Review of saturated flow boiling in small passages of compact heat-exchangers

Intensification of process heat exchangers has led to considerable interest in compact heat exchangers for evaporation. These heat exchangers are characterised by multichannel passages with hydraulic diameters less than about 10 mm. In this article, two different categories of channels composing compact heat exchangers are reviewed: small circular and rectangular channels (including studies of individual channels and multichannel arrangements of parallel channels) and multichannel arrangements of straight perforated and serrated fin passages. The regimes of the two-phase flow which occur in these passages differ from those in larger systems. The typical trends of local heat transfer coefficient in small passages of compact heat exchangers show that it is easy to distinguish between the two dominant mechanisms of boiling, nucleate and convective boiling. The insights relative to heat transfer mechanisms in small passages and the flow patterns associated to convective and nucleate boiling regions are presented. In particular, the effects of vapour quality, mass flux, heat flux, channel geometry/size, fin type, gravity and surface tension in each boiling region are highlighted. Literature correlations valuable in small channels expressing the local boiling heat transfer coefficient as a function of vapour quality, mass flux, heat flux, fluid properties, fluid pressure and channels dimensions are reported. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.