bluuebunny/crossref_metadata_2025 huggingface dataset:
Multi-Disciplinary Optimization of a Turbocharger Compressor
Authors: Childs Edward , Kohr Stephen
Published: January 2021

The design of a turbocharger compressor must meet aerodynamic performance requirements, operate within specified stress and vibration limits, and respond quickly to changes in operating conditions. Design optimization must therefore include static, thermal and modal analysis (including weight and polar moment of inertia calculations) along with aerodynamic analysis (CFD). In some cases, a design optimized for aerodynamic performance only can be optimized separately to meet structural goals, using impeller backface geometry, bore radius and fillet radius inputs, which generally do not impact aerodynamic performance. If, however, impeller geometry inputs such as R1t-R1h, R2, B2 influence both aerodynamic and structural analysis, a coupled optimization is required, and each design must have both CFD and FEA analyses. In this study, a radial compressor with a vaneless diffuser at a single operating point is considered. The aerodynamic parameters for the impeller (BETA1H, BETA1S, BETA2S, main blade count, B2, R2, R1t) and diffuser (Pinch, R3/R2, Rex/R2) comprise in total 10 independent aerodynamic inputs. The aerodynamic objectives are to meet the operating point pressure ratio target and to maximize efficiency. The structural parameters for the backface (shoulder position, shoulder radius, web thickness at outer diameter (OD), OD angle, shoulder angle), bore radius and fillet radius comprise in total 7 independent structural inputs. The main structural objectives are to minimize the polar moment of inertia, and satisfy constraints on allowable maximum stress, deflection and the frequencies of blade vibration (flapping) modes. Successful multi-disciplinary optimization requires both CFD and FEA analysis to complete successfully for each trial design. Initial test runs of the optimization resulted in many geometries for which a valid CFD grid or FEA grid could not be generated. The high percentage of failed runs in the initial DOE impeded the construction of a viable surrogate model. A comprehensive investigation of all failure modes led to prescreening of both CFD and FEA geometry generation, using input constraints. The failure rate was greatly reduced as a result, leading to an improved search. Prior to the geometry screening, the optimizer found a large Pareto frontier between the efficiency and polar moment of inertia objectives. Following the screening, the efficiency and IP objectives became more cooperative. The optimization was carried out using Concepts NREC tools AxCent® and TurboOPT II™, NUMECA Fine/Turbo, and ESTECO modeFRONTIER®.
Squeeze Film Damper Suppression of Thermal Bow: Morton Effect Instability
Authors: Shin Dongil , Palazzolo Alan B. , Tong Xiaomeng
Published: January 2021

The Morton Effect (ME) is a synchronous vibration problem in turbomachinery caused by the non-uniform viscous heating around the journal circumference, and its resultant thermal bow and ensuing synchronous vibration. This paper treats the unconventional application of the SFD for the mitigation of ME-induced vibration. Installing a properly designed squeeze film damper (SFD) may change the rotor’s critical speed location, damping and deflection shape, and thereby suppress the vibration caused by the ME. The effectiveness of the SFD on suppressing the ME is tested via linear and nonlinear simulation studies employing a 3D thermo-hydrodynamic (THD) tilting pad journal bearing, and a flexible, Euler beam rotor model. The example rotor model is for a compressor that experimentally exhibited an unacceptable vibration level along with significant journal differential heating near 8,000 rpm. The SFD model includes fluid inertia and is installed on the non-drive end bearing location where the asymmetric viscous heating of the journal is highest. The influence of SFD cage stiffness is evaluated.
Torsional Fatigue of Heat-Treated Direct Metal Laser Sintered Inconel 718
Authors: Siddiqui Sanna F. , Gordon Ali P.
Published: January 2021

Rapid prototyping and design advancements in aviation gas turbine engine components are being made possible through the laser-fusion powder-bed additive manufacturing processes. However, with improvements in design, a comprehensive understanding of the fatigue performance of these components under realistic stress states characteristic of their in-service conditions is critical in ensuring their durability. Inconel 718 (IN718) is commonly employed as the substrate material for gas turbine engine components, and has been reported to yield improved axial mechanical properties when developed through the direct metal laser sintering (DMLS) process. However, the torsional fatigue properties of heat-treated Inconel 718 is yet unexplored. Given the nature of gas turbine engine components to endure multiaxial stresses (axial and torsional), this study is novel in that it reports the torsional fatigue properties of heat-treated DMLS Inconel 718. Completely reversed torsional fatigue tests under angle of twist control are performed on specimens manufactured along the horizontal, diagonal and vertical build orientations, providing the first set of torsional fatigue properties of additively manufactured heat-treated Inconel 718, contributing to a critical understanding of the performance of these components under cyclic shear.
Online Detection of Combustion Instabilities Using Supervised Machine Learning
Authors: McCartney Michael , Indlekofer Thomas , Polifke Wolfgang
Published: January 2021

