Heat Transfer and Pressure Drop During Laminar Annular Flow Condensation in Micro-Channels

There is wide disagreement between experimental investigations and correlations for heat transfer during condensation in micro-channels. The major problem is the fact that the vapor-side resistance is usually appreciably smaller than that on the coolant side so that methods where only the overall resistance is measured, and the vapor-side heat transfer coefficient obtained by subtraction of resistances, are prone to large uncertainty. A few more recent correlations, based mainly on data for R134a, are in fair agreement when predictions for R134a and for the same conditions are compared. The fact that wide discrepancies are found when the correlations are used for fluids with widely different properties indicates that some or all of the correlations do not capture all of the essential mechanisms. That closely similar predictions are found when the correlations are applied to R134a indicates that the datasets used in the different studies were in essential agreement. Similar comments apply for pressure drop. The special case of annular laminar condensate flow permits wholly theoretical solution without recourse to empirical input. For this mode of condensation and for specified fluid, channel geometry, flow parameters and tube wall temperatures, local heat transfer-coefficient, and local pressure gradient can be calculated as well as local quality and void fraction. The theory is outlined in the article, and recent developments are discussed. Comparisons with the correlations for heat transfer and pressure gradient are given. For the heat transfer coefficient, the results of the annular flow theory are in surprisingly good agreement with the correlations when applied to R134a. For ammonia, the theoretical results lie between the widely spread values obtained from the correlations. Results for pressure gradient given by the annular laminar flow model are generally lower than those given by the correlations.