Numerical Analysis on the Impact of Interstage Flow Addition in a High-Pressure Steam Turbine

This study analyzes the fluid dynamic characteristics of an ultrasupercritical (USC) high-pressure turbine with additional steam supplied through an overload valve between the second and third stages. The mixing between the main and admission flows causes complex flow phenomena such as swirl and changes of velocity vectors of the main flow. This causes a pressure drop between the second-stage outlet and third-stage inlet, which could potentially affect the performance of the turbine. First, a single-passage computational analysis, which is usually preferred in predicting the performance of multistage turbomachines, was performed using a simple model of an admission flow path and a single passage (SP) for the second and third stages of the turbine. However, the actual flow in the overload valve is supplied through the admission flow path, which has the shape of a casing that circumferentially surrounds the turbine, after flowing in two directions perpendicular to the turbine axis. This necessitates full-passage computational analyses of the two stages and the flow paths of the admission flow. To achieve this, we implemented a full three-dimensional (3D) geometric model of the admission flow path and conducted a full-passage computational analysis for all the flow paths, including those of the second and third stages of the turbine. The focus of analysis was on the pressure drop due to the admission flow. The results of the single and full-passage analyses were compared, and the effects of two different methods were analyzed.