Turbomachinery Applications Research Articles

Time-of-flight estimation in acoustic pyrometry: sensitivity to pulse characteristics

Acoustic pyrometry is a non-intrusive measurement technique that may have several applications in turbomachinery. This methodology estimates the gas temperature by measuring the time of flight of an acoustic wave moving through a medium. It can be accomplished by placing a sound source (emitter) and a set of microphones (receivers) on opposite sides of a section. The emitter generates a sound pulse, and the receivers detect it. Since the emitter-receiver distances are known and fixed, the average temperatures of the paths traversed by the acoustic pulse can be computed by estimating the time-of-flight through deconvolution techniques. However, despite the straightforward principle, an acoustic wave suffers a variation of amplitude when propagating within a medium because of energy losses and ambient noise. Hence, time-of-flight estimation becomes a critical task, especially when considering high-frequency waves or short distances between sensors. It is then fundamental to select proper acoustic waves to maximise the cross-correlation between the signals of the emitter-receiver couples, thus improving the accuracy of the time-of-flight measurements and, consequently, the estimation of the spatial temperature distribution within a specific area. This study is a preliminary investigation, based on a modelling approach, to estimate the impact of different acoustic waves on the accuracy of the time-of-flight measurement. The results of this analysis will be useful to design and setup an acoustic pyrometry application.

Instrumentation Interference in a Transonic Linear Cascade

A major challenge when testing high-speed flows in turbomachinery applications concerns instrumentation intrusiveness. The current work exposes the impact of aerodynamic probes, placed upstream and downstream of the testing article, on the aerodynamics of a transonic low-pressure turbine blade investigated in a linear cascade at engine-representative outlet Mach and Reynolds numbers. The effect of a probe placed downstream of the cascade can be perceived as far as to the inlet of the cascade. The quantification of the probe impact is presented on the blade loading as well as on the instrumentation placed upstream and downstream of the cascade used to monitor the operating conditions during testing. It is shown that the presence of the probe can be partially compensated. The impact of this aerodynamic compensation on the cascade loss is reported.

Design of a small pressure chamber to evaluate probe calibrations at varying Reynolds numbers

The Chair of Turbomachinery and Flight Propulsion (LTF) at the Technical University of Munich (TUM) operates a wind tunnel for probe calibration (Caljet). This paper describes the design and the validation process of a pressure chamber. The pressure chamber is used to calibrate probes for turbomachinery applications at comparable conditions to a 3.5-stages axial compressor test rig. This allows the study of Reynolds number and Mach number effects individually. The pressure chamber is designed as an additional component for the Caljet to enable probe calibration at higher mass flow densities and higher Reynolds numbers. Therefore, the original free stream jet is encased by this chamber, nevertheless the outlet of the calibration wind tunnel remains open to the environment. After a first pre-design study, numerical simulations are carried out to confirm expected flow conditions in the measurement area. Ansys Fluent is used for RANS simulations with air modeled as compressible gas. The numerical results are validated with experimental investigations that simultaneously provide a deeper understanding of the flow behavior. Therefore, a five-hole probe, a hot-wire probe, and a Fast Response Aerodynamic Probe are utilized. It is shown that the capability of the jet meets the expectations and can be used for the calibration of hot-wire probes at conditions comparable to a state-of-the-art 3.5-stages axial compressor.

A Review on Modeling and Stability Aspects of Gas Foil Bearing Supported Rotors

Gas foil bearing rotor (GFBR) systems have received significant interest in the field of rotordynamics and vibration analysis. GFBR systems have a wide range of high-speed turbomachinery applications. Due to the high speed, these machines are susceptible to rigorous vibration and instability. Gas foil bearings instigate large amplitude of vibrations at startup and shut down and severe subsynchronous motions during high-speed operations. Over the years, numerous work has been done in the field of high-speed rotors supported on gas foil bearings. Significant improvement has been observed in the stability and feasibility of the GFBR systems. However, accurate model predictions of gas foil bearing still remain a challenge for its widespread usage in high-speed turbomachinery. A comprehensive review needs to be done to study the previous work and pave the way for future research. The current review is majorly divided into three sections. Firstly, various models used for the performance prediction of gas foil bearings are compiled. After that, major causes of instability that manifest during the experiments and practice with gas foil bearing supported rotors is illustrated. Lastly, the developmental attempts made to inhibit the instability is summarised. This paper presents an overall picture of the current engineering scenario and future prospects of the GFBR systems.

