Influence of equivalence ratio on turbulent burning velocity and extreme fuel consumption rate in lean hydrogen-air turbulent flames

Abstract

• The extreme fuel consumption rate (FCR) exhibits a high degree of universality. • The time-averaged extreme values are very close to the peak critically strained laminar flames. • This work supports a corner-stone hypothesis of the leading point concept of premixed turbulent burning. Unsteady three-dimensional Direct Numerical Simulations of seven statistically one-dimensional, planar, highly turbulent, complex-chemistry, lean H 2 -air flames are performed using either mixture averaged or equidiffusive model of molecular transport. The equivalence ratio is varied from 0.35 to 0.70 and the Karlovitz number Ka is varied from 3 to 565. Normalized turbulent burning velocities U T /S L are strongly increased when using the mixture-averaged model, with an increase by a factor of 4.1 being documented even at Ka as high as 565. Here, S L is the laminar flame speed. Moreover, the increase in U T /S L is significantly more pronounced in leaner flames, which are characterized by a thinner reaction zone and a larger Zel’dovich number. Furthermore, U T /S L is increased by the turbulence length scale. The extreme (maximum over the computational domain at a single instant) local values of fuel consumption rate (FCR) exhibit a high degree of universality, i.e., in all studied cases and at all instants, these rates are close to the peak values of FCR obtained from the counterpart critically strained, twin, counter-flow laminar premixed flames. This finding appears to directly support a corner-stone hypothesis of the leading point concept of premixed turbulent burning, thus, suggesting the use of characteristics of the critically strained laminar premixed flames as input parameters for models of turbulent combustion of lean H 2 /air mixtures.