Numerical Simulation On Condensation In Presence Of Non-condensable Gas In Vertical Tubes

Condensation in presence of non-condensable gas (NCG) is common in many industrial industries including nuclear power generation, seawater desalination, refrigeration and petrochemicals. Due to the deterioration effects of NCG on condensing heat transfer, it has become a hot topic in the area of vapor condensation to deeply investigate the mechanism of condensation in presence of NCG and develop a appropriate mathematical model.A mechanistic model of vapor condensation in presence of NCG in a vertical tube, based on the governing equations of liquid-vapor two phases flow, is developed. Thermal and mechanical nonequilibria between the two phases are considered in the model. An equation for mass conservation of noncondensable in the vapor-NCG mixture is included. Void fraction is used to determine flow patterns of annular, churn-slug and bubblely flows and their transitions. The effect of NCG on the combined heat and mass transfer at the gas-liquid interphase is accounted for using the stagnant film model (SFM). The gas-liquid interfacial heat, mass and momentum transfer processes are calculated by flow regime-dependent correlations.The numerical simulation programmed by the Fortran is performed on condensation with NCG present inside a vertical tube to quantitatively investigate the mechanism of the adverse effects of NCG on condensing heat transfer. The results show that the good agreement between the experimental data and simulation results based on the combined SFM model and two-fluid representation of phasic conservation equations proves the accuracy of this developed model for simulating heat and mass transfer process of different flow regimes in a vertical tube.When the inlet NCG mass concentration increases from 0.001 to 0.05, the condensation rate reduces about 50%-70%. Increasing the inlet NCG mass concentration leads to an decrease in the thermodynamic nonequilibrium at the gas-liquid interphase, which results in decreasing heat transfer temperature difference and thermal driving force of phase change. The process of condensation with NCG present is controlled by the heat transfer resistances of both phases and gas-side mass transfer resistance. These heat and mass transfer resistances increase with increasing inlet NCG mass concentrations. The gas-side heat transfer includes convective sensible heat and latent heat. When the inlet NCG mass concentration increases from 0.001 to 0.05, the percentage of convective sensible heat transfer at the gas-side obviously increases from 0.002% to 0.3%. Increasing the inlet vapor-NCG mixture velocity contributes to weakening the deterioration effect of NCG on condensing heat transfer, which becomes more obvious at higher inlet NCG mass fractions. The condensation rate increases about 20% when the inlet vapor-NCG mixture velocity increases from 9m/s to 11.2m/s. The mechanistic model and simulation analysis play an important role in developing high efficient heat exchangers.