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

Abstract

This paper is devoted to the general correlation of turbulent burning velocities in terms of straining rates for premixed flames propagating in intense turbulence. This problem was investigated by the Leeds group led by Professor Bradley and many other researchers. We present here a new methodology based upon a downward propagating premixed CH4–air flame through a nearly isotropic turbulent flow field with a pair of specially designed ion probes for quantitative measurements of turbulent burning velocities. The improvements are that the flame propagation is not influenced by the ignition source and the unwanted turbulence from walls, effects of buoyancy and pressure rise due to burning are minimized, and a greater parameter range than hitherto is covered. The results show that both the turbulent burning velocity bending and the vitality of turbulent premixed flames are certain and surprising. 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 laminar burning velocity. Across the transition, the slope n changes from positive 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 Klimov–Williams criterion that separates corrugated flamelets from distributed reaction zones. Interestingly, no global quenching of premixed turbulent flames is observed, even at u ' S L ≊ 40, a value significantly higher than in most previous measurements. At a fixed u '/ S L, values of the S T/ S L data vary with the equivalence ratio ϕ. This indicates that the common expression of the form S T/ S L = 1 + C ( u '/ S L) n cannot be applicable generally, because values of the constant C are different for different mixture compositions. It is found that all of the present data with different values of ϕ can be approximated by a simple expression, ( S T – S L)/ u ' ≊ 0.06 Da 0.59, where Da is the Damkohler number. Hence a better correlation of turbulent burning velocities in terms of straining rates for premixed turbulent (methane–air) combustion is proposed.