Abstract

Microchannel heat transfer governs the performance of the microchannel heat sink, which is a recent technology aimed at managing the stringent thermal requirements of today's high-end electronics. The present thesis investigates the single-phase and flow boiling heat transfer characteristics in microchannel systems. A state-of-the-art test facility has been constructed for the experimental investigations. It is composed of a closed loop flow setup, a microscopic imaging system and an automated data acquisition system. A technique for using un-encapsulated thermochromic liquid crystals (TLC's) to measure the local heat transfer coefficient in microchannel geometries is developed. A unique localized calibration of the TLC material is employed to minimize the effects of lighting non-uniformity and variable coating thickness. Measurements are carried out in three different microtubes, and in two innovative parallel and radial microchannel heat sinks with 3.5 cm 2 footprint areas. The working fluids are distilled water, FC-72 and air. Local single-phase heat transfer and frictional pressure drop measurements are presented for the laminar, transitional and turbulent flow regimes, in microtubes down to 0.25 mm. Local flow boiling heat transfer coefficient data are presented for a, quality up to 0.3, and for the conditions investigated, suggest a nucleation dominated region. Flow boiling oscillation characteristics in two parallel silicon microchannel heat sink configurations aimed at micro/nano-spacecraft thermal management are investigated. One is a standard heat sink with 45 parallel channels, whereas the second is similar, except with cross-linked paths at three locations. The oscillation amplitudes are relatively large and identical in frequency for the fluid pressure and temperature. Oscillation properties for the standard heat sink are correlated for different heat fluxes, while a first glimpse of the cross-linked heat sink performance under flow boiling instability conditions is presented. Optimization of a radial inflowing microchannel heat exchanger has been investigated numerically, after preliminary fabrication and experimentation trials. Unique to this optimization was consideration of channels with axially varying cross-sections. Three-dimensional conjugate analysis shows that when constrained by a fixed channel outlet area, increasing the channel inlet area will improve the thermal performance. Overall, a method utilizing un-encapsulated TLC thermography for local heat transfer measurements in microgeometries has been developed, while investigating the flow and heat transfer characteristics in microchannel systems. The present work along with the advances in MEMS based manufacturing is expected to lead to the creation and development of a number of miniaturized technologies, from DNA analysis to power-plants-on-chips