Characterization of Properties of Laser Powder Bed Fusion Three-Dimensional-Printed Inconel 718 for Centrifugal Turbomachinery Applications

AbstractThis paper presents the results of a comprehensive effort to characterize the properties of Inconel 718 produced by a form of laser powder bed fusion (LPBF) additive manufacturing (AM) or three-dimensional (3D)-printing, subsequently subjected to hot isostatic pressing (HIP) and heat treatment according to standards F3055-14a and AMS 5663, respectively. Material property data, while broadly available for traditional Inconel 718 presentations (e.g., forgings or castings) is currently lacking for the 3D-printed material. It is expected that while limited in size, the experimental data sets presented provide sufficient information to glean the capability of LPBF Inconel 718. These include: (1) chemical composition, electron backscatter diffraction (EBSD), and X-ray energy dispersive spectroscopy (XEDS) characterization of 3D-printed material structure; (2) tensile properties—0.2% yield stress, ultimate stress, modulus of elasticity, and elongation to failure—based on 108 samples, as functions of temperature and sample print orientation; (3) creep rupture data including the Larson-Miller parameter, based on 21 samples; and (4) high cycle fatigue data based on 21 samples as a function of temperature. Results are compared to available standards and/or data for forged, cast, and other AM Inconel 718. A key observation of this study, based on the EBSD results, is that while the material appears to approach full recrystallization following heat treatment, there is a detectable fraction of the material that does not fully recrystallize, resulting in a material with mechanical properties (e.g., yield stress and creep rupture) measurably lower than those of forgings, but higher than those of castings.

Methodology for Counter Torque, Power Loss, and Frictional Heat for Brush Seals Under Eccentric Transients

Turbomachinery brush seals are used for achieving close clearance control with the aid of flexible bristles. Frictional forces between bristle tips and rotor surfaces induce counter torque, power loss, and heat generation during turbomachinery transients; all of these determine the overall efficiency, operation life, and stability of turbomachinery application. As a result of manufacturing tolerances, rotor dynamics-induced shaft deflections, and additional gyroscopic loads in aviation engines, eccentric rotor rub occurs toward brush seals in transients, causing partial rotor–seal contact with nonlinear radial interference and uneven tip force distribution. There is an inherent error in the current power loss and frictional heat rate analyses, as well as in counter torque methodologies, since they do not account for the uneven tip force distribution. In this study, a nonlinear radial interference profile of eccentric rotor contact conditions has been derived and its effect on contact angle has been examined with a comparative analysis with concentric rotor expansion. The methodology for counter torque, power loss, and generated frictional heat rate analyses has been detailed, and eccentric condition formulations have been derived with the surface integration of dynamic bristle tip force functions adopted from brush seal dynamic stiffness tests. Analysis of nonlinear interference and uneven contact force profiles has been conducted based on the combined effects of assembly conditions and eccentricity levels. Results obtained using the developed closed-form equations for eccentric rotor contact are compared with concentric rotor expansion and simple eccentric rotor rub analyses, which do not account for uneven tip forces.

