Hua Sheng Wang | h.s.wang@qmul.ac.uk and John W. Rose | j.w.rose@qmul.ac.uk
School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, UK.
Abstract: The problem of boiling and condensation in channels is extremely complex. For practical purposes correlations of experimental data for heat transfer and pressure drop are generally needed. Because of their empirical basis and the number of variables involved, correlations are not reliable when applied to fluids and conditions dissimilar from those of the data on which correlations are based. For microchannels (typical dimension around 1 mm) recent experiments have shown annular flow to be the most common flow regime. This is a rare case where a wholly theoretical solution is possible. For annular flow condensation in non-circular section microchannels surface tension driven condensate flow transverse to the vapor stream (i.e. towards the corners of the channel) is important. A theory, based on the well-established Nusselt approximations and including the transverse surface tension pressure gradient, is outlined and some of the more important results discussed. The theory is in fair agreement with correlations, based on data for R134a, which give similar predictions for this fluid. However, predictions of the correlations differ widely when applied to ammonia. Preliminary comparisons are made with limited experimental data where local measurements along microchannels are available. In the light of the comparisons it is considered that the theory, which has no empirical input, should be more reliable for design purposes than correlations for fluids and operating conditions for which no experimental data are available
Keywords:condensation, microchannel, annular flow, surface tension, theory, heat-transfer measurements.