Annals of the Assembly for International Heat Transfer Conference 13

ISBN 1-56700-225-0 / CD 1-56700-226-9

Year 2006

Alberto Cavallini
Dipartimento di Fisica Tecnica, Universita di Padova, via Venezia 1,I-35131 Padova, Italy

Luca Doretti
Dipartimento di Fisica Tecnica, Universita di Padova, via Venezia 1,I-35131 Padova, Italy

F. Fantini
Dipartimento di Fisica Tecnica, University of Padova, Padova, Italy

Claudio Zilio
Dipartimento di Fisica Tecnica, Universita di Padova, via Venezia 1,I-35131 Padova, Italy

Enhanced tubes have already been widely used for air-conditioning and refrigeration applications as they ensure a large heat transfer enhancement with a relatively low pressure drop increase. During condensation enhanced tubes show a heat transfer enhancement, compared to equivalent smooth tubes under the same operating conditions, that is partly due to the mere increase in the effective exchange area, and additionally to the turbulence induced in the liquid film by the enhanced surface (fins) and to the surface tension effect on the liquid drainage.
There is agreement in the literature that the mechanisms of heat transfer and pressure drop are intimately linked with the prevailing two-phase flow regime.
During condensation inside horizontal tubes, the two-phase flow may be dominated by vapour shear or gravity forces. While annular flow pattern is associated with high vapour shear, stratified, wavy and slug flows appear when gravity is the controlling force. In a fully developed annular flow pattern, there is a thin condensate film on the entire tube wall, while the gas phase flows in the central core, and heat transfer is governed by vapour shear and turbulence. Very poor evidence about the effect of microfins both in helical and "herringbone" shapes on flow patterns during condensation is given in the open literature. Thus, to investigate the two phase flow pattern during condensation, a special test section was built. Experimental observations for herringbone tube with three fluids (R236ea, R134a, R410A) in a wide range of operative conditions (mass flux and vapour quality) are reported in this paper.
For the study of the main flow patterns, in particular focusing on the stratified/annular mode transition, the visualisation experimental data are analysed with reference to parameters like dimensionless vapour velocity and Martinelli parameter, that are commonly used in most available flow pattern maps.

CSN-11 pages DOI: 10.1615/IHTC13.p25.110 Download article

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