Unsteady Aerodynamics of Low-Pressure Steam Turbines Operating Under Low Volume Flow

2014 ◽  
Vol 136 (9) ◽  
Author(s):  
Benjamin Megerle ◽  
Ivan McBean ◽  
Timothy Stephen Rice ◽  
Peter Ott
Keyword(s):  
Unsteady Flow ◽  
Steam Turbine ◽  
Measurement Data ◽  
Unsteady Aerodynamics ◽  
Low Pressure ◽  
Development Project ◽  
Rotating Stall ◽  
Steam Turbines ◽  
Blade Vibration ◽  
Low Volume

Nonsynchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Currently extensive validation of new blade designs is required to clarify whether they are subjected to the risk of not admissible blade vibration. Such tests are usually performed at the end of a blade development project. If resonance occurs a costly redesign is required, which may also lead to a reduction of performance. It is therefore of great interest to be able to predict correctly the unsteady flow phenomena and their effects. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. 3D computational fluid dynamics (CFD) has been applied to simulate the unsteady flow in the air model turbine. It has been shown that the simulation reproduces well the characteristics of the phenomena observed in the tests. This methodology has been transferred to more realistic steam turbine multistage environment. The numerical results have been validated with measurement data from a multistage model LP steam turbine operated with steam. Measurement and numerical simulation show agreement with respect to the global flow field, the number of stall cells and the intensity of the rotating excitation mechanism. Furthermore, the air model turbine and model steam turbine numerical and measurement results are compared. It is demonstrated that the air model turbine is a suitable vehicle to investigate the unsteady effects found in a steam turbine.

Unsteady Aerodynamics of Low-Pressure Steam Turbines Operating Under Low Volume Flow

2013 ◽  
Author(s):  
Benjamin Megerle ◽  
Timothy Stephen Rice ◽  
Ivan McBean ◽  
Peter Ott
Keyword(s):  
Unsteady Flow ◽  
Steam Turbine ◽  
Measurement Data ◽  
Unsteady Aerodynamics ◽  
Low Pressure ◽  
Rotating Stall ◽  
Stage Model ◽  
Steam Turbines ◽  
Multi Stage ◽  
Low Volume

Non-synchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Currently extensive validation of new blade designs is required to clarify whether they are subjected to the risk of not admissible blade vibration. Such tests are usually performed at the end of a blade development project. If resonance occurs a costly redesign is required, which may also lead to a reduction of performance. It is therefore of great interest to be able to predict correctly the unsteady flow phenomena and their effects. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. 3D CFD has been applied to simulate the unsteady flow in the air model turbine. It has been shown that the simulation reproduces well the characteristics of the phenomena observed in the tests. This methodology has been transferred to more realistic steam turbine multi stage environment. The numerical results have been validated with measurement data from a multi stage model LP steam turbine operated with steam. Measurement and numerical simulation show agreement with respect to the global flow field, the number of stall cells and the intensity of the rotating excitation mechanism. Furthermore, the air model turbine and model steam turbine numerical and measurement results are compared. It is demonstrated that the air model turbine is a suitable vehicle to investigate the unsteady effects found in a steam turbine.

Numerical Investigation of Exhaust Diffuser Performances in Low Pressure Turbine Casings

2011 ◽  
Author(s):  
Tadashi Tanuma ◽  
Yasuhiro Sasao ◽  
Satoru Yamamoto ◽  
Shinji Takada ◽  
Yoshiki Niizeki ◽  
...  
Keyword(s):  
Steam Turbine ◽  
High Performance ◽  
Stokes Equations ◽  
Three Dimensional ◽  
Measurement Data ◽  
Boundary Layer Separation ◽  
Low Pressure ◽  
Tvd Scheme ◽  
Steam Turbines ◽  
Exhaust Hood

