THEORETICAL AND EXPERIMENTAL INVESTIGATIONS ON FLOW AND THERMAL CHARACTERISTICS OF MICRO PULSATING HEAT PIPES

Abstract

In this study, experimental and theoretical investigations are performed to reveal a relationship between heat transfer and flow behavior of plugs/slugs in a micro pulsating heat pipe (MPHP). A silicon-based MPHP with 5 turns and a hydraulic diameter of 667 µm is fabricated using MEMS techniques. Experiments are performed at various levels of heat input with a bottom-heating mode and ethanol is used as a working fluid at a fixed filing ratio of 55%. Flow visualization is conducted together with a temperature measurement. In the five-turn MPHP, five liquid slugs and five vapor plugs are observed to have harmonic oscillation with two features: First, each liquid slug exhibits a harmonic motion with a phase difference of 2π/5 between adjacent slugs. Secondly, two menisci located at both ends of each vapor plug are observed to be asymmetrically distributed: the time-averaged position of one meniscus (higher-meniscus) is always located higher than that of the other (lower-meniscus). The former and the latter characteristics represent (1) dynamic behavior and (2) static behavior of plugs/slugs in the MPHP, respectively. To find a link between heat transfer and flow behavior, heat transfer to each vapor plug is numerically calculated. From the numerical simulation, the relationship between heat transfer and flow behavior is identified as follows: (1) A nonzero net heat transfer rate to each vapor plug via evaporation/condensation governs dynamic behavior and (2) the heat input determines static behavior. Both dynamic and static behavior may affect the heat transport capability of the MPHP. However, it is static behavior rather than dynamic behavior that determines the maximum heat transport capability of the MPHP. Therefore, a model for the (time-averaged) vapor distribution is suggested and validated with experimental results. The suggested model is shown to be useful for predicting the heat input at which the MPHP attains its maximum thermal performance.