| The heat absorber is the core module in the solar thermal utilization system,its photo-thermal conversion efficiency is the main indicators to evaluate the performance of the heat absorber,and affects the heat utilization rate of the whole system.Porous media—a foam metal has a better enhanced heat exchange performance,and is an ideal endothermic material,because of its excellent three-dimensional leading flow structure,better thermal conductivity,higher porosity and specific surface area.Air heat absorber is widely applied because of its wide sources of working medium,no pollution,and no temperature rise restriction at the outlet;however,it is affected greatly by the focused radiant energy,it is easy to form the hot spots and is damaged.By taking copper foam as a heat receiver and combining with an air heat absorber,this paper is beneficial to improve the photo-thermal conversion efficiency of the heat absorber and reduce the thermal damage of the heat absorber.In this paper,copper foam with the pore density of 10 PPI,20PPI and 30 PPI respectively and the porosity of 95.12%,94.51% and 95.46% respectively is used as the main heat receiver of the dish concentrating collector.By use of the experiment,numerical calculation and theoretical analysis methods,the flow characteristics when an air flowing through the foamed metal,and the photo-thermal conversion efficiency and enhanced heat transfer performance of the foamed copper dish concentrator collector have been studied and analyzed;as follows:(1)In order to study the flow characteristics when an air flowing through the foamed metal,this paper applies three equivalent models including the face-centered cube,honeycomb and cube,the numerical simulation is conducted under the different flow velocity,pressure drop,friction factor and other flow characteristics by use of FLUENT;a set of duct pressure testing system has been built,and the single and cascade superposition experiments has been studied for three different parameters of copper foam.The results have shown that the pressure drop has a quadratic linear relationship with the inlet velocity,and the friction factor after correction has a negative correlation with the velocity;The single equivalent model cannot accurately predict the flow characteristics of foam structure;the inner disturbance of the foam metal after the cascade superposition is enhanced,and the unit pressure drop is increased.(2)In order to study the enhanced heat transfer performance by using copper foam as the heat receiver,this paper has designed a detachable dish concentrating collector,and a photo-thermal conversion experimental platform is built.The photo-thermal conversion efficiency of foam metal heat absorbers under the different pore density and working medium flow rate has been studied;The temperature increment ? in per unit incident power and the efficiency increment rate ? in per unit volume of foam metal are put forward to be used to describe the enhanced heat transfer performance of foam metal.The results have shown that the photo-thermal conversion efficiency of the heat absorber increases with the increasing of pore density under the same flow rate;under the same pore density,the efficiency increases with the increasing of the flow rate,and the maximum efficiency value is 87.61%,which is 35.04% higher than that without foam metal;the efficiency increment rate ? in per unit volume increase with the increasing of the flow rate and pore density;The temperature increment ? in per unit incident power is continually decreasing with the increasing of the flow rate.The synergetic effect of the velocity field and temperature field inside the foam metal under the different flow rates has been analyzed by using the field synergetic principle,it has been shown that the field synergy coefficient decreases with the increasing of the Reynolds numbers under the same porosity;the field synergy numbers inside the foam metal increase in fluctuation. |
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