The Control of Shroud Leakage Loss by Reducing Circumferential Mixing

2006 ◽  
Author(s):  
Budimir Rosic ◽  
John D. Denton
Keyword(s):  
Significant Proportion ◽  
Tangential Velocity ◽  
Secondary Flows ◽  
Main Stream ◽  
Leakage Flow ◽  
Flow Angle ◽  
Blade Row ◽  
Stator Blade ◽  
The Difference ◽  
Mixing Losses

Shroud leakage flow undergoes little change in the tangential velocity as it passes over the shroud. Mixing due to the difference in tangential velocity between the main stream flow and the leakage flow creates a significant proportion of the total loss associated with shroud leakage flow. The unturned leakage flow also causes negative incidence and intensifies the secondary flows in the downstream blade row. This paper describes the experimental results of a concept to turn the rotor shroud leakage flow in the direction of the main blade passage flow in order to reduce the aerodynamic mixing losses. A three-stage air model turbine with low aspect ratio blading was used in this study. A series of different stationary turning vane geometries placed into the rotor shroud exit cavity downstream of each rotor blade row was tested. A significant improvement in flow angle and loss in the downstream stator blade rows was measured together with an increase in turbine brake efficiency of 0.4%.

Control of Shroud Leakage Loss by Reducing Circumferential Mixing

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
Budimir Rosic ◽  
John D. Denton
Keyword(s):  
Significant Proportion ◽  
Tangential Velocity ◽  
Secondary Flows ◽  
Main Stream ◽  
Leakage Flow ◽  
Flow Angle ◽  
Blade Row ◽  
Stator Blade ◽  
The Difference ◽  
Mixing Losses

Shroud leakage flow undergoes little change in the tangential velocity as it passes over the shroud. Mixing due to the difference in tangential velocity between the main stream flow and the leakage flow creates a significant proportion of the total loss associated with shroud leakage flow. The unturned leakage flow also causes negative incidence and intensifies the secondary flows in the downstream blade row. This paper describes the experimental results of a concept to turn the rotor shroud leakage flow in the direction of the main blade passage flow in order to reduce the aerodynamic mixing losses. A three-stage air model turbine with low aspect ratio blading was used in this study. A series of different stationary turning vane geometries placed into the rotor shroud exit cavity downstream of each rotor blade row was tested. A significant improvement in flow angle and loss in the downstream stator blade rows was measured together with an increase in turbine brake efficiency of 0.4 %.

Aerodynamic Analysis of Steam Turbine Feed-Heating Steam Extractions

2014 ◽  
Vol 136 (11) ◽  
Author(s):  
Budimir Rosic ◽  
Cosimo Maria Mazzoni ◽  
Zoe Bignell
Keyword(s):  
Steam Turbine ◽  
Rotor Blade ◽  
Feed Water ◽  
Steam Turbines ◽  
Flow Angle ◽  
Steam Extraction ◽  
Heating Steam ◽  
Blade Row ◽  
Circumferential Distribution ◽  
Stator Blade

Feed-heating in steam turbines, the use of 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 of a rotor blade row. The slot is connected to a plenum, which runs around the outside of the turbine annulus. Steam flows to the feed-heaters through a pipe connected usually to the bottom of the plenum. The steam extraction is driven by a circumferentially nonuniform pressure gradient in the plenum. This causes the mass flow rate of steam extracted to vary circumferentially, which affects the main passage flow downstream of the extraction point. The flow in the extraction plenum and the influence of 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 downstream 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 of the annulus. This difference in extraction rates modifies the flow angle and loss circumferential distribution downstream of the stator blade row. This study finds out that the distribution of 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 of the extraction slot and plenum.

scholarly journals Reducing Secondary Flow Losses in Low-Pressure Turbines: The “Snaked” Blade

2019 ◽  
Vol 4 (3) ◽  
pp. 28
Author(s):  
Matteo Giovannini ◽  
Filippo Rubechini ◽  
Michele Marconcini ◽  
Andrea Arnone ◽  
Francesco Bertini
Keyword(s):  
Detailed Comparison ◽  
Low Pressure ◽  
Secondary Flows ◽  
Wall Region ◽  
Flow Angle ◽  
Pressure Loss Coefficient ◽  
Flow Losses ◽  
Parametric Description ◽  
Stator Blade ◽  
The Impact

