Turbomachinery Test Research Articles

Development and Design Validation of an Inflow-Settling Chamber for Turbomachinery Test-Benches

At Leibniz University of Hannover, Germany, a new turbomachinery test facility has been built over the last few years. A major part of this facility is a new 6 MW compressor station, which is connected to a large piping system, both designed and built by AERZEN. This system provides air supply to several wind tunnel and turbomachinery test rigs, e.g., axial turbines and axial compressors. These test rigs are designed to conduct high-quality aerodynamic, aeroelastic, and aeroacoustic measurements to increase physical understanding of steady and unsteady effects in turbomachines. One primary purpose of these investigations is the validation of aerodynamic and aeroacoustic numerical methods. To provide precise boundary conditions for the validation process, extremely high homogeneity of the inflow to the investigated experimental setup is imminent. Thus, customized settling chambers have been developed using analytical and numerical design methods. The authors have chosen to follow basic aerodynamic design steps, using analytical assumptions for the inlet section, the “mixing” area of a settling chamber, and the outlet nozzle in combination with state-of-the-art numerical investigations. In early 2020, the first settling chamber was brought into operation for the acceptance tests. In order to collect high-resolution flow field data during the tests, Leibniz University and AERZEN have designed a unique measurement device for robust and fast in-line flow field measurements. For this measurement device, total pressure and total-temperature rake probes, as well as traversing multi-hole probes, have been used in combination to receive high-resolution flow field data at the outlet section of the settling chamber. The paper provides information about the design process of the settling chamber, the developed measurement device, and measurement data gained from the acceptance tests.

Implementation of a high-order spatial discretization into a finite volume solver: Applications to turbomachinery test cases using an eddy-viscosity turbulence closure

In this study, the implementation of a high-order spatial discretization method into a Finite Volume solver is presented. Specific emphasis is put on the analysis of the performance over selected turbomachinery test cases. High-order numerical discretization is achieved by adopting the cell-centered Least-Square reconstruction, which is implemented in the in-house solver HybFlow. The validation of the adopted methodology is performed by assessing the solution of a turbulent flat plate with zero pressure gradient, using a eddy-viscosity transitional model. The test case also evidences the effect of the discretization of gradient-based source terms when a high-order reconstruction methodology is used. In the second part of the paper, the solver is used for the solution of relevant two-dimensional turbomachinery test cases, assessing the impact of 2nd and 3rd order reconstruction on the prediction of the aerodynamics and the heat transfer for respectively a low-pressure blade and a high-pressure turbine vane. It is shown how a high-order reconstruction allows for obtaining a better prediction of turbomachinery aerodynamics, with lower number of elements. The benefits over heat transfer predictions in high Reynolds number conditions are instead limited to the reduction of heat transfer coefficient spikes in under-resolved regions of the blade. Eventually, the methodology is validated for a three-dimensional low-pressure turbine cascade with realistic boundary layer inflow conditions.

Improvements in Probabilistic Strategies and Their Application to Turbomachinery

This paper discusses various strategies for probabilistic analysis, with a focus on typical engineering applications. The emphasis is on sampling methods and sensitivity analysis. A new sampling method, Latinized particle sampling, is introduced and compared to existing sampling methods. While it can increase the quality of surrogate models, an optimized Latin hypercube sampling is mostly preferable as it shows slightly better results. In sensitivity analysis, the difficulty lies in correlated input variables, which are typical in engineering applications. First, the Sobol indices and the Shapley values are explained using an intuitive example. Then, the modified coefficient of importance is introduced as a new sensitivity measure, which can be used to reliably identify input variables without functional influence. Finally, these results are applied to a turbomachinery test case. In this case, the flow field of a compressor row is investigated, where the blades are subjected to geometric variability. The profile parameters used to describe the geometric variability are correlated. It is shown that the variability of the maximum camber and the thickness of the leading edge have a decisive influence on the variability of the isentropic efficiency.

