The structure and characteristics of the two-phase closed thermosyphon with working trap are described.A mathematical model is presented to calculate the unsteady-state start-up process of a room temperature heat pipe.The model is based on the special structure and operation conditions of the heat pipe

which is simplified to that the steam temperature in the tube changes with time only.The relation of steam temperature and time is obtained by numerical calculation and used as the temperature boundary condition of the measurement trap.The temperature

velocity and pressure distribution in the working trap are then solved by FLUENT.The main effect of ambient air on the thermometer well occurs at the orifice

while the temperature is uniform at other locations.The calculation is in good agreement with experimental data

and the maximum deviation is 3.7 K when the start-up time is 100 s.The simulation and experiments indicate that water fluid filling volume has little effect on the start-up performance of the heat pipe.The mathematical model and the research methods provide a new approach for the study of special heat pipes.