Experimental Research And Numerical Simulation Of Heat Transfer Performance Of Two-Phase Closed Thermosyphons

In this paper,the classification,working principle and characteristics of heat pipes are briefly introduced.The research progress of heat transfer enhancement of two-phase closed thermosyphons is introduced from two aspects of intrinsic parameters and operating conditions.At present,many researchers have done a lot of researches on two-phase closed thermosyphon.However,due to the factors such as the difficulty of experimental visualization,many factors and synergies,it is difficult to make study deeply about the heat transfer mechanism of twophase closed thermosyphon,and there are many divergences in the research.In this paper,two copper-water two-phase closed thermosyphons with the same overall dimensions were fabricated and tested for their effectiveness.One was an ordinary two-phase closed thermosyphon and the other has an interposer structure in the evaporator.The two-phase closed thermosyphon has a total length of 610 mm,an invalid length of 10 mm,an evaporation length of 200 mm,an adiabatic length of 100 mm and a condensation length of 300 mm.The shell is made of T2 copper tube with an inner diameter of 8mm and a wall thickness of 1mm.The experimental platform of heat transfer performance of two-phase closed thermosyphon was set up.The influences of input power,dip angle and plug-in structure on the heat transfer performance of two-phase closed thermosyphon were studied.The input power was set as 200 W,275W,350 W,425W,500 W,tilt angle is set to 30o,45o,60o,75o,90o.At the same time,the numerical model of two-phase closed thermosyphon is established in this paper.The reliability of the numerical model is verified.The heat transfer performance and internal heat and mass transfer and flow of two-phase closed thermosyphon are simulated.The numerical simulation is used to analyze the experimental results from the influencing factors of heat transfer performance,and the heat transfer mechanism is obtained.The results show that the model proposed in this paper can simulate the internal flow and heat transfer in a two-phase closed thermosyphon.The visualization module can be used to analyze the heat transfer performance and the internal heat transfer mechanism of the two-phase closed thermosyphon further research and prediction.The thermal resistance of evaporator of twophase closed thermosyphon decreases firstly and then increases with the increase of input power.The total resistance and the thermal resistance of condenser monotonically decrease with the increase of input power.From the visualization of the simulation results,it is found that the formation,collision and combination of bubbles are the main reasons that the input power affects the heat transfer performance and mechanism of the evaporator.As the heating power increases,the heat transfer mechanism of the evaporator gradually changes from mixed convection to nucleate boiling.The heat transfer performance of the evaporator will be the best when the heat transfer mechanism is nucleate boiling.The vapor velocity of the condenser increases with the increase of input power,thus under the action of gas-liquid interface shear force,weakening the film thickness,reduce the thermal resistance of the liquid film and enhance the convective heat transfer between the vapor and the copper wall,and improve the condensation heat transfer in the condenser.This is one of the reasons that affect the heat transfer performance of the condenser.Under the premise of no deterioration of heat transfer,the optimum inclined angle is 60o for the heat transfer performance of evaporator,and the thermal resistance of evaporator of two-phase closed thermosyphon is reduced by 12.1% compared with that of 90o.From the visualization of the simulation results,it can be found that under the condition of inclined angle,different driving forces are applied to the bubles on upper and lower sides of the evaporator of the two-phase closed thermosyphon,which affects the merger and detachment of bubbles on the upper and lower side walls,heat transfer performance of closed thermosyphon in evaporator.The thermal resistance of the condenser of two-phase closed thermosyphon reached the minimum value when the inclined angle was 45°.The thermal resistance of condenser reduced by 10.7% on average compared that with the inclined angle of 90°.The total thermal resistance reached the minimum value at 60o.The thermal resistance of the evaporator decreased by an average of 8.8% under the conditions of 90o angle with four input powers.For the two-phase closed thermosyphon with an interposer,the performance of evaporator decreases first and then increases with the increase of input power.When the input power is 275 W,the thermal resistance of the evaporator reaches the minimum,and the total heat resistance reaches the minimum value.From the simulation result Visualization,it can be found that under the condition of low input power,the interposer structure forms more gasification cores in the evaporation section,and under the condition that the input power is too high,the structure of the interposer is unfavorable to the bubble overflow in the evaporation section,leading to the formation of large bubbles in the evaporation section,resulting in the phenomenon of drying up and the occurrence of boiling limits.This is the main reason that the structure of the interposer affects the heat transfer performance of the two-phase closed thermosyphon.