LOW PRESSURE STEAM TURBINE EXHAUST FLOW - PART 2: INVESTIGATION OF THE CONDENSER NECK FLOW FIELD AND 1D MODELLING

Since many years the diffuser and exhaust of low pressure (LP) turbines are in focus of turbine development and accordingly broadly discussed within the scientific community. The pressure recovery gained within the diffuser significantly contributes to the turbine performance and therefore care is taken in flow investigation as well as optimization within this part of the turbine. However, on a plant level the component following the LP turbine is the condenser, which is connected by the condenser neck. Typically the condenser neck is not designed to provide additional enthalpy recovery. For plant design reasons, often numerous built-ins like stiffening struts, extraction pipes, steam dump devices and others are placed into the neck. Here it is vital to keep the pressure losses low, in order not to deteriorate performance, previously gained within the diffuser. Each mbar of total pressure loss in a condenser can reduce the plant power output up to 0.1%. As discussed in the first publication (Part 1), 3D CFD enables a deep insight into the flow field, which is costly with respect to computational time and resources. But there are phases during project execution, when geometry and/or boundary conditions are not fixed and quick estimation of pressure loss and recovery in the condenser neck is needed for benchmark of designs or design changes (e.g. tender phase). Here ID modelling approaches can help to close the gap. Analysis of available 1D correlation of flow around obstacles has shown that these need to be adapted to the flow conditions in a condenser neck of a steam, nuclear or combined-cycle power plant. Therefore, the fluid fields, calculated and discussed in the first publication (Part 1), were analyzed regarding pressure loss created by single obstacles and interaction of built-ins of different size, number and shape. Furthermore, a 1D velocity to be used for ID calculation was derived from the 3D velocity field. In addition, vacuum correction-curves were implemented to cover the range of possible operating conditions. This publication discusses the development of a 1D model for calculation of pressure loss in a condenser neck and the validity of such calculations against measurement and 3D CFD data.