Numerical Investigations of an Axial Exhaust Diffuser Coupling the Last Stage of a Generic Gas Turbine

2018 ◽  
Author(s):  
Marius Mihailowitsch ◽  
Markus Schatz ◽  
Damian M. Vogt
Keyword(s):  
Gas Turbine ◽  
Dynamic Pressure ◽  
Load Condition ◽  
Coupled System ◽  
Operating Point ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Tip Leakage ◽  
Last Stage ◽  
Gap Width

It is well known that the last stage of a turbine and the subsequent diffuser should be viewed at and designed as a coupled system rather than as single standalone components. The turbine outlet flow imposes the inlet conditions to the diffuser, whereas the recovered dynamic pressure in the diffuser directly controls the turbine back pressure. With changing operating point, the turbine outflow can vary significantly. This results consequently in large variations of the diffuser performance. A major role in the coupled system of turbine and diffuser can be attributed to the tip leakage flow. While it is desirable to minimize the tip leakage with regard to the turbine, a higher leakage mass flow can often be beneficial for the diffuser performance. As there is currently a trend towards aggressive and hence shorter diffusers which are particularly prone to separation, the question arises where the optimum for this tradeoff problem lies. To investigate the performance in the coupled turbine/diffuser system, a generic last stage with shrouded rotor and axial exhaust diffuser have been designed. The components are representative for heavy duty stationary gas turbine applications. Results are presented for three different operating points representing part-load, design-load and over-load condition. Three different seal gap widths are taken into account to control the leakage flow. The results indicate that an operating point dependent optimum gap width can be found for the coupled system efficiency whereas the maximimum turbine performance is always achieved with a minimum gap width.

Numerical Investigations of an Axial Exhaust Diffuser Coupling the Last Stage of a Generic Gas Turbine

2018 ◽  
Vol 141 (3) ◽  
Author(s):  
Marius Mihailowitsch ◽  
Markus Schatz ◽  
Damian M. Vogt
Keyword(s):  
Gas Turbine ◽  
Dynamic Pressure ◽  
Coupled System ◽  
Operating Point ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Tip Leakage ◽  
Last Stage ◽  
Inlet Conditions ◽  
Gap Width

It is well known that the last stage of a turbine and the subsequent diffuser should be viewed at and designed as a coupled system rather than as single standalone components. The turbine outlet flow imposes the inlet conditions to the diffuser, whereas the recovered dynamic pressure in the diffuser directly controls the turbine back pressure. With changing operating point, the turbine outflow can vary significantly. This results consequently in large variations of the diffuser performance. A major role in the coupled system of turbine and diffuser can be attributed to the tip leakage flow. While it is desirable to minimize the tip leakage with regard to the turbine, a higher leakage mass flow can often be beneficial for the diffuser performance. As there is currently a trend toward aggressive and hence shorter diffusers which are particularly prone to separation, the question arises where the optimum for this tradeoff problem lies. To investigate the performance in the coupled turbine/diffuser system, a generic last stage with shrouded rotor and axial exhaust diffuser has been designed. The components are representative for heavy duty stationary gas turbine applications. Results are presented for three different operating points representing part-load (PL), design-load (DL), and over-load (OL) condition. Three different seal gap widths are taken into account to control the leakage flow. The results indicate that an operating point-dependent optimum gap width can be found for the coupled system efficiency, whereas the maximum turbine performance is always achieved with a minimum gap width.

scholarly journals Unsteady flow mechanisms in the last stage of a transonic low pressure steam turbine—multistage effects and tip leakage flows

2017 ◽  
Vol 1 ◽  
pp. F4IW8S
Author(s):  
Ilias Papagiannis ◽  
Asad Raheem ◽  
Altug Basol ◽  
Anestis Kalfas ◽  
Reza Abhari ◽  
...  
Keyword(s):  
Shock Wave ◽  
Steam Turbine ◽  
Leading Edge ◽  
Bow Shock ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Tip Leakage ◽  
Bow Shock Wave ◽  
Last Stage ◽  
Two Stages

