Turbomachinery Applications Research Articles

Development of a Three‐Dimensional Geometry Optimization Method for Turbomachinery Applications

This article describes the development of a method for optimization of the geometry of three‐dimensional turbine blades within a stage configuration. The method is based on flow simulations and gradient‐based optimization techniques. This approach uses the fully parameterized blade geometry as variables for the optimization problem. Physical parameters such as stagger angle, stacking line, and chord length are part of the model. Constraints guarantee the requirements for cooling, casting, and machining of the blades.The fluid physics of the turbomachine and hence the objective function of the optimization problem are calculated by means of a three‐dimensional Navier‐Stokes solver especially designed for turbomachinery applications. The gradients required for the optimization algorithm are computed by numerically solving the sensitivity equations. Therefore, the explicitly differentiated Navier‐Stokes equations are incorporated into the numerical method of the flow solver, enabling the computation of the sensitivity equations with the same numerical scheme as used for the flow field solution.This article introduces the components of the fully automated optimization loop and their interactions. Furthermore, the sensitivity equation method is discussed and several aspects of the implementation into a flow solver are presented. Flow simulations and sensitivity calculations are presented for different test cases and parameters. The validation of the computed sensitivities is performed by means of finite differences.

Influence of fuel volatility and spray parameters on combustion characteristics and NO x emission in a gas turbine combustor

The main thrust in research in the field of gas turbine combustion centers around a clean emission, a low liner wall temperature, a desirable exit temperature distribution for turbo-machinery applications along with a fuel economy of the combustion process. An attempt to meet the challenge has been made in the present paper in developing a computational model based on stochastic separated flow analysis of a typical diffusion controlled spray combustion of liquid fuel in a gas turbine combustor to study the influences of fuel volatility and different spray parameters on combustion and emission characteristics. A κ– ε model with wall function treatment for near wall region has been adopted for the solution of conservation equations in gas phase. The initial spray parameters are specified by a suitable probability distribution function size distribution and a given spray cone angle. A radiation model for the gas phase, based on first order moment method, has been adopted in consideration of the gas phase as a gray absorbing–emitting medium. Formation of thermal NO x as a post-combustion reaction process, is determined from Zeldovich mechanism. It is recognized that the combustion efficiency is reduced drastically with a decrease in fuel volatilities at lower spray cone angle. For a given fuel, there is a significant reduction in combustion efficiency at a lower spray cone angle and lower initial SMD. The pattern factor of exit temperature distribution is reduced with a decrease in initial SMD, for all fuels. The influence of spray cone angle on pattern factor is contrasting in nature for fuels with higher and lower volatilities. The pattern factor decreases with an increase in spray cone angle for a higher volatile fuel, whereas, the reverse happens in case of lower volatile fuels. It is found that the bulk exit NO x increases with a decrease in fuel volatility for any given set of values of spray parameters. An increase in spray cone angle increases bulk exit NO x especially for lower volatile fuels, and an increase in initial SMD increases the bulk exit NO x for lower volatile fuels only.

Design of Friction Dampers for Mistuned Bladed Disks

AbstractFriction damping devices such as underplatform dampers are widely used in turbomachinery applications to reduce blade vibration amplitudes. In this paper, a model is presented briefly to calculate the forced response of mistuned bladed systems with friction dampers. Some results will give an insight in the vibrational behavior of frictionally damped bladed disks and how the damping gain is affected by the damper properties.

Multi-Element Unstructured Methodology for Analysis of Turbomachinery Systems

In recent years unstructured mesh techniques have become popular for computational e uid dynamics analysis of external aerodynamic-type problems. The main advantages of such an approach include mesh generation over complex domains, grid adaptation in localized areas, and accuracy in efe ciently identifying complexities in local e ow physics. A hybrid unstructured methodology is used to carry out simulations for predominantly internal e ow turbomachinery applications. Issues related to skewness and other constraints of tetrahedral meshes are addressed in the context of turbomachinery-based propulsive e ows that exhibit a rich variety of length scales and timescales, as well asinteresting e ow physics. The unstructured framework permits the generation of a contiguous grid without internal boundaries between different components of a turbomachinery system and provides good local resolution in regions where the e ow physics becomes important. The increased numerical stability resulting from these factors coupled with the parallel solution framework yields an efe cient solution procedure for complex turbomachinery e ows. Numerical results are presented and compared against experimental measurements for a transonic diffuser‐ volute cone guration and a high Reynolds number pump.

