Flow And Heat Transfer Characteristics Of Supercritical Fluids For Tower Solar Thermal Power Generation

Tower solar thermal power generation technology is one of the main approaches for large-scale development and utilization of solar energy,and the technology is crucial to the energy structure optimization and the low-carbon development.In the tower solar thermal power plants,the energy transportation process of solar-thermal-electricity relies on heat-transfer fluids.Therefore,it is quite important to develop high performance of heat transfer fluids in order to promote the technology development in tower solar thermal power utilization.In recent years,with the development of supercritical technology,supercritical fluids are receiving more and more attention in the field of tower solar thermal power generation technology,because the transportation properties of supercritical fluids is excellent,which can increase the efficiency and reduce investment and operating costs.In the process of energy conversion,the process of flow and heat transfer supercritical fluid are quite important,because the efficiency of the whole system is affected by the flow and heat transfer characteristics.However,the thermo-physical properties of supercritical fluids vary strongly in the critical region and pseudo-critical regions,causing the complex regularities of heat transfer process.The studies on the flow and heat transfer of supercritical fluids are still far from perfect.This thesis focuses on the flow and heat transfer characteristics of supercritical carbon dioxide(scCO2)and supercritical hydrocarbon,and test loops for measuring the flow and heat transfer characteristics are designed.The heat transfer characteristics of supercritical fluids flowing in tubes are investigated experimentally under different working conditions.The main contents and conclusions of this thesis are summarized as follow:1.The convection heat transfer of supercritical carbon dioxide flowing in tubes was experimentally studied.The wall temperature and heat transfer coefficients under conventional and strong heating conditions were obtained.The effects of pressure,mass flux,heat flux are discussed.Under conventional heating conditions,heat transfer would be enhanced when the wall temperature is higher than pseudo-critical value while bulk temperature is lower than pseudo-critical value.The film temperature is employed as the criteria to predict the peak region of heat transfer.When the film temperature is approaching to pseudo-critical value,the heat transfer peak would occur.In the pseudo-critical region,heat transfer enhancement effect is more obvious when pressure approaching the pseudo-critical value.Under strong heating conditions,buoyancy effect is quite obvious,and heat transfer deterioration occurs in the whole test section,which can be explained by the theory of pseudo film boiling.Available heat transfer correlations for scC02 are compared with the experimental data;however,they cannot well capture the data,especially in pseudo-critical regions.Therefore,a new correlation is proposed for scCO2 under high q/G conditions,in which buoyancy effect and variations in thermophysical properties are both taken into account.2.The heat transfer characteristics of supercritical hydrocarbon flowing in tubes were experimentally investigated.The heat transfer features and mechanisms of supercritical hydrocarbon are discussed.The heat transfer regions of supercritical hydrocarbon are divided into laminar region,turbulent region,pseudo nucleate boiling region,pseudo film boiling region,and chemical reaction region,in which the heat transfer mechanisms are analyzed respectively.Heat transfer enhancement and heat transfer deterioration for supercritical hydrocarbon are observed,and the theory of pseudo boiling is employed to explain the phenomena.In the pseudo nucleate boiling region,the low density fluid can move into bulk region and bulk fluid is can supplied to the heating wall due to the buoyancy effect,by which heat transfer is enhanced.However,in pseudo film boiling region,the effect of buoyancy force is weakened,and the transportation process for mass transfer also impair.As a result,the low-density fluids gather around the wall surface,and gradually form a continuous "vapor film",inducing heat transfer deterioration.The pressure drops of supercritical hydrocarbon flowing in typical piping components are obtained.The pressure drop is closed related with the thermophysical properties and heat transfer process.3.When high pressure liquid is leaked into outer environments through an orifice,the flow in the orifice can easily reach as critical flow,according to the critical flow principle.In critical flow,the leakage flow rate is determined by the upstream conditions,while the downstream conditions do not make any effect.In this thesis,the critical flow of supercritical hydrocarbons through a straight orifice and a shrink orifice are measured.A number of data of critical flow are obtained,and the critical flow functions are calculated.The critical mass flow is positive with inlet pressure,and is negative with inlet temperature.The data of critical mass flow are correlated using the equation of Gcr=?Ap0/(?).The model of critical mass flow for supercritical hydrocarbon is developed based on the experimental data.