Research On Thermal Conversion Characteristics Of The Double-layer Solar Absorber

The volumetric solar absorber is widely used in solar heat utilization technology which had the advantages of light weight and high thermal efficiency.However,the uneven distribution of the radiant energy flux at the entrance of the porous absorber results in local high temperature of the absorber and large external radiation heat loss,which affects the thermal efficiency and operational stability of the absorber.Therefore,it is of research significance to change the structure of the porous absorber,promote the transmission of concentrated solar radiation in the absorber,achieve gradient absorption,and reduce hightemperature infrared radiation loss.For this reason,this paper proposes a double-layer solar absorber with quartz glass and porous,which increases the gradient of the absorption coefficient of concentrated solar radiation.The research method combined experimental measurement and numerical simulation is used to study the radiation-convection coupled heat transfer characteristics of the double-layer absorber under the incident solar radiation.Aiming at the numerical simulation of the coupled heat transfer in the double-layer absorber,Monte Carlo Method is used to track the radiation transmission of the incident concentrated solar radiation in the absorber and the high temperature infrared radiation transmission of the porous,using P1 Approximately solve the radiative transmission of the porous medium solid framework itself,consider the heat transfer temperature difference between the porous flow and the solid phase,and use Local Thermal Non-Equilibrium to solve the convective heat transfer in the porous region.The radiative transfer solving program is selfedited by Fortran language,combined with Fluent software,and the radiative transfer and local heat balance heat transfer calculation modules are loaded with the help of User Defined Functions to realize the numerical calculation of radiation-convection coupled heat transfer.Experimental measurement is an important way to study the thermal conversion performance of the double-layer solar absorber,and it is also the key to verify the reliability of the numerical simulation.In this paper,the thermal radiation physical properties and coupled heat transfer characteristics of the double-layer solar absorber are measured and studied respectively.A sample model of the double-layer solar absorber was constructed,and the thermal radiation properties were measured by a spectrophotometer.Design and build a coupled heat transfer experiment device,by measuring the temperature and flow of the working fluid,indirectly measure and analyze the heat transfer performance of the solar absorber.A numerical model of the double-layer solar absorber that consists of quartz glass multitubes and porous was constructed,and the radiation-convection coupled heat transfer characteristics in the solar absorber were analyzed through numerical simulation and experimental measurement,and the startup process of the the double-layer solar absorber was studied.The influence of the non-uniform inlet of the working fluid,the structure parameters of the quartz glass multi-tubes and the porous on the heat transfer performance of the solar absorber was investigated.The pore area is approximated to establish a two-dimensional axisymmetric numerical model of the double-layer solar absorber with glass orifice and porous,and the characteristics of absorbed incident concentrated solar radiation and coupled heat transfer is studied,the factors of the heat transfer performance of the double-layer solar absorber were investigated separately.Finally,the comparison and analysis of the quartz glass-porous double-layer solar absorber and the monolayer porous medium heat-absorbing core are carried out from the aspects of thermal efficiency,safety and manufacturing technology.Through the research,the reliability of the numerical simulation of the coupled heat transfer of the double-layer solar absorber is verified,and the heat transfer performance of the glass sleeve type and the glass orifice type porous double-layer solar absorber core is confirmed,which providing reference for the design and optimization of volumetric porous solar absorber.