Study On The Local Flow Control And Heat Transfer Enhancement In Parabolic Trough Solar Collectors

Solar energy has been widely used as a renewable resource due to its abundant and environmentally-friendly features.Solar energy utilization mainly includes photothermal,photovoltaic,photochemistry and photobiology.Solar thermal utilization has developed rapidly due to its stable performance and high efficiency.The parabolic trough solar collector is one of the important ways of solar thermal utilization,but the uneven heating of collector tubes leads to uneven wall temperature and local high temperature,which restricts its development and promotion.In order to solve the above problems,this paper focuses on the parabolic trough solar collectors,based on the theory of local flow control and heat transfer enhancement,using numerical simulation and experimental,aiming at verifying the feasibility of vortex generators as the local flow control technique.The design and development of vortex generators,and the effect of flow control on heat transfer enhancement were carried out.The main research contents and conclusions of this paper are as follows:(1)An experimental platform was built to test the heat transfer and flow performance of uniformly heated collectors,and the applicability of the system was demonstrated by comparing the experimental results of smooth circular tubes with the predicted results of empirical equations.In addition,based on the Monte Carlo Ray Tracing method,the design of new parabolic trough solar collectors using air as the working mass was carried out.(2)For uniformly heated collectors,the curved wing vortex generators(CWVGs)based flow control method was proposed.Flow and heat transfer characteristics in round pipes with CWVGs were numerically and experimentally evaluated.It was found that CWVGs formed eight pairs of counter-rotating vortices inside the pipe,controlling the movement of fluid toward the heated wall,enhancing fluid mixing and improving heat transfer performance but also introducing additional pump work.In addition,the width ratio(WR)and pitch ratio(PR)of CWVGs were investigated.The results showed that the heat transfer performance and flow resistance increased by 1.41-2.6 and 2.64-15.82 times,respectively,and the thermal enhancement factor(TEF)was increased and then decreased with the increase of the width ratio and pitch ratio of CWVGs.The best overall thermal performance(TEF = 1.27)was achieved at Re = 5962,WR = 0.20,and PR = 1.0.(3)For non-uniformly heated collectors,the rectangular winglet vortex generators(RWVGs)based flow control method was proposed.This paper focused on the effect of circumferential local and full arrangement of RWVGs on the performance of collectors.The results demonstrated that after applying local flow control,vortices were formed in the tube,which enhanced fluid disturbance and mixing,and improved convective heat transfer performance The circumferential full arrangement of RWVGs formed two pairs of vortices in the pipe,the upper part of the rotating vortices restricted the movement of cold fluid to the lower heating wall.And the pair of vortices formed in the circumferential local way was extended to the entire flow channel to control the full mixing of hot and cold fluids,which improved the temperature uniformity,and the heat transfer performance were better than the full arrangement.After that,the height ratio(HR)and pitch ratio(PR)of RWVGs were optimized for the circumferentially localized arrangement.The results showed that the maximum temperature was decreased by 7.44K-150.24 K,and the heat transfer performance and flow resistance was increased by 1.32-2.56 and1.58-9.27 times,respectively,the thermal enhancement factor was 0.95-1.28,with the maximum value obtained at HR = 0.2 and PR = 1.57.This paper shows that the application of local flow control can improve the convective heat transfer performance of the collectors and effectively reduce the circumferential temperature difference of the tube wall.It is confirmed that the longitudinal vortex is one of the characteristics of the optimal heat transfer flow field,and further reveals the mechanism of heat transfer enhancement.In addition,the findings in this paper can provide references for further work on parabolic trough solar collectors.