| The increasing heat generation rates in integrated circuit (IC) chips pose severe thermal management challenges for the semiconductor industry. The cooling capacity of conventional heat sinks will soon reach their limit and novel methods for heat dissipation from ICs need to be developed. Two-phase microchannels and microjets have received attention because they promise compact and efficient cooling solutions.; This dissertation focuses on microchannel cooling technologies during incipient boiling, where bubbles nucleate, grow, and depart from nucleation sites on channel walls. Understanding bubble dynamics and determining departure criteria are critical in these devices such that local dry-out and subsequently poor cooling in regions can be avoided. Silicon microchannels with hydraulic diameters less than 400 mum were fabricated with heaters and sensors. When heating power was applied, bubbles formed due to heterogeneous nucleation and grew from the channel side-walls. In order to understand bubble dynamics, micron-resolution particle image velocimetry (muPIV) was used to obtain two-dimensional liquid velocity fields surrounding the nucleating bubbles. However, the limited information from the data requires the development of a hybrid method to reconstruct the three-dimensional geometry and associated three-dimensional velocity field. The combination of experiments and numerical simulations with this methodology yields important information such as bubble geometry, growth rates, contact angles, and forces that contribute towards the understanding of the physical mechanisms behind growth and departure.; Some recent efforts on two-phase microjet impingement cooling are also presented. In this project, microjet test structures were designed, fabricated, and characterized using a custom heater device.{09}A theoretical model developed for single-phase single jet impingement shows good agreement with the data. Heat removal of over 90 W/cm2 using a 4-microjet array was demonstrated, which suggests microjet impingement is a promising cooling solution. Insights into microjet hydrodynamics were also obtained with flow visualizations using white light microscopy and muPIV.; These current studies show promise towards developing optimized MEMS two-phase heat sinks for future IC chip cooling. The heat sinks are intended for the eventual integration into a closed-loop electroosmotically pumped cooling system. |
