A numerical study of the thermal and fluid-dynamic behaviour of the two-phase flow in ducts under condensation or evaporation phenomena is presented. The numerical simulation has been developed by means of the finite volume technique based on a one-dimensional and transient integration of the conservative equations (continuity, momentum and energy). The discretized governing equations are solved using the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) which allows back flow phenomena.\udSpecial emphasis is performed on the treatment of the transition zones between the single-phase and two-phase flow. The empirical inputs of single-phase and two-phase flow, including sub-cooled boiling\udand dry-out, have been adapted by means of adequate splines in the transition zones where the heat transfer correlations available in the literature are not suitable. Different numerical aspects have been evaluated with the aim of verifying the quality of the numerical solution. The mathematical model has been validated by comparison with experimental data obtained from literature considering condensation\udand evaporation processes. This comparison shown the improvements in the numerical solution not only in the transition zone but also in all condenser and evaporator ducts, when the special treatment for transitions\udis used. Illustrative results on double-pipe heat exchanger are also presented.
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