Turbomachinery Applications Research Articles

Improved Prediction of Pump Performance Through the Rotation–Curvature Correction—An Experimental and Numerical Study of a Low-Specific-Speed Helico-Axial Compression Cell in Series

Abstract Within turbomachines, turbulence production and redistribution are affected by system rotation and streamline curvature. However, the most frequently used turbulence models do not account for these effects. In the present paper, we calibrate a rotation–curvature correction to the shear stress transport (SST) turbulence model to improve the accuracy of pump performance predictions through computational fluid dynamics (CFD) for a wide range of relative flow rates. The new formulation was achieved through comparison of experimental and numerical results obtained for a low-specific-speed (nondimensional specific speed ≈ 0.7) helico-axial compression cell in series. CFD results revealed secondary flows and strong rotor–stator interactions. Steady-state simulations with the standard SST turbulence model were unable to accurately predict pump performance because of such inherently unsteady features. Unsteady simulations improved the predicted performance, but the head coefficient was up to 10% higher than test results at part-load operation. Through calibration of a rotation–curvature correction, the error in the predicted head coefficient was essentially eliminated for relative flow rates above 50% relative flow. Below 47% relative flow, a rotating stall-phenomenon was identified. The stall cell propagated at a rate of 0.4 times the impeller angular frequency, and we identified a propagation mechanism related to a circumferential variation in impeller tip leakage flow (TLF) rate. The presented turbulence model formulation can improve performance predictions in turbomachinery applications where leakage flows are significant, and forms a basis for future work on extended modeling of increasingly complex operating conditions.

Rotationally Induced Local Heat Transfer Features in a Two-Pass Cooling Channel: Experimental–Numerical Investigation

Turbine blades for modern turbomachinery applications often exhibit complex twisted designs that aim to reduce aerodynamic losses, thereby improving the overall machine performance. This results in intricate internal cooling configurations that change their spanwise orientation with respect to the rotational axis. In the present study, the local heat transfer in a generic two-pass turbine cooling channel is investigated under engine-similar rotating conditions (Ro={0…0.50}) through the transient Thermochromic Liquid Crystal (TLC) measurement technique. Three different angles of attack (α={−18.5°;+8°;+46.5°}) are investigated to emulate the heat transfer characteristics in an internal cooling channel of a real turbine blade application at different spanwise positions. A numerical approach based on steady-state Reynolds-averaged Navier–Stokes (RANS) simulations in ANSYS CFX is validated against the experimental method, showing generally good agreement and, thus, qualifying for future heat transfer predictions. Experimental and numerical data clearly demonstrate the substantial impact of the angle of attack on the local heat transfer structure, especially for the radially outward flow of the first passage, owing to the particular Coriolis force direction at each angle of attack. Furthermore, results underscore the strong influence of the rotational speed on the overall heat transfer level, with an enhancement effect for the radially outward flow (first passage) and a reduction effect for the radially inward flow (second passage).

GTP-24-1666 - Temperature Margin Calculation During Finite Element Creep Simulations (GT2024-121776)

Abstract Creep deformation in turbomachinery applications is a highly non-linear phenomenon where +/- 15°C may halve or double the average rupture life. Even in well-controlled laboratory conditions, this stochastic nature means that identical tests may differ by a factor of two. Analysts using implicit finite element user creep subroutines may not appreciate the uncertainty of their solution, or the design changes required to account for uncertainty in either the boundary conditions or the solution itself. Designing to a maximum strain limit does not consider the sudden acceleration of tertiary creep rates. Applying a time factor may extend the simulation time by multiple factors. This paper presents a methodology where two estimations of creep damage are made in parallel to the creep simulation at actual operating temperatures. These two estimations consider the predicted change in stress relaxation, at higher or lower temperatures, to estimate the temperature change required to reach the onset of creep, at any given node in the model, and at any given time in the analysis. This margin calculation is expanded to consider the uncertainty in creep prediction. A time-based scatter factor is incorporated into the temperature margin calculation to provide a minimum temperature margin that includes model uncertainty. The analyst can also consider the uncertainty of the temperature prediction and boundary conditions to produce a robust creep prediction in a real-world simulation. The methodology is validated through the FEA of example cases and applied to a creep-limited 2nd stage turbine blade.

