(1-25) Entropy Analysis of Microscopic Diffusion Phenomena in Diesel Sprays((FS-2)Fuel Sprays 2-Diesel sprays)

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Rapid mixing of fuel and air is an essential factor in improving combustion and emissions of diesel engines. Thus extensive investigation has been made to increase turbulence in the diesel combustion process by increasing swirl ratios, injection velocity, and modifying combustion chamber configurations. However, the relationship between the microscopic 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 the degree and size scale of heterogeneity. This paper investigates local diffusion phenomena with focusing on scales of fuel cloud and eddies based on a newly developed entropy method. The entropy analysis is based on the concept of statistical entropy, and it identifies the degree of homogeneity in the fuel concentration. The entropy values increase with the progress of uniformity in the diffusion process. Figure A1 is an example of entropy values comparing two different states of heterogeneity. The pictures show tracers mixed in fuel jets and were taken by a laser sheet. The entropy value of the N_2 jet image is higher than the value of the C_2H_4 image, indicating the higher degree of diffusion. By analyzing the speed of change in the entropy values, the diffusion intensity of the fuel cloud can be estimated, and it is also possible to identify size scales of the heterogeneity. Using the entropy analysis, the microscopic structure in turbulent jets and diesel sprays was investigated. Figure A2 is a picture of a turbulent jet simulating a diesel spray. In the experiment, a fluorescent compound is injected in to water. The PIV method was used for the analysis of the velocity distribution, and the diffusion intensity was obtained by the analysis of the local entropy. The results show that the diffusion intensity is the highest in the vicinity of the nozzle exit, and the heterogeneity scale is the smallest here. The heterogeneity scale increases gradually along the spray axis towards the downstream, with smaller size scales in the large clouds. In the downstream region, small-scale structures diffuse and become unclear, while large scale structures clearly remain. The paper details the microscopic structure of the heterogeneity in diesel sprays, and it demonstrates availability of the entropy method.