Optical-thermal Conversion Characteristics And Convective Heat Transfer Loss Mechanism Of Cylindrical Receiver With Built-in Helically Coiled Tubes

The continuous deterioration of the ecological environment and the reduction of fossil fuels have become the two prominent factors restricting human development since the 21st century,and several advantages of solar energy,including abundant reserves,easy access and environmental friendliness,make it one of the effective methods to solve the above problems.Compared to other three Concentrated Solar Power types（Parabolic Trough Collector,Linear Fresnel Reflectors,Heliostat Field Collectors）,the Parabolic Dish Collector（PDC）is recognized as the most efficient system for energy conversion.In the entire PDC system,the cavity receiver is a pivotal component for optical-thermal conversion,whose ability to intercept solar radiation energy and convert solar energy to the thermal energy of Heat Transfer Fluid directly affects the efficiency of the whole system.Therefore,there is an urgent need to grasp accurately the evolution law of the optical-thermal conversion characteristics of the receiver in the PDC system,and to understand the mechanism of different parameters on the heat dissipation side of the receiver,in order to finally realize the efficient conversion of light to heat.so as to realize the efficient optical-thermal conversion.In the context of the present moment,this paper develops a coupling model based on Monte Carlo ray trace（MCRT）and CFD to investigate the optical-thermal conversion characteristics of cylindrical cavity receiver with built-in helically coiled tubes.In addition,the convective heat transfer loss mechanism of the receiver under the influence of concentrator structure also be explored detailedly.The main research results are as follows:For the optical-thermal characteristics:（1）the solar radiation heat flux q on the outer wall of the coiled tubes presents a bimodal distribution,and the two peaks are respectively around Z=0.05 m and near the bottom of the cavity.When the tube diameter is increased from 8 mm to 18 mm（DNI=650 W/m²）,the optical efficiencyη_opt、thermal efficiencyη_the、optical-thermal conversion efficiencyη_opt-the of the receiver are reduced by 0.38%,0.3%,1.62%,respectively.Therefore,reducing the tube diameter is conducive to improving the integral performance of receiver.（2）When the aspect ratio of the helically coiled tube H/D increases,theη_opt and total heat loss Q_loss increase obviously while theη_the decrease.In the process of H/D change,η_opt-the presents a trend of increasing first and then decreasing.When H/D=1.8,theη_opt-the reaches a“peak value”,and the solar radiation utilization rate is the highest at the moment.（3）As inlet mass flow rate m increases,the convective heat transfer between the Heat Transfer Fluid（HTF）and the inner wall of the coil gradually increases.Meanwhile,HTF outlet temperature T_out decrease gradually,but theη_the andη_opt-the both increase.In the initial stage when m changes from 0.005 kg/s to 0.025 kg/s,η_the andη_opt-the change obviously,and then continue to increase m has a little effect on increasingη_the and ηopt-the.For the convective heat transfer loss characteristics under the influence of the concentrator:（1）The flow characteristics around the PDC system under different receiver inclinationαand wind incident angleφare different obviously.The presence of the concentrator in a windy environment makes it easy for air to form a flow vortex in the flow field.During the variation ofαfrom 0°to 90°,the effect of the concentrator on the air flow near the receiver gradually decreases.Atα=0°,the low-speed vortex flow near the receiver aperture is almost stagnant.at which time the concentrator has the greatest influence on the flow at the receiver aperture.Whenα=90°,the concentrator has little effect on the flow at the receiver aperture,and the flow velocity at this time near the receiver aperture is close to wind velocity V.The convective heat loss Q_cv shows a trend of decreasing first and then increasing with the increase ofαunder the effect of concentrator.（2）When theαis small,with the increase of V,most of Q_cv under most wind incident angles increase first and then decrease.there exists a minimum value between 0m/s≤V≤4 m/s.However,whenα=60°andα=90°,the convective heat loss curve almost shows a linear increasing trend with the change of V.（3）There exists a significant fluctuation for Q_cv With the increase ofφ.whenα=0°,the curve fluctuation is the most obvious.At this time,the effect ofφis the most complicated under the interference of the concentrator.With the increase ofα,the influence of the concentrator on the air flow near the receiver aperture is weakened.At this time,the curve change gradually slows down gradually,and the influence ofφbecomes weakened until it has no effect on heat loss whenα=0°.