In this paper we investigate how supervised machine learning can be leveraged to improve online prediction of thermoacoustic combustion instabilities using dynamic pressure readings. We discuss the current state of the art tools for the online detection of combustion instabilities, namely the Hurst exponent and Auto-Regressive model, the precursors they detect and how they go about doing so. We show that the generality of these tools comes at the cost of predictive power, which can be recovered using supervised machine learning. To demonstrate this we apply two different supervised machine learning approaches (using Hidden Markov Models and Automatic Machine Learning) to the classification of the state of a combustor given the dynamic pressure readings. We then observe the changes in predictive power when different information is added or removed from the signal. We find that the HMM based approach reduces to the AR model when the signal is normalised. We also find that the performance of a model trained on a signal transformed using the Detrended Fluctuation Analysis (DFA) can be met by a model trained on the Hurst exponent and the DFA transformation at a single (short) scale.
Reconstructing Compressor Non-Uniform Circumferential Flow Field From Spatially Undersampled Data: Part 1 — Methodology and Sensitivity Analysis
Authors: Lou Fangyuan , Key Nicole L.
Published: January 2021

The flow field in a compressor is circumferentially non-uniform due to the wakes from upstream stators, the potential field from both upstream and downstream stators, and blade row interactions. This non-uniform flow impacts stage performance as well as blade forced vibrations. Historically, experimental characterization of the circumferential flow variation is achieved by circumferentially traversing either a probe or the stator rows. This involves the design of complex traverse mechanisms and can be costly. To address this challenge, a novel method is proposed to reconstruct compressor nonuniform circumferential flow field using spatially under-sampled data points from a few probes at fixed circumferential locations. The paper is organized into two parts. In the present part of the paper, details of the multi-wavelet approximation for the reconstruction of circumferential flow and use of the Particle Swarm Optimization algorithm for selection of probe positions are presented. Validation of the method is performed using the total pressure field in a multi-stage compressor representative of small core compressors in aero engines. The circumferential total pressure field is reconstructed from 8 spatially distributed data points using a triple-wavelet approximation method. Results show good agreement between the reconstructed and the true total pressure fields. Also, a sensitivity analysis of the method is conducted to investigate the influence of probe spacing on the errors in the reconstructed signal.
Optimization of the Workflow of Multistage Axial Compressors Using Modern Gas Dynamics Computing Systems
Authors: Popov Grigorii , Goriachkin Evgenii , Baturin Oleg , Matveev Valerii , Egorov Igor , Marchukov Evgenii
Published: January 2021

Current developmental level of computers and numerical methods of gas dynamics makes it possible to optimize compressors using 3D CFD models. Design variants for the compressor can be automatically generated that best suit all the design requirements and limitations.

However, the methods and tool for optimizing compressors are not sufficiently developed for the successful application. The problems lie in the large size of the calculation model, the solution time and the requirements for computer resources.

In present study, a method for finding the optimal configuration of the blades of multi-stage axial compressors using 3D CFD modeling and commercial optimization programs as the main tools was developed. The basic parameters of the compressor (efficiency, pressure ratio, mass flow rate, etc.) can be improved using the created method correcting the shape of the blade profiles and their relative position. The method considers presence of various constraints.

When developing the method, special attention was paid to the creation of an algorithm for parameterizing the blade shape and a program based on it, which can automatically change the shape of the axial compressor blades. They were used by the authors during optimization as a tool that converts variable parameters into the “new” blade geometry.

Recommendations were also found on the rational settings for the CFD models used in the optimization of axial compressors.

The paper provides a brief overview of several works related to the optimization of multi-stage gas turbine axial compressors for various purposes (number of stages from 3 to 15), successfully performed using the developed method. As a result, an increase was achieved in efficiency, pressure ratio and stability margins.
Penetration and Mixing Characteristics of Inclined Jets in Crossflow at Low Momentum Flux Ratios
Authors: Vishwanath Rahul B. , Wabel Timothy M. , Steinberg Adam M.
Published: January 2021

This study investigates the factors affecting low momentum jets that are injected at an angle relative to a crossflow stream, which is relevant to film-cooling technologies. Quantitative measurements of the jet fluid concentration were obtained based on planar laser induced fluorescence (PLIF) from acetone vapor that was seeded into the jet. The jets were injected at four different axial locations downstream of the leading edge of a flat plate, resulting in different boundary layer thicknesses at the injection location. At each location, the jet-to-crossflow momentum flux ratio was varied from 0.5–5. The jet centerline trajectories were affected not only by the momentum flux ratios, but also by the approaching crossflow boundary layer thickness, with the jets penetrating the least for the thickest boundary layers. Measurements of the jet fluid concentration along the jet centerline showed an exponential decay rate of −1.3 across all cases. However, the behavior in the immediate vicinity of the jet depended on the boundary layer thickness, with thicker boundary layers resulting in a slower decay. Hence, the concentration profiles were shifted relative to the injection point depending on the injector position on the plate. The concentration profiles perpendicular to the jet axis were self-similar when scaled with the profile half-width.
Implications of Phase Change on the Aerodynamics of Centrifugal Compressors for Supercritical Carbon Dioxide Applications
Authors: Persico Giacomo , Toni Lorenzo , Gaetani Paolo , Bellobuono Ernani Fulvio , Romei Alessandro , Valente Roberto
Published: January 2021