Improving Accuracy of Airfoil Trailing Edge Noise Models with Turbulent Flow Anisotropy for Cambered Airfoil

In turbomachinery applications such as wind turbines, and aircraft airframe, flow-induced noise and vibration are significant noise sources. There are five main types of airfoil self-noise, with trailing edge noise (TEN) being the most dominant high-frequency broadband noise. The Amiet TE noise model is a low-order analytical model that requires the surface pressure (SP) near the trailing edge for TEN modelling. The accuracy with which the SP spectrum is calculated determines the accuracy of the TEN modelling. The SP spectrum is calculated using the physics-based TNO-Blake (TB) Model and its recently produced improved version. Most studies have focused on using the TB model for symmetric airfoil, with little attention paid to modelling the TEN of cambered airfoil. We use TB model variants on the SP spectrum prediction in this study, which are enforced with the Amiet TEN model for far-field noise modelling. The SP models' source comes from a high-fidelity Large-Eddy-Simulation simulation (LES) based on CABARET method. The simulation is carried out using the NACA65410 airfoil with a chord length of 0.3m and an angle of attack of 0,2, 4, 6, and 8 degrees at a Reynolds number of 5.2e5. The SP spectrum and velocity spectrum used to improve the TEN model's accuracy for cambered airfoil.

Analysis of Turbomachinery Averaging Techniques

Abstract In this paper, various averaging techniques commonly used in turbomachinery applications are analyzed. It is shown how the work average relates to Miller’s mechanical work potential and that it is, in a certain way, consistent with Hartsel’s cooled turbine efficiency. It is found that a key to understand these approaches is to analyze the impact that entropy variations at inflows have on them. Second-order asymptotics of mixing entropy are used to establish a close relationship between flux and work averages. It is found that the mixing entropy asymptotic due to entropy modes is identical for both averages. The work average, along with Miller’s mechanical work potential analysis, is as optimistic as the entropy average for vorticity and acoustic modes, but as pessimistic as the flux averaging for entropy variations. This explains why mechanical work potential-based analysis is pessimistic about the inflow and thus optimistic about the efficiency of a turbine for high entropy variations in the inflow, e.g., in the presence of hot streaks or film cooling. Radial averaging techniques are discussed and their impact on turbine performance is shown. Our findings are illustrated by means of the analysis of steady and unsteady flow simulations of a 1.5 stage turbine configuration.

A Fluid–Structure Interaction Tool Using a Van der Pol-Based Reduced-Order Model for Buffet and Nonsynchronous Vibrations

Abstract This paper builds upon a two-degree-of-freedom Van der Pol oscillator-based reduced-order model for studying the mechanisms around nonsynchronous vibrations (NSV) in turbomachinery. One degree tracks the fluid motion utilizing a combination of a traditional Van der Pol oscillator and a Duffing oscillator; the other degree of freedom is a mass on a spring and a damper, in this case, a cylinder. Thus, this model can be considered one of fluid–structure interaction. The cubic stiffening from the Duffing oscillator proved to improve the match to experimental data. Using this model to study the time history of the fluid and the structure oscillation, additional parameters are extracted to understand the underlying mechanisms of frequency lock-in and limit cycle oscillation. First, the phase shift between the vortex shedding and the structural motion is calculated when it locks-in and then unlocks. Second, the work done per cycle is analyzed from the contributions of the mass, spring, and damping forces to determine the dominant contributor when locking-in versus unlocking. Third, the phase portrait is plotted on a Poincaré map to further study the locked-in versus unlocked responses. This model is then validated against not only experimental data but also computational simulation results and previous reduced-order models. The finalized model can now serve as a preliminary design tool for turbomachinery applications. For more realistic and accurate modeling, the third degree of freedom in the form of an airfoil pitching motion will be added in a separate paper as well.

