Experimental investigation on the thermodynamic state of vapor plugs in pulsating heat pipes

Abstract

In the present study, the thermodynamic state of vapor plugs in micro pulsating heat pipes (MPHPs) is experimentally investigated. The experiments are performed using the silicon-based MPHP with 10 turns. The rectangular micro-channels engraved on the silicon wafer form a closed loop and have the width and the depth of 1 mm and 0.5 mm, respectively. The overall dimensions of the MPHP are 50 x 33.5 x 1.7 mm(3). Ethanol is used as the working fluid and the filling ratio is 47% on the average. A transparent glass wafer is bonded to the silicon wafer for flow visualization. The temporal variations in temperature and pressure of a vapor plug in the MPHP are obtained using micro-thermocouples and pressure transducers. The measured temperature and pressure data are fully synchronized in real time with the flow characteristics obtained through a high-speed photography. It is experimentally found that the thermodynamic state of a vapor plug in the MPHP depends on the presence of the liquid film surrounding the vapor plug. If a vapor plug is surrounded by the liquid film, the vapor plug is saturated and there is no temperature difference along the vapor plug. On the other hand, if a vapor plug is in direct contact with the dry wall without the liquid film, the vapor plug is superheated and there is a difference in the vapor temperature between the evaporator section and the adiabatic section. These results can pave the way to a thorough understanding of the thermo-hydrodynamic coupling in PHPs. (C) 2019 Elsevier Ltd. All rights reserved.