Low pressure (LP) exhaust hoods are an important component of steam turbines. The aerodynamic loss of LP exhaust hoods is almost the same as those of the stator and rotor blading in LP steam turbines. Designing high performance LP exhaust hoods should lead further enhancement of steam turbine efficiency. This paper presents the results of exhaust hood computational fluid dynamics (CFD) analyses using last stage exit velocity distributions measured in a full-scale development steam turbine as the inlet boundary condition to improve the accuracy of the CFD analysis. One of the main difficulties in predicting the aerodynamic performance of the exhaust hoods is the unsteady boundary layer separation of exhaust hood diffusers. A highly accurate unsteady numerical analysis is introduced in order to simulate the diffuser flows in LP exhaust hoods. Compressible Navier-Stokes equations and mathematical models for nonequilibrium condensation are solved using the high-order high-resolution finite-difference method based on the fourth-order compact MUSCL TVD scheme, Roe’s approximate Riemann solver, and the LU-SGS scheme. The SST turbulence model is also solved for evaluating the eddy viscosity. The computational results were validated using the measurement data, and the present CFD method was proven to be suitable as a useful tool for determining optimum three-dimensional designs of LP turbine exhaust diffusers.

Numerical and Experimental Investigation of the Aerodynamic Excitation of a Model Low-Pressure Steam Turbine Stage Operating Under Low Volume Flow

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
Benjamin Megerle ◽  
Timothy Stephen Rice ◽  
Ivan McBean ◽  
Peter Ott
Keyword(s):  
Steam Turbine ◽  
Low Pressure ◽  
Volume Flow ◽  
Operating Conditions ◽  
Single Stage ◽  
Tip Clearance ◽  
Working Fluid ◽  
Steam Turbines ◽  
Excitation Mechanism ◽  
Low Volume

The diversification of power generation methods within existing power networks has increased the requirement for operational flexibility of plants employing steam turbines. This has led to the situation where steam turbines may operate at very low volume flow conditions for extended periods of time. Under operating conditions where the volume flow through the last stage moving blades (LSMBs) of a low-pressure (LP) steam turbine falls below a certain limit, energy is returned to the working fluid rather than being extracted. This so-called “ventilation” phenomenon produces nonsynchronous aerodynamic excitation, which has the potential to lead to high dynamic blade loading. The aerodynamic excitation is often the result of a rotating phenomenon, with similarities to a rotating stall, which is well known in compressors. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. The analysis revealed that the rotating excitation mechanism observed in operating steam turbines is reproduced in the model turbine. A 3D computational fluid dynamics (CFD) method has been applied to simulate the unsteady flow in the air model turbine. The numerical model consists of the single stage modeled as a full annulus, along with the axial-radial diffuser. An unsteady CFD analysis has been performed with sufficient rotor revolutions to obtain globally periodic flow. The simulation reproduces the main characteristics of the phenomenon observed in the tests. The detailed insight into the dynamic flow field reveals information on the nature of the excitation mechanism. The calculations further indicate that the LSMB tip clearance flow has little or no effect on the characteristics of the mechanism for the case studied.

Detached Eddy Simulation of Rotating Instabilities in a Low-Pressure Model Steam Turbine Operating Under Low Volume Flow Conditions

2021 ◽  
Author(s):  
Ilgit Ercan ◽  
Damian M. Vogt
Keyword(s):  
Steam Turbine ◽  
Low Pressure ◽  
Volume Flow ◽  
Steam Turbines ◽  
Detached Eddy Simulation ◽  
Flow Conditions ◽  
Flow Physics ◽  
Part Load ◽  
Eddy Simulation ◽  
Low Volume

Abstract Rotating instability (RI) in steam turbines is a phenomenon occurring during operation at very low volume flow conditions. Whereas RI is well-known in compressors, it is rather uncommon in turbines, where it is limited to the last stages of low-pressure steam turbines. The phenomenon has been studied numerically by means of viscous 3D CFD simulations employing mainly URANS equations. Given the possible difficulties to accurately predict heavily separated flows using such methods, this paper deals with the question whether the more sophisticated Improved Delayed Detached Eddy Simulation (iDDES) model is applicable in an industrial environment and whether it is capable of capturing the complex unsteady flow physics in a more realistic manner. For this purpose, the commercial CFD solver STAR-CCM+ is employed. A three-stage low-pressure model steam turbine featuring a non-axisymmetric inlet and an axial-radial diffuser is used as a test object. In order to capture the asymmetry, the model spans the full annulus and comprises the inlet section, all three stages, the diffuser as well as the exhaust hood. URANS and iDDES simulations have been performed at various low-volume flow part-load operating points and compared to test data. Unsteady pressure fluctuations at the casing as well as time-resolved probe traverse data have been used to validate the simulations. It is found that both models capture the overall flow physics well and that the iDDES model is superior at the most extreme part-load operating condition. In addition to the model accuracy and applicability of the CFD tool used, the paper discusses the challenges encountered during simulation setup as well as during initialization.

