Thermofluid dynamic study of flat-plate closed-loop pulsating heat pipes

Meandering-tube pulsating heat pipes (PHPs) have already found some applications in cooling power/microelectronic components. Reliable experimental data for PHPs is very limited, especially in the wake of the ongoing miniaturization process and efforts to combine electronic components and heat spreaders on the chip level. In fact, the basic operation of PHPs is far from being fully understood, and there are no available design tools for given microelectronics cooling requirements. A logical next step for further applications of PHPs in microelectronics cooling is to design integral structures, i.e., PHPs as an integral part of thermal spreader/substrates, having typical dimensions as applicable for multichip modules or printed circuit boards (PCBs). In this article, experimental results for flat-plate, closed-loop PHP structures (rectangular and circular, D-hyd approximate to 2.0 mm) which are integrally machined into an aluminum substrate (115 x 36 x 3 mm(3)) and covered by a glass plate for visualization are presented. Working fluids employed are water and ethanol. To verify certain observed phenomena, another PHP setup is made consisting of 10 glass tubes (ID 2.0 mm) connected by copper U-turns. As input parameters, fill rate, heat load, and tilt angle are varied. Axial temperature distribution is recorded with thermocouples. By means of a high-speed video camera and a thermo-camera, the evaporation/condensation processes and the individual liquid-vapor plug characteristics are studied. The results enable explanations for operational characteristic and performance limits, which are influenced by capillary resistance, gravity, and thermofluid dynamic effects. Preliminary design rules are also discussed.