Investigation On Heat Transfer Characteristics Of Supercritical CO₂ Cooling Flow In Horizontal Tube And Performance Of Power Cycle

Carbon dioxide?CO₂?,a non-flammable natural working fluid that has no ozone depletion potential?ODP?and negligible global warming potential?GWP?,has been extensively studied in recent years.To improve the prediction accuracy of the supercritical carbon dioxide?CO₂?heat transfer coefficient in different tubes,the heat transfer characteristics of in-tube supercritical CO₂ were studied in this paper.The influence of pressure,mass flux and diameter were investigated by simulations and experiments.Then,some power cycle systems were optimized in the final section.The analysis results show that the optimizations on heat exchange processes in electrothermal energy storage systems?ETES?and comprehensive discussion on the thermal efficiency and cooling process in supercritical CO₂ power system will improve the performances of the power systems.It is difficult to simulate the thermophysical properties because of the dramatic variation in the vicinity of pseudo-critical temperature.To ensure the rationality of numerical analysis,the thermophysical properties were added into the simulations by a line-piece function.On the other hand,the supercritical CO₂ were proposed by pump and heater in the experiments.The efffects of pressure,mass flux and diameter on heat transfer were studied,which are caused by the variable thermophysical properties,flow states,buoyancy and so on.Both the simulations and experiments take the proofs that the influences of buoyancy in different tubes are different,which means except for the variable thermophysical properties and Reynolds number,the influence of diameter should been considered in the supercritical heat transfer process.As for the improvement of heat transfer correlation,this paper compared and analyzed the dimensionless numbers of various factors,and introduced the buoyancy force and the dimensionless number of diameter into the novel correlation to improve the prediction accuracy.The numerical correlation was fitted by the numerical data and compared with the heat transfer coefficients,the RMS deviation is 8.70%.However,the numerical correlation didn't perform well in the experimental data prediction.Thus,the experimental correlation was presented by the format of numerical correlation and experimental data.The RMS deviation of the experimental data prediction is about 16.76%.After improvement of supercritical CO₂ correlation,optimizations on ETES and supercritical CO₂ power cycle were proposed.Since the phase change heat accumulator is used in the latent heat ETES,the optimization was presented on the structure of heat exchanger theoretically and experimentlly;The entransy dissipation principle was employed to improve the performance of the heat transfer process in a subcritical ETES;the comprehensive optimization was finally presented in a supercritical CO₂ power system,the peak thermal efficiency was obtained in the thermodynamic analysis of the power system,the thermal efficiency of the system varies with the inlet pressure of the expander,and a peak thermal efficiency is obtained under the optimal inlet pressure.The results also show the optimal inlet pressure changes with the variation of inlet pressure.On the other hand,the cooler length was calculated by the heat transfer correlation proposed in previous sections,and the length decreased with the increase of inlet pressure.Therefore,to optimize the power cycle,the thermal efficiency and cooler length should been comprehensively optimized.