Thomas Polklas

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

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

Impact of Secondary Flow on the Accuracy of Simplified Design Methods for Steam Turbine Stages

The subject of the presented paper is the validation of a design method for HP and IP steam turbine stages. Common design processes have been operating with simplified design methods in order to quickly obtain feasible stage designs. Therefore, inaccuracies due to assumptions in the underlying methods have to be accepted. The focus of this work is to quantify the inaccuracy of a simplified design method compared to 3D Computational Fluid Dynamics (CFD) simulations.Short computing time is very convenient in preliminary design; therefore, common design methods work with a large degree of simplification. The origin of the presented analysis is a mean line design process, dealing with repeating stage conditions. Two features of the preliminary design are the stage efficiency, based on loss correlations, and the mechanical strength, obtained by using the beam theory. Due to these simplifications, only a few input parameters are necessary to define the primal stage geometry and hence, the optimal design can easily be found. In addition, by using an implemented law to take the radial equilibrium into account, the appropriate twist of the blading can be defined.However, in comparison to the real radial distribution of flow angles, this method implies inaccuracies, especially in regions of secondary flow. In these regions, twisted blades, developed by using the simplified radial equilibrium, will be exposed to a three-dimensional flow, which is not considered in the design process. The analyzed design cases show that discrepancies at the hub and shroud section do exist, but have minor effects. Even the shroud section, with its thinner leading-edge, is not vulnerable to these unanticipated flow angles.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

Integration of an Automatic Optimizer Functionality Into the Design Process of Industrial Steam Turbines

Industrial steam turbines are mostly tailor made machinery, varying in a wide range of specifications. This feature introduces high requirements on the design process which has to be flexible, efficient and fast at the same time. Given live steam and design parameters as input, the geometry corresponding to the valid design scheme can be calculated together with the required thermodynamic, aerodynamic and mechanical characteristics. By variation of design parameters a design may be achieved which optimizes both, efficiency and cost. The optimization task is formulated mathematically, e.g. crucial optimization parameters, criteria for evaluation of different designs and other required constraints are selected. The structure of the resulting optimization problem is analyzed. Based on this analysis a modular optimization system design is proposed. The choice of Genetic Algorithms and Adaptive Particle Swarm Optimizer as optimization methods is discussed, recommendations for their proper use are given. A bicriterial optimization approach for a simultaneous optimization of efficiency and cost is developed.Copyright

Experimental and Numeric Investigations on a Steam Turbine Test Rig in Part Load Operation

Subject of the paper is to present a purpose-built steam turbine test rig and its prospects for complex analysis of thermodynamic, aerodynamic and mechanical problems. The turbine at a scale of 1:1 was equipped with extensive instrumentation. Pressure and temperature rakes behind the control stage at several circumferential positions, rakes before and behind the HP, IP and LP stages and additional 5-hole probes to measure the complete flow field. Also tip timing measurement of the last LP stages were carried out to analyze their mechanic behavior in critical operating points, for example extreme part load or high condenser pressure. The turbine and its equipment are discussed, different operating points are analyzed and some comparisons of measurement results and numeric calculations are presented. The measurements create an additional database for future design optimizations.

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