Laboratory Simulation of Flamelet and Distributed Models for Premixed Turbulent Combustion Using Aqueous Autocatalytic Reactions

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Abstract
Flamelet models have been widely applied to predict premixed turbulent combustion because of their simplicity for the description of chemical features in a turbulent flow field. We had used an aqueous autocatalytic reaction which produced an irreversibly propagating front with characteristics closely matching many of those assumed by flamelet models, to simulate premixed turbulent combustion. We then studied experimentally the influence of turbulence on this reaction-diffusion propagating front. The turbulence was generated by a pair of vertically vibrating grids in a chemical tank and was found to be nearly stationary and isotropic in the core region between the two grids, as verified by laser Doppler velocimetry. Visualization of these turbulent propagating fronts in the nearly isotropic region was obtained using the chemically reacting, laser-induced fluorescence (LIF) technique. In this paper, these planar LIF images were then processed to extract the mean reaction progress variable (c¯), the variance, and the probability density function (pdf) of the progress variable. Markstein numbers of these chemical fronts are probably close to unity. Results of the progress variable pdf revealed a bimodal distribution at a low turbulent Karlovitz number (Ka) with nearly zero probability of intermediate values of c. At higher Ka, the pdfs seemed to show significant probability of partially reacted fluid, in support of the theoretical description proposed by Pope and Anand (1984). Values of the turbulent burning velocities (ST ) were also extracted from successive images. When the turbulent Karlovitz number is less than 5, measurements of front propagation rates ( UT ≡ ST / SL ) as a function of the normalized turbulent intensity (U ≡ u′/ SL ) show a roughly linear increase of UT with U. Values of UT are found to be much lower than those proposed by Bray (1990) and by Pope and Anand (1984), but in good agreement with a Huygens propagation model employing renormalization group analysis. At Ka > 10, UT departs from the linearity and behaviour or these propagating fronts possibly suggests that modes analogous to distributed combustion are observed. For high Ka, values of UT are compared to a classic model. These results are also compared to premixed gaseous experiments.

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