Pressure and Thermal Characterisation of Dynamic Instabilities During Flow Boiling in Micro/Mini-channels at Different Azimuth Orientations

Abstract

• Flow boiling in microchannels at different azimuthal orientation are investigated. • 3 instability types were identified: two-phase mixing, minor and major reverse flow. • Flow instability improves the average heat transfer coefficient by up to 52% • Top-heated channels had the greatest increase in heat transfer coefficient. • Instability frequency increased with mass flux increases and heat flux decreases. Fluid instability was investigated experimentally during flow-boiling of Perfluorohexane (FC-72) in a flat horizontal micro/mini channel with a hydraulic diameter of 909 μ m and an aspect ratio of 10 (5 mm x 0.5 mm). One-sided heating at different azimuth channel orientations ( θ ) in terms of gravity were considered, which ranged from bottom-heating ( θ = 0°) to top-heated ( θ = 180°) in 30° increments. Mass fluxes of 10, 20 and 40 kg/m 2 s were considered at a saturation temperature of 56 °C. Flow instability and the resulting thermal and pressure responses were identified and studied via high-speed video, infrared thermography, and pressure measurements. Various mass flux and heat flux combinations at each channel orientation leading to flow instabilities categorised into two-phase mixing, minor reverse flow and major reverse flow, were studied. Increased mass flux resulted in more frequent and more severe flow instability, irrespective of the channel orientation. Increased heat flux resulted in an increased number of operating conditions that were susceptible to flow instability, but the instabilities occurred at lower frequencies. In general, orientations that had a horizontal heated surface ( θ = 0° and 180°) were most susceptible to flow instability, while the intermediate azimuthal rotations ( θ = 30°, 60° and 90°) exhibited flow instability only at the highest mass flux. It was also found that flow instability often resulted in improved heat transfer performance, particularly in regions of the channel that were occupied by liquid. Where-as no direct relationship between the mass flux and the heat transfer performance improvement could be identified, it was found that better heat transfer improvements were obtained at higher heat fluxes. At the highest mass flux, cases that were horizontal and heated from above ( θ = 180°) exhibited heat transfer coefficient improvements (during instability) of up to 77% and 275% in the single-phase and two-phase regions respectively compared to baseline stable conditions.