Experimental Investigation of Flow Boiling in Saw-Toothed Silicon Microchannels
Type of DegreePhD Dissertation
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Modern-day microprocessors consist of over one billion integrated circuits on silicon chips as small as a human fingernail. Normal operation of this circuitry produces an enormous amount of heat on a very small footprint. Dissipating this heat is a very challenging task, perhaps the largest roadblock to continued increases in computing technology. Microchannel heat sinks utilizing either single-phase flow or phase-change are an effective means of cooling stacked 3-D microelectronics. A roadblock to practical implementation of microchannels is the presence of flow instabilities. The saw-toothed microchannel heat sinks are proposed to address this issue. This study will discuss the design and fabrication of the microchannel test sections, experimental set up and testing results. Deep reactive-ion etching (DRIE) is used to produce channels comprised of asymmetric saw-toothed structures that alter the local flow structure within the microchannel. All experiments are conducted using the dielectric fluid, FC-72. Each microchannel array has a footprint of 1 cm x 1 cm, comprised of thirty-four channels etched into a silicon wafer. A series of thin film serpentine copper heaters is fabricated on the other side of the silicon wafer to provide a uniform heat flux boundary condition. Experimental information is presented for a range of mass fluxes from 444 to 1776 kg/m2s, and inlet subcooling from 5oC to 20oC. A high-speed camera is used to visualize the flow images of the boiling in the channels. The effects of geometry, step direction and height, mass flux, and inlet subcooling on boiling curves, flow patterns, pressure drops, heat transfer coefficients, onset of nucleate boiling, and vapor qualities are discussed in this paper. The saw-toothed steps enhance the heat transfer performance by greater than 30% across the entire range of heat input and mass flux tested, and up to 100% at a mass flux of 1776 kg/m2s and an input power of 16 W, but there is also 125% pressure penalty associated with this geometry. The forward-facing configuration leads to a larger bubble population in the channels, causing more effective mixing. These microchannel structures offer the promise of improved thermal performance without the complex fabrication processes associated with nanostructured or re-entrant geometries.