Consumption speed and burning velocity in counter-gradient and gradient diffusion regimes of turbulent premixed combustion

Flame surface density, local consumption speed, and turbulent burning velocity of turbulent premixed V-shaped flames were investigated experimentally. A novel experimental apparatus was developed, which allows for producing relatively weak, moderate, and intense turbulence conditions. The experiments were performed for three turbulence intensities of about 0.02, 0.06, and 0.17, corresponding to weak, moderate, and intense turbulence conditions, respectively. For each turbulence condition, three mean bulk flow velocities of 4.0, 6.2, and 8.3 m/s, along with two fuel–air equivalence ratios of 0.6 and 0.7 were tested. The results show that intensifying the turbulence conditions significantly affects the flame front characteristics. Specifically, with increasing the turbulence intensity, the mean-progress-variable at which the flame surface density features a maximum decreases from values greater than 0.5 to values smaller than 0.5. This was argued to be linked to the switch of the tested experimental conditions from regime of counter-gradient to that of gradient diffusion. Spatially-averaged values of the local consumption speed and burning velocity were investigated in the present study. The results show that, for all experimental conditions tested, the local consumption speed varies between the unstretched laminar flame speed and 1.5 times the unstretched laminar flame speed. However, the spatially-averaged burning velocity is significantly dependent on the experimental conditions tested. Specifically, the result show that, for relatively intense turbulence conditions, the spatially-averaged turbulent burning velocity is proportional to the root-mean-square of the streamwise velocity.