Design Of Turbomachines Research Articles

An Efficient Approach for the Frequency Analysis of Nonaxisymmetric Rotating Structures: Application to a Coupled Bladed Birotor System

The improvement of efficiency in the design of turbomachines requires a reliable prediction of the vibrating behavior of the whole structure. The simulation of blades vibrations is decisive and this is usually based on elaborated finite element model restricted to the bladed-disk. However, the blades dynamic behavior can be strongly affected by interactions with other parts of the engine. Global dynamic studies that consider these other parts are required but usually come with a high numerical cost. In the case of a birotor architecture, two coaxial rotors with different rotating speeds can be coupled with a bearing system. The mechanical coupling between the shafts generates energy exchange that alters the dynamic behavior of the blades. The equations of motion of the whole structure that take into account the coupling contain periodic time-dependent coefficients due to the difference of rotational speed between both rotors. Equations of this kind, with variable coefficients, are typically difficult to solve. This study presents a preprocessing method to guarantee the elimination of time-dependent coefficients in the birotor equations of motion. This method is tested with a simplified finite element model of two bladed-disks coupled with linear stiffnesses. We obtain accurate results when comparing frequency analysis of preprocessed equations with time-integration resolution of the initial set of equations. The developed methodology also offers a substantial time saving.

Thermo-Mechanical Modeling of Abradable Coating Wear in Aircraft Engines

In modern turbomachine designs, the nominal clearances between rotating bladed-disks and their surrounding casing are reduced to improve aerodynamic performances of the engine. This clearance reduction increases the risk of contacts between components and may lead to hazardous interaction phenomena. A common technical solution to mitigate such interactions consists in the deposition of an abradable coating along the casing inner surface. This enhances the engine efficiency while ensuring operational safety. However, contact interactions between blade tips and an abradable layer may yield unexpected wear removal phenomena. The aim of this work is to investigate the numerical modeling of thermal effects within the abradable layer during contact interactions and compare it with experimental data. A dedicated thermal finite element mesh is employed. At each time-step, a weak thermo-mechanical coupling is assumed: thermal effects affect the mechanics of the system, but the mechanical deformation of the elements has no effect on temperatures. Weak coupling is well appropriated in the case of rapid dynamics using small time-step and explicit resolution schemes. Moreover, only heat transfer by conduction is considered in this work. To reduce computational times, a coarser spatial discretization is used for the thermal mesh comparing to the mechanical one. The time-step used to compute the temperature evolution is larger than the one used for the mechanical iterations since the time constant of thermal effect is larger than contact events. The proposed numerical modeling strategy is applied on an industrial blade to analyze the impact of thermal effects on the blade's dynamics.

A Dynamic Approach for Faster Performance Measurements on Hydraulic Turbomachinery Model Testing

During the design and optimization of hydraulic turbomachines, the experimental evaluation of hydraulic performances beyond the best efficiency point and for off-design conditions remains essential to validate the simulation process and to finalize the development. In this context, an alternative faster method to measure the efficiency of hydraulic turbomachines using a dynamic approach has been investigated. The so-called “sliding-gate” dynamic measurement method has been adapted and implemented on the hydraulic test rig of the HES-SO Valais//Wallis, Sion, Switzerland. This alternative approach, particularly gainful for small-hydro for which the investment devoted to development is limited, has been successfully assessed on two cases for drinking water networks energy recovery. A 2.65 kW double-regulated laboratory prototype of a tubular axial micro-turbine with two independent variable speed counter-rotating runners and a 11 kW multi-stage centrifugal pump-as-turbine (PAT) with variable speed have been selected. The hydraulic efficiency results obtained by dynamic measurements are compared to the ones obtained by the classical steady point-by-point method. This dynamic method, suitable not only for hydraulic machinery, allows: (i) reducing significantly (up to 10×) the time necessary to draw the complete efficiency characteristics of a hydraulic machine; (ii) rapidly detecting the hydrodynamic instabilities within the operating range of the machine.

