Understanding of the thermo-hydrodynamic coupling in a micro pulsating heat pipe

In this study, experimental and theoretical investigations are performed to identify the thermo-hydrodynamic characteristics of a micro pulsating heat pipe (MPHP). Specifically, the relationship between the heat input and the vapor distribution observed in the MPHP is revealed. A silicon-based MPHP with five turns and a hydraulic diameter of 667 μm is fabricated using MEMS techniques. Experiments are performed, using ethanol as a working fluid at a filling ratio of 55%, in a vertical orientation with a bottom-heating mode. Flow visualization is conducted together with a temperature measurement. In the MPHP, two menisci located at both ends of each vapor plug are observed to be asymmetrically distributed: the position of one meniscus (up-header) is always located higher than that of the other (down-comer) and is linearly increased with an increasing heat input. At a critical value of the heat input, the position of the up-header meniscus reaches its upper limit and cannot be increased further. At this upper limit, the thermal performance of the MPHP reaches its maximum and cannot be increased further. This suggests that physically the (asymmetric) vapor distribution is the key factor that determines the heat transport capability of the MPHP. To theoretically explain the relationship between the heat input and the vapor distribution in the MPHP, a model for the asymmetric vapor distribution is developed. Based on the model, a correlation for predicting the positions of vapor menisci is proposed. Finally, the proposed correlation is shown to be useful for predicting the heat input at which the MPHP attains its maximum thermal performance.