Construction of complex high order subharmonic vibrations in a nonlinear rotor system I: Eigenvalue dynamics and prediction

In real turbomachinery testing, we always confront strange high order vibrations which scatter a few points or draw some local lines in the experimental bifurcation diagrams. Such vibrations maybe ignored due to the lack of experiences and knowledge while they can be important in design and operation due to the nonsmooth jumping and instability. This paper proposes a new method for predicting the complex high order subharmonic vibrations for a nonlinear rotor system with light rub. The nonlinearity is induced by the contact of the bristles and the rotor. A continuous nonlinear rotor system is established based on the geometric and bending force diagrams. We discretize the complex orbits and built implicit mappings between the adjacent nodes. A directional eigenfunctional matrix is constructed which maps each nodes and passes the perturbation along the orbits for semi-analytical prediction of the high order subharmonic vibrations. Stability criterion is defined. Numerical bifurcation diagrams and harmonic contours are presented for quick glance of the global subahrmonic vibrations. Two local subharmonic vibrations of odd orders are succesfuly predicted. Local high order subharmonic vibrations of “full tree” and “half tree” are predicted and achieved accurately. An experimental platform is established to compare the semi-analytical predictions with experimental results. The research delivers a new perspective for predicting and computing the high order subharmonic vibrations in nonlinear rotor system.

Annular Dump Diffuser and Deswirl System for Back-Pressure Control in Engine-Scale Transonic Annular Cascade

Abstract A common requirement for turbomachinery testing facilities is the ability to independently control Mach number and Reynolds number. In practice, this means independent control of the inlet total pressure and exit static pressure in a test facility. In this paper, we present a solution to this problem with particular applicability to large-scale annular test facilities. We describe the design and commissioning of a combined annular dump diffuser and deswirl system for back-pressure control in environments with high-whirl transonic flow. The particular application was an annular cascade of nozzle guide vanes from a current civil engine. The purpose was to provide independent control of Mach number and Reynolds number, by controlling the back-pressure in the intermediate annular plenum which forms the dump diffuser. The dump diffuser is necessary to facilitate optical and probe access (without interaction effects) and to reduce the risk of exit static pressure disturbances (due to particular duct design). The system has been installed and validated in the engine component aerothermal (ECAT) facility at the University of Oxford. In this implementation of the system, the high-whirl transonic flow from the nozzle guide vanes passes through a short, parallel annular duct, and is dumped into an annular plenum, before being re-accelerated into a deswirl vane ring. The deswirl ring turns the flow to the axial direction. The flow is then discharged through a variable choke plate into a silencer. Conditioning the flow to have zero whirl at the choke plate reduces the sensitivity of the choke plate effective blockage to the whirl angle. The design, deswirl vane aerodynamic performance, and overall system performance are assessed with detailed experiments and 3D unsteady computational fluid dynamics predictions. The control of high-whirl transonic flow is notoriously challenging, and the deswirl system has application to exhaust conditioning in a number of applications including annular cascade experiments and rocket turbo-pump exhaust systems.

Design analysis of a novel orifice control valve for turbomachinery testing facilities

In testing facilities for turbomachinery applications, the main component for the regulation of the flow is the control valve. In order to fulfill the flow rate requirements, the performance of the control valve should be highly accurate in a specified flow range. The robustness against variations or disturbances, is a major issue to be considered. A control valve incorporated in any testing facility has to meet certain requirements. Thus, operating aspects, such as the flow rate range and the valve pressure drop, along with control prerequisites, regarding flow stability, repeatability and robustness of the valve, are considered. The purpose of this study is to establish a new designing approach for control valves. Orifice plates are used as a guideline for this study because of the geometrical similarities with the control valve. A sensitivity analysis determines the most influential parameters and prioritizes the requirements. With emphasis on the cross-sectional area and by neglecting its shape, a simplified 2-D model is optimized. Eventually, a CFD model, verified by experiments, is used for the analysis of the effects of geometrical parameters. The established workflow weighs the requirements according to each application and indicates the method to reach the desired goals. CFD simulations verify the 2-D model and assist in the fine adjustment of the control valve’s characteristics. With an average deviation of 3% between the 2-D and the 3-D model, the simplified 2-D model proves to be sufficient for setting the valve’s characteristics, setting the CFD simulations necessary, only in applications of extreme flow rates and temperature conditions.

Structured multi-block grid partitioning using balanced cut trees

An algorithm to partition structured multi-block hexahedral grids for a load balanced assignment of the partitions to a given number of bins is presented. It uses a balanced hierarchical cut tree data structure to partition the structured blocks into structured partitions. The refinement of the cut tree attempts to generate equally shaped partitions with a low amount of additional surface. A multi-block load balancing approach is presented that guarantees to satisfy an upper bound of load imbalance. The partition quality of the algorithm is compared to established recursive edge bisection approaches and an unstructured partitioning using METIS. Two generic and two turbomachinery test cases demonstrate the superior quality and fast runtime of the present algorithm at generating load balanced structured partitions.