This paper presents an innovative design for reducing the impact of secondary flows on the aerodynamics of low-pressure turbine (LPT) stages. Starting from a state-of-the-art LPT stage, a local reshaping of the stator blade was introduced in the end-wall region in order to oppose the flow turning deviation. This resulted in an optimal stator shape, able to provide a more uniform exit flow angle. The detailed comparison between the baseline stator and the redesigned one allowed for pointing out that the rotor row performance increased thanks to the more uniform inlet flow, while the stator losses were not significantly affected. Moreover, it was possible to derive some design rules and to devise a general blade shape, named ‘snaked’, able to ensure such results. This generalization translated in an effective parametric description of the ‘snaked’ shape, in which few parameters are sufficient to describe the optimal shape modification starting from a conventional design. The “snaked” blade concept and its design have been patented by Avio Aero. The stator redesign was then applied to a whole LPT module in order to evaluate the potential benefit of the ‘snaked’ design on the overall turbine performance. Finally, the design was validated by means of an experimental campaign concerning the stator blade. The spanwise distributions of the flow angle and pressure loss coefficient at the stator exit proved the effectiveness of the redesign in providing a more uniform flow to the successive row, while preserving the original stator losses.

The Influence of the Clearance of Shrouded Rotor Blades on the Development of the Flowfield and Losses in the Subsequent Stator

2000 ◽  
Author(s):  
Patrick Peters ◽  
Volker Breisig ◽  
Andre Giboni ◽  
Christian Lerner ◽  
Heiner Pfost
Keyword(s):  
High Efficiency ◽  
Suction Side ◽  
Numerical Approach ◽  
Rotor Blades ◽  
Leakage Flow ◽  
Flow Angle ◽  
Flow Solver ◽  
Shrouded Rotor ◽  
Additional Losses ◽  
Mixing Losses

Although turbines are commonly noted for their high efficiency this efficiency can be improved further. The importance of the leakage flow for the losses of turbine stages makes this flow a promising candidate to be examined for loss reduction: first it decreases the workflow through the rotor and second, the suction side incidence of the re-entering leakage flow in the subsequent stator causes additional losses. As an essential parameter for leakage flow the clearance and its influence on the losses is investigated by experimental and numerical approach as well. The measurements of the experimental part were carried out on a 1.5 stage axial model turbine of an enlarged scale. The rotor of the turbine consists of shrouded blades with two teeth on the shroud. The clearance was varied from s/D = 0.0007 up to 0.004. Linear traverse measurements with a pneumatic 5-hole probe were taken in front of and behind the second stator. The re-entering leakage flow extremely influences the flow properties in radial direction (up to 50% of the span) at the inlet of the following stator. So the flow angle α1 deviates up to 90° of the flow angle in the mid-span, depending on the clearance. Along with mixing losses, this suction side incidence leads to an increase of the losses within the stator. In addition numerical investigations of the flow field were done with the commercial flow solver CFX-TASCflow. The interaction between the leakage flow and the secondary losses of the following stator is shown, and a comparison with available loss correlations is carried out in this paper.

On the Interactions of a Rotor Blade Tip Flow With Axial Casing Grooves in an Axial Compressor Near the Best Efficiency Point

2018 ◽  
Vol 141 (1) ◽  
Author(s):  
Huang Chen ◽  
Yuanchao Li ◽  
Joseph Katz
Keyword(s):  
Rotor Blade ◽  
Leading Edge ◽  
Secondary Flows ◽  
Maximum Efficiency ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Flow Angle ◽  
Tip Leakage ◽  
Blade Tip ◽  
The Impact

Experiments in a refractive index-matched axial turbomachine facility show that semicircular skewed axial casing grooves (ACGs) reduce the stall flowrate by 40% but cause a 2.4% decrease in the maximum efficiency. Aiming to elucidate mechanism that might cause the reduced efficiency, stereo-PIV measurements examine the impact of the ACGs on the flow structure and turbulence in the tip region near the best efficiency point (BEP), and compare them to those occurring without grooves and at low flowrates. Results show that the periodic inflow into the groove peaks when the rotor blade pressure side (PS) overlaps with the downstream end of the groove, but diminishes when this end faces the suction side (SS). Entrainment of the PS boundary layer and its vorticity generates a vortical loop at the entrance to the groove, and a “discontinuity” in the tip leakage vortex (TLV) trajectory. During exposure to the SS, the backward tip leakage flow separates at the entrance to the groove, generating a counter-rotating circumferential “corner vortex,” which the TLV entrains into the passage at high flowrates. Interactions among these structures enlarge the TLV and create a broad area with secondary flows and elevated turbulence near the groove's downstream corner. A growing shear layer with weaker turbulence also originates from the upstream corner. The groove also increases the flow angle upstream of the blade tip and varies it periodically. Accordingly, the circulation shed from the blade tip and strength of leakage flow increase near the blade leading edge (LE).