Closed Joule-Bryton cycles operating with carbon dioxide in supercritical conditions (sCO2) are nowadays collecting a significant scientific interest, due to their high potential efficiency, the compactness of their components, and the flexibility that makes them suitable to exploit diverse energy sources. However, the technical implementation of sCO2 power systems introduces new challenges related to the design and operation of the components. The compressor, in particular, operates in a thermodynamic condition close to the critical point, whereby the fluid exhibits significant non-ideal gas effects and is prone to phase change in the intake region of the machine. These new challenges require novel design concepts and strategies, as well as proper tools to achieve reliable predictions.

In the present study, we consider an exemplary sCO2 power cycle with main compressor operating in proximity to the critical point, with an intake entropy level of the fluid lower than the critical value. In this condition, the phase change occurs as evaporation/flashing, thus resembling cavitation phenomena observed in liquid pumps, even though with specific issues associated to compressibility effects occurring in both the phases. The flow configuration is therefore highly nonconventional and demands the development of proper tools for fluid and flow modeling, which are instrumental for the compressor design. The paper discusses the modeling issues from the thermodynamic perspective and then highlighting the implications on the compressor aerodynamics. We propose tailored models to account for the effect of the phase change in 0D mean-line design tools as well as in fully 3D computational fluid-dynamic (CFD) simulations. In this way, a design strategy is build-up as a combination of mean-line tools, industrial design experience, and CFD for detailed flow analysis. The application of the design strategy reveals that the potential onset of the phase change might alter significantly the performance and operation of the compressor, both in design and in off-design conditions.
Experimental and Numerical Study of Laminar-Turbulent Transition on a Low-Pressure Turbine Outlet Guide Vane
Authors: Jonsson Isak , Deshpande Srikanth , Chernoray Valery , Thulin Oskar , Larsson Jonas
Published: January 2021

This work presents an experimental and numerical investigation on the laminar-turbulent transition and secondary flow structures in a Turbine Rear Structure (TRS). The study was executed at engine representative Reynolds number and inlet conditions at three different turbine load cases. Experiments were performed in an annular rotating rig with a shrouded low-pressure turbine upstream of a TRS test section. The numerical results were obtained using the SST k–ω turbulence model and the Langtry-Menter γ–θ transition model. The boundary layer transition location at the entire vane suction side is investigated. The location of the onset and the transition length are measured using IR-thermography along the entire vane span. The IR-thermography approach was validated using hot-wire boundary layer measurements. Both experiments and CFD show large variations of transition location along the vane span with strong influences from endwalls and turbine outlet conditions. Both correlate well with traditional transition onset correlations near midspan and show that the transition onset Reynolds number is independent of the acceleration parameter. However, CFD tends to predict an early transition onset in the midspan vane region and a late transition in the hub region. Furthermore, in the hub region, CFD is shown to overpredict the transverse flow and related losses.
Particle Image Velocimetry in a High-Pressure Turbine Stage at Aerodynamically Engine Representative Conditions
Authors: Inman Daniel , Cuadrado David G. , Andreoli Valeria , Fisher Jordan , Paniagua Guillermo , Aye-Addo Papa Aye N. , Bhatnagar Lakshya , Lozano Francisco , Meyer Terrence , Bloxham Matthew
Published: January 2021

Particle Image Velocimetry (PIV) is a well-established technique for determining the flow direction and velocity magnitude of complex flows. This paper presents a methodology for executing this non-intrusive measurement technique to study a scaled-up turbine vane geometry within an annular cascade at engine-relevant conditions. Custom optical tools such as laser delivery probes and imaging inserts were manufactured to mitigate the difficult optical access of the test section and perform planar PIV. With the use of a burst-mode Nd: YAG laser and Photron FASTCAM camera, the frame straddling technique is implemented to enable short time intervals for the collection of image pairs and velocity fields at 10 kHz. Furthermore, custom image processing tools were developed to optimize the contrast and intensity balance of each image pair to maximize particle number and uniformity, while removing scattering and background noise. The pre-processing strategies significantly improve the vector yield under challenging alignment, seeding, and illumination conditions. With the optical and software tools developed, planar PIV was conducted in the passage of a high-pressure stator row, at mid-span, in an annular cascade. Different Mach and Reynolds number operating conditions were achieved by modifying the temperature and mass flow. With careful spatial calibration, the resultant velocity vector fields are compared with Reynolds Averaged Navier Stokes (RANS) simulations of the vane passage with the same geometry and flow conditions. Uncertainty analysis of the experimental results is also presented and discussed, along with prospects for further improvements.
Experimental Heat Transfer and Boundary Layer Measurements on a Ceramic Matrix Composite Surface
Authors: Wilkins Peter H. , Lynch Stephen P. , Thole Karen A. , Quach San , Vincent Tyler
Published: January 2021

