Ronald Mailach

Numerical Analysis of Partial Admission Flow in an Industrial Steam Turbine

A partially admitted control stage is a typical feature of an industrial steam turbine. Its purpose is to provide efficient part-load operation and to reduce losses caused by an adverse blade height to tip gap ratio by closing segmental arcs of the inlet annulus. On the other hand partial admission naturally causes circumferential nonuniformity of the flow, because the flow enters the control stage rotor over only a portion of the annulus. This induces not only unsteady blade forces but also additional losses in comparison to a full-admission turbine. So the advantage of partial admission is reduced. In order to analyze partial admission flow effects a 3D CFD model of an industrial steam turbine needs to be developed. It consists of three parts: i) The nozzle groups covering only a portion of the annulus and the rotor of the impulse-type control stage, ii) a cross-over channel directing the flow to a reduced diameter, and iii) the downstream reaction-type turbine stages. The results show considerable flow nonuniformity downstream of the cross-over channel which affects performance of the adjacent full-admission stages. Different operating points of the turbine are investigated. Circumferential periodicity is utilized to minimize computational cost of the simulation. Customary guidelines to CFD-simulation are taken into account and simulation parameters are carefully checked for their influence on the results: turbulence models, meshing parameters and boundary conditions are varied. The influence of gap flow is checked. The results are finally compared to experimental data to check simulation quality.Copyright

Losses of Steam Admission in Industrial Steam Turbines Depending on Geometrical Parameters

Due to the range of applications, industrial steam turbines show a compact and modular design including several branches for the admission and/or extraction of process steam. In conjunction with a flexible operation and partial load conditions, it is important to estimate the losses appearing at those branches sufficiently.Therefore, the results of an extended parameterized numerical study of a T-junction with steam admission are discussed in the first part of the current paper. This study, carried out with a 3D RANS CFD-solver, is used to determine the additional secondary loss, which is caused by deflection of the admitted steam and mixing with the main flow. At this, the loss distribution depends on geometrical parameters of the T-junction such as the area ratio of branch to main pipe diameter and the curvature of the transition piece. The secondary loss, calculated as a function of total pressure loss and local wall shear, is compared with measurement data from literature.In the second part of the paper the loss calculation procedure is adapted from theoretical computations to two actual industrial steam turbine configurations. First, a 3-stage segment of a high speed turbine which includes a circumferential slot for steam admission is examined. Therefore, flow ratios from 0 to 50 % of admitted steam, compared to main flow, are numerical performed. Second, a 2.5-stage low speed turbine segment with two asymmetrical branches and a fix flow ratio of 40% for the first branch, respectively 80% for the second branch is considered. All invested configurations illustrate how geometrical parameters affect the secondary loss distribution as well as the mixing process within subsequent turbine stages.© 2014 ASME

Optimization of Axial Thrust Balancing Swirl Breakers in a Centrifugal Pump Using Stochastic Methods

Hydraulic axial thrust balancing in single-stage radial pumps is a frequently applied procedure to reduce the bearing load to a reasonable level. It leads to considerable efficiency loss, which gives reason to investigate the optimization potential of the common balancing methods. This paper’s focus is on so-called casing ribs, which are used in the default design of an examined industrial pump. Radial vanes on the casing wall of the front impeller side chamber work as swirl breakers by decreasing rotating flow whereby the static pressure at the shroud increases and counteracts the resulting axial thrust. The objective is to retain the reduction of axial thrust and to improve the internal efficiency simultaneously.Therefore a CFD model of the industrial radial pump is created with Ansys CFX. Sufficient numerical quality is ensured whereby consistency is verified by a mesh study. The model is validated by integral values of the characteristic curve and axial thrust measurements as well as by experimental transient static pressure measurement at different locations of the pump flow. Probes are placed in the suction port, the volute and the impeller side chambers, where most balancing methods are implemented.Since the side chamber contains a complex flow, the effect of geometry changes is hard to predict. For this reason a stochastically based sensitivity analysis using a comprehensively parameterized geometry of the front side chamber domain with the included casing ribs is carried out. For this purpose 110 design points are calculated and evaluated with support of the software Optislang. Correlations of parameters are suggested and important parameters regarding the objective are identified. Some reasonable model simplifications are conducted to reduce the computational time. According to the acquired findings a local optimization is executed using the best sample of the sensitivity analysis as start design. An evolutionary algorithm method determines a best design with an efficiency improvement of 0.26 percentage points. It is discussed in detail conclusively.Copyright