Spanwise Mixing in Axial-Flow Turbomachines

1982 ◽  
Vol 104 (1) ◽  
pp. 97-110 ◽  
Author(s):  
G. G. Adkins ◽  
L. H. Smith
Keyword(s):  
Secondary Flow ◽  
Boundary Layers ◽  
Axial Flow ◽  
Separated Flow ◽  
Secondary Flows ◽  
Main Stream ◽  
Flow Measurements ◽  
Mixing Coefficient ◽  
Blade Row ◽  
End Wall

Flow measurements taken in multistage axial-flow turbomachines suggest that substantial spanwise mixing of flow properties often occurs. In addition, measured blade row turnings often show considerable deviation from two-dimensional cascade theory, particularly in the end-wall regions. An approximate method is presented with which both of these effects can be included in design through-flow calculations. The method is based on inviscid, small-perturbation secondary flow theory. Frictional effects are not directly included but secondary flows caused by annulus wall and blade boundary layers are included in an approximate way. The secondary flow model includes effects of 1) main-stream nonfree-vortex flow, 2) end-wall boundary layers, 3) blade end clearances, 4) blade end shrouding, and 5) blade boundary layer and wake centrifugation. The spanwise mixing phenomenon is modeled as a diffusion process, where the mixing coefficient is related to the calculated spanwise secondary velocities. Empirical adjustments are employed to account for the dissipation of the secondary velocities and interactions with downstream blade rows. The induced blade row overturnings are related to the calculated cross-passage secondary velocities. The nature of the assumptions employed restricts the method to design-point-type applications for which losses are relatively small and significant regions of separated flow are not present.

scholarly journals Three Dimensional Flow in Radial-Inflow Turbines

1988 ◽  
Author(s):  
M. Zangeneh-Kazemi ◽  
W. N. Dawes ◽  
W. R. Hawthorne
Keyword(s):  
Secondary Flow ◽  
Three Dimensional ◽  
Flow Distribution ◽  
Secondary Flows ◽  
Wake Flow ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Flow Angle ◽  
Numerical Predictions ◽  
Absolute Flow

The flow through an impeller of a low speed radial-inflow turbine has been analysed using a fully three-dimensional viscous program and good correlations with instantaneous measurements of casing static pressure and exit flow distribution have been obtained. The flow at the exit of the turbine shows a pronounced non-uniformity with a wake region of high absolute flow angle near the casing. The predictions show that the flow is fully attached inside the impeller, while secondary flows can be observed especially in the exducer moving low momentum fluid towards the casing-suction corner. The presence of these secondary flows is discussed with reference to classical secondary flow theory. However, the comparison of measurements and numerical predictions indicate that the wake flow pattern is only partly due to the secondary flow. It is shown that in fact the tip leakage flow also plays a significant role in the wake generation and correspondingly some modelling of the leakage flow is essential in any attempted numerical simulations.

Improvement of Steam Turbine Stage Efficiency by Controlling Rotor Shroud Leakage Flows—Part II: Effect of Axial Distance Between a Swirl Breaker and a Rotor Shroud on Efficiency Improvement

2018 ◽  
Vol 141 (4) ◽  
Author(s):  
Chongfei Duan ◽  
Hisataka Fukushima ◽  
Kiyoshi Segewa ◽  
Takanori Shibata ◽  
Hidetoshi Fujii
Keyword(s):  
Structural Reliability ◽  
Tangential Velocity ◽  
Maximum Pressure ◽  
Leakage Flow ◽  
Axial Distance ◽  
Steam Turbines ◽  
Turbine Stage ◽  
Entire Flow ◽  
The Difference ◽  
Stage Efficiency

