浮升力和流动加速对超临界CO2管内流动传热影响

基于单相流体的概念,超临界流体的异常传热行为已经被研究很多年了,但是关于其流动传热机理仍没有统一的认识.本文通过理论分析和实验研究了超临界二氧化碳在竖直管内向上流动过程中,浮升力和流动加速效应对其流动结构和传热过程的影响.结果表明,没有确凿的实验证据表明超临界流体的异常传热行为是浮升力和流动加速直接导致的,存在的估计浮升力和流动加速效应准则均是在常物性流体的基础上,做了大量假设得出的,不同的研究者采用浮升力和流动加速准则分析超临界流体的传热恶化得出的结论不一致.最后,基于拟沸腾理论分析超临界流体的传热恶化过程,提出超临界沸腾数区分了超临界流体正常传热与恶化传热的转换边界,为超临界流体流动传热研究提供新思路,超临界沸腾数对建立用于不同技术的超临界流体动力循环的最佳运行条件具有重要意义.

Effects of buoyancy and acceleration on heat transfer of supercritical CO2 flowing in tubes

Based on the concept of single-phase fluid,the abnormal heat transfer behavior of supercritical fluid has been investigated for many years.However,there is no unified understanding of the mechanism of its flow and heat transfer.In this paper,we first review the reported effects of buoyancy and acceleration on supercritical fluids,and then study the effects of buoyancy and acceleration on the flow structure and heat transfer for the upward vertically flowing of supercritical CO2 fluid in a tube theoretically and experimentally.The results show that there is no conclusive experimental evidence that the abnormal heat transfer behavior of supercritical fluid is directly related to buoyancy and flow acceleration,and the existing criteria for estimating buoyancy and acceleration effect are based on the constant physical fluid and a lot of assumptions,as a result,different conclusions are obtained,though the same prediction method is used.Finally,we investigate the heat transfer deterioration of supercritical fluids based on the pseudo-boiling theory,and the proposed supercritical-boilingnumber distinguishes the normal heat transfer deterioration from heat transfer deterioration of supercritical fluid.Our work paves a new way to understanding the supercritical fluid flow and heat transfer mechanism.The supercritical-boiling-number is important for establishing the optimum operating conditions for the supercritical fluid power cycle used in different technologies.