Study on Relative Scales between Fuel Clouds and Turbulence in a Transient High-Speed Combustion

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Rapid mixing of fuel and air is essential to improve combustion in diffusion flames, and an understanding of the microscopic structure in the diffiision process is important. However, the relationship between the structure of the heterogeneous distribution of fuel clouds and the local turbulence structure is not well understood. Additionally there is no appropriate index for the analysis of degree and scale of the heterogeneity. This research investigates a method for identifying the homogeneity degree and the scale of clouds : in this study large-scale cluster of fluids, for example fuel vapor (or air), is termed "cloud". The method is based on the entropy concept of statistical dynamics, and it identifies the homogeneity degree of the fuel concentration. Entropy values increase with the progress of uniformity in diffusion processes, and by analyzing entropy values it is possible to identify the scale of the heterogeneity.The microscopic structure of turbulent jets and diesel spray flames were investigated using entropy analysis. The results show that the diffusion intensity is highest at the vicinity of the nozzle exit, and that the heterogeneity scale increases downstream along the spray axis, with smaller size scales remaining inside larger clouds. Downstream, small-scale structures diffuse while large-scale structures remain clear. The analysis was applied to a spray flame with jet impingement. The jet impingement clearly enhanced the combustion, and the entropy analysis indicated that the impingement broke fuel clouds into a number of clearly distinguishable clouds and this resulted in faster diffusion and faster completion of combustion. The research demonstrates the possibility of the entropy method.