Analysis of Contact Mechanics for Rotor-Bristle Interference of Brush Seal

Brush seals have proven to be an attractive alternative to labyrinth seals for turbomachinery applications. This innovation in seal technology utilizes both the high temperature capability of special-alloy wire and the flexural adaptability of fibers to accommodate a wide range of operating conditions that are encountered during service. The effectiveness of the seal is principally derived from the bristles ability to endure forces imparted by both the fluid and shaft, and yet maintain contact between the filament tips and the surface of the rotor. Consequently, contact forces generated along the interface of the fiber tip and rotor are an important consideration for both the design and performance of the rotor-seal assembly. This paper focuses on evaluating brush seal forces that arise along the surface of the rotor due to the dimensional disparity or interference between the rotor-fiber. Filament tip contact forces are computed on the basis of an in-plane, large deformation mechanics analysis of a cantilever beam, and validation of the model is assessed by using an electronic balance for measuring the shear and normal force exerted by a bristle tip onto a flat, hardened surface. Formulation of the mechanics problem is briefly reviewed, and includes the effect of Coulombic friction at the interface of the fiber tip and rotor. Filament contact force is used as a basis for computing bearing stress along the fiber-rotor interface. Results are reported for a range of brush seal design parameters in order to provide a better understanding of the role that seal geometry, friction, and bristle flexural rigidity play in generating rotor contact force.

Vibration Damping by Friction Forces: Theory and Applications

In this paper, we deal with the vibrational behavior of mechanical structures interconnected by contacts with friction. The focus is set on the utilization of friction forces that are generated in the contact interfaces with the objective to increase damping and to reduce vibration amplitudes in order to prevent structures from failures owing to high resonance stresses. We present a comparison and classification of different contact models that are most commonly used, including the derivation of a three-dimensional contact model under consideration of rough surfaces. We give different solution methods for problems with non-linear friction elements. The effectiveness of friction damping devices is pointed out by a single-degree-of-freedom friction oscillator, beam-like structures with frictional interfaces and different underplatform dampers in turbo-machinery applications. It can be shown that in many practical applications friction damping devices provide a remarkable decrease of vibration amplitudes.

Flow instability and its control in compression systems

This paper reviews the development in the research of flow instability and its control over the recent ten or more years. This development was largely stimulated by the novel idea of active control of the aerodynamic instability in compressors. Three topics are covered in the paper, which appeared as the major themes towards the goal of stability enhancement. The first topic is the pre-stall behavior of rotating stall, which plays a vital role in designing the control scheme and discovering the convenient route to find the causal factors of flow disturbances potentially leading to stall. The second topic is the mechanism of blade passage flow during stall and its inception, which is the basic knowledge needed to manipulate the blade design for the stability improvement and eventually to predict the unsteady performance of the compressor system. The third topic is the recent trend of the control strategy based on the learning of active vs. passive methods. To introduce to the discussion of these topics, a brief description of the history of the recent development is given at the beginning of the paper. In discussing each topic, future works are also highlighted to enhance the further development of this long-standing problem in turbomachinery research and application.

Transonic injection in interaction with transverse compressible flow.

An extensive study devoted to modelling blade cooling was undertaken at CEAT a few years ago in collaboration with SNECMA. For the turbomachinery applications, an experimental configuration of a turbulent boundary layer with heat transfer was studied for compressible and incompressible flows. The research presented here is a part of that study and this paper reports on the experimental results of an investigation concerned with a row of transonic jets interacting with a transverse flow. In many applications, the cooling layer does not emerge onto the surface from a tangential slot but comes from a slot normal to or inclined to what is otherwise a flush surface. In this case the freestream interacts with the coolant flow. The secondary (jet) flow is introduced at an angle of 45 degrees to the mainstream flow direction. Visualization studies using the surface flow patterns and surface temperature flow patterns are reported and discussed.

The harmonic adjoint approach to unsteady turbomachinery design

AbstractIn recent years, there has been rapid progress in aerodynamic optimization methods which use adjoint flow analysis to efficiently calculate the sensitivity of steady‐state objective functions to changes in the underlying design variables. This paper shows that the same adjoint approach can be used in turbomachinery applications in which the primary concern is blade vibration due to harmonic flow unsteadiness.The paper introduces the key engineering concepts and discusses the derivation of the adjoint analysis at the algebraic level. The emphasis is on the algorithmic aspects of the analysis, on the iterative solution method and on the role played by the strong solid wall boundary condition, in particular. The novel ideas are exploited to reveal the potential of the approach in the minimization of the unsteady vibration of turbomachinery blades due to incident wakes. Copyright © 2002 John Wiley & Sons, Ltd.