Influence of blade lean together with blade clocking on the overall aerodynamic performance of a multi-stage turbine

In the present work, an in-house flow solver for multi-stage turbomachines is first applied to compute the unsteady flows in a subsonic 1.5-stage axial turbine with different clocking configurations. The influence of the wake location of the first stator on the overall adiabatic efficiency is presented and analyzed, demonstrating the possibility to improve the aerodynamic performance of the turbine by properly leaning the stator blade. Four different blade lean schemes are then configured based on the given clocking configuration with peak efficiency at 50% span. The overall adiabatic efficiency and the spanwise distributions of aerodynamic performance parameters of different lean configurations are presented and compared to those of the baseline configuration. The adiabatic efficiency of the optimal lean configuration is about 0.35% larger than that of the baseline configuration, while the maximum adiabatic efficiency difference among different clocking configurations is only 0.086%. The improvement on the overall aerodynamic performance of the multi-stage turbine contributed by blade lean is significantly remarkable. The blade lean and blade clocking can be carefully assembled in the unsteady flow regime during the design or optimization phases of multi-stage turbomachines.

소형 터보샤프트 엔진의 구성품 설계

Aerodynamic design of turbomachines inside a small turboshaft engine has been performed. The engine is a twin spool turboshaft engine, which consists of a gas generator and axial type power turbine followed by. The gas generator consists of a centrifugal compressor-radial turbine spool. To minimize specific fuel consumption of the engine, heat exchange occurs between compressor outlet air and power turbine exhaust gas. To supply sufficiently decelerated and radially de-swirled flow to the heat exchanger, two-staged cascade diffuser was introduced at the compressor diffuser row. Mass flow rate is controlled by the radial turbine nozzle throat. The design results showed that aerodynamic performances are higher than design requirements. For this reason, stage matching between adjacent components is required. In this study, stage matching between the radial turbine and the power turbine was considered. CFD result revealed that flow capacity of the axial turbine is high for adequate stage matching, so flow path has been adjusted to produce required power both from radial and axial turbines.

Effects of Cutting Parameters over Turning of UDIMET® 720 Superalloy in a Broaching Process Simulation

The nickel-based alloy “UDIMET® 720” has been used for several years to manufacture components in turbomachines. Its physical properties allow it to keep high mechanical strength when subjected to high temperatures. However, those properties also cause its low machinability. Turbomachine design requires the use of rotating parts assembled and submitted to high thermo-mechanical stresses. Fir-tree slots machined by broaching are used to create the bond between turbine discs and blades. Currently, the broaches are made from high speed steel which is restricted to low speed and feed hence limiting the productivity. In order to go faster, a solution is to use carbide tools. However, this material is more sensitive to shock and it is then necessary to precisely identify cutting conditions for the optimal design of broaches. Furthermore, cutting phenomena are different and can affect the machined surface integrity for this critical workpiece.A large experimental campaign has been done by turning to reduce tool costs and to highlight the effects of cutting parameters on tool wear and machined surface quality. In order to optimize the productivity, the effect of cutting conditions on cutting forces is taken into account. To determine the broach geometry composed of several teeth, the effects of rake angle, cutting edge preparation and clearance angle are presented. In order to simulate the broaching process, a specific test has been designed on a lathe. This allows us to include specific disturbances like interrupted cut, temperature variation, lubrication discontinuity, and difficulty to liberate the chip.

Integrated Design and Multi-objective Optimization of a Single Stage Heat-Pump Turbocompressor

Small-scale turbomachines in domestic heat pumps reach high efficiency and provide oil-free solutions, which improve heat-exchanger performance and offer major advantages in the design of advanced thermodynamic cycles. An appropriate turbocompressor for domestic air based heat pumps requires the ability to operate on a wide range of inlet pressure, pressure ratios, and mass flows, confronting the designer with the necessity to compromise between range and efficiency. Further, the design of small-scale direct driven turbomachines is a complex and interdisciplinary task. Textbook design procedures propose to split such systems into subcomponents and to design and optimize each element individually. This common procedure, however, tends to neglect the interactions between the different components leading to suboptimal solutions. The author proposes an approach based on the integrated philosophy for designing and optimizing gas bearing supported, direct driven turbocompressors for applications with challenging requirements with regards to operation range and efficiency. Using experimentally validated reduced order models for the different components an integrated model of the compressor is implemented and the optimum system found via multi-objective optimization. It is shown that compared to standard design procedures, the integrated approach yields an increase of the seasonal compressor efficiency of more than 12 points. Further, a design optimization based sensitivity analysis allows to investigate the influence of design constraints determined prior to optimization such as impeller surface roughness, rotor material, and impeller force. A relaxation of these constrains yields additional room for improvement. Reduced impeller force improves efficiency due to a smaller thrust bearing mainly, whereas a lighter rotor material improves rotordynamic performance. A hydraulically smoother impeller surface improves the overall efficiency considerably by reducing aerodynamic losses. A combination of the relaxation of the three design constraints yields an additional improvement of six points compared to the original optimization process. The integrated design and optimization procedure implemented in the case of a complex design problem thus clearly shows its advantages compared to traditional design methods by allowing a truly exhaustive search for optimum solutions throughout the complete design space. It can be used for both design optimization and for design analysis.

