CORRELATIONS FOR THE PREDICTION OF BOILING HEAT-TRANSFER IN SMALL-DIAMETER CHANNELS

This paper describes aspects of the work relating to boiling in single , small-diameter tubes as part of a study of compact two-phase heat ex changers. In order to realise the energy-saving potential of compact h eat exchangers for evaporating duties it is necessary to establish des ign procedures. A test facility was commissioned which was used to mea sure pressure drop and boiling heat transfer coefficients for R141b fl owing through tubes 500 mm long with diameters of 1.39-3.69 mm. Establ ished correlations predicted the heat transfer coefficients reasonably well for the largest tube but performed badly when applied to the sma ller tubes. It would appear that simple nucleate pool boiling correlat ions, such as that of Cooper, best predict the data. While under some conditions increasing quality leads to an increasing heat transfer coe fficient, it is suggested that intermittent dry-our occurs at very low quality in single narrow channels, thus reducing the average heat tra nsfer coefficient below that expected from the pool boiling correlatio ns. (C) European Communities 1997. Published by Elsevier Science Ltd.