Unsteady Flow in Extraction Modules of Industrial Steam Turbines

Industrial steam turbines are designed for application in power-, process- and chemical engineering. Particular modules ensure the optimum integration into power plants and other engineering processes. Extraction modules allow the controlled extraction of large steam quantities on certain and constant enthalpy levels. Valves regulate the amount of steam extracted from the turbine expansion path. Depending on the valve lift, different flow separation phenomena can occur peripherally inside the valves, causing undesired large unsteady fluid forces on the valve head and seat. Due to the compact design of the industrial steam turbines, these unsteady jets can influence the rotor dynamics as well as the blade loading of the adjacent stages. These fluctuations should be understood and avoided in order to enhance the reliability of steam turbines.In the present study the unsteady flow phenomena due to separation occurring circumferentially inside the valve of extraction modules are investigated numerically. First, the commercial 3D RANS CFD-solver (ANSYS CFX 14) is validated in the application to experimental results. Subsequently, the various flow patterns of the examined valve design are analyzed on a standalone numerical valve model in an extensive study.In order to assess the impact of these unsteady flow separations on other components, the complete extraction module is simulated in combination with the adjacent stages. The transient simulation results show pressure fluctuations downstream of the valves resulting in an unsteady load of the control valves, the shaft and the blading.Copyright

Comparison of Transient Blade Row Methods for the CFD Analysis of a High-Pressure Turbine

Usually, in a turbine an uneven number of blades are selected for vane and blade rows to reduce the level of interaction forces. To consider all unsteady flow phenomena within a turbine the computation of the full annulus is required causing considerable computational cost. Transient blade row methods using few passages reduce the numerical effort significantly. Nevertheless, those approaches provide accurate results. This contribution presents three different unsteady approaches to compare the accuracy and the computational effort, using a full annulus unsteady CFD simulation as a reference. The first approach modifies the blade-to-blade ratio whereas the second method scales the circumferential flow pattern to reach spatial and temporal periodicity. Third approach is based on time-inclining method to overcome unequal blade pitches with less numerical effort. All unsteady CFD simulations are carried out for the transonic test turbine VKI BRITE EURAM using the commercial CFD solver ANSYS CFX 14.5. The resulting unsteady pressure disturbances and blade forces of the different transient blade row methods are compared to each other as well as to experimental data. Finally, the accuracy and the computational costs are discussed in more detail.Copyright

An Efficient Workflow for Accurate Flutter Stability Analyses and Application to a State of the Art Compressor Rotor

During the aeromechanical design process of turbomachinery blading, one of the main goals is to improve the blade loading which may lead to a higher risk of flutter. To avoid flutter induced blade failure during operation, the final blade design has to fulfill certain aero mechanical requirements. These refer to the permitted static and dynamic stress levels as well as the aeroelastic stability constraint of flutter for the whole operating range. In this contribution, an efficient workflow for three-dimensional viscous flutter stability analyses will be presented using the three-dimensional viscous flow solver TBLOCK and the open-source software package CalculiX for FE modal analyses. For this purpose, the workflow is applied to the first compressor rotor of a state of the art gas turbine. The flutter analysis is performed for several operating points to predict an accurate flutter envelope for the whole operating range of the investigated compressor stage. To reduce the numerical effort, only the first two mode shapes are considered with respect to different shaft speeds. In addition, phase-shifted boundary conditions are applied to all flutter calculations using the traveling wave mode domain taking all possible inter-blade phase angles into account. The results of the flutter analysis show no indications for flutter within the projected operating range of the rotor and for the considered mode shapes. In conclusion, the described workflow is able to determine the critical flutter stability boundaries of the investigated compressor rotor with reasonable numerical effort.Copyright

Influence of Shroud Cavity Jet and Steam Admission Through a Circumferential Slot on the Flow Field in a Steam Turbine

Steam turbines for industrial application are often constructed according to modular design concepts. This allows interchangeable combinations of modules including steam admission and extraction. Prior to field tests the flow in a typical stage configuration of such a steam turbine is predicted numerically. Focus of the current work is the axial gap between high pressure and intermediate pressure part containing a circumferential slot. Mass flow used for axial thrust balancing re-enters the blade channel through this slot. Another exceptional feature appears at the high pressure vane carrier: For manufacturing reasons the last rotor shroud next to and upstream of the gap is not fully enclosed by the vane carrier. This results in a turbulent jet at the exit of the rotor shroud cavity mixing with both the blade channel flow as well as the incoming flow from the slot.A commercial 3D RANS CFD-solver (ANSYS CFX 12) is used to predict the mixing of the different flow partitions within the stage gap. Therefore, the last stage of the high pressure part, the gap with the slot and the first stage of the intermediate pressure part are modeled and solved numerically. The amount of flow through the circumferential slot is varied to discern the influences of the specific flow partitions. Additionally, a modification of the vane carrier helps to analyze radial distribution of incoming flow for the downstream vane row as well as scoring global loss characteristics. As the simulation results indicate, flow parameters up- and downstream and also fluctuations crossing the gap are affected by flow through the slot. Furthermore, the computed flow field shows locations appropriate for a traversing probe system to be used in the test facility.Copyright

