Numerical Study On Flow And Heat Transfer Characteristics Of Molten Salt Steam Generator

Molten salt steam generator is widely used in solar thermal power station,which transfers the heat carried by the high temperature molten salt to the water on the secondary side.The high temperature and high pressure steam produced in the steam generator enters the steam turbine to work and drives the generator to generate electricity.Subcooled boiling of water may occur if the feed water temperature is lower than the saturation temperature.The velocity of water and steam,void fraction and heat transfer coefficient significantly change because of the bubble generation on the secondary side,therefore,it is necessary to study subcooled boiling in the research of steam generator.In addition,the single-phase convection heat transfer occurs when molten salt flows in the tube,the heat transfer coefficient is generally lower than the secondary side,so an effective way to improve the heat transfer performance is to enhance the flow and heat transfer performance of the primary side.The complex heat and mass transfer mechanism of the steam generator become more clear through the study of the above problem,and it make a difference for stable operation and energy saving of steam generator.A molten salt natural circulation steam generator is designed for the matched steam turbine based on a 1MWe solar thermal power station,and the parameters are calculated such as molten salt mass flow,heat exchange area,the number and structure of heat transfer tubes and the shell structure of steam generator.The numerical calculation about heat exchange of steam generator is carried out by use of ANSYS FLUENT to study the single-phase convection on the primary side and the two-phase flow subcooled boiling on the secondary side.The effects of secondary side inlet subcooling and velocity of water on the flow and heat transfer characteristics such as working fluid temperature change,void fraction,heat transfer coefficient are studied,and the performance of the transvers groove tube with different structure parameters on the primary side is investigated.The RPI subcooled boiling model is used to simulate the subcooled boiling of water and the Euler two-fluid model is used to simulate the gas-liquid two-phase flow on the secondary side.The results show that increased inlet subcooling lead to increasing length of subcooled region and increasing gas and liquid phase velocity;increased inlet subcooling and velocity makes working fluid temperature rise rate,the void fraction and secondary side convection heat transfer coefficient decreasing,whereas the primary side convection heat transfer coefficient change less.The heat exchange performance and the friction coefficient of the transversally corrugated tube with different groove depth,pitch and groove width are different.No.8 tube(e=0.5mm,P=5mm,a=2mm)performs best in enhancing heat exchange performance,No.1 tube(e=0.2mm,P=5mm,a=1mm)has the worst enhancing heat exchange performance;No.4 tube(e=0.5mm,P=5mm,a=1mm)has the largest resistance coefficient,while No.1 tube(e=0.2mm,P=5mm,a=1mm)has the smallest resistance coefficient.The calculate results is helpful to analyzing the flow and heat exchange and improving the heat exchange efficiency of steam generator.