The paper examines the special case of annular laminar flow pressure drop, or more precisely pressure gradient, during condensation in microchannels. This is the only flow regime permitting wholly theoretical solution without having recourse to experimental data. Solutions are obtained and comparisons made with empirical formulae for void fraction (needed to calculate the momentum pressure gradient) when obtaining the friction pressure gradient from experimentally measured or “total” pressure gradient. To date calculations and comparisons are restricted to one fluid (R134a), one channel section and one flow condition. For the case considered it is found that earlier approximate models for estimating void fraction agree quite well with the theoretical annular flow solutions. There is, however, significant difference between momentum pressure gradients obtained from approximate models used in the earlier investigations and that given by the theoretical annular flow solution which is (numerically) higher than all of them. The annular flow solution indicates that the momentum pressure gradient is not small in comparison with the friction pressure gradient. The friction pressure gradient in the annular flow case is appreciably smaller than given by the earlier correlations.

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