Study on thermo-hydrodynamic characteristics of micro pulsating heat pipes in a vertical orientation

Abstract

In this thesis, experimental and theoretical analyses are performed to understand the thermo-hydrodynamic coupling in micro pulsating heat pipes (MPHPs) in a vertical orientation. Specifically, a link is identified between the heat input and the thermally-driven flow inside MPHPs.

First, flow and thermal characteristics of MPHPs are investigated experimentally. Five-turn MPHPs with hydraulic diameters of 667 μm are fabricated using MEMS techniques. Flow visualization is conducted together with temperature measurement. Five liquid slugs and five vapor plugs are observed to have harmonic oscillation with two features: (1) Two menisci located at both ends of each vapor plug are observed to be asymmetrically distributed: the time-averaged position of a up-header meniscus is always located higher than that of a down-comer meniscus. (2) Each liquid slug oscillates with a phase difference of 2π/5 between adjacent slugs. Flow characteristics of MPHPs are decomposed into static (time-averaged) behavior and dynamic (oscillating) behavior.

Next, theoretical investigations are performed to find a link between the heat input and flow behavior of MPHPs. First, static behavior is evaluated by suggesting a model for the asymmetric vapor distribution. Based on the model, a correlation is proposed for predicting the time-averaged (equilibrium) positions of vapor menisci. Finally, the suggested correlation is shown to be useful for predicting the heat input at which MPHPs attain their maximum thermal performances. Second, dynamic behavior is analyzed by adopting the vapor spring-liquid mass model. Based on the model, a closed-form expression for the oscillating motion of the slugs is proposed. To quantitatively explain the oscillating mechanism, a link is found between the spring motion of and heat transfer to the vapor plug: Expansion and contraction of the vapor plug are shown to result from continuous evaporation and condensation at the liquid film enveloping the vapor plug. To mathematically express the relationship between the net heat transfer rate by evaporation/condensation and the spring motion of the vapor plug, a semi-analytic model for the vapor spring constant is proposed. Based on the model, a correlation is proposed for determining the oscillation frequency and validated with experimental results.

Finally, thermo-hydrodynamic coupling in MPHPs is interpreted physically as follows: First, the heat input determines the equilibrium positions of plugs/slugs. Second, the net heat transfer rate to each vapor plug governs the oscillation of each liquid slug around its equilibrium position. Both static and dynamic behavior may affect the thermal performances of MPHPs. However, it is static behavior rather than dynamic behavior that determines the maximum heat transport capabilities of MPHPs.