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

This paper presents the analysis results of the fluid dynamic characteristics when steam is supplied through an overload valve to the second and third stages of an ultra-supercritical (USC) high-pressure turbine. Firstly, a single-passage computational analysis by using a simple model of an admission flow path, and a single passage for the second and third stages of the USC high-pressure turbine was performed. Computational analysis was conducted to determine the fluid dynamic characteristics exhibited when the second-stage outlet flow, that is, the main flow between the second-stage outlet and third-stage inlet, and the admission flow are being mixed. The mixing causes complex flow phenomena such as swirl, and the velocity vector of the main flow changes. This, in turn, causes a pressure drop between the second-stage outlet and third-stage inlet, potentially affecting the performance of the turbine. The actual flow in the overload valve is supplied through the admission flow path, which has the shape of a casing circumferentially surrounding the turbine, after passing through the valve and flowing in two directions perpendicular to the turbine axis. This necessitates full-passage computational analyses of the entire turbine and the flow paths of the overload valve. To achieve this, we implemented a full 3-D geometric modeling of the admission flow path, and conducted full-passage computational analyses of the fluid dynamic characteristics of all the flow paths including those of the second and third stages of the USC high-pressure turbine, focusing on the pressure drop occurring in the flow path of the overload valve. Furthermore, the results by the single and full-passage computational analyses were compared and the effects of two different methodological approaches on the results of the computational analysis were analyzed.