Experimental And Numerical Study On Flow And Heat Transfer Characteristics For Supercritical Carbon Dioxide In Vertical Tubes

The"double carbon"target puts forward new requirements for the national existing energy structure,developing efficient and compact coal-fired power generation system is imperative.Compared with traditional steam cycle,supercritical carbon dioxide（S-CO₂）Brayton cycle has the advantages of high cycle efficiency and compact structure,which is of great research value.A comprehensive understanding of the flow and heat transfer behavior of S-CO₂ in the tubes is of great significance for the safe operation and design of the water wall in the S-CO₂ Brayton cycle coal-fired power generation system.In this paper,a study on flow and heat transfer characteristics for S-CO₂ in tubes is carried out combined experimental and numerical method to reveal the heat transfer mechanism of S-CO₂.The results provide technical guidance for the design of the heat exchange components and safe operation of the S-CO₂ Brayton cycle coal-fired power generation system.The supercritical carbon dioxide generation and flow heat transfer characteristics experimental apparatus was designed and established,under the working condition among the inlet temperature from 15 to 45℃,pressure from 6.5 to 12MPa,mass flus from 0 to 848.83kg/m²s,heat flux from 0 to 150 k W/m².Based on the experimental apparatus,experiment on flow and heat transfer characteristics for S-CO₂ in vertical tubes was carried out,and the effects of operating parameters including inlet temperature,inlet pressure,mass flux,heat flux,and flow direction on heat transfer were studied.The results show that with the increase of mass flux,the heat transfer performance of S-CO₂ will be enhanced effectively,and the heat transfer deterioration will be weakened or even eliminated.Wall temperature will increase obviously with the increase of inlet temperature,pressure and heat flux,heat transfer deterioration will be aggravated.Compared with vertical upward flow,vertical descending flow has lower wall temperature and larger convective heat transfer coefficient,and has better heat transfer performance,which can reduce the degree of heat transfer deterioration.The theory of buoyancy and flow acceleration caused by physical property changes cannot clearly explain the heat transfer mechanism of S-CO₂.Based on the pseudo-boiling theory,the rapid growth of the gas-like film and the large temperature gradient of the gas-like film are the main reasons for the deterioration of heat transfer in S-CO₂.Due to the lack of experimental measurement methods,the flow field structures and the properties of S-CO₂ in the test section can not be directly obtained,which limits the research on the heat transfer mechanism of S-CO₂.The numerical simulation study of heat transfer for S-CO₂ flowing vertical tubes is carried out to deeply understand the heat transfer behavior of S-CO₂.The results of the comparison of the parameter distributions in normal heat transfer and heat transfer deterioration working condition cross-sections show that the inhomogeneity of physical parameter distribution is the main reason for the heat transfer deterioration of S-CO₂,and the"M"-shaped velocity curve is the sign of heat transfer recovery.Based on the thermal resistance heat transfer mechanism proposed by the quasi-boiling theory,the heat transfer mechanism of S-CO₂ was revealed from a quantitative point of view.The results show that the thermal resistance of the gas-like phase region dominates the heat transfer of S-CO₂,and the significant increase of the thermal resistance of the gas-like phase region is the main reason for the deterioration of the heat transfer of S-CO₂.A heat transfer database of S-CO₂ was established combined with the experimental and numerical results.Based on the deep neural network model established in this paper,the high-precision transfer correlations for convective heat transfer coefficient and critical heat flux were proposed respectively,considering the effects of thermophysical properties and operating parameters on S-CO₂ heat transfer.In terms of the convective heat transfer correlations(P:7.36～20.8MPa,G:119.09～386.22kg/m²s,q_w:5.25～122.74k W/m²,T_in:20.1～40.0℃),the average relative error,arithmetic mean error and root mean square error are-4.22%,7.03%and9.58%in vertical upward flow,and-5.21%,5.22%and 5.39%in vertical downward flow.As for the correlation of critical heat flux under wide range of working conditions(d_in:0.27～22mm,P:7.5～21.14MPa,G:50～2716kg/m²s,q_w:2.9～549k W/m²),the accuracy reaches 89.76%under non-heat transfer deteriorating working conditions,98.46%under heat transfer deteriorating working conditions,and 95.61%under all working conditions.The results show that the model can accurately predict the heat transfer behavior of S-CO₂,which provides a reference for the design and safe operation of the heat transfer components of the S-CO₂Brayton cycle coal-fired power generation system.