Thermally Induced Oscillatory Two-Phase Flow in a Mini-Channel: Towards Understanding Pulsating Heat Pipes

Research on Pulsating Heat Pipes (PHP) has received substantial attention in the recent past, due to its unique operating characteristics and potential applications in many passive heat transport situations. Reliable design tools can only be formulated if the nuances of its operating principles are well understood; at present, this is rather insufficient for framing comprehensive models. In this context, this paper reports experimental data on self-sustained thermally driven oscillations in a 2.0 mm ID capillary tube sub-system, consisting of only one vapor slug and one liquid plug (‘unit-cell’). Understanding such a sub-system/‘unit-cell’ is vital, as it represents a primary unit of a multi-turn PHP. Experiments have been performed with two fluids, i.e. Pentane (BP = 36.1°C) and Methanol (BP = 64.7°C) at different evaporator (40°C to 65°C) and condenser temperatures (−5°C to 15°C) respectively. High speed videography and spectrum analysis reveals that self-sustained thermally driven flow oscillations are observed for both fluids, albeit the dominant periodicity is different. Oscillation frequencies vary from 1.5 Hz to 4.2 Hz approximately, depending on the fluid, operating pressure and temperature. Increasing the difference of temperature between the evaporator and condenser sections leads to enhanced driving force for creating flow oscillations. The resulting phase velocities cause interfacial instabilities, resulting in the formation of secondary bubbles which break-off from the main meniscus. Results of this study can be compared to numerical models and will be useful to understand the physics of multi-turn PHPs.