Numerical Simulation On Heat Transfer Characteristics Of Supercritical Carbon Dioxide In Circular Tubes

With the advantages of easily reaching the supercritical state,high cycle efficiency and small equipment size,supercritical carbon dioxide power cycle has broad application prospects in the power generation fields of coal,nuclear energy and solar energy.In order to ensure the safe and efficient operation of heat exchange equipment during a cycle,researches on flow and heat transfer characteristics of supercritical CO2 in a tube are particularly important.Based on the viewpoint of single-phase fluid,many researchers believe that the heat transfer behaviors of supercritical fluids,including heat transfer deterioration and heat transfer enhancement,are caused by the effect of buoyancy or flow acceleration.However,so far no scholar has been able to propose a unified discriminant parameter for judging the influence of buoyancy or flow acceleration effect on heat transfer.Even if the same discriminant parameter is used,the thresholds used by different scholars may be different.Recently,some scholars have experimentally and theoretically verified that supercritical fluids exist in two states:gas-like and liquid-like states,and proposed pseudo-boiling theory,which paves a new road for the study of supercritical fluid heat transfer characteristics.Based on the concept of single-phase fluid and the assumption of constant physical properties,the convective heat transfer performance of fluid cooled and heated in tubes remains consistent.However,existing studies have shown that the heat transfer performance of supercritical fluids under cooling and heating conditions is significantly different.In order to explore the similarities and differences of convective heat transfer characteristics of supercritical CO2 in the tube under cooling and heating conditions,the flow and heat transfer characteristics of supercritical CO2 cooled and heated in a vertical tube were numerically studied.Firstly,the ability of various low Reynolds number turbulence models to predict heat transfer coefficients of supercritical CO2 was evaluated,and it was found that the SST k-ω model has the highest prediction accuracy.Based on the hypothesis of "pseudo-phase transition",a partition model of supercritical fluid convective heat transfer is established according to the simulation results.Taking the pseudo-critical temperature Tpc as the boundary,the fluid in the tube can be divided into a gas-like zone and a liquid-like zone.For cooling conditions,the fluid near the wall and in the core area are a liquid-like film layer and superheated steam respectively;for heating conditions,the fluid near the wall and in the core area are a gas-like film layer and subcooled fluid,respectively.Further studies have shown that,compared with heating conditions,the heat transfer coefficients of supercritical CO2 under cooling conditions are significantly greater.This is mainly because the heat transfer performance of liquid-like fluid near the wall under cooling conditions is better than that of gas-like fluid near the wall under heating conditions,and the stronger turbulent kinetic energy in the core area is also an important factor to promote heat transfer under cooling conditions.Finally,the heat transfer characteristics of supercritical CO2 cooled in a horizontal tube were numerically studied.The results show that when the wall temperature is less than pseudo-critical temperature Tpc,supercritical CO2 in the tube will change from a gas-like phase to a liquid-like phase,and a liquid-like film will be formed on the wall.Affected by the buoyancy force,the gas-like-liquid-like interface and the turbulent kinetic energy distribution of fluid in the tube cross-section will be obviously nonuniform,causing the heat transfer coefficient of the top generatrix higher than that of the bottom generatrix.In addition,the effects of heat flux and mass flux G on heat transfer coefficient were analyzed.The calculation results show that as the cooling process progresses,the circumferential average heat transfer coefficient in a horizontal tube will have a maximum value near Tpc,and the influence of qw on heat transfer is mainly reflected on the right side of the heat transfer coefficient peak.When the heat flux qw is large,the great thermal conductivity near the wall and the high turbulent kinetic energy in the core area can promote the heat transfer from the core area to the tube wall.A higher mass flux of supercritical CO2 can suppress the increase in liquid-like film thickness and strengthen the turbulent intensity of fluid in the tube,and the heat transfer performance will be enhanced accordingly.This study provides a theoretical basis for the design and safe operation of coolers and heaters for supercritical CO2 power cycles.