Study On Flow And Heat Transfer Properties And Thermal Expansion Of High Temperature Molten Salt Device

Molten salts usually refer to molten salts of inorganic salts or their mixtures.Common salts are chlorides,nitrates,fluorides,and so on.Molten salt has the advantages of wide operating temperature range,large heat capacity,small viscosity at high temperature,low vapor pressure and high thermal stability,and it is cheap and easy to obtain.It is an excellent heat storage and heat transfer medium.Therefore,molten salt is used to form near atmospheric high temperature molten salt heat transfer system,which has great potential in the field of high temperature heating and energy.In this paper,two kinds of complex structure commonly used in the high temperature molten salt system,heating plate(commonly used in disc heat exchanger)and rotary joint(commonly used in blade heat exchanger)as the research object,the flow and heat transfer characteristics(such as temperature,pressure,entropy generation distribution etc.)of high temperature molten salt in complex structure was studied.The theoretical analysis and numerical simulation of high temperature thermal deformation and thermal stress was performed and leakage characteristics of high temperature rotary joint were studied.The main contents of the research include the following aspects:First,the numerical simulation of the complex structure hollow heating plate of a high temperature molten salt disk heat exchanger is taken as an example.The influence of the inlet and outlet structure,circumferential fin and guiding plate on the flow and heat transfer characteristics of the molten salt in the flow of the heating plate was discussed.According to the Constructal Law,the general optimization method for complex structure is proposed.The structure of the traditional heating plate was optimized,and the preheating time of the optimized heating plate and the original heating plate was compared with the dynamic simulation.The flow and transfer situation of the heating disk was improved obviously and the reheating time was shortened a lot after the optimization.The thermal expansion and thermal stress of the molten salt heating plate are simulated,and the problems to be paid attention to in the design of molten salt heating plate are put forward.Second,the effects of thermal expansion on the leakage of rotary joint(another complex structure as the object)were investigated.The CFD method is used to simulate the temperature field and flow field of the molten salt in the rotary joint,and the data of the leakage of the rotary joint is obtained according to the temperature field data.Aiming at the serious problem of high temperature leakage of rotary joints,a circulating water cooling method is proposed.The influence of circulating cooling water on the leakage of rotary joints is investigated by numerical simulation.And it was found that the cycling water cooling system cut down the leakage efficiently.Third,on the basis of the above work,the high temperature molten salt rotary joint is improved and optimized in two aspects.On the one hand,the material and machining precision of the rotary joint are improved;on the other hand,the circulating water cooling rotary joint scheme which has been simulated and verified is introduced.It is tried to improve its high temperature performance without making the structure of the rotating joint too complex.A rotary joint test rig for circulating water cooling is set up,and the performance test of the preliminary design of circulating water cooling rotary joint scheme is carried out.The testing results showed that even if the cycling cooling system was closed,the new rotary joint worked in 450? molten salt environment without any problem.When working with the cycling water cooling system,the outer shell of the rotary joint was cooled efficiently.