Analysis of an Elasto-Hydrodynamic Seal by Using the Reynolds Equation

This paper reports numerical studies of an Elasto-Hydrodynamic (EHD) seal, which is being developed for supercritical CO2 (sCO2) turbomachinery applications. Current sCO2 turbomachinery suffers from high leakage rates, which is creating a major roadblock to the full realization of sCO2 power technology. The high leakage rates not only penalize the efficiencies but also create environmental concerns due to greenhouse effects caused by the increased CO2 discharge to the atmosphere. The proposed EHD seal needs to work at elevated pressures (10–35 MPa) and temperatures (350–700 °C) with low leakage and minimal wear. The unique mechanism of the EHD seal provides a self-regulated constriction effect to restrict the flow without substantial material contact, thereby minimizing leakage and wear. This work utilizes a physics-based modeling approach. The flow through the gradually narrowing seal clearance is modeled by the well-known Reynolds equation in EHD lubrication theory, while the deformation of the seal is modeled by using the governing equations of three-dimensional solid mechanics. As for the solution methodology, COMSOL’s Thin-Film Flow and Solid Mechanics modules were employed with their powerful capabilities. The numerical results were presented and discussed. It was observed that the Reynolds equation fully coupled with the surface deformation was able to successfully capture the constriction effect. The maximum and minimum leakages were calculated to be 2.25 g/s and 0.1 g/s at P = 5.5 MPa and P = 11 MPa for the design seal, respectively. It was interesting to observe that the seal leakage followed a quadratic trend with increasing pressure differential, which can become advantageous for high-pressure applications such as sCO2 power generation technology.

1D analytic and numerical analysis of multilayer laminates and thin film heat transfer gauges

The impulse response method is widely used for heat transfer analysis in turbomachinery applications. Traditionally, the 1D method assumes a linear time invariant, isotropic, semi-infinite block and does not accurately model the behaviour of laminated materials. This paper evaluates the error introduced by the single layer assumption and outlines the required modifications for multilayer analysis. The analytic solution for an N layer, semi-infinite laminate is presented. Adapted multilayer basis functions are derived for the impulse response method and used to evaluate the impact of uniform, isotropic assumptions. A numerical solution to the laminate problem is also presented. A penta-diagonal inversion algorithm, for a modified Crank-Nicolson scheme, is evaluated for fast stable implementation of multilayer simulation. The scheme shows comparable performance to the impulse response, whilst removing the requirement for linear time invariance. The methods are demonstrated in the case of analysing a thin film gauge, used in laboratory analysis of heat transfer in a turbine nozzle guide vane. Thin film gauge manufacturing techniques have advanced significantly in recent years. Advanced multilayer constructions are now used however, post-processing commonly relies on outdated single layer methods. This paper provides a universal methodology, required to analyse modern-day multilayer heat transfer measurements.

Lattice Boltzmann simulation of the ERCOFTAC pump impeller

The analysis of unsteady flows is becoming more and more important in the field of hydraulic turbomachinery applications, both for stable and especially for unstable operating conditions and other transient phenomena. Unsteady Reynolds-averaged Navier-Stokes (RANS) methods for incompressible flows, implemented via finite volume or finite element approaches, reach their limit in both accuracy and acceptable computational time. The accuracy mainly suffers due to the strong dependence of the solution on turbulence model variants. Furthermore, incompressibility of the flow and the linked implicit pressure equation makes large eddy simulations (LES) of Navier-Stokes-based methods for such flows prohibitively expensive. An alternative to Navier-Stokes fluid solvers are methods based on the solution of the discrete Boltzmann equation, so-called Lattice Boltzmann Methods (LBM). The purpose of this work is to present first results of an ongoing project involving the simulation of unsteady hydraulic turbomachines at high Reynolds numbers. For this purpose, the ERCOFTAC pump impeller was selected as a test case. Results obtained with the current implementation of the in-house code suggest, that the present methodology could provide a viable alternative for the analysis of unsteady flow phenomena in hydromachines.

Experimental Investigation of a Large Tilting-Pad Journal Bearing—Comparison of a Flooded and Non-Flooded Design

In tilting-pad journal bearings (TPJB), power loss corresponds to the internal friction in the shearing of the oil. Besides the lubrication gap, intermediate spaces between the pads account for a notable amount of frictional losses. Against the background of increasing demands for efficiency and sustainable use of resources, the reduction of power loss takes a key position in the further development of bearings. In our research, we compare two bearing lubrication concepts of a five-pad TPJB. Our objective is to work out the influence of different lubrication methods and bearing housing designs on the bearing operation characteristics. We conduct experimental testing of a 500 mm TPJB in two different bearing configurations with respect to the lubrication concept: an oil-flooded and non-flooded bearing design. In the flooded bearing design, oil is supplied via spray-bars and axial seals ensure the inter-pad spaces to be completely filled with oil. The non-flooded design comes without axial seals but oil drain channels to avoid oil accumulation in the bearing. In the latter design, oil is fed in via leading edge grooves (LEG). For the non-flooded bearing design, the experimental data show that the unloaded pads are not completely filled with oil and therefore, no pressure build-up occurs. The absence of additional load on the lower pads compared to the flooded design results in an increase of minimum film thickness. With the non-flooded design, power loss at high speeds is reduced to almost half. As a result, the efficiency of the entire turbomachinery application can be considerably improved.

