Flow patterns and flow pattern maps for microchannels

Dense packaging of electronic components generates very high heat fluxes and therefore results in challenges for proper thermal management of such components. Microchannel based evaporators with phase changing liquids are regarded as a promising solution for such high heat flux cooling applications. Due to confinement of flow and differences in the relative importance of governing phenomena, the two-phase flow and heat transfer characteristics of microchannels have been shown to be different from those of conventional sized channels. The fact that microchannel is an attractive cooling option but at the same time there is a clear lack of understanding of related hydrodynamic and thermal transport phenomena which provides an impetus for microchannel research. This paper presents the flow patterns and flow pattern maps obtained for an experimental study of R134a during flow boiling in a horizontal microchannel. The microchannel was a fused silica tube, the outer surface of which was coated with thin, transparent and electrically conductive layer of Indium-Tin-Oxide (ITO). The microchannel was 781 μm in internal diameter and 191 mm in heated length. Operating parameters during the experiments were: mass flux 100-400 kg/m² s, heat flux 5-45 kW/m², saturation temperature 25 and 30 °C. A High speed camera was used with a close up lens to capture the flow patterns evolved along the channel. Flow pattern maps are presented in terms of superficial gas and liquid velocity and in terms of Reynolds number and vapor quality plots. The results are compared with some flow pattern maps for conventional and micro scale channels available in literature.