Performance Measurements of a Low Specific Speed TurboClaw® Compressor

Low specific speed compressors have been historically based on positive displacement machines. Attempts to bring advantages of turbomachinery such as oil free, low parts counts, low cost of manufacture, and reliability to low flow rate applications have not been sparse, but the principle difficulty has always been that the conventional turbomachine design operates at ultra-high speed to deliver low volume flow rates. This is synonymous with low efficiency due to higher losses (windage, surface finish, and tip clearances). The innovative TurboClaw® design is a low specific speed turbomachinery with forward swept impeller geometry. It owes its high efficiency and operational stability to careful design of its nearly tangential forward swept blading and diffuser geometry.

The continuous adjoint method for the design of hydraulic turbomachines

This article presents the development and application of the continuous adjoint method for designing/optimizing the shape of hydraulic turbomachines. The Reynolds-averaged flow equations are solved in the rotating reference frame and the terms arising from the differentiation of the Coriolis and centripetal forces are taken into account in the formulation of the adjoint equations. The objective functions presented in this article can be used for achieving (a) the optimal collaboration of the runner impeller with the draft tube, by controlling the meridional and circumferential velocity profiles at the exit of the runner, (b) the operation at the desired hydraulic head and/or (c) the cavitation suppression. All of them are used to improve an existing Francis runner. It is important to note that the objective function related to cavitation is, by definition, non-differentiable and a way to effectively handle it, is proposed. The continuous adjoint method is presented in its most general form and could readily be adapted to other objective functions in the field of hydraulic turbomachines.

Non-contact test set-up for aeroelasticity in a rotating turbomachine combining a novel acoustic excitation system with tip-timing

Due to trends in aero-design, aeroelasticity becomes increasingly important in modern turbomachines. Design requirements of turbomachines lead to the development of high aspect ratio blades and blade integral disc designs (blisks), which are especially prone to complex modes of vibration. Therefore, experimental investigations yielding high quality data are required for improving the understanding of aeroelastic effects in turbomachines. One possibility to achieve high quality data is to excite and measure blade vibrations in turbomachines. The major requirement for blade excitation and blade vibration measurements is to minimize interference with the aeroelastic effects to be investigated. Thus in this paper, a non-contact—and thus low interference—experimental set-up for exciting and measuring blade vibrations is proposed and shown to work. A novel acoustic system excites rotor blade vibrations, which are measured with an optical tip-timing system. By performing measurements in an axial compressor, the potential of the acoustic excitation method for investigating aeroelastic effects is explored. The basic principle of this method is described and proven through the analysis of blade responses at different acoustic excitation frequencies and at different rotational speeds. To verify the accuracy of the tip-timing system, amplitudes measured by tip-timing are compared with strain gage measurements. They are found to agree well. Two approaches to vary the nodal diameter (ND) of the excited vibration mode by controlling the acoustic excitation are presented. By combining the different excitable acoustic modes with a phase-lag control, each ND of the investigated 30 blade rotor can be excited individually. This feature of the present acoustic excitation system is of great benefit to aeroelastic investigations and represents one of the main advantages over other excitation methods proposed in the past. In future studies, the acoustic excitation method will be used to investigate aeroelastic effects in high-speed turbomachines in detail. The results of these investigations are to be used to improve the aeroelastic design of modern turbomachines.