Turbulent Scale Resolving Modelling of Rotating Stall in Low-Pressure Steam Turbines Operated Under Low Volume Flow Conditions

2015 ◽  
Author(s):  
Benjamin Megerle ◽  
Timothy Stephen Rice ◽  
Ivan McBean ◽  
Peter Ott
Keyword(s):  
Flow Field ◽  
Large Scale ◽  
Stokes Equations ◽  
Low Pressure ◽  
Quantitative Agreement ◽  
Rotating Stall ◽  
Navier Stokes ◽  
Steam Turbines ◽  
Navier Stokes Equations ◽  
Low Volume

Non-synchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Unsteady computational fluid dynamics (CFD) has been applied to simulate the rotating stall phenomenon in two model turbines. It is shown that the investigated flow field presents a challenge to conventional Reynolds-averaged Navier–Stokes equations simulations. The modelling has been enhanced by applying scale-resolving turbulence modelling, which can simulate large-scale turbulent fluctuations. With this type of simulation a qualitative and quantitative agreement between CFD and measurement for the unsteady and time averaged flow field has been achieved. The results of the numerical investigation allow for a detailed insight into the dynamic flow field and reveal information on the nature of the excitation mechanism. It is concluded that the CFD approach developed can be used to assess LSMB blade designs prior to model turbine tests to check whether they are subjected to vibration under LVF caused by rotating stall.

Numerical and Experimental Investigation of the Aerodynamic Excitation of a Model Low-Pressure Steam Turbine Stage Operating Under Low Volume Flow

2012 ◽  
Author(s):  
Benjamin Megerle ◽  
Timothy Stephen Rice ◽  
Ivan McBean ◽  
Peter Ott
Keyword(s):  
Steam Turbine ◽  
Low Pressure ◽  
Volume Flow ◽  
Operating Conditions ◽  
Single Stage ◽  
Tip Clearance ◽  
Working Fluid ◽  
Steam Turbines ◽  
Excitation Mechanism ◽  
Low Volume

The diversification of power generation methods within existing power networks has increased the requirement for operational flexibility of plants employing steam turbines. This has led to the situation where steam turbines may operate at very low volume flow conditions for extended periods of time. Under operating conditions where the volume flow through the last stage moving blades (LSMBs) of a low-pressure (LP) steam turbine falls below a certain limit, energy is returned to the working fluid rather than being extracted. This so-called “ventilation” phenomenon produces non-synchronous aerodynamic excitation, which has the potential to lead to high dynamic blade loading. The aerodynamic excitation is often the result of a rotating phenomenon, with similarities to rotating stall, which is well known in compressors. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. Detailed analysis revealed that the rotating excitation mechanism observed in operating steam turbines, is reproduced in the model turbine. 3D CFD has been applied to simulate the unsteady flow in the air model turbine. The numerical model consists of the single stage modeled as a full annulus, as well as the axial-radial diffuser. An unsteady CFD analysis has been performed for sufficient rotor revolutions such that the flow is globally periodic. It has been shown that the simulation reproduces well the characteristics of the phenomenon observed in the tests. The detailed insight into the flow field allows the drawing of conclusions as to the nature of the excitation mechanism. One result is that the LSMB tip clearance flow is found to have very little or no effect on the characteristics of mechanism for the case studied.

