An unsteady time-marching technique has been developed that can be employed for any type of wet steam flow : nucleating or non-nucleating, subcritical or supercritical, steady or unsteady. It is robust, accurate, simple and fast. The scheme uses a novel technique that performs the integration of the droplet growth equations along the fluid path lines rather than the more usual method which involves freezing the gas dynamic flowfield instantaneously in order to perform the integration. This allows simultaneous solution of all the relevant equations and thus the correct coupling between the vapour-phase gas dynamics and the relaxation effects due to the droplets is maintained. The scheme maintains a polydispersed droplet spectrum which is essential for modelling the nucleation zone accurately. Calculations based on the present scheme show good agreement with experimental measurements, steady and unsteady, reported in the literature.

It is also shown how the unsteady condensation process due to supercritical heat addition may give rise to a polydispersed droplet spectrum. This has a direct bearing on a possible explanation of the polydispersity measured in steam turbines ; a polydispersity which cannot be predicted with existing steady flow calculation methods.