Title: Micro-Evaporator Design and Investigation of Phase Change in Tailored Microchannels
Description: Microchannels show promising potential for implementation in next generation high heat flux cooling schemes. Promising research has been conducted in the area of MEMS cooling devices, taking advantage of the increased heat transfer characteristics in microfabricated structures. While significant advances in microchannels can be found in the literature, little work is being done to develop microchannels with non-uniform cross sections that can evaporate fluid without the presence of the bubbles at the exit flow. This thesis presents an experimental study of flow evaporation in micro-evaporators with tailored microchannel walls, demonstrating the ability to provide a stable flow of evaporated fluid for energy conversion and chip cooling applications. The design and modeling approach, microfabrication process and the full testing of the micro-evaporators are a part of this study and are all presented. Two mechanisms are proposed to stabilize the internal flow evaporation. The first mechanism is to establish a temperature gradient along the channel to separate the room temperature inlet fluid from the steam exit flow.

The second mechanism is to change the direction of the surface tension forces acting on the meniscus to fix its position along the channel. The test device used in this work consists of a silicon wafer with through-etched complex microchannels that is anodically bonded to a glass wafer on each side. Inlet and exit holes for the fluid are machined in the glass wafers. Water is forced through the chip while it is heated on the exit side of the three layer chip. The qualitative nature of the two-phase flow along the shaped channels is observed through the glass cover wafer, for different flow rates and wall temperatures. The work also provides a comparative study between different channel designs, different boundary conditions which reveal the benefits of the shaped microchannels with temperature gradient. The temperature gradient achieved with different thickness of channel walls shows agreement with the modeling results. Also, the benefit of having multiple expansions in the channels was demonstrated.

By using these two mechanisms the onset of water evaporation was fixed along the channel. The behavior of the meniscus in a microchannel with complex geometries and with a temperature gradient along it is quite encouraging in the perspective of the phase change in microchannels. These results of this research provide a design basis for a broad range of micro thermal systems, or Power MEMS, for chip cooling and micro power generation.