A Test Rig for LDV and Seeding Characterization for High Speed Turbomachinery Application: Experimental Results

A Test Rig for LDV and Seeding Characterization for High Speed Turbomachinery Application: Experimental Results

Influence of fuel volatility on combustion and emission characteristics in a gas turbine combustor at different inlet pressures and swirl conditions

The practical challenges in research in the field of gas turbine combustion mainly centre around a clean emission, a low liner wall temperature and a desirable exit temperature distribution for turboma-chinery applications, along with fuel economy of the combustion process. An attempt has been made in the present paper to develop a computational model based on stochastic separated flow analysis of typical diffusion-controlled spray combustion of liquid fuel in a gas turbine combustor to study the influence of fuel volatility at different combustor pressures and inlet swirls on combustion and emission characteristics. A κ-ɛ model with wall function treatment for the near-wall region has been adopted for the solution of conservation equations in gas phase. The initial spray parameters are specified by a suitable probability distribution function (PDF) size distribution and a given spray cone angle. A radiation model for the gas phase, based on the first-order moment method, has been adopted in consideration of the gas phase as a grey absorbing-emitting medium. The formation of thermal NO x as a post-combustion reaction process is determined from the Zeldovich mechanism. It has been recognized from the present work that an increase in fuel volatility increases combustion efficiency only at higher pressures. For a given fuel, an increase in combustor pressure, at a constant inlet temperature, always reduces the combustion efficiency, while the influence of inlet swirl is found to decrease the combustion efficiency only at higher pressure. The influence of inlet pressure on pattern factor is contrasting in nature for fuels with lower and higher volatilities. For a higher-volatility fuel, a reduction in inlet pressure decreases the value of the pattern factor, while the trend is exactly the opposite in the case of fuels with lower volatilities. The NOx emission level increases with decrease in fuel volatility at all combustor pressures and inlet swirls. For a given fuel, the NOx emission level decreases with a reduction in combustor pressure and an increase in inlet swirl number.

Computational modeling for honeycomb-stator gas annular seal

Gas annular seals are commonly adopted for leakage control in turbomachinery applications. Honeycomb seals are attractive from the viewpoints of leakage control as well as rotordynamic stability. To improve our understanding of thermo-fluid-physics in such seals, a computational capability is developed for low Mach number, compressible, turbulent flows. The emphases of the present study include (i) development of an original numerical scheme with periodic boundary conditions for flows around repeated geometries, (ii) evaluation of a low Reynolds number version of the k– ε turbulence model suitable for the operating conditions of honeycomb seals, and (iii) 3-D computations to assess the implications of the numerical predictions for practical configurations including velocity, pressure, temperature characteristics and loss mechanisms.

Interference and Turning of In-Parallel Wakes

Multiple in-parallel wakes are studied for 'internal' configurations, modelling contained motion periodic about a central axis, and 'external' ones set in a stream. The former configurations (but not the latter ones) induce a substantial pressure gradient and wake shapes that are controlled, in an unusual manner, by the local lateral momentum balance. Particular attention is given to the pressure losses and the overall turning of the relatively thick wakes for the internal case, based on slender-flow theory throughout. The links with experiments using external flow to simulate internal conditions for turbomachinery applications are discussed, especially in terms of first- and second-order effects for small wake deficits and the differences between the upstream and the downstream lateral periods for many external wakes.

Numerical Investigation of Unsteady Transitional Flows in Turbomachinery Components Based on a RANS Approach

This paper presents results of the numerical simulation of periodically unsteady flows with focus on turbomachinery applications. The unsteady CFD solver used for the simulations is based on the Reynolds averaged Navier–Stokes equations. The numerical scheme applies an extended version of the Spalart–Allmaras one-equation turbulence model coupled with a transition correlation. The first example of validation consists of boundary layer flow with separation bubble on a flat plate, both under steady and periodically unsteady main flow conditions. The investigation includes a variation of the major parameters Strouhal number, amplitude, and Reynolds number. The second, more complex test case consists of the flow through a cascade of turbine blades which is influenced by wakes periodically passing over the cascade. The computations were carried out for two different blade loadings. The results of the numerical simulations are discussed and compared with experimental data in detail. Special emphasis is given to the investigation of boundary layers with regard to transition, separation and reattachment under the influence of main flow unsteadiness.

Modelling of compressible periodic flows with application to turbomachinery seals

Gas annular seals are commonly adopted for leakage control in turbomachinery applications. Due to repeated geometric patterns of the seals, it is desirable to develop suitable strategies to treat a small portion of the flow domain and extrapolate the information to the entire seal. This paper presents such a technique. The following issues are addressed: validation of results obtained using periodic boundary conditions accounting for turbulent and compressible flows; effect of the rotor rotation and inlet swirl ratio on flow development in the seal; extrapolation of the full-length results from limited periodic simulations; and distribution of the friction factor on contributing pressure and shear stress components. The computational techniques presented can help address the design-related issues more economically, and with better insight into the fluid physics.

