On the Effects of Unsteadiness on the Condensation Process in Low-Pressure Steam Turbines

Abstract

The condensation process in a turbomachine is in reality an essentially random and unsteady phenomenon. On a time-averaged basis, the condensation zone is spread over a much greater distance in the flow direction than a simple steady-flow calculation would indicate. The droplet growth rate also shows different characteristics which are observed in experiments measured in real low-pressure steam turbines. These differences are mainly introduced by the large-scale temperature fluctuations which are caused by the segmentation of blade wakes by successive blade rows. Furthermore, the additional losses by condensation have to be reconsidered for an unsteady simulation. This paper describes a time-accurate Eulerian/Lagrangian two-phase model which is implemented within the DLR in-house code TRACE [1]. The phases are coupled through appropriately generated source terms for heat, mass and momentum. For the subcooled thermodynamic properties of steam the local formulation of IAPWS-IF97 [2, 3] is used. The implementation has been validated in a previous publication of the author [4] using one and two-dimensional experiments of Laval nozzles and a cascade blade from literature. The focus of this work is on the unsteady Phenomena which are investigated in a stage of an industrial low-pressure steam turbine.