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

Fluid instability was investigated experimentally during flow-boiling of Perfluorohexane (FC-72) in a flat hori-zontal micro/mini channel with a hydraulic diameter of 909 mu m and an aspect ratio (width to depth) of 10 (5 mm x 0.5 mm). One-sided heating at different azimuth channel orientations (theta) in terms of gravity were considered, which ranged from bottom-heating (theta = 0 degrees) to top-heated (theta = 180 degrees) in 30 degrees increments. Mass fluxes of 10, 20 and 40 kg/m2s were considered at a saturation temperature of 56 degrees 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 (theta = 0 degrees and 180 degrees) were most susceptible to flow instability, while the intermediate azimuthal rotations (theta = 30 degrees, 60 degrees and 90 degrees) 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. Whereas 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 (theta = 180 degrees) 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.

Automated summary

The fluid instability during flow-boiling of Perfluorohexane in a flat horizontal micro/mini channel was experimentally investigated. The results showed that the flow instability was most severe when the heated surface was horizontal and increased with higher mass and heat fluxes. Flow instability often led to improved heat transfer performance.