Steady-State Segmented Thermofluid Network Simulations of a Loop Heat Pipe Operating With Four Different Working Fluids

Abstract

A segmented network thermofluid model for the simulation of a loop heat pipe (LHP) operating under steady-state conditions is presented, with special emphasis on quasi one-dimensional models and semi-empirical correlations for the related multiphase phenomena. Attention is focused on an LHP with one flat-evaporator, a vapor transport line, one condenser, a liquid transport line, and a compensation chamber. The evaporator consists of the following parts: An upper piece, machined out of a stainless steel plate, with vapor-transport grooves of rectangular cross section on its bottom face; a lower piece, also machined out of a stainless steel plate, with a cavity of rectangular cross section that serves as the liquid pool in the evaporator during the operation of the LHP; and a rectangular wick sandwiched between the upper and lower pieces. The wick is a sintered powder metal plate made of stainless steel. The condenser is a horizontal tube that is fitted with excellent thermal contact inside a large high thermal conductivity metallic sleeve that is maintained at a fixed sink temperature. The vapor-transport line, the condenser, and the liquid-transport line are divided into control volumes or cells. Quasi one-dimensional models are used to impose balances of mass, momentum, and energy on each of these cells. The variation of fluid thermophysical properties and multiphase phenomena, such as the change in quality and pressure drops in the two-phase regions, are suitably accounted for in this model. Four different working fluids, ammonia, distilled water, ethanol and isopropanol, are considered, and the results obtained for a representative range of steady-state operating conditions of the LHP are presented and comparatively discussed.