| As an important solar low-temperature heat utilization device,flat-plate solar collector is widely used in building heating field,including solar air collector and solar water collector.However,in summer,the solar air collector will be idle due to high temperature;in winter,the solar water collector will have the risk of freezing due to low temperature.The flat plate solar collector with single collector is difficult to meet the heat collection needs in different seasons,and the instantaneous heat collection efficiency is low,which significantly reduces the economic benefits of the system.Therefore,this thesis designs a flat plate solar dual-function collector,which has both air and water heat collection functions.Through the free switching of multiple working modes,it can effectively improve the utilization rate and heat collection performance of the flat-plate solar heat collector.In this thesis,numerical simulation method is used to evaluate the outlet temperature of working fluid,instantaneous heat collection efficiency and total heat loss coefficient.The influence of several key parameters and different heat transfer enhancement measures on the thermal performance of solar dual-function collector is analyzed when air,water and air-water heat collection modes are used.The specific research contents and conclusions are as follows:(1)The main body of flat-plate solar dual-function collector is built on the basis of conventional solar water collector.The effects of the distance between the collector and drain tubes,the distance between the glass cover plate and the heat absorber plate and the thickness of the heat absorber plate on the thermal performance of the solar dual-function collector are studied respectively.It is found that when the distance between the heat collecting pipes is 120.0mm,the distance between the plates is 22.5mm and the thickness of the heat absorber plate is 0.8mm,best for the comprehensive performance of dual-function of solar collector.Based on the optimal structure parameter model,the effects of air/water inlet temperature,air/water inlet mass flow,solar radiation intensity,ambient wind speed and ambient temperature on the thermal performance of the solar dual-function collector are studied.At the same time,the significance of the influence of working fluid inlet parameters and environmental parameters on the collector performance is analyzed.(2)Change the relative flow direction of air and water,and compare and analyze the thermal performance of solar dual-function heat collector under different flow direction.The results show that: when the flow direction of air and water is opposite,namely "counter current",solar dual-function heat collector has higher thermal performance.The best combination of inlet and outlet settings is "air east-water West" direction.(3)Structural optimization of double-layer glass cover for solar dual-function collector shows that the optimum air layer thickness between the two-layer glass cover is 15.0mm.In air heating mode,compared with the single-layer glass cover,the total heat loss coefficient of dual-function solar collector with double-layer glass cover is significantly reduced.(4)Enhanced heat transfer of double-layer glass cover solar dual-function collector was studied by using straight ribs and nanofluids from air side and water side respectively.The research shows that adding straight ribs to the upper air channel and the lower air channel respectively weakens and strengthens the heat collection.The recommended height of rib of the lower air channel is 12.5mm.Graphite oxide nanofluids replaces pure water as liquid working fluid,the heat gain and heat collection efficiency of water are significantly increased.The higher the concentration of graphene oxide,the better the heat collection performance.While the 0.2% graphene oxide nanofluids have the best comprehensive performance. |
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