Experimentally measured burning rates of premixed turbulent flames

Abstract

Integrated burning rates of premixed flames are measured in turbulent opposed axisymmetric flows. The measurement technique is based on the conservation of reactant mass flowing in and out of a control volume. The flow velocities are measured by laser Doppler velocimetry (LDV) using an oil seed to track only reactants. Simultaneous Mie scattering from the LDV probe volume is used to measure the mean progress variable. The mean mass flux of reactants is calculated by combining the velocity and progress variable data. Sixteen experiments were conducted to study the effects of varying turbulent kinetic energy, bulk strain rate, and equivalence ratio of methane-air flames. Experiments were compared in terms of integrated mean rates of creation of products across the flame brush, which have been expressed as turbulent burning speeds. The turbulent burning speed was observed to increase with increased levels of turbulent kinetic energy but also decrease because of the effects of turbulent and bulk straining on the flame. Mixtures near stoichiometric were sufficiently robust that their turbulent burning speed increased monotonically within the range of turbulence tested. Leaner mixtures showed an initial increase in burning speed that became impaired at higher turbulence and bulk strain rates. The integrated burning rates were shown to be two to six times less than that estimated from measured propagation speeds. These results emphasize the differences that exist between propagation speed and burning rate in diverging flows. A refinement to this experiment is proposed to allow the local mean rate of creation of products to be measured.