超临界压力下二氧化碳在水平管内传热异常特性研究

Study on the Abnormal Heat Transfer Characteristics of CO_(2) Under Supercritical Pressure in Horizontal Tubes

运用FLUENT软件对超临界压力下CO_(2)在直径4 mm、管长1 000 mm水平管内的传热过程开展数值研究。首先分析了热流密度对传热的影响,发现在质量流量140 kg/(m^(2)·s)条件下,上下母线将产生较大壁温差,使传热行为具有异常特性;其次对比和评价了2种浮升力准则,分析了压力对浮升力的影响;最后通过典型截面物性分布揭示了水平管上下母线不同的传热机理。结果表明:热流密度越大,传热恶化现象越明显,浮升力效应越强;BP判别式能更好地预测上述工况下的浮升力分布,且壁温差大小可反映浮升力的强弱;增大压力可使CO_(2)的物性变化更加平缓,浮升力效应随着压力增大而减弱。

The FLUENT software was used to numerically simulate the heat transfer characteristics of CO_(2) in a horizontal tube with the diameter of 4 mm and the length of 1000 mm under supercritical pressure. First, the effect of heat flux on heat transfer was analyzed, indicating that under the condition of mass flow of 140 kg/(m^(2)·s), the top and bottom walls presented a large temperature difference and showed the abnormal characteristics of heat transfer behaviour. Then two kinds of buoyancy criteria were compared and evaluated, and the effect of pressure on buoyancy was analyzed. Finally, the heat transfer characteristics of the horizontal tube walls were revealed via a study on the distributions of profiles’ physical properties. Results show that the larger the heat fluxes are, the more obvious the deterioration of heat transfer and the stronger the buoyancy effect will be achieved. The BPcriterion can better predict the buoyancy distribution under above conditions. Besides, the wall temperature difference can reflect the strength of buoyancy. Increasing pressure can mitigate the changes of CO_(2) physical properties and decrease the effect of buoyancy.