Experimental Study On Natural Circulation Flow And Heat Transfer Of Supercritical Carbon Dioxide In Horizontal Tube

The power conversion system using supercritical carbon dioxide(S-CO2)as the working fluid has the characteristics of high thermodynamic cycle efficiency,compact system and strong flexibility.Therefore,S-CO2 power system has become a research hotspot in recent years.However,due to the drastic changes in the thermophysical properties of S-CO2 in the pseudo-critical region,its basic research faces many challenges,such as the unclear heat transfer mechanism in the pseudo-critical region,incomplete consideration of natural circulation influencing factors,and lack of highprecision heat transfer correlation and high-precision friction factor correlation,etc.It can be seen that further exploration of the natural circulation characteristics,heat transfer characteristics,and pressure drop characteristics of S-CO2 in the pseudocritical region play a key role in the wide application of S-CO2 in nuclear power generation,waste heat recovery,and low-temperature refrigeration.This article first introduces the construction process of the S-CO2 natural circulation experimental bench,and gives the specific experimental steps and schemes.Subsequently,S-CO2 related experiments were carried out under different pressures and temperatures,and the uncertainty of the experimental data was evaluated.Finally,through the processing of experimental data and the analysis of experimental phenomena,the flow,heat transfer and pressure drop laws of S-CO2 are explored.The main research contents and results are as follows:In terms of natural circulation of S-CO2,the variation curve of circulation flow with heating power under the action of flow resistance and driving force is analyzed.The sensitivity of inlet temperature,system pressure,temperature difference between hot and cold sections,and height difference between cold and heat sources to the steadystate circulating flow is explored.The flow instability observed in the experiment is discussed,and the stable power threshold under different pressures is obtained according to the experimental data.Regarding the convective heat transfer of S-CO2,a database of convective heat transfer of horizontal heating tubes is established,and the effects of buoyancy effect,bulk flow acceleration and physical property changes on heat transfer are analyzed.Based on the experimental data,the collected heat transfer correlations are evaluated and a new heat transfer correlation with better accuracy is proposed.In terms of the resistance characteristics of S-CO2,the variation law of each partial pressure drop with heat flux under the conditions of constant thermophysical properties and variable thermophysical properties is analyzed.Based on the experimental data,the seven common correlations of friction factor in the literature were evaluated,and the accuracy was found to be insufficient to meet the needs of engineering practice.For this reason,on the basis of considering the coupling effect of density,viscosity and Prandtl number,a new friction factor correlation with higher accuracy is fitted.