Design of a Microbreather for Two-Phase Microchannel Devices by Brentan R. Alexander SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN MECHANICAL ENGINEERING

Multiphase flows in microchannels are encountered in a variety of microfluidic applications. Two-phase microchannel heat sinks leverage the latent heat of vaporization to offer an efficient method of dissipating large heat fluxes in a compact device. In microscale methanol-based fuel cells, the chemical reactions produce a twophase flow of methanol solution and carbon dioxide gas. Differences in the underlying physics between microscale and macroscale systems, however, provide a new set of challenges for multiphase microscale devices. In thermal management devices, large pressure fluctuations caused by the rapid expansion of vapor are prevalent in the flow channels. In fuel cells, the gaseous carbon dioxide blocks reaction sites. In both of these cases, dry-out is a problem that limits device performance. We propose a design for a microscale breather that uses surface chemistry and microstructures to separate gas from a liquid flow to improve two-phase microchannel performance. To better understand the physics and governing parameters of the proposed breather, we have designed and fabricated test devices that allow crosssectional visualization of the breathing events. We have conducted various experiments to examine the effects of device channel hydraulic diameters ranging from 72 μm to 340 μm and liquid inlet flow rates ranging from 0.5 cm/s to 4 cm/s on the maximum gas removal rate. We demonstrated a maximum breather removal rate of 48.1 μl/min through breather ports with a hydraulic diameter of 4.6 μm connected to a microchannel with a hydraulic diameter of 72 μm, and a liquid inlet flow velocity of 0.5 cm/s. A model was developed that accurately predicts the exponential dependence of the maximum gas removal rate on a non-dimensional ratio of the pressure across the breather ports compared to the pressure drop in the main channel caused by the venting bubble. These results serve as design guidelines to aid in the development of more efficient and sophisticated breathing devices. The successful implementation of a microchannel with an efficient breather will allow for new technologies with higher heat removal capacities or chemical reaction rates that can be effectively used by industry. Thesis Supervisor: Evelyn N. Wang Title: Assistant Professor of Mechanical Engineering