Abstract In this paper, an unsteady investigation of the last two stages of a low-pressure steam turbine with supersonic airfoils near the tip of the last stage’s rotor blade is presented. Goal is the investigation of multistage effects and tip leakage flow in the last stage of the turbine and to provide insight on the stator-rotor flow interaction in the presence of a bow-shock wave. This study is unique in a sense of combining experimental data for code validation and comparison with a numerical simulation of the last two stages of a real steam turbine, including tip-cavity paths and seals, steam modelling and experimental data used as inlet and outlet boundary conditions. Analysis of results shows high unsteadiness close to the tip of the last stage, due to the presence of a bow-shock wave upstream of the rotor blade leading edge and its interaction with the upstream stator blades, but no boundary layer separation on stator is detected at any instant in time. The intensity of the shock wave is weakest, when the axial distance of the rotor leading edge from the upstream stator trailing edge is largest, since it has more space available to weaken. However, a phase shift between the maximum values of static pressure along the suction side of the stator blade is identified, due to the shock wave moving with the rotor blades. Additionally, the bow-shock wave interacts with the blade shroud and the tip leakage flow. Despite the interaction with the incoming flow, the total tip leakage mass flow ingested in the tip-cavity shows a steady behaviour with extremely low fluctuations in time. Finally, traces of upstream stage’s leakage flow have been identified in the last stage, contributing to entropy generation in inlet and outlet of last stage’s stator blade, highlighting the importance of performing multistage simulations.

Axial Turbine Tip-Leakage Losses at Off-Design Incidences

2004 ◽  
Author(s):  
R. Willinger ◽  
H. Haselbacher
Keyword(s):  
Load Condition ◽  
Turbine Rotor ◽  
Rotor Blades ◽  
Turbine Cascade ◽  
Tip Leakage Flow ◽  
Turbine Rotor Blade ◽  
Tip Leakage ◽  
Blade Row ◽  
Axial Turbines ◽  
Gap Width

The tip-leakage losses in axial turbines with unshrouded rotor blades can account for as much as one third of the total losses. Various effects are influencing the tip-leakage flow and losses. This paper presents results of an experimental investigation concerning off-design incidences. Off-design incidences occur when the turbine operates at conditions different from the rated load condition. A low speed cascade wind tunnel has been used for the investigation. The geometry of the turbine cascade is an up-scale of the tip section of a low-pressure gas turbine rotor blade row (“Yaras–Sjolander cascade”) with a tip gap width of 2.5% of the chord length. The applied inlet flow angles consist of the design value as well as four off-design incidences in the range ±20°. Total pressures, static pressures and flow angles were obtained by traversing of a pneumatic five-hole probe in a plane about 0.3 axial chord lengths downstream of the turbine cascade. Based on the experimental results, a tip-leakage loss model is presented which can take into account off-design incidences. The model is applied to the present turbine cascade as well as to the turbine cascade of Yamamoto [1]. Due to its underlying concept, the model is able to predict, in addition to the losses, the flow underturning near the endwall caused by the tip-leakage vortex.

Effects of the Last Stage Rotor Tip Leakage Flow on the Aerodynamic Performance of the Exhaust Hood for Steam Turbines

2013 ◽  
Author(s):  
Jun Li ◽  
Zhigang Li ◽  
Zhenping Feng
Keyword(s):  
Static Pressure ◽  
Aerodynamic Performance ◽  
Pressure Recovery ◽  
Leakage Flow ◽  
Steam Turbines ◽  
Exhaust Hood ◽  
Tip Leakage Flow ◽  
Tip Leakage ◽  
Last Stage ◽  
Recovery Performance

The static pressure recovery coefficient of the exhaust hood has significant impact on the aerodynamic performance of the low pressure cylinder for steam turbines. Numerical investigations on the aerodynamic performance of the exhaust hood and full last stage with consideration of the rotor tip leakage were presented in this paper. Three-dimensional Reynolds-Averaged Navier-Stokes (RANS) solutions and k–ε turbulent model were utilized to analyze the static pressure recovery performance of the exhaust hood using the commercial CFD software ANSYS-CFX. Effect of the last stage rotor tip leakage flow on the aerodynamic performance of the downstream exhaust hood was conducted by comparison of the computational domains for the exhaust hood and full last stage with and without tip clearance. The numerical results show that the last stage rotor tip leakage jet can suppress the flow separation near the diffuser wall of the exhaust hood and improve its static pressure recovery performance. The detailed flow fields of the exhaust hood with and without consideration of the rotor tip leakage flow were also illustrated and corresponding flow mechanism was discussed.

