Experimental studies of freely propagating turbulent premixed kernels in low speed channel flow

To gain new insight into highly turbulent flame dynamics, a facility is built that allows for growth and propagation of spherical flame kernels in a convective mean flow without mean-shear. The high turbulent intensities are achieved using an active vane grid, based on the design by Makita.1 A small-scale divergent duct is developed to support the mean flow with turbulent characteristics similar to that investigated by Comte-Bellot and Corrsin.2, 3 The goal is to generate nearly homogeneous (in planes parallel to the grid), isotropic and weakly decaying turbulence. The passive and active vane grid result in turbulence intensities of Tu = 4: 1%-11: 4% and Taylor-Reynolds numbers of Re_λ =179-402 at the first position of interest with a flow Mach number of Ma ≈ 0: 089. Isotropic turbulence is achieved over a wide range of scales smaller than the duct. For combustion studies, the facility is designed to allow the injection of methane into the mean flow upstream of the turbulence generator. Flame kernels are generated via a laser ignition system and the flame development studied. Measurements are initially made in the cold flow with hot-wire probes. Subsequently high resolution, low-speed planar PIV, high-speed stereo PIV and OH-PLIF is conducted, separately and simultaneously in the cold and reacting flow cases. The growth rate of the kernels yields the normalized turbulent flame speeds. The approach to normalization and data points by Chaudhuri et al.4 are used to validate and compare our results. It is found that the data points collapse well using correlations that allow for thermo-diffusive effects, and the flames exhibit large amounts of stretching and wrinkling due to the imposed turbulence. With the ability to compare CTA and PIV results, we further validate our optical measurement techniques, which is required for the evaluation of flowfield properties in reacting flows.