Design Optimization of a Low Pressure Steam Turbine Radial Diffuser Using an Evolutionary Algorithm and 3D CFD

2012 ◽  
Author(s):  
Tom Verstraete ◽  
Johan Prinsier ◽  
Alberto Di Sante ◽  
Stefania Della Gatta ◽  
Lorenzo Cosi
Keyword(s):  
Design Optimization ◽  
Steam Turbine ◽  
Differential Evolution Algorithm ◽  
Low Pressure ◽  
Pressure Recovery ◽  
Strong Impact ◽  
Optimized Design ◽  
Steam Turbines ◽  
Recovery Coefficient ◽  
Lp Turbine

The design of the radial exhaust hood of a low pressure (LP) steam turbine has a strong impact on the overall performance of the LP turbine. A higher pressure recovery of the diffuser will lead to a substantial higher power output of the turbine. One of the most critical aspects in the diffuser design is the steam guide, which guides the flow near the shroud from axial to radial direction and has a high impact on the pressure recovery. This paper presents a method for the design optimization of the steam guide of a steam turbine for industrial power generation and mechanical drive of centrifugal compressors. This development is in the frame of a continuous effort in GE Oil and Gas to develop more efficient steam turbines. An existing baseline exhaust and steam guide design is first analyzed together with the last LP turbine stage with a frozen rotor full 3D Computational Fluid Dynamics (CFD) calculation. The numerical prediction is compared to available steam test turbine data. The new exhaust box and a first attempt new steam guide design are then first analyzed by a CFD computation. The diffuser inlet boundary conditions are extracted from this simulation and used for improving the design of the steam guide. The maximization of the pressure recovery is achieved by means of a numerical optimization method that uses a metamodel assisted differential evolution algorithm in combination with a 3D CFD solver. The profile of the steam guide is parameterized by a Bezier curve. This allows for a wide variety of shapes, respecting the manufacturability constraints of the design. In the design phase it is mandatory to achieve accurate results in terms of performance differences in a reasonable time. The pressure recovery coefficient is therefore computed through the 3D CFD solver excluding the last stage, to reduce the computational burden. Steam tables are used for the accurate prediction of the steam properties. Finally, the optimized design is analyzed by a frozen rotor computation to validate the approach. Also off-design characteristics of the optimized diffuser are shown.

Sequential vs Multidisciplinary Coupled Optimization and Efficient Surrogate Modelling of a Last Stage and the Successive Axial Radial Diffuser in a Low Pressure Steam Turbine

2014 ◽  
Author(s):  
Kevin Cremanns ◽  
Dirk Roos ◽  
Arne Graßmann
Keyword(s):  
Steam Turbine ◽  
Design Process ◽  
Energy Demand ◽  
Three Dimensional ◽  
Flow Simulation ◽  
Low Pressure ◽  
Steam Turbines ◽  
Low Pressure Turbine ◽  
Pressure Steam ◽  
Last Stage

In order to meet the requirements of rising energy demand, one goal in the design process of modern steam turbines is to achieve high efficiencies. A major gain in efficiency is expected from the optimization of the last stage and the subsequent diffuser of a low pressure turbine (LP). The aim of such optimization is to minimize the losses due to separations or inefficient blade or diffuser design. In the usual design process, as is state of the art in the industry, the last stage of the LP and the diffuser is designed and optimized sequentially. The potential physical coupling effects are not considered. Therefore the aim of this paper is to perform both a sequential and coupled optimization of a low pressure steam turbine followed by an axial radial diffuser and subsequently to compare results. In addition to the flow simulation, mechanical and modal analysis is also carried out in order to satisfy the constraints regarding the natural frequencies and stresses. This permits the use of a meta-model, which allows very time efficient three dimensional (3D) calculations to account for all flow field effects.

