-ignited explosions of turbulent premixed methane-air mixtures have shown the amount of unburnt gas behind the visible flame front to be quite large, particularly at high turbulent velocities. All measurements were made during the prepressure period. The explosion bomb was fitted with four rotary variable-speed fans that controlled the isotropic r.m.s. turbulent velocity. The true mass burning rate was measured by two different techniques. One involved measurements of flame speed and burning velocity with high speed cine schlieren photography. The other involved the intensity of emission from the CH radical. The two techniques gave values that were in good agreement with each other. A burning velocity based upon the actual mass rate of burning might be but a quarter of the burning velocity as conventionally defined. The increase in burning velocity during any one explosion is attributed to the influence upon the flame-front of an ever-broadening frequency band of the turbulent spectrum, and hence of the r.m.s. turbulent velocity associated with this band. Experimental dimensionless correlations are given for curves of constant flame strain that relate the mass burning velocity and the frequency band r.m.s. velocity. In terms of gasoline engine performance, an initial increase in turbulent velocity increases the mass of gas burnt at a given time after ignition. However, there are diminishing returns in this respect eventually a turbulent velocity is attained beyond which the pumping losses must be unacceptably high.