Dynamic Models of Mechanical Seals for Turbomachinery Application

One of the primary causes of mechanical face seal failure is rotor vibration. Traditional dynamic seal models often cannot fully explain failure mechanisms. The dynamic models of seals proposed in this paper, including those developed by the authors, are valuable for predicting seal dynamics during operation in specific turbomachinery and for explaining the causes of seal failure. The single-mass dynamic model is suitable for analyzing the dynamics of contact mechanical face seals and simply designed dry gas seals. The two-mass dynamic model is used to investigate the operational dynamics processes of classical dry gas seals under complex loading conditions. The three-mass dynamic model is used to study various complex types of mechanical face seals. This model can determine the normal operating condition range and explain leakage mechanisms in the presence of excessive rotor vibrations.

Modeling Stochastical Particle Rebound based on High Velocity Experiments

Abstract Solid particle erosion is a major deterioration process which contributes to performance deterioration of modern axial compressors. The prediction of this deterioration process requires the correct computation of particle movement through the machine and their resulting impacts on the effected components. Especially for particles with high Stokes numbers the movement is determined mainly by the particle wall interaction, which is described by coefficients of restitution. Today, they are derived from experiments featuring high particle velocities and target materials, which are representative for turbomachinery applications. In this study, an already published rebound model is optimized for particle materials and velocities within high pressure compressors. The statistical spread of the rebound experiment is evaluated and the implementation into the rebound model is shown, which improves the prediction capability of the model. The model is implemented into a CFD software and numerical simulations are performed. The model is applied to a cylinder test specimen within a sand blast facility. The simulation shows the importance of the stochastics of the rebound, which is often neglected in particle wall models. Moreover, the numerical study shows requirements for the test specimen and its positioning in the experimental setup, which are prerequisites for the derivation of the coefficients of restitution using two dimensional particle evaluation equipment.

A Review on the Dynamic Performance Studies of Gas Foil Bearings

Gas foil bearings have important and wide applications in high-speed turbomachinery, generating low frictional lubricating gas film in series with underlying elastic foil structures to support the rotor system. Their dynamic performance is of vital significance in maintaining the rotor stability as well as in depressing rotor vibrations. This paper conducts a comprehensive review on dynamic performance studies conducted on gas foil bearings, including research on the dynamic stiffness and damping coefficients, bearing stability, nonlinear vibrations of the rotor–bearing system, and active methods for controlling rotor dynamic motions. This review provides clear observations towards the developments and iterations of the models, methods, and experiments of these studies.

1D analytic and numerical analysis of thin film heat transfer gauges and infra-red cameras in non-planar applications

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 with planar surfaces and does not accurately model the true geometric behaviour. This paper evaluates the error introduced by the planar assumption and outlines the required modifications for accurate freeform surface analysis. Adapted cylindrical basis functions are defined for the impulse response method and used to evaluate the impact of the 1D planar assumption. The analytic solutions for both convex and concave surfaces are presented. A penta-diagonal algorithm, for a modified numerical Crank-Nicolson scheme, is also evaluated for fast stable implementation of curved geometry simulation. The scheme shows comparable performance to the impulse response, whilst removing the requirement for linear time invariance. The new methods are demonstrated in the case of aerothermal analysis for heat transfer in a turbine nozzle guide vane. A 3D ANSYS simulation is used as a benchmark and further highlights the importance of the curvature effects. The methods are extended using curvature mapping for infra-red camera data, enabling direct 3D thermal simulation or the fast calculation of freeform curvature. This paper defines the methodology to analyse heat transfer measurements on non-planar geometry.

Analysis of tensile strength of thin ABS specimens 3-D printed in various build orientations for turbomachinery applications

Analysis of tensile strength of thin ABS specimens 3-D printed in various build orientations for turbomachinery applications

Using Fluid Curtains to Improve Sealing Performance in Turbomachinery Applications

Abstract The results from an investigation into the physics of how fluid curtains can be applied to improve the aerodynamic performance of conventional turbomachinery shaft and rotor seals are described in this paper. Computational fluid dynamics and testing on two experimental facilities are used in the study. In the first part of the work, computational fluid dynamics simulations validated against experimental test data demonstrate the fundamental mechanism by which the presence of the curtain can act to reduce leakage flow through conventional seals. These results are consolidated into a single performance carpet map, showing how the leakage reduction performance and the curtain supply pressure needed to achieve it vary with changes in values of key geometrical parameters. In the second part of the work the effect of swirl in the seal inlet flow, as is often encountered in turbomachinery applications, on the performance of the fluid curtain is investigated experimentally. Test results show that if the swirl momentum in the inlet flow is greater than the momentum of the curtain flow, the performance benefit from applying the curtain is greatly diminished. Overall, the results provide some fundamental design rules for applying fluid curtains to enhance turbomachinery sealing performance for the general type of leakage path geometry (cylindrical channel, 45-deg jet angle, curtain upstream of a conventional seal) and working fluid type and conditions (air, ambient temperature, subsonic leakage channel flow) used in the study.

