多尺度分形柱群对流换热特性的尺度效应

Scale effect of convection heat transfer performance in fractal cylinders

针对呈Sierpinski地毯配置的多尺度分形柱群结构,采用有限体积法,Ansys Fluent软件进行数值模拟,研究了其流动换热特性的尺度效应.研究表明:在分形结构中,流体流动流线呈现弯曲和自相似的特征.分形圆柱群与方柱群的总换热量随尺度的变化趋势基本类似,但是分形圆柱群换热性能较好.同时将分形柱群换热量拟合为尺度的幂函数,结果显示:随着分形柱群尺度增加,换热量的指数逐渐增大.分形柱群的对数换热量与尺度近似成线性增长关系.圆柱群的平均换热系数较高,换热性能较好.研究结果可帮助进一步了解多尺度中换热的机理,分形结构对换热更为有利,这意味着在列管式换热器中,有可能采用分形柱群组合,达到增强换热的目的.

Unsteady flows passing a dual-scale fractal structure were simulated using the finite element method and the Ansys Fluent software. Convection heat transfer process around Sierpinski carpet,a kind of fractal structure,is carefully studied and discussed. Both square cylinders and circular ones are investigated in detail. The crooked streamlines passing these kinds of fractal structures exhibit self-similariy. For the relation between the total heat flux and scales,the circular cylinders show the same trend as that of square cylinders. It is found that the heat transfer performance of the circular cylinders is better than that of circular ones owing to larger heat exchange area. The empirical dimensionless correlations for different fractal structure are fitted successfully,which indicate that the total heat flux is a power function of scales. The exponents of correlations increase with the fractal scales. Linear correlation at the forth degree is obtained,which means good heat exchange performance of multi-scale structure. This research would be helpful to understanding heat exchange mechanism for the multi-scale structure well. This means that in the shell and the tube heat exchangers,adopting the combination of fractal column group is likely to achieve the purpose of enhancing heat transfer.