Study On Forced Convective And Mixed Convective Heat Transfer With Molten Salt

The urgent energy situation encourages the rapid development of ConcentratingSolar Power (CSP) technology. As a key part of solar thermal power, high temperatureheat transfer and storage system has a direct effect on the system’s total efficiency andits application prospects. Molten salt has become the preffered heat transfer fluid forheat transfer process in solar thermal power plants with its advantages of broadutilizable range of temperature, low vapor pressure, high thermal stability and lowcost. Forced convective heat transfer takes place in the core components of solarthermal power plant, such as solar receiver and molten salt heat exchanger. Moreover,mixed convective heat transfer phenomenon under condition of non-uniform heatingflux also would happen in the molten salt receiver and parabolic trough concentratingcollectors. Therefore, it will help to understand heat transfer mechanism of the moltensalt receiver and heat exchanger and improve performance of the core components bystudying forced and mixed convective heat transfer rules with molten salt. Atheoretical basis and calculation method of designing molten salt receiver and heatexchanger will be provided by the research.Although experimental studies on forced convection of molten salt had beencarried out from1950s to1970s in United States, neither fitting general correlations,or comparisons between numbers of classic equations and experimental data werefound. In addition, no reports on mixed convection of molten salt in horizontal ducthave been found in any open literature.Under the financial support of National Basic Research Program of China withGrant Numbers2010CB227103, the paper firstly studied mixed convective heattransfer process of the molten salt in the horizontal tube under one sided heating fluxwith numerical simulation menthod. The flow and heat transfer characteristics ofmolten salt were carefully analyzed, and how the local Nusselt number varies withdimensionless distance under three different surface heating fluxes were alsoexplained. We summarized the variation among Nusselt number, Richardson numberand Reynold number, and obtained the comparison between simulated data and mixedturbulent convection correlation. The results show that, under non-uniform heatingflux conditions, the density difference of molten salt caused by larger temperaturedifference would give rise to effect of buoyancy under gravity which results in thesecondary vortex strengthening the process of convective heat transfer. Under threedifferent heating fluxes, the local Nusselt number increases with the increasing flowdistance in fully developed turbulent flow region, while the trend of growth isdifferent from each other. The deviation between simulated data and average Nusseltnumber is within±20%, and the order of dimensionless number is Nulateral> Nubottom> Nutop.Secondly, mixed convective heat transfer experiments have been transformedbased on the original experiment. The constantan film and custom heater were chosenas heating elements in the experiment. Although they were not successed, the failureanalysis may provide some helpful suggestions for further study.In the end, on basis of the experimental data of LiNO3and Hitec as well as theexisting data from literatures, the fitting correlations for transitional flow regime andfully developed turbulent flow were obtained. Then comparisons were made betweenthe accepted empirical correlations and the present data and existing data, and resultssuggested that the well-known correlations are applicable for high-temperature moltensalt. Moreover, the fitting curve of three different sets of structural parameters and ageneral correlation have been obtained for transversally corrugated tubes.