Flow-boiling instabilities in high aspect-ratio rectangular minichannels under single-sided heating at different inclinations

Abstract

Flow boiling can potentially enhance the heat transfer performance of cooling systems for industrial and everyday microelectronics. Flow instability is difficult to prevent, and its impact on heat transfer performance, whether beneficial or detrimental, is poorly defined, slowing its adoption. To better understand the phenomenon, experimental pressure instabilities were analysed during flow-boiling of Perfluorohexane (FC-72) in a high aspect ratio rectangular minichannel at different inclinations under one-sided heating conditions to simulate heating from microelectronics. The flow passage had a width to height aspect ratio of 10 (5 mm × 0.5 mm) and a hydraulic diameter of 909 µm. Inclination angles ranged from -20° (downward flow) to 0° (horizontal flow) and up to +90° (vertical upward flow), for mass fluxes of 10 kg/m2∙s, 20 kg/m2∙s and 40 kg/m2∙s at a fluid saturation temperature of 56.0 °C for a subcooling degree between 36.0 °C and 38.3 °C. Compared to in the absence of instabilities, it was found that instability-driven cooling improved heat transfer performance and decreased the heated surface temperature by up to 15.0 °C (in the case of 60° upward flow at a mass flux of 10 kg/m2∙s and heat flux of 11.3 kW/m2). Observed instability types included major reverse flow (severe pressure-drop oscillation), minor reverse flow (mild pressure-drop oscillation) and two-phase mixing. Reverse flow and two-phase mixing instability maps were created, which show that an increase in mass flux decreased the intensity of the reverse flow from major to minor as less vapour was generated, contributing to the pressure changes within the channel. An analysis of the pressure at the inlet and outlet showed that the mass flux effect on the frequency of reverse flow and two-phase mixing events varied depending on inclination. The nucleation site location substantially affected the instability frequency at an inclination of +30°. Channels inclinations of 0°, +30°, +45° and +60° experienced the most instability-driven cooling, while at +90°, instabilities reduced as bubbles were eased toward the channel outlet via buoyancy. For horizontal and all of the upward inclined channels, the heat transfer improvements became more significant as heat flux increased. The results provide guidelines for how flow instabilities can enhance the heat transfer performance during flow boiling while at the same time avoiding surface dry-out.

HIGHLIGHTS • Flow boiling in microchannels at different inclinations are investigated. • 3 instability types were identified: two-phase mixing, minor and major reverse flow. • Flow instability improves heated surface temperatures by up to 15 °C. • Two-phase mixing and major reverse flow frequency increased with heat flux. • The average heat transfer improvement from two-phase mixing increased with heat flux.