Development and Evaluation of Generic Test Pieces for Creep Property Assessment of Laser Powder Bed Fusion Components

Abstract Metal laser powder bed fusion (PBF-LB/M) allows for high degrees of design freedom and the manufacture of high-temperature Ni-based materials, such as IN738LC. The PBF-LB/M microstructure is dependent on several factors, including process parameters, component geometry, build orientation and postprocessing steps (e.g., heat treatment). The correlation between the resulting microstructure and these parameters is material specific and not yet fully understood. In this study, the development of a specimen extraction cube (SEC), based on a generic component with design aspects related to turbomachinery applications, is presented. The SEC allows for the extraction of three samples, one for each of the build orientations: 0 deg (perpendicular to build direction), 45 deg (diagonal) and 90 deg (parallel to build direction). Specimens extracted from the SEC are mechanically tested and compared to witness samples manufactured in 0 deg, 45 deg, and 90 deg build orientation. Particular focus is placed on correlating measured properties and their variations with heat treated microstructures. Creep testing was performed using 240 MPa and a temperature of 850 °C. Microstructural differences and hence differences in mechanical properties are found in extracted and witness samples.

Parametric study for model calibration of a friction-damped turbine blade with multiple test data

Model updating using multiple test data is usually a challenging task for frictional structures. The difficulty arises from the limitations of nonlinear models which often overlook the uncertainties inherent in contact interfaces and in actual test conditions. In this paper, we present a parametric study for the model calibration process of a friction-damped turbine blade, addressing the experimentally measured response variability in computational simulations. On the experimental side, a recently developed test setup imitating a turbomachinery application with mid-span dampers is used. This setup allows measuring multiple responses and contact forces under nominally identical macroscale conditions. On the computational side, the same system is modeled in a commercial finite element software, and nonlinear vibration analyses are performed with a specifically developed in-house code. In numerical simulations, the multivalued nature of Coulomb’s law, which stems from the inherent variability range of static friction forces in permanently sticking contacts, is considered to be the main uncertainty. As the system undergoes vibration, this uncertainty propagates into the dynamic behavior, particularly under conditions of partial slip in contacts, thus resulting in response variability. A deterministic approach based on an optimization algorithm is pursued to predict the limits of the variability range. The model is iteratively calibrated to investigate the sensitivity of response limits to contact parameters and assembly misalignment. Through several iterations, we demonstrate how uncertain initial contact conditions can be numerically incorporated into dynamic analyses of friction-damped turbine blades. The results show a satisfactory level of accuracy between experiments and computational simulations. This work offers valuable insights for understanding what influences test rig response and provides practical solutions for numerical simulations to improve agreement with experimental results.

Low-Noise Airfoils for Turbomachinery Applications: Two Examples of Optimization

Low-Noise Airfoils for Turbomachinery Applications: Two Examples of Optimization

PDMS diaphragm based miniature fiber-optic Fabry-Perot dynamic pressure sensor for turbomachinery application.

Small-sized, highly sensitive dynamic pressure sensors are crucial in the field of turbomachinery application. In this paper, a fiber-tip structure dynamic pressure sensor utilizing a small piece of glass tube as the air cavity and PDMS material as the diaphragm was fabricated. It has the advantage of being small in size with the diameter of 125µm while having high sensitivity of 26.26pm/kPa. The fabrication process was described in detail, which is simple and cost-effective. The sensor characteristics were studied theoretically and experimentally. Quasi-square pressure signal of different frequencies generated by the siren disk were measured by the sensor and compared with that obtained from the commercial piezoresistive pressure sensor to verify the accuracy of the proposed sensor. The R2 of the four pairs of pressure waveforms were 0.94, 0.81, 0.93, and 0.96, respectively. Stability testing of the sensor was also performed, showing that the sensor can work reliably under dynamic pressure environment.

