A comparison of modelling techniques for polydispersed droplet spectra in steam turbines

Within steam turbine flows, condensation phenomena give rise to complex droplet spectra that can span more than two orders of magnitude in size. To predict the behaviour of the two-phase flow and the resulting losses, the interactions between the vapour phase and droplets of all sizes must be accurately calculated. The estimation of thermodynamic losses and droplet deposition rates, in particular, depend on the size range and shape of the droplet spectrum. These calculations become computationally burdensome when a large number of droplet groups are present, and it is therefore advantageous to capture the complete droplet spectrum in a compressed form. This paper compares several methods for reducing the complexity of the droplet spectrum: A single representative droplet size (equivalent monodispersion), the moment method (including various growth rate approximations), the quadrature method of moments, and spectrum pruning. In spectrum pruning, droplet groups are individually nucleated, but their number is subsequently reduced by combining groups together in a manner that preserves droplet number, wetness fraction, and the shape of the initial spectrum. The various techniques are compared within a Lagrangian framework by tracking the two-phase behaviour along predefined pressure-time trajectories. Primary and secondary nucleation, droplet evaporation, and a representative turbomachinery case are modelled. The calculations are compared in terms of speed, accuracy, and robustness. It is shown that both the moment methods and spectrum pruning provide an appreciable improvement in accuracy over the use of an 'equivalent' monodispersion without compromising calculation speed. Although all the examined methods are adequate for primary nucleation and droplet growth calculations, spectrum pruning and the quadrature method of moments are most accurate over the range of conditions considered