FE Analysis Of Runner Blade For Small Bulb Turbine

The stress analysis of the rotor blade being the key phase of the turbomachine design requires much attention and careful determination of the blade loading to get realistic results. So due to the advantages of finite element analysis in the field of mechanical engineering it is widely used as simulation. The runner blades of the turbine are very critical part because its transfer fluid energy to convert the rotary or mechanical energy. The turbine runner blades are subjected to different load e.g. fluid or gas forces, inertia load, centrifugal forces. The primary reason of break down in turbine is the failure of blade. So, the proper blade design is require. The finite element analysis of bulb turbine is using software Creo. Initial studies focused on the modeling, meshing and physics definition of the problem. The results were analyzed for rotor and found location of several critical sections subjected to high stress concentration causing failure of rotor blade. Also found the effect of rotor speed on stress and displacement. Hence, the proper mechanical design of the turbo machine blade plays a vital role in the proper functioning of the turbo machine.

Turbomachine design choice for the gas-turbine cycle of NPP with HTGR

The most characteristic from the standpoint of turbomachine design reactor plants with HTGR and a closed gas-turbine cycle are briefly described. The operational particulars of an energy conversion system with single- and multishaft turbomachines in a closed gas-turbine cycle are examined. On the basis of an analysis of the particulars of single- and multishaft turbomachines, several variants of single-shaft turbomachines with rotational speeds 3000 and 4400 rpm and multishaft turbomachines with a high-rpm turbocompressor (6000 rpm) and turbogenerator with rotational speed 3000 rpm with a freewheel clutch or electric starting motor are presented. The effect of the configuration of the turbomachine on the static and dynamic parameters of the reactor facility in normal operating regimes and in emergency situations is analyzed. The present article analyzes a turbomachine design for the characteristic MGR-T (Russia) and PBMR (South Africa) projects in order to determine a rational configuration that secures high efficiency and satisfies operational reliability and safety requirements for the energy conversion system in a closed gas-turbine cycle with HTGR. The energy conversion system of a 600MW(t) MGR-T reactor includes a vertically oriented single-shaft turbomachine, recuperative heat exchanger, and water-cooled preliminary and intermediate coolers [1]. A single-shaft turbomachine on a full electromagnetic suspension consisting of a generator and turbocompressor made it possible to assemble an energy conversion system contained in a single sealed vessel, ensuring compactness and minimizing hydraulic losses in the first loop. However, the single-shaft high-capacity structure with rotation speed equal to the grid frequency (50 Hz) has a large number of compressor steps and, correspondingly, large rotor mass with blade systems. At the conceptual design stage of a 265 MW(t) PBWR reactor, a three-shaft design consisting of two vertical lowand high-pressure turbocompressors on electromagnetic bearings with rotational speeds 14200 and 15200 rpm, respectively, and a 3000 rpm turbogenerator were studied. The main plant equipment was placed in separate vertical vessels [2]. This turbomachine configuration decreased the rotor mass and, correspondingly, the load on the electromagnetic bearings, but the placement in separated vessels, connected by pipelines, increased the first-loop pressure limits and the hydraulic resistance. HTGR with Gas-Turbine Cycle. A setup with HTGR and a gas-turbine cycle must operate stably and efficiently at a 100% load in a prescribed range (from 15 to 100% N nom ) with speed to 10%/min and allow plant startup and shutdown. For reliable and safe operation during emergencies, it must be possible to reduce the load to the internal needs level, the admissible run-out revolutions of the turbomachine must not be exceeded in the event the generator is suddenly disconnected from the grid, it must be possible to maintain the rotation frequency of the turbomachine for a definite period of time and then restore it to the required level, and it must be possible to stop the reactor and turbomachine quickly and safely in the event of power loss and internal failures in the turbomachine and its systems.

