Prediction of steam condensation in the presence of noncondensable gases using a CFD-based approach

We integrate in the ANSYS CFD code Fluent a model for wall condensation from a vapor-noncondensable gas mixture. The condensation phenomenon is modeled from first principles as sink terms for the mass, momentum, species and energy conservation equations. The condensation rate is obtained by requiring the condensate-gas interface to be impermeable to the noncondensable gas. The model assumes in addition that the thermal resistance of the liquid film is negligible, and hence the predictions are only valid for relatively large mass fractions of the noncondensable gas (above 0.1). When the condensation rates are high, a best-estimate suction correction factor is proposed for CFD codes that impose the no-slip boundary conditions at the wall surfaces. In such a way, the enhancement in the heat transfer due to suction is accounted for. We first simulate condensation in laminar and turbulent forced flows along a cold flat plate. More challenging simulations are subsequently conducted for the case where vapor is introduced into closed vessels containing a noncondensable gas and in which stand condensing surfaces held at constant cold temperature. The flow transient is computed until steady conditions are reached, at which point the condensation flow rate equals the injected steam flow rate. Overall, the predicted heat transfer rates are in good agreement with available analytical solutions as well as experimental correlations. CFD Best Practice Guidelines are followed to a large extent. In particular, a hierarchy of grids is used to ensure mesh-independence of the results, and second order accuracy is adopted. (C) 2013 Elsevier B.V. All rights reserved.