Study On The Convective Patterns And Heat Transfer Characteristics Of Nanofluid Under Concentrated Solar Radiation

The medium-temperature solar collection has broad application in the fields of industrial heating,seawater desalination,driving refrigeration and air conditioning and medium temperature thermal power generation,and has significant energy saving advantages.However,the conventional medium-temperature solar collector has disadvantages such as thermal stress deformation damage caused by large circumferential temperature difference of the heat absorbing tube under the concentrated solar radiation,poor durability of the absorbing coating,vacuum failure,etc.,which hinders its popularization and application.In this paper,a nanofluid-based solar collector vapor generator(NSVG)is proposed,which adopts a transparent double-layer all-glass vacuum tube.The steam generating tube is coaxially arranged in the tube,and the nanofluid with strong light absorption property is filled between the steam generating tube and the glass tube.The nanofluid absorbs solar energy and transfers heat to the working fluid within the steam generating tube to produce steam.Under concentrated solar radiation,the nanofluid absorbs solar radiation to form the non-uniform volume "internal heat source",which cause complex natural convection and exhibits a special temperature distribution.Furthermore,the convective patterns also change with the working conditions,and the coupled effect of flow fields and temperature fields collectively affect the heat transfer and heat collection performance of the NSVG.Therefore,this paper focuses on the convection patterns and coupled heat transfer characteristics of nanofluid under non-uniform concentrated solar radiation.The main work is as follows:Firstly,using the Monte Carlo ray tracing method(MCRT),considering the factors such as the non-parallelism of the solar light and the change of the incident angle,the threedimensional optical model of the trough solar concentrator was established and the model is verified.The radiation energy flux distribution on the surface of the collector tube was simulated under different working conditions.The results showed that when the solar incident angle is 0°,the radiant heat flux was unevenly distributed in the circumferential direction of the heat collecting tube,and was constant along the collector tube direction.When the incident angle was gradually increased,the radiation intensity decreased and and a significant end loss occurs.Secondly,based on the analysis of the mechanism of heat collection and heat transfer in nanofluid,the radiation energy flux distribution on the surface of the collector tube was used as the boundary condition to establish the mathematical model of non-uniform heat collecting,convection and heat transfer in nanofluid.The thermal model was used to carry out CFD simulation calculation,and the simulation results of temperature distribution and heat collection efficiency of the collector were compared with the existing research to verify the correctness of the model.Thirdly,the flow fields of heat transfer fluid in the annular space of NSVG and conventional coated solar vapor generator(SVG)were obtained by simulation,and then the two kinds of flow patterns were compared and analyzed.The Rayleigh number Ra was used to characterize the natural convection intensity.And the changes of flow pattern with various influencing factors were analyzed.The results showed that the nanofluid concentration,solar radiation intensity and the size of the annular space had a great influence on the flow pattern of the nanofluid,while the inlet temperature and flow velocity of the evaporation medium had little effect on the flow pattern.Finally,based on the study of the flow pattern of nanofluid,the coupling characteristics of the flow field and temperature field in the annular space were discussed.The efficiency factor F' was used to quantitatively evaluate the influence of the temperature distribution characteristics of the collector on the heat collecting performance,and the variation law of coupling characteristics under different influencing factors was analyzed.The results showed that under concentrated solar radiation,the convective patterns and temperature distribution characteristics of the nanofluid in the NSVG due to the non-uniform volume "internal heat source" made the flow velocity at the evaporation tube to be larger,compared with the conventional SVG.What's more,the high temperature zone was closer to the main center of annular apace in NSVG,which enhanced the heat transfer to the evaporation medium and was beneficial to the improvement of the heat collection performance.