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 for p/p_CT > 0.44. An improved predictive model called the “equivalent Reynolds number model” is introduced 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 predicts the vapor shear component. An existing theoiy of Adamek and Webb is applicable for the surface tension contribution. The vapor shear model is also applicable to evaporation inside tubes; however, it is not applicable after diyout. 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%.