Progresses in Particle-Laden Flows Simulations in Multistage Turbomachinery With OpenFOAM

Abstract Numerical simulations of particle-laden flows have received growing attention in the last decade, due to the broad spectrum of industrial applications in which discrete phases prediction is of interest. Among these, ingestion of particles by turbomachinery is an area that is seeing vivid research and studies. The most common technique to tackle this kind of problem is the Eulerian–Lagrangian method, in which individual particles are tracked inside the domain. At the same time, in multistage turbomachinery simulation interfaces are needed to couple the flow solution in adjacent domains in relative motion. In this work, an open-source extension for Lagrangian simulations in multistage rotating machines is presented in the foam-extend environment. First, a thorough discussion of the implementation is presented, with particular emphasis on particle passage through general grid interfaces (GGI) and mixing planes. Moreover, a mass-conservative particle redistribution technique is described, as such a property is requested at interfaces between multiple reference frame (MRF). The peculiarities of the algorithm are then shown in a relevant test case. Eventually, three turbomachinery applications are presented, with growing complexity, to show the capabilities of the numerical code in real-life applications. Simulation results in terms of erosion and impacts on aerodynamic surfaces are also presented as examples of possible parameters of interest in particle-laden flow computations.

A Conceptual Alternative Machine Learning-Based Method for Mesh Sensitivities Calculation in a Turbomachinery Blades Optimisation Framework

In gradient-based aerodynamic optimisation, the functional gradient required by the optimiser can be obtained as a product of adjoint-based functional sensitivities to volume grid nodes and the volume mesh sensitivities to the design parameters. For turbomachinery applications, it is desirable to use the actual blade design variables as degrees of freedom for the optimisation process, but this can lead to tedious programming tasks. As an alternative, a Machine Learning (ML) model is created to mimic and differentiate the blade geometry and the mesh generation processes. The typical ML forward pass is followed by a back differentiation operation enabling the computation of the volume mesh derivatives with respect to design parameters. The model is tested by comparing the ML-predicted and reference grid, and the modelled sensitivities are verified through algorithmic differentiation. The modelled mesh sensitivities are successfully employed for adjoint-based reverse engineering and design optimisation problems on a turbine blade.

Flow in Pillow-Plate Channels for High-Speed Turbomachinery Heat Exchangers

In numerous turbomachinery applications, e.g., in aero-engines with regenerators for improving specific fuel consumption (SFC), heat exchangers with low-pressure loss are required. Pil low-plate heat exchangers (PPHE) are a novel exchanger type and promising candidates for high-speed flow applications due to their smooth profiles avoiding blunt obstacles in the flow path. This work deals with the overall system behavior and gas dynamics of pillow-plate channels. A pillow-plate channel was placed in the test section of a blow-down wind tunnel working with dry air, and compressible flow phenomena were investigated utilizing conventional and focusing schlieren optics; furthermore, static and total pressure measurements were performed. The experiments supported the assumption that the system behavior can be described through a Fanno–Rayleigh flow model. Since only wavy walls with smooth profiles were involved, linearized gas dynamics was able to cover important flow features within the channel. The effects of the wavy wall structures on pressure drop and Mach number distribution within the flow path were investigated, and a good qualitative agreement with theoretical and numerical predictions was found. The present analysis demonstrates that pressure losses in pillow-plate heat exchangers are rather low, although their strong turbulent mixing enables high convective heat transfer coefficients.

