Analysis On Characteristics Of Complex Multiphase Flow And Heat Transfer In The Evacuated Tube With Application

Saline water resource utilization technology based on solar energy integrates energy saving,desalination,fresh water gathering,zero emission.It has the advantages of alleviating water shortage,improving saline soils while reducing carbon emissions,and helps to achieve comprehensive and sustainable economic and social development.In this thesis,the nonuniform heat transfer mechanism of the walls of evacuated tubes was investigated.The thermodynamic characteristics of the multiphase flow inside the tube under the influence of heat flux distribution,mass velocity and evacuated tube structure were explored.The research provides the technical basis for the treatment of saline water by solar thermal methods.The main results and understandings are as follows:The non-uniform heat transfer mechanism involving conduction,convection,and radiation between the inner and outer walls of the evacuated tube under direct solar irradiation was revealed.The distribution function of radiation heat flux on the surface of inner and outer tubes under direct solar radiation was derived theoretically,and the multi-form heat transfer between different parts was analyzed.The theoretical model of non-uniform heat transfer on multi-layer walls of evacuated tube was established.The high thermal insulation material was inserted into the evacuated tube to investigate the heat transfer properties of the evacuated tube walls under actual solar radiation.The accurate circumferential temperature data of the wall surface was obtained to verify the non-uniform heat transfer model.Heat flux distribution and selective coating are the main factors affecting the heat transfer of evacuated tubes.The unbalanced heat flux distribution leads to non-uniform heat transfer on the tube wall,while the circumferential heat conduction of the coating will adjust the temperature distribution of the inner tube wall to make it uniform.Considering the fluid-solid coupling heat transfer between the fluid and the multilayer tube walls,the feasibility of heating the liquid phase to the boiling state in series with multi-stage evacuated tubes was explored.The three-dimensional numerical simulation of flow and heat transfer in horizontal evacuated tubes under the influence of heat flux distribution at low Reynolds number was carried out.The effects of uniform heat flux,forward and backward nonuniform heat flux on single-phase forced convective heat transfer in the tube were analyzed in terms of pressure drop,convective heat transfer,temperature distribution,and secondary flow.The applicability of different multiphase flow models in flow boiling simulation was verified and compared with each other.The effects of the above three wall heat fluxes on the multi-phase flow boiling heat transfer in the tube were discussed from the aspects of cross-section vapor volume fraction,flow pattern,pressure drop,wall temperature distribution,and heat transfer coefficient.The influence mechanism of wall heat flux inhomogeneity on the single-phase or multiphase flow and heat transfer in horizontal tubes was revealed.The Nusselt number of low Reynolds number single-phase forced convection heat transfer in tubes under the direct solar radiation was calculated and analyzed by introducing correction coefficient.The asymmetric U-type evacuated tube was designed,and the structural asymmetry was optimized by multi-parameter comparison.The numerical simulation results show that the design can effectively connect the adjacent evacuated tubes,enhance the axial flow rate while maintaining the hot and cold fluid stratification in the water tank,and solve the problem of stagnation zone at the bottom of the evacuated tube of the solar heating unit and the serious mixing of hot and cold fluid in the water tank.The dimensionless thermal stratification number based on temperature difference was proposed,which can accurately describe the change of temperature distribution contours in the water tank and quantify the thermal stratification phenomenon.