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

快速混合燃料和空气是必不可少的，以改善燃烧的扩散火焰，并在扩散过程中的微观结构的理解是重要的。然而，燃料云的非均质分布结构与局地湍流结构之间的关系尚不清楚。此外，还没有合适的指标来分析异质性的程度和规模。本研究探讨了一种识别云的均匀度和尺度的方法：在本研究中，如燃料蒸气（或空气）的大规模流体簇被称为“云”。该方法基于统计动力学中的熵概念，对燃油浓度的均匀度进行了识别。熵值随扩散过程均匀性的增加而增加，通过分析熵值可以确定扩散过程的非均匀性尺度。采用熵分析方法研究了湍流射流和柴油喷射火焰的微观结构。结果表明：在喷嘴出口附近扩散强度最大，沿喷雾轴向下游扩散不均一性尺度增大，较大的云内分布着较小的尺度；下游，小规模结构扩散，而大规模结构仍然清晰。该分析应用于具有射流撞击的喷射火焰。射流撞击明显增强了燃烧，熵分析表明，撞击将燃料云分解成许多明显可区分的云，这导致了更快的扩散和更快的燃烧完成。研究证明了熵法的可行性。

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.