Experimental And Numerical Investigation On Molten Salt Heat Exchangers For The TMSR Proiect

In order to develop the next generation nuclear power system in China,a TMSR(Thorium Molten Salt Reactor)project was officially launched in 2011 by Chinese Academy of Sciences.A TMSR system is designed to use high temperature molten fluoride salts(600～700℃)as the primary and secondary coolants,which has such excellent technology advantages as inherent safety,better economy,minimum nuclear waste,nuclear nonproliferation and so on.Heat exchangers using the molten fluoride salts as working fluids are the key components in the TMSR system,since they have great impact on the system’s safety and economy.There are urgent requirements to develop large-scale and practical molten salt heat exchangers for the TMSR project.Both the conventional tube and shell and compact heat exchangers are considered to be used in a TMSR system.But they are still a little far from application,because they all lack comprehensive heat transfer performance data.Based on the TMSR project,this paper will focus on the experimental and numerical investigation on molten salt heat exchangers.An experimental system for molten salt heat exchangers will be built and various kinds of molten salt heat exchangers will be tested by this system.Based on the experimental data,practical and large-scale molten salt heat exchangers will be designed for the TMSR system,which will be further numerically investigated to ensure the system’s safety.In the first part of the paper,an experimental system is designed and built to test the heat transfer performance of molten salt heat exchangers.The system consists of four major loops,including two molten salt loops,a closed gas loop and a water cooling loop.The system is designed for molten salt to salt and molten salt to gas heat exchangers with the conventional tube and shell and compact configurations.The system can work reliably under high temperature and is more perfect than the existing molten salt heat transfer experimental devices,which are usually only designed for single tubes.In order to improve the accuracy of measurement results in the system,measurement uncertainties in the system are assessed by the uncertainty theory.It is indicated that larger gas flow rates and larger temperature differences of molten salt between the inlet and outlet are recommended to decrease the uncertainties in the measurement results.The experimental system provides a reliable platform for the molten salt heat exchangers of the TMSR project and is undoubtedly significant in science.In the second part of the paper,three tube and shell molten salt heat exchanger prototypes are designed and tested respectively,according to the near term needs of the TMSR project.The tests are innovative since experiments of this kind were mostly conducted in a single tube before.The test results show great heat balance in the system,which means that the system is reliable enough.Heat transfer characteristics for molten salt in both the tube and shell sides are analyzed and three main conclusions are obtained:(1)In the tube side,molten salt acts like normal fluids in the transitional and turbulent flow.But in the laminar flow,the data is 30%higher than the traditional empirical correlation.(2)Heat transfer characteristic of molten salt in the shell side with baffles is obviously different than that of oil.It can be concluded that the existing design correlation maybe not suitable for molten salt in the shell side with baffles.(3)Thermal resistance of a molten salt to gas heat exchanger is mainly caused by gas.Molten salt in the tube bundle will risk solidification when a molten salt to gas heat exchanger is in operation if the molten salt pump stops accidently.Based on the operating experience,preliminary optimizations for the experimental system are discussed in three aspects:system layout,electric heaters and thermal insulation and heat tracing system.In the third part of the paper,a 10 MW air cooling system is designed for a 10 MW solid fuel molten salt experimental reactor.The 10 MW molten salt to air heat exchanger in the air cooling system is numerically investigated with the help of Ansys FLUENT,in order to the ensure the system’s safety.The investigation focuses on the molten salt solidification characteristic and the flow distribution of molten salt in tube bundle.First,Solidification&Melting model in FLUENT is experimentally proved to be able to predict the solidification process of molten salts precisely.After that,the behavior of molten salt in the 10 MW molten salt heat exchanger is numerically investigated with the help of FLUENT when the molten salt pump stops accidently.It is shown that molten salt in the tube bundle will start to solidify in 15 seconds in the pump stopping accident,which is a major thread to the system’s safety.Second,flow distribution of molten salt in the tube bundle is numerically investigated.A large amount of meshes and limited computing resource are two major challenges which are overcame by using the porous media condition.It is shown that flow distribution in the tube bundle is uniform and safe enough.