Research On Heat Transfer Enhancement Of Flat-panel Solar Water Heater By Vortex Generator

The flat plate solar water heater uses clean energy solar energy as the working energy source,which has the advantages of green environmental protection,durability,rational use of renewable resource,and is widely used in all aspects of production and life.In order to save energy and improve the heat transfer efficiency of the flat plate solar water heater,this paper improves the ordinary rectangular smooth channel flat plate solar water heater by changing the rectangular channel into a sudden expansion channel,so that the fluid can flow and separate at the sudden expansion point,and then reattach at the downstream of the channel.The heat transfer is enhanced by breaking the flow boundary layer near the downstream heat exchange wall,and at the same time,vortex generators are placed in the sudden expansion channel,the combination of the two can jointly improve the heat exchange efficiency of flat panel solar water heaters,and this combination can achieve the goal of energy conservation in practical engineering applications.On this basis,the influence of the position,height to bottom length ratio,and attack angle of the vortex generator on the heat transfer performance of the sudden expansion channel was further analyzed,and the optimal structural parameters of the vortex generator were found,which has certain reference significance for the design of the flow channel structure of solar water heaters.The major work in this paper is as follows:(1)When the ordinary rectangular smooth channel becomes a sudden expansion channel,the flow separation occurs when the fluid flows to the sudden expansion of the channel.The longitudinal vortex generated by the vortex generator placed in the channel makes the cold and hot fluid strongly mixed.The combined effect of flow separation and longitudinal vortex destroys the boundary layer and improves the heat transfer efficiency.When the sudden expansion channel is combined with a pair of delta wing vortex generators,the heat transfer effect is the best.Compared with the ordinary rectangular smooth channel without built-in vortex generators,the average Nusselt number number(Nu)is increased by 102.3%.(2)As the distance D between the trailing edge of the vortex generator and the edge of the suddenly expanded step in the suddenly expanded channel increases,the longitudinal vortex generated by a pair of delta wing vortex generators continuously attenuates and the average Nu decreases in the process of transferring to the downstream of the channel.When the angle of attack of the vortex generator β=45°,the ratio of height to bottom length is A/B=0.456,the average Nu of the sudden expansion channel is the highest at D=0mm,and compared with D=30mm,the enhanced heat transfer evaluation factor JF has increased by12.9%.(3)When the ratio A/B of the height of the vortex generator to the length of the bottom edge gradually increases,the longitudinal vortex intensity in the sudden expansion channel first increases and then decreases.The heat transfer performance is best at A/B=0.456.Angle of attack of vortex generator β=45°,the distance between the trailing edge and the edge of the sudden expansion step is D=0mm,the maximum enhancement heat transfer evaluation factor JF increases by 18.8% compared to A/B=1.(4)Angle of attack of vortex generator β increases from 15° to 60°,the heat transfer performance in the sudden expansion channel first increases and then decreases,The heat transfer effect is best at β=45°,and the fluid temperature inside the channel is higher,and the fluid temperature inside the channel is higher.The angle of attack β=45° has a maximum increase of 20.7% in JF compared to β=15°.(5)According to the above rules,a relatively optimal parameter is proposed.When Reynolds number Re=466,the angle of attack of vortex generator β=45°,the ratio of height to bottom length A/B=0.456,and the distance between the trailing edge of the vortex generator and the edge of the sudden expansion step D=0mm,the maximum JF value can reach 1.24,which improves the heat transfer efficiency of the flat plate solar water heater.