出口高度对过渡流态后向台阶传热特性的影响

Influence of outlet height on heat transfer characteristics for transitional flow over a backward-facing step

建立了过渡流状态下2维不可压缩后向台阶流的非定常数值计算模型,研究了后向台阶的出口高度H对底面时均努塞尔数Num、流道中流场温度场分布和近壁面流动传热参数等的影响.结果表明:减小H,主峰附近的Num提高,双峰现象明显;增加H,主峰下游的Num提高,主峰和次峰的间距增大;H越小,流道中正旋涡、负旋涡的相互作用越强烈,冷流体和热流体间的对流换热增强,Num增加,但下游底面的温度梯度减小,Num减小;Num的主峰位置与表面摩擦系数为0的位置不保持重合;法向速度分量、温度随着时间的波动程度影响Num的分布.

A two-dimensional incompressible and unsteady numerical model of the backward-facing step was established for transitional flow. The influences of outlet height on time averaged Nusseh number （ Num ） , distributions of flow field and temperature field, flow and heat transfer parameters near wall were also investigated. The results show that when outlet height is reduced, the Num near the main peak position is enhanced with obvious bimodal phenomenon. When outlet height is increased, the Num downstream the main peak position is raised with widen space between main and minor peak. The smaller the outlet height is, the more intense the interaction between positive vortex and negative vortex is, and the convective heat transfer between cold fluid and thermal fluid is strengthened with increased Num. The temperature gradient on the downstream bottom wall is decreased with decreased Num. The Num main peak position and the location for the surface friction coefficient of zero are not always overlapped. The distribution of Num is affected by the fluctuation level of normal velocity component and temperature over time.