Three-dimensional spatial flamelet statistics for premixed turbulent combustion modeling

Abstract

Flamelet models are now commonly used to predict premixed turbulent combustion. These include the Bray-Moss-Libby (BML) model in which flamelet statistics and, thus, mean reaction rate are deduced from a mean crossing frequency. Recently, Shy et al, applied experimentally an aqueous autocatalytic reaction system which produced propagating chemical fronts with characteristics that closely match those assumed by flamelet models to simulate premixed turbulent combustion. In this work, full spatial statistics of these fronts propagating in a nearly isotropic turbulence that is generated by a pair of vibrating grids in a chemical tank are measured for the first time. Visualization is via a high-speed, successive planar chemically reacting, laser-induced fluorescence imaging of a synchronized swept laser beam, combined with a high-speed data acquisition system. It was found that the integral length scale of flame wrinkling ( y ), evaluated along contours of a reaction progress variable is nearly constant only for values of between 0.1 to about 0.7, while y decreases gradually as increases further to the product side. Its averaged magnitude ( y ) from all values of is measured to be larger than the integral length scale of turbulence ( L y ). Values of y decrease with a turbulent Karlovitz number ( Ka ), and y approaches L I when Ka >4. This result differs with previous gaseous flame results, suggesting that the BML model may need a precise closure for y . It is found that for Ka >1, islands of reactants are present but extensive disconnected regions do not exist, and most of their sizea are probably not greater than L 1 . The overall mean cosine value of the flamelet crossing angle is measured to be 0.65 in contrast to 0.5 found for Bunsen flames. Other parameters of interest, such as contours, crossing frequencies, and actual crossing angle are also reported.