Assessment of counterflow to measure laminar burning velocities using direct numerical simulations

The laminar burning velocity is a fundamental property that is extensively used in the study and modelling of premixed combustion processes. A counterflow flame configuration is commonly used to measure this quantity for different combustion systems. In this procedure, the burning velocities are typically measured at various low stretch conditions and the unstretched burning velocity is extrapolated from these measurements. This extrapolation is done assuming a theoretically one-dimensional system along the centre-line. We analyse the validity of this assumption by performing DNS studies with finite rate chemistry of the experimental counterflow configuration. The extrapolation process using one-dimensional computations is performed on the DNS data and the extrapolated value is compared to the computed laminar burning velocity for the chemical mechanism used. We show that the assumption works well if the nozzle exit velocity has a nearly top-hat profile. For non-uniform velocity profiles, it is shown that the temperature curvature at the centre-line becomes important. This effect cannot be captured by the one-dimensional formulation. Thus, experimental studies measuring laminar burning velocity need to ensure that the nozzle velocity profile is very close to uniform. The extrapolation to zero stretch using 1D counterflow simulations can be performed in different ways. Based on the results obtained in this paper, a simple and accurate extrapolation method is proposed.