考虑管壁导热的方管内熔盐混合对流传热数值模拟

Numerical simulation of mixed convective heat transfer of a molten salt in a square tube with a thermally conductive wall

熔盐因具有传热能力强、工作温度高、使用温度广、系统压力低、经济适用等优点,成为太阳能热发电系统蓄热工质的理想选择。熔盐在实际应用中会因加热过程的非均匀性产生存在于固体表面和流体间的温差,造成流体工质中的密度梯度,因此出现重力导致的浮升力效应,其叠加到主流流动方向上即形成混合对流。管壁导热会对熔盐混合对流传热过程产生一定的影响。本文对熔盐在水平方管内非均匀加热条件下的单面加热的混合对流过程进行了数值模拟研究,在考虑壁厚的情况下研究了方管单面加热熔盐混合对流传热特性,分析了无量纲参数间的变化关系,并将结果与流型判定图和经典关联式进行对比。结果表明,非均匀加热时,浮升力效应会造成随流动距离增加主流核心区域的形状发生改变,且更加靠近加热壁面。Nu数随Re数、Ri数的增大而增大,局部Nux数随流动距离的深入先减小后增大。与忽略管壁导热数值模拟结果相比,主流核心区形状更加均匀,局部Nux更高且回升位置更加提前,流动特性和传热特性基本保持一致。

Molten salts has been used an a regenerative working medium for solar thermal power because of their good heat transfer ability, high working temperature and temperature range, low system pressure and favorable economics. Non-uniform heating process, however, often results in temperature difference between solid heating surface and the heat transfer fluid, leading to the buoyancy effect and hence mixed convection. Heat conduction through the solid tube wall is likely to exert an effect on the mixed convective heat transfer. This paper numerically studies this effect, specifically the mixed convection process of a molten salt heated by a horizontal square tube surface under a non-uniform heating condition. The wall thickness was considered in the study. The relationship between various dimensionless numbers was analyzed. The results were compared with the flow pattern diagram and classical correlations. The results showed that there was buoyancy effect in the non-uniform heating process with the core zone located near the heating wall and the shape of the core zone changing with the increased flow distance. The Nu number increases with the increase of Re number and Ri number, and Nux decreases first before rises with the depth of flow distance.Compared with the numerical simulation results for a zero wall thermal conductivity, the shape of the mainstream core region was more uniform, the local Nux was higher and the rising position was earlier, and the flow and heat transfer characteristics were consistent. The simulation results were in good agreement with the classical correlation.