Identification of Dynamic Force Coefficients for an Additively Manufactured Hermetic Squeeze Film Bearing Support Damper Utilizing a Pass-Through Channel

Abstract The following paper presents breakthrough experimental results for a new hermetic squeeze film damper (HSFD) concept that is integrally designed within an externally pressurized tilting-pad radial gas bearing support. The flexibly damped gas bearing module was designed for a 7.2″ (183 mm) diameter shaft and fabricated using direct metal laser melting (DMLM); also known as additive manufacturing. The bearing and HSFD were sized based on ongoing studies for oil-free supercritical carbon dioxide (sCO2) power turbines in the 8.5 MW–10 MW power range. The development of the new damper concept was motivated by past dynamic testing on HSFD, which generated frequency-dependent stiffness and damping force coefficients. In efforts to eliminate the frequency dependency, a new HSFD architecture was conceived that adds accumulator volumes and a pass-through channel to previously conceived HSFD flow network designs. The other motivation for the work is the need to develop a cost-effective and reliable oil-free bearing technology that is scalable to large power turbomachinery applications. There were several objectives for the following work. The first objective was to successfully design and fabricate a single piece bearing-damper using additive manufacturing, while dimensionally controlling critical design features. The paper discusses the manufacturing steps and shows cut-ups that reveal adequate clearance control capability with internal damper clearances. The second objective was to perform experimental testing with the new HSFD design in efforts to extract stiffness and damping coefficients for excitation frequencies within 20–160 Hz and peak vibration amplitudes between 0.25 mils (6.35 microns) to 1 mil (25.4 microns). The test results for a single HSFD bearing module indicated that the design modifications to the HSFD architecture were successful in eliminating nearly all the frequency dependencies for the stiffness force coefficient. The dynamic tests yielded a stiffness coefficient that varied between 112 klb/in. (19.6 MN/m) and 96 klb/in. (16.8 MN/m). The damping force coefficient however, exhibited relatively more variation with frequency with values residing between 175 lb-s/in. (31 kN-s/m) to 214 lb-s/in. (37 kN-s/m). Finally, the paper advances a three-dimensional fluid-structure interaction (FSI) model using transient finite element analysis (FEA) coupled to a computational fluid dynamics (CFD) model. The FSI analysis performed between 20 Hz and 80 Hz was used to predict the stiffness and damping of the HSFD using a quarter-section model of the damper. The FSI analysis was able to support test results by showing only a 6–7.4% change in the magnitude of force coefficients. Stiffness predictions agree reasonably well with experiments whereas damping is underpredicted.

Numerical heat transfer investigation of a single impingement jet with combined V-ribs and preliminary optimization of V-rib configuration in initial crossflow

The heat transfer performance of a single jet impingement configuration with V-ribs on the impingement plate and V-ribs on the target plate was numerically investigated for turbomachinery applications. The geometrical parameters of the impingement wall ribs were kept constant. The effects of the height and the rib-to-jet distance of the target wall ribs were analyzed in detail. The studied jet Reynolds number was Re = 35,000, the separation distance was H/D = 3, and the velocity of jet-to-crossflow ratio was R = 1–5. Based on the numerical results, taking the heat flow rate as the objective, the rib geometry was optimized by the Differential Evolution (DE) algorithm. The optimized rib configuration was then numerically simulated to validate whether the DE algorithm can be utilized for similar applications. The results of the optimized configuration were compared to those of the sample cases to verify the effectiveness of the optimization. The pressure differences between the inlet of the main jet and the outlet, and between the inlet of the initial crossflow and the outlet were computed to evaluate the pressure loss introduced by the ribs. Thus, this work provides another perspective on designing high efficient anti-crossflow cooling configurations for turbomachinery applications.

A Numerical Test Rig for Turbomachinery Flows Based on Large Eddy Simulations With a High-Order Discontinuous Galerkin Scheme—Part II: Shock Capturing and Transonic Flows

Abstract In the second paper of this three-part series, we focus on the simulation of transonic test cases for turbomachinery applications using a high-order discontinuous Galerkin spectral element method (DGSEM). High-fidelity simulations of transonic compressors and turbines are particularly challenging, as they typically occur at high Reynolds numbers and require additional treatment to reliably capture the shock waves characterizing such flows. A recently developed finite-volume subcell shock capturing scheme tailored for the DGSEM is applied and evaluated with regard to the shock sensor. To this end, we conduct implicit large eddy simulations of a high-pressure turbine cascade from the public literature and a transonic compressor cascade measured at the German Aerospace Center, both at a high Reynolds number above 106. Based on the results, we examine modal-energy and flow-feature based shock indicator functions, compare the simulation data to experimental and numerical studies, and present an analysis of the unsteady features of the flows.