Towards Automatic Blocking of Shapes using Evolutionary Algorithm

This work focuses on the use of evolutionary algorithm to perform automatic blocking of a 2D manifold. The goal of such a blocking process is to completely partition a 2D region into a set of conforming and non-intersecting quadrilaterals to facilitate the generation of an all-quadrilateral, or more preferably an ideal quadrilateral mesh configuration covering the closed 2D region. However, depending on the input shape, the optimal blocking strategy is often unclear and can be very user-dependent. In this work, a novel approach based on evolutionary algorithm is adapted to search for a potential set of such ideal configurations. Based on a selection within a set of candidate vertices from a pre-computed pool, blocking configurations can be derived and ranked based on the collective quality of its blocks. The quality of a block is computed based on objective functions relating to its interior angles and opposite length ratios. Using multi-dimensional ranking criteria, inferior solutions can be slowly filtered away with each successive generation. Based on observations on a range of turbomachinery test cases, it is possible to derive and improve near-optimal blocking configurations by utilizing a large number of generations.

Reduction and coupling of substructures via Gram–Schmidt Interface modes

The paper presents a novel reduction method in the class of component mode synthesis techniques. The method is here called Gram–Schmidt Interface (GSI), since it is based on the Gram–Schmidt algorithm. The GSI reduction method addresses problems where the Craig–Bampton reduced order model of a substructure has still a large number of interface degrees of freedom that should be further reduced. Starting from the pre-existing reduction method based on the Characteristic Constraint Modes, the GSI technique goes further, being suitable to reduce a Craig–Bampton model that retains as a master a wide variety of boundary DoFs (interface, active, etc.). The GSI method is here proved to be effective when assembling adjacent substructures having non-compatible meshes at the interfaces. This is possible by performing the interfaces coupling in the modal space of the GSI modes instead in the space of physical degrees of freedom. The technique is here applied to a turbomachinery test case consisting of a blade coupled with a disk’s sector treated in cyclic symmetry conditions. It is also demonstrated how the GSI method appears useful in the reduction of cyclic symmetric structures, representing a valid alternative of the method developed by Tran.

Harmonic Balance developments in OpenFOAM

The Harmonic Balance method for simulating unsteady nonlinear periodic flows is presented in this paper. Its recent, yet rapid development was motivated by the possibility of significant CPU time consumption savings compared to conventional transient methods, while providing sufficient accuracy.Using the Harmonic Balance method, a single time-continuous primary variable is decomposed into a number of steady state snapshots throughout the representative period, depending on the number of resolved harmonics. Decomposition is done following the Fourier series, yielding the mutually coupled equations where time derivation term is replaced by a coupling source term. Therefore, the decomposition relates to temporal evolution and accounts for transient effects.The numerical method is based on the Finite Volume (FV) method for computational fluid dynamics and was implemented in the open-source software foam-extend. Along with the mathematical model, this work presents the additional derivation of temporal coupling over the interface for cases with more than one domain. With proper mathematical procedure, no additional time-interpolation between interfaces is needed, presenting a novelty in the field.The method is validated in two distinct fields: turbomachinery and naval hydrodynamics. Turbomachinery test cases consist of 2D ERCOFTAC centrifugal pump and a 3D Aachen turbine, for which the comparison with conventional transient and steady state results is performed. For the ERCOFTAC case, CPU time comparison is presented as well. Two phase variant of the Harmonic Balance method is validated on regular wave propagation test case, including the comparison with transient results, which is followed by a DTMB ship hull on waves test case. The two-phase variant of Harmonic Balance represents a novelty in the field of method application. The overall results show the approach is both efficient and accurate, while offering versatility in the area of application.

Sparse Representation Based Frequency Detection and Uncertainty Reduction in Blade Tip Timing Measurement for Multi-Mode Blade Vibration Monitoring.

The accurate monitoring of blade vibration under operating conditions is essential in turbo-machinery testing. Blade tip timing (BTT) is a promising non-contact technique for the measurement of blade vibrations. However, the BTT sampling data are inherently under-sampled and contaminated with several measurement uncertainties. How to recover frequency spectra of blade vibrations though processing these under-sampled biased signals is a bottleneck problem. A novel method of BTT signal processing for alleviating measurement uncertainties in recovery of multi-mode blade vibration frequency spectrum is proposed in this paper. The method can be divided into four phases. First, a single measurement vector model is built by exploiting that the blade vibration signals are sparse in frequency spectra. Secondly, the uniqueness of the nonnegative sparse solution is studied to achieve the vibration frequency spectrum. Thirdly, typical sources of BTT measurement uncertainties are quantitatively analyzed. Finally, an improved vibration frequency spectra recovery method is proposed to get a guaranteed level of sparse solution when measurement results are biased. Simulations and experiments are performed to prove the feasibility of the proposed method. The most outstanding advantage is that this method can prevent the recovered multi-mode vibration spectra from being affected by BTT measurement uncertainties without increasing the probe number.

