A Study On Heat Transfer Enhancement Of Parabolic Trough Solar Collector

As a renewable energy to replace traditional fossil fuels,solar energy has developed a variety of photothermal conversion methods in decades of research,development and operation.Nowadays,with the continuous updating and iteration of photothermal conversion equipment,the traditional thermal mass exchange of fluid with low thermal conductivity can not keep up with the actual demand.The research on new thermal mass exchange with high thermal conductivity and good heat transfer performance has become a hot spot.Starting with the thermophysical parameters of heat transfer fluid,this paper investigates and arranges the current mainstream calculation methods of physical parameters,and obtains the calculation models of thermal conductivity,specific heat capacity and dynamic viscosity of Al₂O₃-Syltherm-800 nanofluid.By combining the two-phase Mixture model and Discrete ordinates radiation model,the thermal performance of parabolic trough solar collector is analyzed.The accuracy and applicability of the application of nanofluid and foam metal are verified according to the change of heat transfer and flow characteristics.On this basis,the effects of volume fraction,inlet temperature and Reynolds number of nanofluid on heat transfer were studied and compared with pure base fluid.The results show that nanofluid can effectively reduce the wall temperature gradient of collector tube,and the peak temperature can be reduced by 114.75 K at most.With the decrease of wall temperature,the heat loss per unit length of the pipe is also significantly improved.When the inlet temperature is 600 K,the heat loss in the high Reynolds number range can be reduced by 90.13%at most.In addition,it is also found that the main factor affecting the pump power consumption is the inlet temperature.Under the conditions of this study,the pump power consumption decreases with the increase of inlet temperature,with a maximum reduction of 93.21%from400 K to 600 K.It is found that the Nusselt number increases by 5.28%～62.63%for all volume fractions and Reynolds numbers considered.The friction factor is reduced by 18.50%～56.19%,and the comprehensive thermal hydraulic evaluation index PEC varies from 1.06 to 1.70.After using nanofluids,the thermal efficiency of the collector can be increased by 47.94%.In addition,this study also analyzes the entropy generation rate.The results show that the 5%nanofluid volume fraction collector tube with inlet temperature of 400 K has the lowest entropy generation rate,and its entropy generation rate can be reduced by 71.08%.In order to further reduce the wall temperature gradient and improve the heat transfer performance and thermal mechanical properties,the metal foam is placed in the collector tube and numerically simulated.The circumferential temperature gradient on the wall of the collector tube decreased significantly and the peak temperature decreased by 11.30%as a result of the metal foam placement.The heat transfer and fluid friction properties of the collector tube also changed greatly due to the participation of metal foam.Due to the high thermal conductivity of the metal foam,the Nusselt number increased by 5.5 to 11.4 times than that of the smooth tube.Because of the viscous resistance and inertia resistance caused by the internal structure of the foam metal,the friction factor increased 15.11 times to 64.77 times at the entrance temperature400 K.The maximum value of PEC is 4.21,which occurs under the conditions of Reynolds number 10000,bore diameter 10 PPI and filling height 0.65H.Due to the flow resistance caused by the metal foam,the thermal efficiency and the efficiency of exergy are lower than that of the smooth tube under the present conditions,and the falling ranges are 0.285%to 0.936%and14.82%to 86.12%respectively.Fig.45 Table 6 Ref.98...