On the role of circulation effectiveness in the thermal performance of micro heat pipes

Due to the rapid advances in miniaturization of electronic components, micro heat pipe manifests itself as one of the effective high-heat-flux removal devices in electronics cooling. It is a micro-scale cooling device with a hydraulic diameter of the order of 100 µm. Benefiting from the phase-change heat transfer of its working fluid, micro heat pipe possesses extremely high thermal conductance, which is typically multiple times of that of solid material. Due to the difficulties associated with experimentation in a sealed micro-scale device like micro heat pipe, analytical and numerical investigations of transport processes such as the heat and fluid flow characteristics are crucial in the thermal performance analysis. In this thesis, a detailed steady-state mathematical model is developed based on the physical phenomena of micro heat pipe, particularly to investigate the role of circulation effectiveness of working fluids in analyzing the thermal performance of micro heat pipes. Firstly, the model is established to examine the interrelationship of the thermo-physical properties of the working fluid with the effect of axial conduction in the solid wall on the thermal performance of a micro heat pipe. The circulation effectiveness is properly defined and evaluated under different operating conditions. The theoretical model has been validated by the existing experimental data in the literature. Secondly, the model is extended to facilitate investigation on electrohydrodynamic (EHD) pumping assisted micro heat pipes of which the thermal efficiency is tremendously enhanced. The effect of electrolyte solution concentration on the working fluid’s circulation effectiveness in electroosmotically-driven micro heat pipes is studied. Thirdly, a different type of EHD pumping configuration—dielectric liquid pump—is incorporated in micro heat pipe to investigate the relationship between the circulation effectiveness and the thermal performance. Effective thermal enhancement can be attained by applying dielectric liquid pump operating at high voltage. Lastly, the coupled effects of gravitational force and both EHD pumping forces, i.e. electroosmotic and dielectric pumping forces are investigated in inclined micro heat pipes. The significance of the competing forces in affecting the circulation effectiveness is scrutinized by analyzing the liquid saturation distributions in an inclined micro heat pipe. A well-defined exposition of the circulation effectiveness of the working fluid is proposed at the initial stage and its significance is further justified in later stages. The working fluid’s circulation effectiveness which is a dominant factor in determining the heat transport capacity of micro heat pipe is strongly affected by the electric and gravitational forces. While dielectric liquid pumping does not exhibit significant performance enhancement at low voltage, electroosmotic flow stands out to be a better thermal enhancement approach. In view of the requirement of low applied voltage, electroosmotically-driven micro heat pipes are more efficient and practical than the dielectric-pumping operated counterparts in real applications. This study provides insights on the circulation effectiveness of micro heat pipes and paves the way for the feasible application of EHD flows in micro-scale two-phase heat transfer device.