Aerodynamic Analysis of Steam Turbine Feed-Heating Steam Extractions

Abstract

Feed-heating in steam turbines, the use o f steam extracted from the turbine to heat the feed-water, is known to raise the plant efficiency and so is included in most steam turbine power plant designs. The steam is extracted through an extraction slot that runs around the casing downstream o f a rotor blade row. The slot is connected to a plenum, which runs around the outside o f the turbine annulus. Steam flows to the feed-heaters through a pipe connected usually to the bottom o f the plenum. The steam extraction is driven by a circumferentially nonuniform pressure gradient in the plenum. This causes the mass flow rate o f steam extracted to vary circumferentially, which affects the main passage flow downstream o f the extraction point. The flow in the extraction plenum and the influence o f the steam extraction on the mainstream aerodynamics is analyzed numerically in this paper. A complete annulus with the extraction slot and plenum together with the down stream stator and rotor blade rows is modeled in this study. The results reveal a highly nonuniform steam extraction around the annulus with the highest extraction rates from the bottom nearest the extraction pipe and the lowest at the top o f the annulus. This dif ference in extraction rates modifies the flow angle and loss circumferential distribution downstream o f the stator blade row. This study finds out that the distribution o f steam extraction around the annulus and its influence on the main passage flow could be greatly improved by changing the shape and increasing the volume o f the extraction slot and plenum

Modeling Nonuniform Bleed in Axial Compressors

“…There are several relevant pieces of work in which an approach, similar to ours, has been used to study non-uniformity on the lengthscale of the order of the circumference. Rosic et al [11] investigate non-uniform extraction of steam for feed-heating from a steam turbine. They model a full annulus turbine with one extraction pipe using a RANS CFD solver and show that the non-uniform steam extraction associated with the discrete pipe causes non-uniform flow in the main blade passages.…”