The basic principle of a distinct idea to reduce an aerodynamic mixing loss induced by the difference in tangential velocity between mainstream flow and rotor shroud leakage flow is presented in “Part I: Design Concept and Typical Performance of a Swirl Breaker.” When the swirl breaker is installed in the circulating region of leakage flow at the rotor shroud exit cavity, the axial distance between the swirl breaker and the rotor shroud is a crucial factor to trap the leakage flow into the swirl breaker cavity. In Part II, five cases of geometry with different axial distances between the swirl breaker and the rotor shroud, which covered a range for the stage axial distance of actual high and intermediate pressure (HIP) steam turbines, were investigated using a single-rotor computational fluid dynamics (CFD) analysis and verification tests in a 1.5-stage air model turbine. By decreasing the axial distance between the swirl breaker and the rotor shroud, the tangential velocity and the mixing region in the tip side which is influenced by the rotor shroud leakage flow were decreased and the stage efficiency was increased. The case of the shortest axial distance between the swirl breaker and the rotor shroud increased turbine stage efficiency by 0.7% compared to the conventional cavity geometry. In addition, the measured maximum pressure fluctuation in the swirl breaker cavity was only 0.7% of the entire flow pressure. Consequently, both performance characteristics and structural reliability of swirl breaker were verified for application to real steam turbines.

Blade Lean and Shroud Leakage Flows in Low Aspect Ratio Turbines

2008 ◽  
Author(s):  
Budimir Rosic ◽  
Liping Xu
Keyword(s):  
Aspect Ratio ◽  
Numerical Study ◽  
Three Dimensional ◽  
Secondary Flows ◽  
Leakage Flow ◽  
Industrial Practice ◽  
Leakage Flows ◽  
Low Aspect Ratio ◽  
Stator Blade ◽  
Different Levels

Blade lean, i.e. non-radial blade stacking, has been intensively used over the past in the design process of low aspect ratio gas and steam turbines. Although its influence on turbine efficiency is not completely understood, it has been proved as an effective way of controlling blade loading and secondary flows on blade passage endwalls. Three-dimensional blade designs in modern industrial practice are usually carried out using clean endwalls. The influence of the leakage flows on three-dimensional blade design is traditionally neglected. This paper presents an experimental study where two different stator blades, with different levels of compound lean, were tested in a low speed three-stage model turbine with the shroud leakage flow geometry representative of industrial practice. The experimental measurements were compared with numerical tests, conducted on the same blade geometries. The influence of the compound lean on the stator flow field was analysed in detail. In order to analyse the combined effects of both the stator hub and rotor shroud leakage flow on the blade lean, in the second part of the paper a numerical study on a two stage turbine with both leakage flow paths representative of a real turbine was carried out. Performance of three different stator blade designs (two different levels of compound lean and a straight blade) was investigated. The aim of this study is to understand the mechanism and the consequence of the stator blade lean on stage performance in an environment with leakage flows and associated cavities.

The Influence of Blade Lean on Turbine Losses

1992 ◽  
Vol 114 (1) ◽  
pp. 184-190 ◽  
Author(s):  
S. Harrison
Keyword(s):  
Marked Effect ◽  
Turbine Blades ◽  
Flow Angle ◽  
Blade Loading ◽  
Low Aspect Ratio ◽  
The Third ◽  
Straight Line ◽  
Blade Row ◽  
End Wall ◽  
Mixing Losses

Three linear cascades of highly loaded, low-aspect-ratio turbine blades have been tested in order to investigate the mechanisms by which blade lean (dihedral) influences loss generation. The blades in all three cascades have the same section but they are stacked perpendicular to the end wall in the first cascade, on a straight line inclined at 20 deg from perpendicular in the second, and on a circular arc inclined at 30 deg from perpendicular at each end in the third cascade. Lean has a marked effect upon blade loading, on the distribution of loss generation, and on the state of boundary layers on the blade suction surfaces and the endwalls, but its effect upon overall loss coefficient was found to be minimal. It was found, however, that compound lean reduced the downstream mixing losses, and reasons for this are proposed. Compound lean also has the beneficial effect of substantially reducing spanwise variations of mean exit flow angle. In a turbine this would be likely to reduce losses in the downstream blade row as well as making matching easier and improving off-design performance.

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