中文摘要：

本文对超临界压力CO2在竖直加热圆管内的对流换热进行了实验研究，比较了不同流向、不同热流密度等对流动和换热的影响。实验结果表明，管内径为2mm时，在低进口Re条件下，由于浮升力影响导致层流向湍流提前转变，对流换热增强；与向上流动相比，向下流动更易由层流转变为湍流；向下流动的换热要强于向上流动，表明浮升力对换热有很大影响。对于管内径为0．27mm的微细圆管，当进口Re高于10^4时，浮升力的影响可以忽略，对流换热系数的变化完全由物性的变化尤其是Cp的变化导致。

英文摘要：

Convection heat transfer of supercritical pressure CO2 in vertical tubes was investigated experimentally. The tests investigated the effects of heat flux, buoyancy and flow direction on the convection heat transfer. For low Re number flow （Din=2 mm）, due to the strong influence of buoyancy which result in an early transition from laminar to turbulent flow, the convection heat transfer was enhanced, and the local wail temperature shows a complex and nonlinear fashion for upward flow. For downward flow the transition is earlier, and the convection heat transfer coefficient for downward flow is higher than that of upward flow which indicates that the buoyancy has significant influence. For the tube with the diameter of 0.27 mm, when the inlet flow Re exceeds 104, the experimental result indicates that has not influence. The variation of convection heat transfer coefficient is attributed to the variation of thermal properties, especially the variation of Cp.