Full three-dimensional investigation of structural contact interactions in turbomachines

Minimizing the operating clearance between rotating bladed-disks and stationary surrounding casings is a primary concern in the design of modern turbomachines since it may advantageously affect their energy efficiency. This technical choice possibly leads to interactions between elastic structural components through direct unilateral contact and dry friction, events which are now accepted as normal operating conditions. Subsequent nonlinear dynamical behaviors of such systems are commonly investigated with simplified academic models mainly due to theoretical difficulties and numerical challenges involved in non-smooth large-scale realistic models. In this context, the present paper introduces an adaptation of a full three-dimensional contact strategy for the prediction of potentially damaging motions that would imply highly demanding computational efforts for the targeted aerospace application in an industrial context. It combines a smoothing procedure including bicubic B-spline patches together with a Lagrange multiplier based contact strategy within an explicit time-marching integration procedure preferred for its versatility.The proposed algorithm is first compared on a benchmark configuration against the more elaborated bi-potential formulation and the commercial software Ansys. The consistency of the provided results and the low energy fluctuations of the introduced approach underlines its reliable numerical properties. A case study featuring blade-tip/casing contact on industrial finite element models is then proposed: it incorporates component mode synthesis and the developed three-dimensional contact algorithm for investigating structural interactions occurring within a turbomachine compressor stage. Both time results and frequency-domain analysis emphasize the practical use of such a numerical tool: detection of severe operating conditions and critical rotational velocities, time-dependent maps of stresses acting within the structures, parameter studies and blade design tests.

RANS–URANS in axial compressor, a design methodology

Turbomachinery flows are inherently unsteady. Until now during the design process, unsteadiness has been neglected, with resort merely to steady numerical simulations. Despite the assumption involved, the results obtained with steady simulations have been used with success. One of the questions arising in recent years is can unsteady simulations be used to improve the design of turbomachines? In this work the numerical simulation of a multi-stage axial compressor is carried out. Comparison of Reynolds averaged Navier-Stokes (RANS) and unsteady Reynolds averaged Navier-Stokes (URANS) calculation shows that the unsteadiness affects pressure losses and the prediction of stall limit. The unsteady inflow due to the wake passing mainly modifies the losses and whirl angle near the endwalls. The computational cost of the fully unsteady compared with a steady simulation is about four times in terms of mesh dimension and two orders of magnitude as number of iterations. A mixed RANS–URANS solution has been proposed to give the designer the possibility to simulate an unsteady stage embedded in a steady-state simulation. This method has been applied to the simulation of a four-stage axial compressor rig. The mixed RANS–URANS approach has been developed using sliding and mixing planes as interface conditions. The rotor–stator interaction has been captured physically while reducing the computational time and mesh size.

Fitting a Pitch

This article discusses gas turbine efficiency, which is an essential but often unappreciated aspect of turbomachine design pitch. To an engineer, the pitch of a turbo machinery blade is the angle at a representative blade cross-section between the blade chord line and the plane of the blade’s rotation. An axial flow gas turbine consists of many rows of rotating blades, interspersed with rows of stationary airfoils, called vanes or stators. The gas turbine compressor (whose first row of rotating blades in a jet engine may be a fan) draws in air, which after passing through a combustor to add energy to the air flow, powers the turbine which drives the compressor. Most modern commercial jet engines are turbofan, with a front mounted fan, whose size is indicated by the bypass ratio. During the 1990s, jet engine companies developed and tested variable pitch turbofans, with cycle studies showing between 6 and 14% fuel savings. If fuel savings could spread through the airline industry, changing the pitch could lead to air carriers singing a happier tune.

Computational Analysis of Swept Compressor Rotor Blades

Three-dimensional blading has been used in the process of turbomachine designs. To meet the need for efficient turbine blade designs, CFD predictions of a complex 3D flow field in turbine blade passages had been used. Because the numerous advantages of 3D CFD have been reported in the open literature, many industries already use 3D blading in their turbomachines. In addition, blade lean and sweep have been implemented to increase the blade row efficiency. Experimental studies have shown the advantages of these features. However, most of the experimental results combined other features together, which is difficult to determine the effects of individual features. The development of the CFD technique had made it possible to do three-dimensional turbulent flow analyses in a very short time. In this study, the numerical studies are presented to study the sweep effects of a transonic compressor airfoil. The focus of the paper is to investigate the sweep effects without changing the compressor blade's other features, i.e. keeping the blade outflow angle the same for all the cases. The purpose of the study is to enhance the understanding of the sweep in a transonic compressor rotor blade. The results show that the sweeps redistribute the flow, which may reduce the secondary flow loss depending on the baseline. It is shown that forward sweep reduces the tip loading in terms of static pressure coefficient.

