Study On Heat Transfer Characteristics Of Passive Residual Heat Removal Air Heat Exchanger

The passive residual heat removal system of the molten salt reactor,which is one of the Generation IV reactors,plays an important role in removing the decay heat,lowering the temperature of fuel salt and ensuring the security of fuel drain tank.The natural draft air heat exchanger is the heat exchange equipment between the atmosphere and the passive residual heat removal system.Its function is to transfer the heat in the system into the atmosphere,so that the system can operate safely and reliably.No manual operations or other external powers are required for the air heat exchanger to operate,which makes it have a good passive characteristic.In this paper,heat transfer characteristics of the stainless steel bare tube,stainless steel finned tube,finned tube composited by stainless steel and aluminum and finned tube composited by copper and aluminum were investigated by the method of experiment.The heat transfer modes outside and inside the tube are natural convection heat transfer of the air and condensation heat transfer of the water steam respectively.The experimental results show that the heat transfer powers of finned tubes are obviously higher than that of bare tube in the condition of natural convection,and the effects of material and condensation heat transfer coefficient inside the tube on the heat transfer power of finned tube are very small.Based on this,a natural draft air heat exchanger was designed,which mainly consisted of a finned tube bundle and a chimney.The finned tube bundle was installed at the bottom of the chimney for increasing the natural circulation velocity of air outside the tube bundle by the pumping action of chimney,because of which the heat transfer ability at the air side was improved.The steady heat transfer characteristics of the air heat exchanger in the design condition were studied by the method of numerical simulation using the computational fluid dynamics software ANSYS FLUENT,and the effects of air velocity,environmental temperature,finned tube arrangement,finned tube pitch,finned tube material,fin pitch and fin height on it were analyzed.The natural convection heat transfer process between the finned tube bundle and air was presented from the local and global perspective.Through the works above,some references used to further optimize the design of the air heat exchanger were given.By developing a model to the whole passive residual heat removal system,the transient heat transfer characteristics of the air heat exchanger designed during the operation of the passive residual heat removal system were further studied,and the effects of the environmental temperature,finned tube number and chimney height on it were analyzed.The results show that the air heat exchanger can meet the heat removal requirements of the system in different working conditions.In the initial stage of the operation of the system,the effects on heat removal power of the air heat exchanger are very obvious.With the increase of operation time,these effects gradually become very small due to the automatic adjustment of the air heat exchanger,but the variation of peak pressure in the steam drum is great in the adjustment process.It should be avoided that the peak pressure is higher than the pressure the passive residual heat removal system can withstand in the actual operation,so as not to cause damage to the system.