Quantitative visualizations of flow boiling for a dielectric fluid

Effectively exploiting liquid-to-vapor phase change—boiling—can greatly increase the power density of thermal management systems. Two-phase compact and ultracompact heat exchangers, for example, rely on flow boiling in channels with hydraulic diameters D_h of at most a few mm to maximize heat transfer in a small volume. Dielectric fluorinated solvents, with their low saturation temperatures and low surface tensions, are common coolants for thermal management of microelectronics because they are compatible with silicon devices operating at temperatures (well) below 85 °C. We have recently built a flow facility to study subcooled flow boiling in optically accessible D_h = 3.3 mm borosilicate and sapphire test sections heated over the top, or bottom, wall by an indium tin oxide (ITO) thin-film heater. Flow boiling of the hydrofluoroether HFE-7200, dyed with a custom fluorinated rhodamine, is visualized with illumination at a wavelength of 532 nm at low to moderate vapor quality for flow rates of ~1 cm³/s, heat fluxes of ~5 W/cm², and subcooling of 5 °C or less. Flow characteristics including flow regime maps, void fraction and vapor-phase velocity as a function of these flow parameters are presented and discussed.