管道倾斜角度对超临界CO_2管内换热特性的影响

Effect of Tube Inclination Angel on Heat Transfer Characteristics of Supercritical CO_2 in Tube

在恒热流加热工况下,对超临界CO2在不同倾角的微细圆管内混合对流换热进行了数值模拟。采用FLUENT软件分析了不同倾角时管内截面温度、轴向速度、二次流、上母线传热系数、周向壁面温度和Nuw的变化规律,并引入相对二次流动能定量表示二次流强度。研究发现:倾斜管内顶部流体温度高于底部,周向Nuw在底部高于顶部,速度分布不是中心对称且其峰值出现在管中心轴线下侧;浮升力引发的二次流先增大后减小,且在靠近入口处达到峰值;倾斜管内上母线温度高于下母线,上母线传热系数在拟临界温度附近达到峰值。通过水平管中浮升力判据,得到了浮升力对对流换热的影响规律。

Laminar mixed convection heat transfer of supercritical CO2 in miniature circular tubes with different inclination angles and under constant heat flux heating condition was determined through numerical simulation. The distribution and variation of the cross-sectional temperature, axial velocity, secondary flow, the top generatrix heat transfer coefficient, the circumferential wall temperature and the Nusselt number were analysed by using the FLUENT software for different tube inclination angles, and the relative secondary flow kinetic energy was introduced to quantitatively express the intensity of the secondary flow. The results show that the top fluid temperature is higher than the bottom＇s, the bottom circumferential Nusselt number is higher than thetop^s, the distribution of axial velocity is not centrosymmetric and the velocity reaches peak value in the lower side of the tube center axis. The buoyancy-induced secondary flow first increases, then decreases, and reaches the maximum near the entrance. The top generatrix heat transfer coefficient reaches peak value near the critical temperature and is much higher than the bottom generatrix＇s. The influence of buoyancy on convection heat transfer is obtained according to the horizontal pipe buoyancy criterion.