In situ investigation of liquid films in pulsating heat pipe

Abstract : To understand functioning of the pulsating (or oscillating) heat pipe (PHP), a liquid film deposited by an oscillating meniscus is studied experimentally inside the simplest, single branch PHP, which is a straight capillary sealed from one end. The PHP capillary is of rectangular section of high aspect ratio. The evaporator is transparent so that the films can be studied by two complementary optical methods: grid deflection method and interferometry. We were able to measure the dynamic film profile during the self induced meniscus oscillations. It has been shown that the PHP films have the same origin as those of the Taylor bubbles; their thickness right after deposition is well described by the classical formulas. The film shape in PHPs differs from the classical wedge-shaped film observed in capillary heat pipes because both of the larger thickness and of the receding triple liquid-vapor-solid contact line. The film slope is very weak, with a growing in time ridge adjacent to the contact line. It is shown that this ridge is the dewetting ridge. Its dynamics is defined mainly by the capillary effects. Such results can be generalized to the conventional multi-branch PHP.