In microgeometries, interfacial forces dominate over inertia due to the high surface area to volume ratio. Heat and mass transfer can be controlled by manipulating these interfaces. This can be achieved through the use of multiphase flows to create fluid-fluid interfaces or through nano and micro-manufacturing to change the surface properties at solid-fluid interfaces. I will go over two of my research topics that show how interfaces can be manipulated to enhance heat transfer. First, I will show how the generation of fluid-fluid interfaces can increase convective heat transfer in microchannel heat sinks. Flow in microchannels is usually laminar since the Reynolds number (Re = ρVd/μ) is small. When air bubbles are injected into liquid filled channels they create recirculating wakes because the air bubbles move faster than the liquid. When the bubbles are longer than the channel width, a phenomenon known as segmented flow, the recirculating wakes encompass the liquid filled channels and increase both the heat and mass transfer rate over 100% at the fluid-wall interface. Second, I will show how surfaces with heterogeneous wettability enhance heat transfer. In pool boiling, the heat transfer coefficient is a function of the bubble size and frequency at departure, and the number of active nucleation sites. By selectively changing the surface properties using nano and micro-manufacturing, the three parameters can be controlled. Using this technique the heat transfer coefficient can be increased by over 300%.
Refreshments in CW 119 at 4:15 p.m.