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 exchangers. In order to realise the energy-saving potential of compact heat exchangers for evaporating duties it is necessary to establish design procedures. A test facility was commissioned which was used to measure pressure drop and boiling heat transfer coefficients for R141b flowing through tubes 500 mm long with diameters of 1.39-3.69 mm. Established correlations predicted the heat transfer coefficients reasonably well for the largest tube but performed badly when applied to the smaller tubes. It would appear that simple nucleate pool boiling correlations, such as that of Cooper, best predict the data. While under some conditions increasing quality leads to an increasing heat transfer coefficient, it is suggested that intermittent dry-our occurs at very low quality in single narrow channels, thus reducing the average heat transfer coefficient below that expected from the pool boiling correlations. (C) European Communities 1997. Published by Elsevier Science Ltd.