Analysis of heat pipe startup from the frozen state

The finite element eigenvalue method is used to study the startup phenomenon of a sodium heat pipe initially at the frozen state. The heat pipe has an axial groove structure. The heat conduction equation in the wall and grooves including melting of the working fluid is solved together with the quasi-steady, one-dimensional compressible vapor flow equations. The choked flow phenomenon, transition from molecular to continuum flows, and the transition from kinetic-controlled to flow-transport-controlled evaporation and condensation are considered. Results obtained include the time dependent axial wall temperature distribution, axial vapor temperature and pressure distributions, vapor mass flow rates, and the melting rate of the working fluid. The evaporator wall temperature increases rapidly in the initial transient period but the increase is arrested as soon as the working fluid is melted and starts to evaporate. Also, the maximum difference between the vapor and liquid pressure distributions occurs during the transient period.