A Test Rig for LDV and Seeding Characterization for High Speed Turbomachinery Application: Experimental Results

The accuracy and data rate of Laser Doppler Velocimetry (LDV) are strongly influenced by several parameters. To optimize measurements, a system for setting up the velocimeter, not influenced by the specific environment, is needed. It is necessary to choose several optical parameters, to evaluate insemination techniques (not influenced by specific flow fields) and to compare seeding performance in a right range of velocities. A supersonic wind tunnel was constructed, to evaluate the measurement LDV system in a range of velocities from 50 m/s to 450 m/s using various kinds of seeding. Tests were performed both in accelerating velocity zones and in the presence of shock waves. Comparative tests were made using suspension in water of various materials as seeding: Polystyrene (PSL), microballoons, four kinds of commercial aluminum oxide powder in distillate (pH = 6) and acid water (pH = 2), plus other oxides like TiO_2 and ZrO_2.

A fully distributed unstructured Navier-Stokes solver for large-scale aeroelasticity computations

AbstractThis paper presents the development and validation of a parallel unsteady flow and aeroelasticity code for large-scale numerical models used in turbo machinery applications. The work is based on an existing unstructured Navier-Stokes solver developed over the past ten years by the Aeroelasticity Research Group at Imperial College Vibration University Technology Centre. The single-process multiple-data paradigm was adopted for the parallelisation of the solver and several validation cases were considered. The computational mesh was divided into several sub-sections using a domain decomposition technique. The performance and numerical accuracy of the parallel solver was validated across several computer platforms for various problem sizes. In cases where the solution could be obtained on a single CPU, the serial and parallel versions of the code were found to produce identical results. Studies on up to 32 CPUs showed varying levels of parallelisation efficiency, an almost linear speed-up being obtained in some cases. Finally, an industrial configuration, a 17 blade row turbine with a 47 million point mesh, was discussed to illustrate the potential of the proposed large-scale modelling methodology.

Modern pump-design software systems offer an integrated process with fluid dynamics

To be competitive in both engineering productivity and quality of results, many pump designers are transitioning from traditional empirically-based design methods to methods based upon rigorous fluid dynamics. While some companies are well into the changeover, others are just beginning to assess their needs. Concepts NREC, developer of design software for turbomachinery applications, looks at the role of computational fluid dynamics in the design process.

A conservation-based discretization approach for conjugate heat transfer calculations in hot-gas ducting turbomachinery components

A numerical procedure has been developed for simulation of conjugate heat transfer in generalized coordinates and used in some typical turbomachinery applications. Discretized equations for nodes located exactly on the solid–fluid interface were derived using energy conservation principles, yielding the corresponding temperatures directly, without the need for inter- or extrapolation from adjacent nodes. A finite-volume-based computer code was used along with the SIMPLE algorithm and the k– ϵ turbulence model. The turbulent flow and heat transfer in a stepped labyrinth seal and in an effusion-cooled combustor liner have been studied and results were compared with measured data showing good agreement. In the labyrinth-seal case, the comparisons were in terms of surface temperatures and Nusselt numbers, while for the effusion-cooling case in terms of the streamwise velocity and the film-cooling effectiveness for different blowing and density ratios. In the latter case, two different liner materials were used to study the influence of the thermal conductivity on film-cooling characteristics and the agreement was better for the lowest of the two conductivities. The dominant flow structures could be captured with good accuracy.

Linearized Unsteady Viscous Turbomachinery Flows Using Hybrid Grids

The paper describes the theory and the numerical implementation of a three-dimensional finite volume scheme for the solution of the linearized, unsteady Favre-averaged Navier–Stokes equations for turbomachinery applications. A further feature is the use of mixed element grids, consisting of triangles and quadrilaterals in two dimensions, and of tetrahedra, triangular prisms, and hexahedra in three dimensions. The linearized unsteady viscous flow equations are derived by assuming small harmonic perturbations from a steady-state flow and the resulting equations are solved using a pseudo-time marching technique. Such an approach enables the same numerical algorithm to be used for both the nonlinear steady and the linearized unsteady flow computations. The important features of the work are the discretization of the flow domain via a single, unified edge-data structure for mixed element meshes, the use of a Laplacian operator, which results in a nearest neighbor stencil, and the full linearization of the Spalart–Allmaras turbulence model. Four different test cases are presented for the validation of the proposed method. The first one is a comparison against the classical subsonic flat plate cascade theory, the so-called LINSUB benchmark. The aim of the second test case is to check the computational results against the asymptotic analytical solution derived by Lighthill for an unsteady laminar flow. The third test case examines the implications of using inviscid, frozen-turbulence, and fully turbulent models when linearizing the unsteady flow over a transonic turbine blade, the so-called 11th International Standard Configuration. The final test case is a rotor/stator interaction, which not only checks the validity of the formulation for a three-dimensional example, but also highlights other issues, such as the need to linearize the wall functions. Detailed comparisons were carried out against measured steady and unsteady flow data for the last two cases and good overall agreement was obtained.