On the development of a magnetoresistive sensor for blade tip timing and blade tip clearance measurement systems.

A simultaneous blade tip timing (BTT) and blade tip clearance (BTC) measurement system enables the determination of turbomachinery blade vibrations and ensures the monitoring of the existing running gaps between the blade tip and the casing. This contactless instrumentation presents several advantages compared to the well-known telemetry system with strain gauges, at the cost of a more complex data processing procedure. The probes used can be optical, capacitive, eddy current as well as microwaves, everyone with its dedicated electronics and many existing different signal processing algorithms. Every company working in this field has developed its own processing method and sensor technology. Hence, repeating the same test with different instrumentations, the answer is often different. Moreover, rarely it is possible to achieve reliability for in-service measurements. Developments are focused on innovative instrumentations and a common standard. This paper focuses on the results achieved using a novel magnetoresistive sensor for simultaneous tip timing and tip clearance measurements. The sensor measurement principle is described. The sensitivity to gap variation is investigated. In terms of measurement of vibrations, experimental investigations were performed at the Air Force Institute of Technology (ITWL, Warsaw, Poland) in a real aeroengine and in the von Karman Institute (VKI) R2 compressor rig. The advantages and limitations of the magnetoresistive probe for turbomachinery testing are highlighted.

A disc to air heat flux error and uncertainty analysis applied to a turbomachinery test rig design

This article describes a Monte Carlo simulation-based error and an uncertainty analysis for values of disc to air heat fluxes as part of the design of an experimental axial turbine test rig. This work is of interest for those who study heat transfer and measurement or the design and use of experimental test rigs. An inverse analysis of theoretical disc surface temperatures was performed for different thermocouple configurations to compare the errors and uncertainties resulting from each to establish whether there was any configuration that would return the lowest magnitudes of error and uncertainty and hence influence the location of the proposed instrumentation. It is shown that great care needs to be taken when using an analysis of this kind together with temperature measurements having realistic and typical uncertainty values. This is because such an analysis is purely analytical, and any small fluctuations in the inputs, such as typical thermocouple uncertainties and noise, result in the process of an inverse analysis becoming unstable. This instability has two effects: (a) the returned values of heat flux have an inbuilt bias error and (b) the magnitudes of uncertainty can exceed>100 per cent.

Customer days to demo turbomachinery sealing technology

John Crane has announced a series of Customer Days to celebrate the official launch of the recently completed turbomachinery testing facilities in Slough, UK.

Achieving high parallel performance for an unstructured unsteady turbomachinery CFD code

This paper describes the work done to achieve high parallel performance for an unstructured, unsteady turbomachinery computational fluid dynamics (CFD) code. The aim of the work described here is to be able to scale problems to the thousands of processors that current and future machine architectures will provide. The CFD code is in design use in industry and is also used as a research tool at a number of universities. High parallel scalability has been achieved for a range of turbomachinery test cases, from steady-state hexahedral mesh cases to fully unsteady unstructured mesh cases. This has been achieved by a combination of code modification and consideration of the parallel partitioning strategy and resulting load balancing. A sliding plane option is necessary to run fully unsteady multistage turbomachinery test cases and this has been implemented within the CFD code. Sample CFD calculations of a full turbine including parts of the internal air system are presented.

A Correlation-Based Transition Model Using Local Variables—Part II: Test Cases and Industrial Applications

