Gravity heat pipes garner attention owing to their high thermal conductivities.In this study,the variable saturation temperature and variable physical properties of the working medium were added to the VOF model.The start-up process of a U-shaped gravity heat pipe was studied,and the evolutions of pressure and temperature during the start-up process were thoroughly discussed.The improved model could be better validated by the experimental data.The average deviation was only 5 K,which shows a high accuracy.Based on the simulation results,three stages can be divided:initial,equilibrium,and variation stages.In the initial stage,the temperature increase for the adiabatic and condensation section was hysteretic compared to that of the evaporation section,and small fluctuations of pressure difference between the evaporation and condensation sections could be observed(the average fluctuation was 81.5 Pa).In the equilibrium stage,the temperature increase for the condensation section was obtained.There were more obvious fluctuations in the pressure difference between the evaporation and condensation sections(the average fluctuation was 230.2 Pa),in which geyser boiling could be observed.In the variation stage,both temperature and pressure exhibited fluctuations.The pressure fluctuation was more obvious.
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