Control of Instabilities in Two-Phase Microchannel Flow Using Artificial Nucleation Sites

Microchannel heat sinks offer the promise of an effective and compact technique for heat dissipation from high-powered microelectronics. When combined with phase-change mechanisms, it is possible to extend heat removal capabilities over several future technology nodes on electronics roadmaps. Many studies have documented instabilities in phase-change flows associated with uncontrolled transition from bubbly flows to slug flows at fairly low void fraction. The phase-change thermal transport reported here was carried out on microchannel heat sinks etched in silicon and cooled by the dielectric fluid FC72. Microchannels with a hydraulic diameter of 253 microns were studied. The base of each microchannel is augmented with 20-micron cavities to trigger controlled nucleation activity and help control large-scale instabilities reported in the literature. The cavities significantly impact the flow regime by promoting stable nucleate boiling. Temperature and pressure measurements show stable flow regimes under all combinations leading to saturated channel exit conditions and even for subcooled exit conditions at low-to-moderate heat fluxes. These measurements are confirmed by high-speed photography. Instabilities observed were confined to a sub-set of the subcooled exit flow regime. Within this region, an analysis of the data shows that the frequency of the flow instability is between 8–14 Hz. Instability maps demarcating regions of stable and unstable flow are presented as functions of mass flux (535–2140 kg/m²–s), and inlet subcooling (5–25°C).