Operating characteristics of a dual flat-evaporator loop heat pipe for single heat source cooling in any orientation

Abstract

This paper reports the operating characteristics of a dual flat-evaporator loop heat pipe (LHP) for single heat source cooling in any orientation. The performance of the dual flat-evaporator LHP is compared with that of other single evaporator LHPs used in various orientations. An LHP with two flat evaporators and one natural convection condenser was designed and produced. Capillary structures were made from hydrophilic polytetrafluoroethylene porous membranes, and pure water was used as the working fluid. Two flat evaporators were set on opposite sides of a heat source to maintain the heat source temperature below 130 °C in six typical orientations. Evaporators/compensation chambers (CC) ran parallel to the horizontal plane, in which one evaporator was located completely above its CC, in two orientations. Evaporators/CCs were parallel to the vertical plane in the other orientations. The LHP successfully operated with normal operating patterns in all orientations. In the first two orientations, the unsaturated CC was the lower CC, and was flooded by liquid (deeply subcooled state) owing to gravity. The unsaturated CC contained both vapor and liquid in the other four orientations. In addition to the difference in elevation between the evaporators and condenser, this was the main reason for the difference in LHP operating characteristics in the six orientations. However, LHP performance was relatively uniform at each heat load when LHP orientation changed. The steady-state temperature could be estimated using our proposed numerical model. Below the heat source temperature of 130 °C, 140-W heat was transported and cooled with the lowest thermal resistance of 0.34 K/W.