Investigation of Tip Clearance Flow Effects on an Open 3D Steam Turbine Flutter Test Case

2017 ◽  
Author(s):  
Tianrui Sun ◽  
Paul Petrie-Repar ◽  
Di Qi
Keyword(s):  
Steam Turbine ◽  
Large Scale ◽  
Numerical Models ◽  
Leading Edge ◽  
Tip Clearance ◽  
Leakage Flow ◽  
Steam Turbines ◽  
Tip Leakage Flow ◽  
Tip Leakage ◽  
Last Stage

Blade failure caused by flutter is a major problem in the last stage of modern steam turbines. It is because rotor at this stage always has a large scale in spanwise, which provides low structural frequency as well as supersonic tip speeds. Since most of the unsteady aerodynamic work is done in the tip region, transonic tip-leakage flow that influences the tip region flow could have a remarkable effect on the aerodynamic stability of rotor blades. However, few research had been done on the tip-leakage flow influence on flutter characteristic based on full-scale steam turbine numerical models. In this paper, an open 3D steam turbine stage model designed by Durham University was applied, which was widely analyzed and representative for the last stage of modern industrial steam turbines. The average Mach number at the rotor outlet is 1.1. URANS simulation carried by both numerical software CFX and LUFT code is applied, and the two solvers show an agreement on steady and unsteady results. The numerical results indicate that the influence of tip leakage flow on blade stability is based on two types of flow mechanisms. Both mechanisms act on the suction side of near tip region. The first type of mechanism is produced by the reduction of passage shock near the leading edge, and the other type of mechanism at the rear of blade is caused by the interaction between tip leakage vortex and trailing edge shock of the neighbor blade. In conclusion, tip leakage flow has a significant influence on steam turbine flutter boundary prediction and requires further analysis in the future.

scholarly journals Effects of Tip Leakage Flow on the Aerodynamic Performance and Stability of an Axial-Flow Transonic Compressor Stage

Energies ◽  
2021 ◽  
Vol 14 (14) ◽  
pp. 4168
Author(s):  
Botao Zhang ◽  
Xiaochen Mao ◽  
Xiaoxiong Wu ◽  
Bo Liu
Keyword(s):  
Boundary Layer ◽  
Shock Wave ◽  
Axial Flow ◽  
Leading Edge ◽  
Rotating Stall ◽  
Tip Clearance ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Tip Leakage ◽  
Transonic Compressor

To explain the effect of tip leakage flow on the performance of an axial-flow transonic compressor, the compressors with different rotor tip clearances were studied numerically. The results show that as the rotor tip clearance increases, the leakage flow intensity is increased, the shock wave position is moved backward, and the interaction between the tip leakage vortex and shock wave is intensified, while that between the boundary layer and shock wave is weakened. Most of all, the stall mechanisms of the compressors with varying rotor tip clearances are different. The clearance leakage flow is the main cause of the rotating stall under large rotor tip clearance. However, the stall form for the compressor with half of the designed tip clearance is caused by the joint action of the rotor tip stall caused by the leakage flow spillage at the blade leading edge and the whole blade span stall caused by the separation of the boundary layer of the rotor and the stator passage. Within the investigated varied range, when the rotor tip clearance size is half of the design, the compressor performance is improved best, and the peak efficiency and stall margin are increased by 0.2% and 3.5%, respectively.