Improvement of Flow Conditions for the Stages Subsequent to Extraction Modules in Industrial Steam Turbines

Industrial steam turbines are applied for power generation as well as drive for turbo-compressors. They combine a high level of operational flexibility with highest reliability. Especially in the field of process technology they provide process heat on a certain enthalpy level for other industrial applications. Modular design concepts are used to meet these various requirements like admission or extraction of large steam quantities. Extraction modules use valves to control the amount of steam extracted from the turbine expansion path at constant steam parameters.While extraction steam is taken from the turbine through an outlet flange, the remaining steam passes valves and downstream diffusers, flows into an annular inner casing and finally escapes through the subsequent stages. Depending on the valve lift, different flow separations can occur around the valves, resulting in unsteady transonic jets. Due to the compact and asymmetric design of the inner casing the flow into the subsequent stages is strongly disturbed. Hence, strong unsteady mechanical blade loading can occur in addition to efficiency loss.The current work focusses on the improvement of the flow conditions in the subsequent stage. Experimental results are applied to quantify the viability of the used 3D RANS CFD-solver (ANSYS CFX 14) for these numerical investigations. Compared with the experiment, the distribution of pressure, velocity and incidence angle are well predicted by the numerical code. It is evident that the unsteady transonic jets emerging around the valves have a major influence on the distribution of the parameters considered. Thus, to quantify the impact of a modified inlet chamber design, it is sufficient to simulate the domain starting from the valves.The influence of different design modifications on the flow parameters in comparison with the base design is discussed in detail in an extensive study. The results clarify that horizontal and vertical valve positions, as well as thorough contouring of the radial-axial deflection have a strong influence on the distribution of pressure, mass flux and incidence angle. Hence, in this contribution combinations of the most beneficial modifications are investigated numerically and compared with the base design.Copyright

Grundlagen der Strömungsmaschinen

Stromungsmaschinen sind Fluidenergiemaschinen, die durch eine Arbeitsubertragung zwischen Maschinenbauteilen und einem kontinuierlich durch die Maschine stromenden Fluid charakterisiert sind. Es gibt verschiedenste Bauausfuhrungen. Eine Ubersicht zu den Klassifizierungsmerkmalen sowie wesentliche zugehorige Maschinentypen ist in Bild 1 der Einleitung des Teils „R –Stromungsmaschinen“ gegeben. In Abhangigkeit der Richtung der Energieubertragung zwischen Fluid und Bauteilen wird grundlegend zwischen Kraft- und Arbeitsmaschinen unterschieden. Wird in einer Maschine von einem mit Schaufeln bestuckten Rotor an ein Fluid Arbeit ubertragen und ihm dadurch Energie zugefuhrt, wird diese als Arbeitsmaschine bezeichnet. Fur deren Antrieb ist an der Welle mechanische Leistung aufzuwenden. Wird im Gegensatz dazu dem Fluid Energie entzogen und in mechanische Arbeit umgewandelt, spricht man von einer Kraftmaschine, die entsprechend Leistung an der Welle abgibt.

The Effect of Four Part Gap Geometry Configurations for Variable Stator Vanes in a Compressor Cascade

The article describes numerical investigations on the influence of four different endwall clearance topologies for variable stator vanes to secondary flow field development and the performance of high pressure compressors. The aim of this work is to quantify the characteristics of different clearance configurations depending on the penny-axis position and the penny diameter for a typical operating range. All clearance configurations were implemented to a linear cascade of modern stator profiles. The analysis was introduced using a relative clearance size of 1.3% chord at three stagger angles and two characteristic Reynolds numbers to model the operating range on aircraft engines. 3D numerical calculations were carried out to gain information about the flow field inside the cascade. They were compared with measurements of a 5-hole-probe as well as pressure tappings on the airfoil and the endwall.The CFD shows the clearance characteristics in good agreement with the measurements for the lower and the nominal stagger angle. Small gaps in the rear part of the vane have a beneficial effect on the flow field. In contrast, a clearance in the higher loaded front part of the vane always resulted in increased losses. Otherwise, the significant enhanced performance of a rear part gap, which was measured at the higher stagger angle, was not reflected by the CFD. The reduced mixing losses and the higher averaged flow turning even compared to a configuration without a clearance are not verified with the calculations. Large flow separations at the high stagger angle result in a two to four times higher underturning of the CFD in comparison to the experiments. The clearance effects to the characteristic radial loss distribution up to 40 % bladeheight also deviate from the measurements due to heavy mixing of clearance and reversed separated flow.Copyright