中文摘要：

本文提出了圆管内变密度流体流动与换热充分发展的概念及其相关假设。在所提出的概念和假设基础上给出了摩擦阻力系数Cf与Nusselt数Nu的定义式和计算式。通过对圆管内密度随温度变化的氩气的流动与换热所进行的数值模拟，验证了在本文的计算条件范围内，圆管内变密度流体充分发展流动与换热的概念是合理的，并且CfRe=16， Nu=4．36。

英文摘要：

The concept and its related assumptions of fully developed fluid flow and heat transfer in tube considering variable density are presented in this paper. The definitions and derivations of Cf and Nu, which represent friction factor and Nusselt number respectively, are given on the basis of the concept and assumptions above. The numerical simulation of Argon flow and heat transfer considering density variation with temperature in tube is performed, which proves that the fully developed concept of variable density fluid flow and heat transfer in tube is reasonable in the range of calculation parameters in this paper. The calculation shows that CfRe is 16, and Nu is 4.36.