Advanced Water Droplet Erosion Protection for Modern Low Pressure Steam Turbine Steel Blades

2015 ◽  
Author(s):  
Joerg Schuerhoff ◽  
Andrei Ghicov ◽  
Karsten Sattler
Keyword(s):  
Steam Turbine ◽  
Water Droplet ◽  
Precipitation Hardening ◽  
Turbine Blades ◽  
Low Pressure ◽  
Steam Turbines ◽  
Treatment Facility ◽  
Material Loss ◽  
Pressure Steam ◽  
Steam Turbine Blades

Blades for low pressure steam turbines operate in flows of saturated steam containing water droplets. The water droplets can impact rotating last stage blades mainly on the leading edge suction sides with relative velocities up to several hundred meters per second. Especially on large blades the high impact energy of the droplets can lead to a material loss particularly at the inlet edges close to the blade tips. This effect is well known as “water droplet erosion”. The steam turbine manufacturer use several techniques, like welding or brazing of inlays made of erosion resistant materials to reduce the material loss. Selective, local hardening of the blade leading edges is the preferred solution for new apparatus Siemens steam turbines. A high protection effect combined with high process stability can be ensured with this Siemens hardening technique. Furthermore the heat input and therewith the geometrical change potential is relatively low. The process is flexible and can be adapted to different blade sizes and the required size of the hardened zones. Siemens collected many years of positive operational experience with this protection measure. State of the art turbine blades often have to be developed with precipitation hardening steels and/or a shroud design to fulfill the high operational requirements. A controlled hardening of the inlet edges of such steam turbine blades is difficult if not impossible with conventional methods like flame hardening. The Siemens steam turbine factory in Muelheim, Germany installed a fully automated laser treatment facility equipped with two co-operating robots and two 6 kW high power diode laser to enable the in-house hardening of such blades. Several blade designs from power generation and industrial turbines were successfully laser treated within the first year in operation. This paper describes generally the setup of the laser treatment facility and the application for low pressure steam turbine blades made of precipitation hardening steels and blades with shroud design, including the post laser heat treatments.

Low Pressure Steam Turbine Exhaust Flow: Part 1 — CFD Coupling of LP Turbine and Condenser Neck

2016 ◽  
Author(s):  
Peter Stein ◽  
Dirk Telschow ◽  
Frederic Lamarque ◽  
Nuncio Colitto
Keyword(s):  
Steam Turbine ◽  
Measurement Data ◽  
Low Pressure ◽  
Computational Time ◽  
Feed Water ◽  
Cfd Modelling ◽  
Design Changes ◽  
Plant Level ◽  
Lp Turbine ◽  
Modelling Approach

Since many years the diffuser and exhaust of low pressure (LP) turbines have been in the focus of turbine development and accordingly broadly discussed within the scientific community. The pressure recovery gained within the diffuser significantly contributes to the turbine performance and therefore plenty of care is taken in investigations of the flow as well as optimization within this part of the turbine. However on a plant level the component following the LP turbine is the condenser, which is connected by the condenser neck. Typically the condenser neck is not fully designed to provide additional enthalpy recovery. Due to plant arrangement reasons, often it is full of built-ins like stiffening struts, feed-water heaters, extraction pipes, steam dump devices and others. It is vital to minimize the pressure losses across the condenser neck, in order to keep performance benefit, previously gained within the diffuser. As a general rule, each mbar of total pressure loss in a condenser neck may reduce the gross power output up to 0.1%. While turbines usually follow a modular approach, the condenser is typically designed plant specific. Therefore, on a plant level it is crucial to identify and evaluate the loss contributors and develop processes and tools which allow an accurate and efficient design process for an optimized condenser neck design. This needs to be performed as a coupled modelling approach, as both, turbine and condenser flow interact with each other. 3-D CFD tools enable a deep insight into the flow field and help to locally optimize the design, as they help to identify local losses and this even for small geometrical design changes. Unfortunately these tools are costly with respect to computational time and resources, if they are used to analyze a full condenser neck with all built-ins. Here 1-D modelling approaches can help to close the gap, as they can provide fast feedback, e.g. in a project tender phase, or can allow to quickly analyze design changes. For this they need a proper calibration and validation. This publication discusses the CFD modelling of a LP steam turbine coupled to a condenser neck and the validity of such calculations against measurement data. In the second publication (Part 2) a simplification of the gained information to a 1-D modelling approach will be discussed.

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