The present study identifies and resolves numerical solution difficulties in a state-of-the-art numerical model used for predicting fluid flow and heat transfer in two dimensions in a shell-and-tube steam condenser. In this model, the shell-side flow is assumed to be a homogenous mixture of saturated steam and air and the tube bundle region is modelled using the continuum approach. Discretized equations for conservation of mixture mass and momentum and air mass are obtained using the finite volume method, and solved using a segregated solution approach with the SIMPLEC algorithm.

Improved algorithms, which overcome convergence difficulties, are presented for four areas of the model. First, a more physically realistic vent mass flow calculation is proposed. Second, new linearizations of the air mass and mixture mass conservation equations are made which couple the equations, resulting in improved modelling of the effects of non-condensing gas on the condensation rate. With this new approach, a special implicit inlet velocity correction is implemented to enforce global mass conservation. Third, iteration of calculations in a thermal resistance network is used to yield a more consistent calculation of the condensation rate. Fourth, relaxation and iteration are introduced into the treatment of the pressure-velocity coupling.

A simple test problem is used to discuss the solution difficulties which are identified and to demonstrate the improvement in convergence and increased robustness gained by using the new algorithms. Further testing indicates that a limitation on the time step size exists for more complex problems. Finally, flow predictions for two configurations of a small test condenser demonstrate that the new model yields reasonable results, both in fewer coefficient iterations than the state-of-the-art model, and in cases where a solution was not previously obtained due to convergence difficulties.