A new correlation-based transition model has been developed, which is built strictly on local variables. As a result, the transition model is compatible with modern computational fluid dynamics (CFD) methods using unstructured grids and massive parallel execution. The model is based on two transport equations, one for the intermittency and one for the transition onset criteria in terms of momentum thickness Reynolds number. The proposed transport equations do not attempt to model the physics of the transition process (unlike, e.g., turbulence models), but form a framework for the implementation of correlation-based models into general-purpose CFD methods. Part I of this paper (Menter, F. R., Langtry, R. B., Likki, S. R., Suzen, Y. B., Huang, P. G., and Völker, S., 2006, ASME J. Turbomach., 128(3), pp. 413–422) gives a detailed description of the mathematical formulation of the model and some of the basic test cases used for model validation. Part II (this part) details a significant number of test cases that have been used to validate the transition model for turbomachinery and aerodynamic applications, including the drag crisis of a cylinder, separation-induced transition on a circular leading edge, and natural transition on a wind turbine airfoil. Turbomachinery test cases include a highly loaded compressor cascade, a low-pressure turbine blade, a transonic turbine guide vane, a 3D annular compressor cascade, and unsteady transition due to wake impingement. In addition, predictions are shown for an actual industrial application, namely, a GE low-pressure turbine vane. In all cases, good agreement with the experiments could be achieved and the authors believe that the current model is a significant step forward in engineering transition modeling.

The rotating rake fan mode measurement system

An experimental measurement system was developed and implemented by the NASA Glenn Research Center in the 1990s. The system is a continuously rotating radial rake immersed into the duct. This rotating rake provides a complete map of the acoustic duct modes present in a ducted fan and has been used on a variety of test articles: from a low-speed, concept test rig to a full-scale production turbofan engine. The rotating rake has been critical in developing and evaluating a number of noise reduction concepts as well as providing experimental databases for verification of several aero-acoustic codes. This paper will describe the physical theory (Sofrin) and the analytical techniques (Moore) upon which the rotating rake is based will be described. Data processing and analysis as well as implementation issues will be discussed. Several Rotating Rake systems have been custom built for 3 facilities. In order of complexity of the turbo machinery test article, these are (1) the Advanced Noise Control Fan, (2) various 22-inch fan rigs in the NASA Glenn 9×15 wind tunnel, and (3) a full scale turbofan, the Honeywell TFE-731-60. Descriptions and measurement achievements of these systems will be provided (Heidelberg, Sutliff).

Effects of Flow Instabilities on the Linear Analysis of Turbomachinery Aeroelasticity

The linear analysis of turbomachinery aeroelasticity is based on the linearization of the unsteady flow equations around the mean flow field, which can be determined by a nonlinear steady solver. The unsteady periodic flow can be decomposed into a sum of harmonics, each of which can be computed independently by solving a set of linearized equations. The analysis considers just one particular frequency of unsteadiness at a time, and the objective is to compute a complex flow solution that represents the amplitude and phase of the unsteady flow. The solution procedure of both the nonlinear steady and the linear harmonic Euler/Navier-Stokes solvers of the HYDRA suite of codes consists of a preconditioned fixed-point iteration. The numerical instabilities encountered while solving the linear harmonic equations for some turbomachinery test cases are documented, their physical origin highlighted, and the implementation of a GMRES algorithm aiming at the stabilization of the linear code summarized. Presented results include the flutter analysis of a two-dimensional turbine section and a civil engine fan.

A finite element overlapping scheme for turbomachinery flows on parallel platforms

Two- and three-dimensional turbomachinery flows in stationary and rotating compressor cascades are studied by using a one-level inexact explicit Schwarz method, and a cubic eddy viscosity turbulence closure. The message passing paradigm is used for the parallel implementation of the domain decomposition algorithm, allowing the solver portability on different parallel platforms. A convergence accelerator is proposed, based on a condensed cycle structure that merges the additive Schwarz iterations with the fixed point non-linear ones. The use of a stable finite element formulation on higher-order elements Q2–Q1 is addressed as a mean for retaining non-oscillatory and accurate solutions. Furthermore, the elementwise quadratic approximation is used to enable the exact implementation of higher-order integrals arising in the anisotropic turbulence closure adopted. Numerical campaigns are carried out on IBM SP2 and SP3, and CRAY T3E architectures, in order to demonstrate the portability. The accompanying performance improvement is assessed. Finally, the predicting capabilities are discussed with reference to challenging turbomachinery test cases: a transitional linear compressor cascade, and an isolated compressor rotor designed for non-free vortex operation. Convergence speed-up in such configurations is discussed.

Demonstration of optical fiber probes for high bandwidth thermal measurements in turbomachinery

Interferometric temperature sensors comprising a zinc selenide film on the end of a single-mode optical fiber have been incorporated into rugged probes for measurement of gas temperature fluctuations in aerospace turbomachinery test rigs. Two-wavelength illumination provides signals in quadrature for determination of optical phase change of the signal reflected from the film which is exposed to the gas flow. We present ensemble-averaged results showing 15 K amplitude gas temperature fluctuations up to 70 kHz with a resolution of 1 K. >