Convective heat transfer characteristics of molten salts flowing in complex flow structures: Experimental studies and progress

Abstract

The large-scale utilization of traditional fossil fuels causes global warming and environment pollution, which is constraining the sustainable development of human society. Recently, new energy utilization technologies, including solar power and nuclear power, have witnessed rapid developments because they can relieve these problems. For the new energy utilization, molten salts are widely used. Therefore, having a deep understanding on the convective heat transfer characteristics of molten salts flowing in complex heat transfer structures is a critical issue to realize the high heat-transfer performance for new energy utilization technologies, especially the next-generation advanced nuclear power plant and high-temperature concentrated solar power (CSP) plant. In this paper, experimental studies on the convective heat transfer characteristics of the molten salt are firstly reviewed. It can be concluded that these studies experienced a development process, which is from smooth tubes to enhanced tubes, from tube side to shell side. Meanwhile, it is pointed out that studying on the convective heat transfer characteristics of molten salt flowing in complex structures is of great importance in the future. Then, previous experimental studies by our team on the convective heat transfer characteristics of the molten salt flowing in 5 heat exchangers with complex structures are introduced systematically, including shell-and-tube heat exchangers (STHE) without baffle plate, STHE with large and small hole baffles, STHE with segmental baffles, STHE with rod baffles, and airfoil printed circuit heat exchanger (PCHE). Based on the above experimental studies, corresponding convective heat transfer correlations for the molten salt flowing in the five complex heat transfer structures were obtained; and the applicable temperature and Reynolds number ranges for these correlations were also provided. The results show that the obtained correlations have simple forms and high precision, which can meet the engineering requirement. Moreover, heat transfer performances of the above five heat exchangers are also compared with each other. The results show that Nusselt numbers of the five heat exchangers have the same changing trend with Reynolds number; and the order of the heat transfer performances of the STHEs is: STHE with large and small hole baffles > STHE with rod baffles > STHE with segmental baffles > STHE without baffle plate. In addition, the heat transfer rate per unit volume of the airfoil PCHE is 70 times that of the STHE. To conclude the experimental research status of the friction and heat transfer characteristics of molten salt flowing in typical heat exchangers with complex structures, it can be found that most of the investigations focused on the convective heat transfer performance, lacking the analysis of friction characteristics. Therefore, the friction performances of the heat exchangers with complex structures should be further studied to obtain the comprehensive performance. Moreover, most of the experimental studies on the convective heat transfer performance of molten salts are related with the traditional heat exchangers. Thus, novel heat exchangers, which can achieve high heat transfer performance and low pressure drop, should be investigated and developed to meet the need of high efficiency, high temperature, and high pressure in the next-generation advanced nuclear power plant and high-temperature CSP plant.