Study On Heat Transfer Characteristics Of Supercritical Fluid R410a In U-shaped Elbo

Organic Rankine cycle(ORC)is one of the the most appropriate heat-to-power conversion technology,which has many advantages in terms of system efficiency and power output.Supercritical fluids do not experience a two-phase co-existence zone during the heating period,which can further improve energy efficiency in supercritical ORC.The heat transfer property of the organic fluid is an important factor which affecting the efficiency of the ORC.In this paper,the heat transfer characteristics of supercritical organic fluid R410 a in a U-tube are investigated based on experimental studies,numerical simulations and theoretical analysis,which aim to provide theoretical basis and guidance for the design of heat exchanger with return-bend.This paper firstly improved the supercritical fluid flow heat transfer experiment platform of Kunming University of Science and Technology.The effects of heat flux,mass flux,inlet temperature and pressure on U-tube heat transfer were analyzed,and obtained the basic experimental data of convective heat transfer of organic fluid R410 a in a U-tube.The relationship between the bend-induced enhancement and the property variations of fluid at different positions along the pipe was analyzed and summarized,and a numerical simulation study of the experimental cases was conducted to clarify the turbulence model suitable for the U-tube.The main studies and findings are as follows:(1)Flow heat transfer of supercritical pressure R410 a in a U-tube.Among the operating parameters considered,the mass flux,heat flux,and inlet temperature have a significant influence on the supercritical heat transfer in a U-bend and pressure has only a small effect on the heat transfer.The specific extent of the effect is partially determined by whether there are dramatic fluid property variations or not.The heat transfer coefficient in bending section has a strengthening effect of 20%～60%.The local enhancement of heat transfer is observed within the bend in all the cases,and it is shown to be generally more significant at larger Reynolds numbers.(2)Aside from an increase in the Reynolds number,more dramatic changes in the fluid properties can also contribute to a larger bend-induced enhancement of heat transfer.The bend-affecting upstream length is found to be within 5.75 d.For the downstream straight section,the bend effect is reduced at 11.5d downstream of the bend and further.However,the downstream effect is relatively weak,and a strong enhancement(>10%)is only possible within 5.75 d from the bend outlet.(3)The centrifugal force within the U-tube leads to a decrease in the temperature difference between the top and bottom of the bending section and an increase in the temperature difference between the inside and outside,with the best heat transfer strengthening effect at the top,followed by the outside,bottom and inside.(4)A few existing correlations(Krashnoschekov & Protopopov;Jackson)perform reasonably well in the straight sections upstream and downstream of the U-tube.By introducing an enhancement factor to account for the centrifugal force effect caused by bend,a new heat transfer correlation applicable to supercritical U-bends is proposed.The newly proposed equation correlates 85% of the experimental data with an absolute error less than 10%.