Nonlinear Vibration Analysis of Turbine Bladed Disks With Midspan Dampers

Abstract Friction dampers are one of the most common secondary structures utilized to alleviate excessive vibration amplitudes in turbomachinery applications. In this paper, the dynamic behavior of the turbine bladed disks coupled with one of the special damper designs, the so-called Midspan Dampers (MSDs) that is commonly used in steam turbines of Baker Hughes Company, is thoroughly studied. Friction between the blade and the damper is modeled through a large number of contact nodes by using two-dimensional contact elements with a variable normal load. In the solution procedure, the coupled static/dynamic Harmonic Balance approach is utilized for the first time in the assessment of the dissipation capability of MSDs, computationally shown by predicting the forced response levels of the system at different resonances. Moreover, it is demonstrated that the nonlinear dynamic response is non-unique and it may vary considerably even if all the user-controlled inputs are kept identical. This phenomenon is a novel observation for MSDs, and it is explained by an uncertainty present in the contact forces. Contact conditions corresponding to multiple responses are also investigated to unveil the different kinematics of the damper under the same nominal conditions.

Influence of emitter-receiver number on measurement accuracy in acoustic pyrometry

Acoustic pyrometry is an interesting technique that may find several useful applications in turbomachinery. As the speed of sound is directly related a medium temperature, this measurement technique estimates the temperature of a gas by considering the time of flight of an acoustic wave moving through it. If only an acoustic emitter-receiver couple is used, only the average temperature along the acoustic path can be determined. If multiple emitter-receiver couples laying on the same plane are used, a reconstruction of the temperature map in the section is possible. This estimation is performed by considering that multiple acoustic paths travel across the same sub-portions of the section and, therefore, the temperature of each sub-portion affects the time of flight along several sound paths. Many parameters affect the accuracy of the measurement, and they are related to the physic of the phenomena involved in the measurement, the accuracy of the instrumentation used, the interaction between the acoustic wave and the flow velocity and the hardware set-up. In this study, the impact of some set-up parameters on the accuracy of the measurement was investigated and, in particular, the number of sound emitter-receiver couples and the number of investigation sub-portions in which the section is divided. A reference temperature map has been considered as a benchmark. This study, which is a preliminary investigation on this technique, was useful to assess the capability of this methodology to correctly describe a temperature distribution in an ideal condition. Therefore, it represents a first step in the set-up of an experimental investigation with an acoustic pyrometer..

Application of oil and dye flow visualization in incompressible turbomachinery flows

Flow visualization is one of many available tools in experimental fluid mechanics and is used from the primary stages of fluid mechanics research in order to identify the physical sizes and locations of the flow features under consideration. Most of the fluids used in engineering applications are transparent (water, air, etc) and flow visualization techniques are used in order to make their flow patterns visible. Simple flow visualization experiments are relatively inexpensive and they can be easily implemented providing with a first feeling of the characteristics of the flow domain. Subsequently, flow visualization techniques are of great applicability in complex flow fields and especially in turbomachinery applications where the flow is characterized by three dimensional and secondary flow patterns. In general, fluid motion can be visualized by surface flow visualization, by particle tracer methods or by optical methods. The former flow visualization technique reveals the streamlines of fluid flows around a solid surface. In this paper flow visualization techniques applied in two different cases of experimental testing (fans in crossflow and cascade experiment) are presented. In both cases, the mixture of paint was prepared using a highly volatile light mineral or heavy machine oil of viscosities of approximately100cPand200cP, respectively, together with very fine pigments of Titanium Dioxide (TiO2) or fluorescein sodium in various colors. After the preparation of the mixture, a homogenous thin film was applied onto the whole plate surface by painting it with a soft brush. The air stream which flows over the surface of the plate, modifies the concentration and the homogeneity of the oil film, according to the flow conditions very close to the wall. The film was dried by the airflow and photographed for further consideration while the time taken for drying depended on the wind tunnel velocity as well as, on the pigmentation of the mixture. Successful and un-successful flow visualization tests are herein presented while each case is respectively commented as far as the mixtures, the proportions used and the application onto the rigs are concerned.

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.