Gravity heat pipes have a simple structure

low cost

and good heat transfer performance

and are increasingly used in waste heat recovery devices. The start-up

flow

phase change

and heat transfer characteristics inside a gravity heat pipe are studied through experiments and numerical simulations. A heat pipe whose size parameters are the same as those of actual devices is selected for experiments. The volume of fluid (VOF) multiphase flow model is used to simulate the gas-liquid two-phase flow and the heat transfer inside the heat pipe

revealing the influence of filling rate

inclination angle

condensation section length

cooling wind speed

etc. on the isotherm

heat transfer coefficient

heat transfer resistance

and heat transfer efficiency of the heat pipe. The start-up and heat transfer characteristics of the heat pipe are obtained. The results show that among filling rates ranging from 15% to 35%

heat pipes with a filling rate of 30% have the minimum thermal resistance and the maximum heat transfer efficiency ratio. Among inclination angles ranging from 0° to 15°

heat pipes with an inclination angle of 9° have the lowest thermal resistance and the best isothermal performance. The start-up time of the heat pipe decreases with the increase of vacuum degree

and intermittent boiling occurs at 1 050 s. Under the condition of constant heat transfer power

the cooling water flow rate is inversely proportional to the optimal condensing section length. The optimal axial wind speed outside the tube is 8.4 m/s under the air-cooled condition. The numerical simulation results are in good agreement with the experimental results

and the evolution process of the internal working fluid phase distribution during the start-up process of the gravity heat pipe under the optimal operating conditions has been obtained. The research results can provide guidance for the design and application of heat-pipe heat exchangers in power plants.