An experimental correlation of turbulent burning velocities for premixed turbulent methane-air combustion

This paper is devoted to the general correlation of turbulent burning veloc ities in terms of straining rates for premixed flames propagating in intens e turbulence. This problem was investigated by the Leeds group led by Profe ssor Bradley and many other researchers. We present here a new methodology based upon a downward propagating premixed CH4-air flame through a nearly i sotropic turbulent flow field with a pair of specially designed ion probes for quantitative measurements of turbulent burning velocities. The improvem ents are that the flame propagation is not influenced by the ignition sourc e and the unwanted turbulence from walls, effects of buoyancy and pressure rise due to burning are minimized, and a greater parameter range than hithe rto is covered. The results show that both the turbulent burning velocity b ending and the vitality of turbulent premixed flames are certain and surpri sing. Logarithmic plots of turbulent burning velocities S-T/S-L - 1 against the turbulent intensities u'/S-L reveal a transition, where S-L is the lam inar burning velocity. Across the transition, the slope n changes from posi tive to negative when values of u'/S-L and/or Karlovitz number are greater than some critical values. This transition seems to correspond to the Klimo v-Williams criterion that separates corrugated flamelets from distributed r eaction zones. Interestingly, no global quenching of pre mixed turbulent fl ames is observed, even at u'/S-L approximate to 40, a value significantly h igher than in most previous measurements. At a fixed u'/S-L, values of the S-T/S-L data vary with the equivalence ratio phi. This indicates that the c ommon expression of the form S-T/S-L = 1 + C(u'/S-L)(n) cannot be applicabl e generally, because values of the constant C are different for different m ixture compositions. It is found that all of the present data with differen t values of phi can be approximated by a simple expression, (S-T - S-L)/u' approximate to 0.060Da(0.59), where Da is the Damkohler number. Hence a bet ter correlation of turbulent burning velocities in terms of straining rates for premixed turbulent (methane-air) combustion is proposed.