SoftInWay - Conceptual turbomachinery design and optimization
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Proceedings of ASME Turbo Expo 2004
Power for Land, Sea, and Air
June 14-17, 2004, Vienna, Austria



GT2004-54228


FLOW PHENOMENON IN STEAM TURBINE DISK-STATOR CAVITIES
CHANNELED BY BALANCE HOLES


L. Moroz
SoftInWay, Inc.
35 Corporate Dr., 4th floor, Burlington, MA 01803
A. Tarasov
SoftInWay, Inc.
35 Corporate Dr., 4th floor, Burlington, MA 01803

ABSTRACT

Computational fluid dynamic (CFD) analysis of the region the secondary flow path of high pressure steam turbine was conducted. Region included two adjacent to disc cavities and section of main flow path. The cavities were channeled by balance holes located in the disk. Two geometrical models were considered: single disk cavity and both cavities with balance holes. 2D axisymmetric and 3D analyses were conducted for the first model, however only 3D study was performed for the second one.

Analysis of the single disk cavity has revealed appearance of vortexes in circumferential direction even in case when all boundary conditions and geometry features were close to the axi-symmetric case. The trend of the average pressure distributions along radius was found for 2D and 3D models.

The transient analysis of the two cavity model revealed vortexes movement in circumferential direction with higher than disk rotation velocity. It induces periodic conditions at the inlet and outlet of balance hole and periodic steam mass flow rate through the holes. The flow pattern in the two cavities with balance holes is so complicated that common 1D net flow representation should be considered as significant simplification.

INTRODUCTION

Steam/gas mass flow distributions in the secondary turbine flow path is usually treated with method replacing real hydraulic system with equivalent graph. Branches of such graph are 1D models of flow in elements like tubes, annulus, valves and so on. Existing 1D models are essentially correlation dependencies of hydraulic resistance upon various parameters.

Mass flow rate and pressure distributions in the whole hydraulic system of the secondary flow path are calculated using mass conservation law at each graph node and pressure drops on each branch.

The most of the correlations in simple cases are reliable for the wide range of operation and geometry parameters. The complicated flow such as flow in the disk-stator axial seals couldn’t be treated with the same accuracy as simple ones, and unfortunately its influence on the whole secondary flow path is significant. In reality flow in the axial seal is substantially 3D, especially when adjacent to seal cavities are channeled with balance holes. In spite of that, often such flows are treated with simple methods when whole turbine secondary path is analyzed for the lack of more accurate methods.

Turbine design requires development of reliable and rapid methods of secondary flow path prediction. Such methods must be based on deep understanding of processes in axial disk-stator seals and should extract accurate 1D models applicable for turbine designer needs.

An alternative way is axi-symmetric CFD modeling of the turbine secondary flow path with several additional assumptions related to labyrinth seals, balance holes and others elements that induce 3D effect [1]. This approach promises more accurate results but several assumptions used in reduction of the problem from 3D to 2D axisymmetric formulation need to be validated.

Flow in cavities between rotating disk and fixed stator is a subject of many investigations [2, 3, 4, 6, 7, 8] and remains a point of considerable interest due to a variety of disk cavity and rim seal geometries that are explored in current



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