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ISSN 961-91393-0-5

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Year 2004

• 3Thermal Sciences 2004
Proceedings of The ASME - ZSIS International Thermal Science Seminar II, Bled, Slovenia, June 13-16, 2004

1032 pages

Volume price - $246.00

The Characterization of Flow Regimes with Power Spectral Density Distributions of Pressure Fluctuations During Condensation in Smooth and Enhanced Tubes

Leon Liebenberg
Department of Mechanical and Aeronautical Engineering, University of Pretoria, Pretoria, 0002, South Africa

Josua Meyer
University of Pretoria

ABSTRACT

This paper presents an objective predictor of the prevailing flow regime during refrigerant condensation inside smooth-, micro-fin and herringbone tubes. The power spectral density (PSD) distribution of the fluctuating condensing pressure signal was used to predict the prevailing flow regime, as opposed to the traditional (and subjective) use of visual-only methods, and/or smooth-tube flow regime maps. The prevailing flow regime was observed by using digital cameras and was validated with the use of the conventional smooth-tube flow regime transition criteria, Froude Rate criteria, as well as a new flow regime map that was developed for micro-fin tube condensation. Experimental work was conducted for condensing R-22, R-407C, and R-134a at an average saturation temperature of 40°C with mass fluxes ranging from 300 − 800kg/m²s, and with vapour qualities ranging from 0.05 − 0.15 at condenser outlet, to 0.85 − 0.95 at condenser inlet. Tests were conducted with one smooth-tube condenser, one herringbone tube and three micro-fin tube condensers (with helix angles of 10°, 18°, and 37° respectively). It is shown that the herringbone tube condenser causes a delay in the transition from annular to intermittent flow by at least 22% vapour quality (compared to the smooth-tube), whilst the micro-fin tubes cause a nominal delay of at least 19% (compared to the smooth tube), thus significantly contributing to the enhancement of heat transfer.