Numerical Investigation On Heat Transfer Characteristics Of Supercritical CO₂ In Collector Tube Under Non-Uniform Heat Flux

As a natural working medium,carbon dioxide（CO₂）has the advantages of friendly environment and stable chemical properties.Supercritical CO₂ shows excellent heat and mass transport potential due to its special and drastic changes in thermal physical properties.The Brayton cycle with supercritical CO₂ as the working medium has attracted wide attention in the fields of solar thermal power generation and advanced compression energy storage.In the above system,the heat pipe under the non-uniform irradiation of solar spotlight and flame is a typical working state.Therefore,the study of the convective heat transfer characteristics of supercritical CO₂ under the non-uniform heat flow is of great significance for the optimal design of key heating components and the safe operation of the system.Compared with the conventional vertical arrangement,the solar receiver of the tower heat absorber is arranged by spiral tilt,which has the advantages of uniform heating and enhanced heat transfer.In this thesis,the flow heat transfer characteristics of supercritical CO₂ in a single-sided non-uniformly heated inclined pipe were investigated under the conditions of G=100～1200 kg/（m²·s）,q=50～600 k W/m²,and P=7.5～9 MPa.The effects of incline Angle,heat flux,mass velocity and pressure on the inner wall temperature and heat transfer coefficient of the collector tube were analyzed.Combined with the distribution of thermal physical parameters and the changes of secondary flow and turbulent kinetic energy,the non-uniform heat transfer characteristics and heat transfer mechanism of supercritical CO₂ in the tube were discussed.Based on the adaptability analysis of the existing correlation,a new heat transfer correlation is established.The results show that the peak value of circumferential wall temperature is located at the center line of the phototropic side,the non-uniformity degree of circumferential wall temperature increases with the increase of the incline Angle,the maximum temperature difference is 180 K,and the heat transfer capacity decreases with the increase of the incline Angle.It is recommended that the spiral collector tube should be arranged with a rising Angle of 20°.When the dip Angle is 20°,the secondary flow,turbulent kinetic energy,specific heat capacity at constant pressure and thermal conductivity of the fluid near the optical side wall are all higher than 45°and 70°.The sharp increase of specific heat capacity at constant pressure and thermal conductivity of the fluid near the wall is the main reason for the heat transfer enhancement.Decreasing heat flux density and increasing mass flow velocity are effective methods to reduce the deterioration of heat transfer to the optical side.With the increase of pressure,both the enhancement and deterioration of heat transfer are decreased.The prediction of mean heat transfer of supercritical CO₂ in tubes under unilateral non-uniform heating has been greatly deviated by the existing heat transfer correlation formula.The new heat transfer correlation formula fitted can be used to guide engineering practice.Secondly,the flow heat transfer characteristics of supercritical CO₂ in a horizontal tube with non-uniform heating at the bottom were numerically simulated for the trough or linear Fresnel reflectors.The results show that the non-uniformity degree of circumferential wall temperature increases with the increase of heat flux,and decreases with the increase of mass flow rate and pressure.The maximum circumferential temperature difference is 220 K.Reducing heat flux and increasing mass flow rate can effectively improve the non-uniform distribution of circumferential wall temperature of horizontal tube,and improve the convective heat transfer coefficient and heat transfer capacity of the fluid near the lower bus.The distribution of specific heat capacity and thermal conductivity of fluid at constant pressure near the wall is the main reason for its heat transfer capacity,while the secondary flow and turbulent kinetic energy have little influence on the heat transfer near the upper and lower busbar.Combined with the analysis of the accuracy of the existing heat transfer correlation formula for predicting the working condition in this thesis,for the horizontal pipe with non-uniform heating at the bottom,Liu et al.’s correlation formula has a better prediction accuracy for the mean heat transfer characteristics of supercritical CO₂ in the pipe.