Study on Void Fraction Characteristics of Gas-Liquid Two-Phase Flows in Non-circular Mini-channels
(Square and triangular channels)
Purpose
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For higher heat transfer performance and compactness of heat exchangers used in refrigeration and air conditioning systems, the increase in heat transfer area density by channel diameter reduction is effective.
Channel diameter reduction is also effective in reducing the refrigerant charge amount. Recently, all aluminum heat exchanger using multi-port flat tubes with the channel diameter less than 1 mm had been developed for air-conditioners.
The effect of surface tension on the interface structure of gas-liquid two-phases flows becomes relatively larger against gravity.
Especially, for non-circular channels, the flow characteristics would be much different from those in normal diameter channels due to liquid stagnation in the corner. Although some previous reports showed that the condensing heat transfer coefficient in non-circular tubes was improved due to the formation of thin liquid film region by surface tension, void fraction defined as the volume fraction of gas phase, one of the most important parameter in two-phase flows, in noncircular channels are not well understood yet.
In this study, one-component gas-liquid two-phase flows in square and triangular mini-channels with the hydraulic equivalent diameter of 1 and 2 mm are examined to clarify the effect of the tube diameter, cross-sectional shape, and flow direction on gas-liquid interface structure and void fraction characteristics.
Research content
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| Churn flow observed in 2mm square tube |
Average void fraction against thermodynamics equilibrium quality |
Vertical upward one-component gas-liquid two-phase flows in a square and triangular tubes with the hydraulic diameter of 2 mm were examined.
A fluorocarbon FC-72 (surface tension 8.35 mN/m and boiling point 56.7 ℃) was used as the working fluid. The test section were formed in a piled-up transparent acrylic resin sheets.
Cross-sectional average void fraction was measured by an electrostatic capacitance sensor with plate-type electrodes.
The flow behaviors in the sensing area can be observed in the direction parallel to the electrodes by a high frame rata camera, simultaneously.
The measurement results of flow behavior and time average void fraction
for a square channel with a hydraulic equivalent diameter of 2 mm were
shown in the figure. The flow conditions of the movie were mass flux of
50 kg/(m2･s) and vapor quality of 0.1. Also the vapor flowed like a continuous
phase, it can be observed that the liquid was intermittently falling downward
due to gravity and blown up by the vapor flow. This flow pattern is churn
flow. The churn flow is easily occurred because the liquid film thickness
at corner became thicker and the interfacial shear force is reduced. The
average void fraction was lower than the correlation equation by Cioncolini
et al. for circular tubes. Void fraction in non-circular mini-channels
became lower with decreasing mass flux.
Publications
M. Hikita, T. Yamakita, H. Asano
Study on Void Fraction Characteristics of Gas-Liquid Two-Phase Flows in Horizontal Non-Circular Small Diameter Channel, Japanese Journal of Multiphase Flow 36, pp.47-54 (2022)(in Japanese)
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