Detailed visualization experiments on the low-surface-tension ethanol in a pulsating heat pipe with large internal diameter

Before this article, no visualization research has ever been conducted for a low-surface-tension fluid in a closed loop pulsating heat pipe (CLPHP) with large inner diameter. In this work, a transparent CLPHP is built by bending a glass capillary tube with the inner diameter of 6 mm. Under the vertical orientation, even all the ethanol sinks to the bottom as liquid pools before the test due to its high Bond number of 3.56, two types of pulsation motion can still be observed. Short liquid slugs are repeatedly propelled upward, but only within the original evaporator U bend, which is called local pulsation. Geyser pulsation normally occurs in an ultra-long liquid slug, which is so intense as to redistribute the working fluid over the CLPHP and facilitate the following pulsation motion. Under the horizontal orientation, part of the ethanol drains into sections of stratified film, with the remaining ethanol forming liquid slugs. The local pulsation still appears and acts as a primary drive to trigger a series of pulsation motion. Without the gravity effect, the liquid slug trains can move with ease. At 80 W, the dynamic pulsating behavior disappears and transforms into a static mode with leading thermal performance. The lower stratified liquid film facilitates film evaporation, and the upper vapor passage sends the generated vapor to the condenser without hindrance. This thermosyphon-like heat transfer mode under the horizontal orientation is first observed and is recommended for real applications for its leading thermal performance.