Development of the Tip-Leakage Flow Downstream of a Planar Cascade of Turbine Blades: Vorticity Field

1989 ◽  
Author(s):  
M. Yaras ◽  
S. A. Sjolander
Keyword(s):  
Simple Model ◽  
Turbine Blades ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Vorticity Field ◽  
Trailing Edge ◽  
Tip Leakage ◽  
Blade Row ◽  
Axisymmetric Structure ◽  
Insight Into

The paper presents detailed measurements of the tip-leakage flow emerging from a planar cascade of turbine blades. Four clearances of from 1.5 to 5.5 percent of the blade chord are considered. Measurements were made at the trailing edge plane, and at two main planes 1.0 and 1.56 axial chord lengths downstream of the cascade. The results give insight into several aspects of the leakage flow including: the size and strength of the leakage vortex in relation to the size of the tip gap and the bound circulation of the blade; and the evolution of the components of vorticity as the vortex diffuses laterally downstream of the blade row. The vortex was found to have largely completed its roll-up into a nearly axisymmetric structure even at the trailing edge of the cascade. As a result, it was found that the vortex could be modelled surprisingly well with a simple model based on the diffusion of a line vortex.

Study of the Standard k-ε Model for Tip Leakage Flow in an Axial Compressor Rotor

2016 ◽  
Vol 33 (4) ◽  
Author(s):  
Yanfei Gao ◽  
Yangwei Liu ◽  
Luyang Zhong ◽  
Jiexuan Hou ◽  
Lipeng Lu
Keyword(s):  
Flow Field ◽  
Large Scale ◽  
Turbulent Viscosity ◽  
Axial Compressor ◽  
Leakage Flow ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Tip Leakage Flow ◽  
Tip Leakage ◽  
Compressor Rotor

AbstractThe standard k-ε model (SKE) and the Reynolds stress model (RSM) are employed to predict the tip leakage flow (TLF) in a low-speed large-scale axial compressor rotor. Then, a new research method is adopted to “freeze” the turbulent kinetic energy and dissipation rate of the flow field derived from the RSM, and obtain the turbulent viscosity using the Boussinesq hypothesis. The Reynolds stresses and mean flow field computed on the basis of the frozen viscosity are compared with the results of the SKE and the RSM. The flow field in the tip region based on the frozen viscosity is more similar to the results of the RSM than those of the SKE, although certain differences can be observed. This finding indicates that the non-equilibrium turbulence transport nature plays an important role in predicting the TLF, as well as the turbulence anisotropy.

Flow Structures in the Tip Region for a Transonic Compressor Rotor

2013 ◽  
Vol 135 (3) ◽  
Author(s):  
Juan Du ◽  
Feng Lin ◽  
Jingyi Chen ◽  
Chaoqun Nie ◽  
Christoph Biela
Keyword(s):  
Numerical Simulations ◽  
Reference Frame ◽  
Three Dimensional ◽  
Flow Structures ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Three Dimensional Flow ◽  
Tip Leakage ◽  
Leakage Flows ◽  
Tip Leakage Flows

Numerical simulations are carried out to investigate flow structures in the tip region for an axial transonic rotor, with careful comparisons with the experimental results. The calculated performance curve and two-dimensional (2D) flow structures observed at casing, such as the shock wave, the expansion wave around the leading edge, and the tip leakage flow at peak efficiency and near-stall points, are all captured by simulation results, which agree with the experimental data well. An in-depth analysis of three-dimensional flow structures reveals three features: (1) there exists an interface between the incoming main flow and the tip leakage flow, (2) in this rotor the tip leakage flows along the blade chord can be divided into at least two parts according to the blade loading distribution, and (3) each part plays a different role on the stall inception mechanism in the leakage flow dominated region. A model of three-dimensional flow structures of tip leakage flow is thus proposed accordingly. In the second half of this paper, the unsteady features of the tip leakage flows, which emerge at the operating points close to stall, are presented and validated with experiment observations. The numerical results in the rotor relative reference frame are first converted to the casing absolute reference frame before compared with the measurements in experiments. It is found that the main frequency components of simulation at absolute reference frame match well with those measured in the experiments. The mechanism of the unsteadiness and its significance to stability enhancement design are then discussed based on the details of the flow field obtained through numerical simulations.

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