The effects of volume expansion and laminarization on turbulent mixing in a diffusion flame

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This study aims at the clarification of the non-equilibrium process and turbulence generation in the diffusive combustion. The contents of the present study are summarized as follows :1. Particle image velocimetry(PIV)for measuring velocity vectors was applied to the flow in a methane jet flame and a cold methane jet. Characteristics of turbulent eddies were clarified from the distribution of fluctuating velocity and vorticity. Results show that eddies generated inside flame front entrain the combustion products into the fuel stream, at the same time the dissipation of turbulence eddies takes place by the laminarization due to local heat-release.2. Time-repetitive measurements of gas flow using cross-correlation particle image velocimetry is developed. The velocity vector is accurately estimated from the interpolated cross-correlation function. The application of this method to the flow in a methane jet flame and a nitrogen jet is demonstrated, and the distributions of the ensemble-averaged velocity and turbulence intensity are calculated from the change of velocity vectors with time. The results show that this method may successfully give the feasible information about turbulence characteristics.3. Numerical simulation were performed to predict fluid motions induced by interactions between density and pressure gradients due to the presence of a vortex string situated in a deviated position from the jet flame axis. Deformation and stretching of the flame front take place once the vorticity production becomes stronger than the dissipation due to viscosity. When either the pressure gradient or the density gradient is high, the vorticity production proceeds at a high rate, which promotes the deformation of flame front and accelerates the heat release.