Research On Heat Transfer Characteristics Of Solar Air Collector With Silicon Carbide Foam Ceramic As Heat Receiver

With the increasingly prominent problem of global energy shortage and the continuous promotion of the concept of sustainable development,solar energy has gradually stood out among several renewable energy sources because of its unique advantages such as its wide distribution,convenient utilization and clean and environment friendly.Solar air collector is popular as a heating device which have following characteristics as stable operation,and long service life,and high cost performance.However,due to the lower heat capacity and small thermal conductivity of air,at the same time and the heat exchange area is limited,so both the heat storage and heat exchange capacity of traditional solar air collector are constrained.On the basis of traditional solar heat collecting technology（Permeable collector and bending-flow type collector）and high temperature porous media heat collector,two kinds different structures（D-SAC and S-SAC）of solar air collector with double glass cover plate and using silicon carbide foam ceramic as heat absorption plate were proposed in this paper.Due to the using of silicon carbide foam ceramic,the airflow disturbance is strengthened,the heat transfer area is increased,and the heat collection efficiency is improved.The research content mainly focuses on testing the radiation heat transfer characteristics of silicon carbide foam ceramics by experimental method and simulating the thermal performances of solar air collector with silicon carbide foam ceramics as absorber by numerical method.The specific research conclusions are as follows:（1）The blackness of silicon foam ceramics by experimental testing is 0.877,it shows that the material has excellent thermal radiation absorption capacity.Under steady state conditions,when the light intensity increased from 65.1 W/m~2 to 2002.3 W/m~2,the temperature difference between the upper and lower surface of the foam ceramic increased from 0.7K to 5.6K,and the maximum temperature difference between the upper surface skeleton and the air in the pores was less than 1.5K.The thermal resistance decreased from 1.56K/W to 0.37K/W,and the decreasing range decreased with the increase of light intensity.Under unsteady state conditions,when the light intensity arrived at 900 W/m~2,the temperature of the upper surface was up to 332.5K,the temperature of the lower surface was up to 330.3K and the temperature of the air in the pores was up to 330.4K.Based on the above experimental results,it is considered that the material performance meets the application requirements of solar air collector plate.（2）The three-dimensional physical and mathematical models of the collectors were established,and the finite volume method was used to solve the governing equations.（1）The effects of solar radiation intensity,medium inlet velocity,inlet temperature and ambient temperature on the performance of the collector were analyzed,respectively.The results show that the collector has high a thermal efficiency under the condition of low solar radiation intensity and high inlet velocity.Under the condition of high solar radiation intensity and low inlet velocity,the outlet temperature of the collector is higher.Under the same inlet Reynolds number condition,the thermal efficiency of the solar collectors designed in this paper is increased by 20%～30%compared with the traditional plate solar air collector.The thermal efficiency of the collector decreases with increasing of air inlet temperature,and the decreasing trend is more obvious with increasing of air inlet temperature.The lower air inlet temperature is more conducive to improving the thermal efficiency of the collector.The thermal efficiency of the collector increases with increasing of ambient temperature.The lower ambient temperature would cause a large amount of heat loss,and decreases the thermal efficiency of the collector.The ratio of the temperature difference which is between the air inlet temperature and the ambient temperature and the light intensity was defined as normalized temperature difference Ti*.The relational expressions between the efficiencyηand normalized temperature difference Ti*of the collectorwere obtained by the method of curve fitting asη_D-SAC=0.737-5.175T_i*for D-SAC model andη_S-SAC=0.651-2.66T_i*for S-SAC model,respectively.（2）The effects of porosity,pore density and pore diameter size and the thickness of the foam ceramic on the performance of the collector were analyzed severally,and the structure parameters of collector were optimized.The results show that the collector thermal efficiency and pressure drop increases with increasing of the inlet velocity,but the increasing range of thermal efficiency decreases with the increasing of the inlet velocity,and the pressure drop increases by exponential curve.When the porosity is constant,the pressure drop of the collector increases with increasing of pore density,but the effects of pore density on the thermal efficiency of the collector is negligible.The pore size is the most important factor affecting the collector pressure drop,and the collector pressure drop decreases with increasing of the pore size of foam ceramics.Among the five kinds of foam ceramic materials with different structural parameters used in this paper,the thermal efficiency of the collector is the highest only when the porosity of foam ceramic materials is 0.88 and the pore density is 10PPI.Keeping the inlet and outlet dimensions of the collector unchanged,the effects of the foam ceramics thickness at 25mm,30mm,35mm,40mm and 45mm,on the pressure drop were researched respectively.The conclusion show that the pressure drop of the collector decreases gradually with increasing of the thickness of the foam ceramics.When the thickness is30mm,the heat transfer performance of collector is the highest.The obtained results provide a new approach for the engineering application of silicon foam ceramic materials,and supply a theoretical basis for the optimal design of solar collector with porous media as heat receiver.