Prediction of condensation and evaporation in micro-fin and micro-channel tubes

This paper surveys methods to predict condensation and evaporation in plain, and in micro-fin and micro-channel tubes. Hydraulic diameters as small as 1.0 mm are of interest. To date, the Shah correlation has been well accepted for prediction of condensation in plain tubes. This work shows apparent deficiencies of the Shah equation at higher values of p/p(cr). An improved predictive model based on the two-phase heat-momentum transfer analogy is described. This is called the "equivalent Reynolds number model." Use of the new model requires prediction of the single-phase heat transfer coefficient and the two-phase pressure gradient. For condensation in micro-fin tubes, both vapor shear and surface tension forces contribute to the condensing coefficient. The equivalent Reynolds number model will predict the vapor sheer component. An existing theory of Adamek and Webb is applicable for the surface tension contribution. The vapor shear model is also applicable to evaporation inside tubes. However, this model is not applicable after dryout. A nucleate boiling component will modestly add to the evaporation coefficient. Data are shown that suggests the nucleate boiling contribution is less than 15%, and that this contribution exists only at vapor qualities less than 50%.