Validation of Filtered Rayleigh Scattering Optical Rake Measurement Techniques in Turbomachinery Applications and Boundary Layers

Abstract Filtered Rayleigh scattering (FRS) is a non-intrusive, laser-based optical technique for measuring three-component velocity, static temperature, and static density with high spatial resolution and low uncertainty. FRS can be used to derive total values as well as turbomachinery efficiencies. The Virginia Tech team has been developing this seedless technique for simultaneous planar (or line) measurements to overcome the limitations associated with seed-based laser measurement techniques such as laser Doppler velocimetry (LDV), particle image velocimetry (PIV), and Doppler global velocimetry (DGV) as well as limitations with physical probe rakes such as blockage and wake production. This technique is especially attractive in flow cases or environments where the aforementioned seed-based laser measurement techniques are limited or not possible. A combination of specially designed boundary layer total pressure probe rake measurements, FRS optical rake measurements, and computational fluid dynamics (CFD) results in the inlet of a Honeywell TFE731-2 turbofan are presented. Results show that all three techniques (FRS, probe, and CFD) match within approximately 2% root-mean-square error (RMSE). Inlet efficiency was derived and found to be within 2.3% difference for all three techniques.

Ceramic and Metal Additive Manufacturing of Monolithic Rotors From SiAlON and Inconel and Comparison of Aerodynamic Performance for 300W Scale Microturbines

Abstract The gas turbine industry is continuously developing and testing new materials and manufacturing methods to improve the performance and durability of hot section components, which are subjected to extreme conditions. SiAlON and Inconel 718 are especially desirable for turbomachinery applications due to their high strength and high-temperature capabilities. To demonstrate the viability of additive manufacturing for small-scale turbomachinery for 300W scale microturbines, a monolithic rotor with a design speed of 450,000 RPM containing radial turbine and compressor was developed considering additive manufacturing constraints. The geometry was manufactured from SiAlON and Inconel 718 using lithographic ceramic manufacturing and selective laser melting, respectively. The additive manufacturing and thermal process parameters as well as material characterization are described in detail. Surface and computerized tomography scans were conducted for both rotors. While the metallic rotor showed undesirable printing artifacts and a large number of defects, the ceramic part achieved a level of relative precision and surface quality similar to large-scale production via casting. To compare turbomachinery performance, an aerodynamic test facility was developed allowing to measure pressure ratios and efficiency of small compressors. The rotors were tested in engine-realistic speeds, achieving a compressor rotor pressure ratio of 2.2. The ceramic part showed superior efficiency and pressure ratio compared to the Inconel rotor. This can be explained by lower profile and incidence losses due to a higher fidelity physical representation of the model geometry and better surface finish.

Comparison of LDA and hot-wire measurements at moderate subsonic mach numbers

A detailed comparison of Hot-Wire-Anemometry (HWA) and Laser-Doppler-Anemometry (LDA) measurements in the wake of an airfoil is presented. An evaluation of the capabilities of both techniques in measuring three-dimensional turbulence in turbomachinery applications is made and potential application pitfalls are highlighted. It is shown that the HWA tends to underestimate the turbulence intensity due to damping effects, while the LDA tends to overestimate the turbulence intensity due to the optical setup. The wake flow of a single airfoil at inflow Mach numbers of 0.35 and 0.45 and corresponding Reynolds numbers of 480,000 and 630,000 are used as a reference case. Different HWA probe head designs and various wire diameter are considered, as well as different setups for the LDA measurement device. In a comparison of the two techniques, the turbulence intensity measured differs by up to <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mn>1.3</mml:mn></mml:math></inline-formula> percentage points (pp) in the freestream region and 4.0 pp within the wake. This is primarily due to the resolution of high-frequency fluctuations. For both measuring techniques, potential sources of errors are highlighted, especially regarding the application to turbomachinery flows.

Influences of Laser Incidence Angle and Wall Thickness on Additive Components

Abstract Additive manufacturing (AM), particularly laser powder bed fusion, is growing the ability to rapidly develop advanced cooling schemes for turbomachinery applications. However, to fully utilize the design and development opportunities offered through AM, impacts of the build considerations and processing parameters are needed. Prior literature has shown that specific build considerations such as laser incidence angle and wall thickness influence the surface roughness of additively made components. The objective of this technical brief is to highlight the effects of both laser incidence angle and wall thickness on the surface roughness and cooling performance in micro-sized cooling passages. Results indicate that for any given laser incidence angle, surface roughness begins to increase when the wall thickness is less than 1 mm for the cooling channels evaluated. As the laser incidence angle becomes further away from 90 deg, the surface roughness increases in a parabolic form. Laser incidence angle and wall thickness significantly impact friction factor, while there is less of an influence on the Nusselt number for additively manufactured microchannels.