液氮通过受热U型管传热特性的数值模拟

对单相液氮通过受热竖直U型管的传热特性进行了三维数值模拟.采用结构化网格，并对近壁面处网格加密,对网格无关性和模型有效性进行了验证.将入口速度、出口压力和均匀壁面热流密度作为边界条件及利用标准的k-ε紊流模型和CFX 4.4求解器求解.模拟结果发现：在U型弯头上游直管中，流体温度形成中心低四周高的分布形式；在U型弯头内，流体被二次流重构，温度较低的流体流向管外侧，温度高的流体则流向管内侧；在U型弯头下游直管中，中心低四周高的温度分布逐渐得到恢复.通过分析，曲率比对下游努赛尔数影响明显，其影响区域可用下游努赛尔数达到极小值来衡量；存在一曲率比，使下、上游努赛尔数百分比达到极小值.最后，将模拟得到的平均努赛尔数与不同的经验关联式作了比较.

Numerical Simulation of Heat Transfer of Liquid Nitrogen Through Heated U-Tubes

A three-dimensional numerical simulation of the heat transfer characteristics of single-phase liquid nitrogen flowing through heated vertical U-tube was conducted. The structured grid was used and the mesh near the wall was refined. Besides, the grid independence and model availability was verified. The velocity inlet, pressure export and uniform wall heat flux were used as boundary conditions. In addition, the standard k-ε turbulence model was adopted and solved by CFX 4.4. It is found that the fluid temperature is lower in the center and higher near the wall in the U-bend upstream pipe, while the fluid temperature is reconstructed by the secondary flow in the U bend: the cold core is flowing to the concave surface, and the high temperature fluid is close to the convex surface; and in the downstream of the pipe, the temperature distribution is restored. The impact of curvature ratio on the downstream Nu number is significant and the affected area can be measured by the minimum Nu value. There is a curvature ratio which makes the percentage of the downstream to the upstream Nu to be the minimum. Finally, the comparison of simulation results and the correlation was conducted.