Axisymmetric design of axial turbomachines: An inverse method introducing profile losses

The distributed loss model has been included in a time-marching Euler solver. This predicts the axisymmetric flow field and hub-to-tip stream surface geometry once the loading distribution is prescribed in the blade regions. The loss model replaces shear stresses with a suitable viscous force distribution, which does not work on the relative flow motion, but is only meant to produce entropy along the meridional streamlines. As in a fully inverse computation the flow angles are unknown, empirical loss correlations for single cascades cannot be used. The overall loss level through each blade row will be provided by the one-dimensional stage of the design process, then simplifying assumptions or specific correlations are used to estimate its spanwise distribution. The loss model effects on the flow field and stream surface geometry predicted by the inverse solver have been assessed.

ANALYSIS OF FLOW INSIDE OF A COMPRESSOR BLADE ROW(Fluid Machinery)

Three-dimensional blading has been used in the process of turbomachine designs. To meet the need for efficient turbine blade designs, CFD predictions of a complex 3D flow field in turbine blade passages had been used. Because the numerous advantages of 3-D CFD have been reported in the open literature, many industries already use 3D blading in their turbomachines. In addition, blade lean and sweep have been implemented to increase the blade row efficiency. Experimental studies have shown the advantages of these features. However, most of the experimental results combined other features together, making it difficult to determine the effects of individual features. The development of CFD techniques has made it possible to do three-dimensional turbulent flow analyses in a very short time. In this study, numerical studies are presented to study the sweep effects on a transonic compressor air foil. The focus of the paper is to investigate the sweep effects without changing other compressor blade features i.e. keeping the blade out flow angle the same for all cases. The purpose of the study is to enhance the understanding of the sweep in a transonic compressor rotor blade. The results showed that the sweeps redistributes the flow reducing the secondary flow loss, depending on the baseline. It was shown that forward sweep reduces the tip loading in terms of the static pressure coefficient.

04/02451 Wake and aerodynamics loads in multiple bodies— application to turbomachinery blade rows: Pereira, L. A. A. et al. Journal of Wind Engineering and Industrial Aerodynamics, 2004, 92, (6), 477–491

This two part review covers experiments examining the effects of blade tip gaps encountered in turbomachines and the methods by which the synthesised data are currentl used in turbomachine design and analysis. Data gained since the 1930's are subdivided for convenience into cascade (Part 1) and rotating machinery (Part 2) data, with a further subdivision into diffusing, or compressor type flows and accelerating, or turbine type flows. The overall trend is that an increasing tip gap, whose effect can reach over most or all of the blade height, reduces turbomachine performance. There is some evidence among the compressor and compressor cascade data that an optimum gap exists when the opposing effects of secondary flows and tip leakage with rotor/wall relative movement tend to balance. Turbine data are, in general, more regular than the body of compressor data, possibly because of the enhanced effect of, usually, undefined boundary layers in diffusing flow in the latter. Comment is made in Part 2 on the predictive and design models reported in the liturature

Spatial Dynamics of Tuned and Mistuned Bladed Disks with Cylindrical and Wedge-Shaped Friction Dampers

One of the main tasks in the design of turbomachines like turbines, compressors, and fans is to increase the reliability and efficiency of the arrangement. Failures due to blade cracks are still a problem and have to be minimized with respect to costs and safety aspects. To reduce the maximum stresses, the blades can be coupled via friction damping devices such as underplatform dampers that are pressed onto the blade platforms by centrifugal forces. In this work, a method will be presented to optimize two different types of underplatform dampers in bladed disk applications with respect to a maximum damping effect. In practice, underplatform dampers with various geometric properties—cylindrical and wedge-shaped—are commonly used and lead to different contact conditions. A discretization of the contact areas between the blade platforms and the dampers is applied to be able to investigate nearly arbitrary contact geometries and spatial blade vibrations. The functionality of the two mentioned damper types has been studied in detail under different working conditions of the assembly. The advantages and disadvantages of both damper types are pointed out and strategies are presented to improve the damper design. In this context, the influence of mistuning effects is discussed in terms of statistical mistuning of the blades’ natural frequencies due to manufacturing tolerances as well as systematical mistuning due to a deliberate slight variation of the blade masses or geometries.