Experimental study on the operational characteristics of pulsating heat pipes in a horizontal orientation

Abstract

In this thesis, the operational characteristics of micro pulsating heat pipes (MPHPs) in a horizontal orientation is experimentally investigated. Furthermore, based on the experimental observations, a criterion for normal operation of PHPs in a horizontal orientation is provided as a design guideline. For this, the experiments are performed using the silicon-based MPHP with 10 turns, which is fabricated through MEMS techniques. The rectangular micro-channels engraved on the silicon wafer form a closed loop and have the width and the depth of 1 mm and 0.5 mm, respectively. The overall dimensions of the MPHP are 50 x 33.5 x $1.7 mm^3$. Ethanol is used as the working fluid and the filling ratio is 47% on the average. A transparent glass wafer is bonded to a silicon wafer for flow visualization. The temporal variations in the temperature and pressure of vapor plugs are simultaneously measured using micro-thermocouples and pressure transducers. The measured temperature and pressure data are fully synchronized in real time with the flow characteristics obtained through a high-speed photography. In the first technical chapter, the thermodynamic state of vapor plugs is experimentally examined prior to developing a theoretical model for a vapor plug. Since the equation of state to be used in a model for a vapor plug depends on its thermodynamic state, it is essential to identify the thermodynamic state of vapor plugs. It is experimentally found that the thermodynamic state of a vapor plug in the MPHP depends on the presence of the liquid film surrounding the vapor plug. If a vapor plug is surrounded by the liquid film, the vapor plug is saturated and there is no temperature difference along the vapor plug. On the other hand, if a vapor plug is in direct contact with the dry wall without the liquid film, the vapor plug is superheated and there is a difference in the vapor temperature between the evaporator section and the adiabatic section. Especially, during a stopover period in a horizontal orientation, it is shown that the vapor temperature in the evaporator section is significantly higher than the saturation temperature and almost identical to the wall temperature. In the second technical chapter, a transition between a stopover and a large-amplitude oscillation in a horizontal orientation is investigated, and finally a criterion for normal operation of PHPs even in a horizontal orientation is developed through experimental and theoretical investigations. For this, the MPHP is treated as a mass-spring-damper system and it is postulated that a transition from a stopover to a large-amplitude oscillation in the MPHP occurs with negative damping that induces dynamic instability in vibration systems. To verify this postulate, a spring-damper model for a vapor plug in the MPHP is developed in conjunction with the experimental observations, and then the postulate is confirmed. Finally, a criterion for normal operation of PHPs in a horizontal orientation is suggested in the form of a figure of merit, which is defined as the ratio of the vapor damping coefficient to the fictional damping coefficient. For PHPs filled with ethanol, it is experimentally confirmed that D_h/L_eff has to be larger than approximately 0.03 for normal operation without any stopover even in a horizontal orientation.