A new model for numerical simulations of condensing annular flows in microchannels was previously developed by Nebuloni and Thome [14]. Presently, their condensation model has been extended by including turbulent liquid films and evaporating annular flows. In particular, the treatment of dry patch formation in the evaporating thin film of the annular flow is carried out. As such, the model is able to predict heat transfer and fluid flow characteristics in a large portion of the flow pattern map for microchannels in both condensing and evaporating conditions, for low and high mass velocities. The condensing turbulent liquid film and evaporating two-phase flows were validated against a selection of experimental data. Particular emphasis was placed on the analysis of the fluid flow and heat transfer differences between condensation and evaporation. Discrepancies between the liquid film thickness distribution and liquid film velocities yielded different optimal geometries for evaporation and condensation. The evaporation and turbulent flow data comparisons showed reasonable agreement with the data.