Ceramic matrix composites (CMCs) are quickly becoming more prevalent in the design of gas turbines due to their advantageous weight and thermal properties. While there are many advantages, the CMC surface morphology differs from that of conventional cast airfoil components. Despite a great deal of research focused on the material properties of CMCs, little public work has been done to investigate the impact that the CMC surface morphology has on the boundary layer development and resulting heat transfer. In this study, a scaled-up CMC weave pattern was developed and tested in a low speed wind tunnel to evaluate both heat transfer and boundary layer characteristics. Results from these experiments indicate that the CMC weave pattern results in augmented heat transfer and flow field properties that significantly vary locally when compared to a smooth surface.
Design of Stress Constrained SiC/SiC Ceramic Matrix Composite Turbine Blades
Authors: Boyle Robert J. , Gnanaselvam Pritheesh , Parikh Ankur H. , Ameri Ali A. , Bons Jeffrey P. , Nagpal Vinod K.
Published: January 2021

The structural and aerodynamic performance of a a low aspect ratio SiC/SiC CMC High Pressure Turbine blade was determined. The application was a NASA notional single aisle aircraft engine to be available in the N+3, beyond 2030, time frame. The notional rpm was maintained, and to satisfy stress constraints the annulus area was constrained. This led to a low span blade. For a given clearance low span blade are likely to have improved efficiency when shrouded. The efficiency improvement due to shrouding was found to strongly depend on the axial gap between the shroud and casing. Axial gap, unlike clearance or reaction, is not a common parameter used to correlate the efficiency improvement due to shrouding. The zero clearance stage efficiency of the low aspect ratio turbine was 0.920. Structural analyses showed that the rotor blade could be shrouded without excessive stresses. The goal was to have blade stresses less than 100 MPa (14.5 ksi) for the unshrouded blade. Under some not very restrictive circumstances, such as blade stacking, a one-dimensional radial stress equation accurately predicted area averaged Von Mises stress at the blade hub. With appropriate stacking radial and Von Mises stresses were similar.
On the Effect and Analysis of Fluid-Structural Mode Coupling
Authors: Sadeghi Mani , Yang Ming-Ta , Shang Huan-Min , Grover Eric
Published: January 2021

The consideration of aeromechanics plays a vital role in the design of machines that operate under aerodynamic forces, such as turbomachinery, and aircraft. The structure of those machines is subject to aeromechanical dynamics, including forced response and flutter. The strength of aeromechanical interaction depends on the level of coupling between flow and structure. One effect that can lead to strong coupling is the interaction between eigenmodes of the structure and eigenmodes of the flow near coincidence. This paper examines the impact of modal coincidence on the linear dynamic stability of aeromechanical systems for two illustrative canonical examples, one governed by inviscid acoustics, and one by the eigenmode of a wake. Three commonly used analysis techniques are applied and ranked for various levels of coupling: The 1-way coupled work-per-cycle method, a 2-way coupled non-linear modal FSI analysis in time, and an eigenanalysis of the 2-way coupled linear system, based on a state-space representation. It is demonstrated that all three methods agree for low to moderate levels of 2-way coupling, typical in turbomachinery applications. At higher levels, the work-per-cycle assessment is insufficient, whereas the FSI and eigenvalue analysis agree well.
Integrated Systems Simulation for Assessing Fuel Thermal Management Capabilities for Hybrid-Electric Rotorcraft
Authors: Roumeliotis Ioannis , Castro Lorenzo , Jafari Soheil , Pachidis Vassilios , De Riberolles Louis , Broca Olivier , Unlu Deniz
Published: January 2021

Future aircraft and rotorcraft propulsion systems should be able to meet ambitious targets and severe limitations set by governments and organizations. These targets cannot be achieved through marginal improvements in turbine technology or vehicle design. Hybrid-electric propulsion is being widely considered as a revolutionary concept to further improve the environmental impact of air travel. One of the most important challenges and barriers in the development phase of hybrid-electric propulsion systems is the Thermal Management System (TMS) design, sizing and optimization for addressing the increased thermal loads due to the electric power train.

The aim of this paper is to establish an integrated simulation framework including the vehicle, the propulsion system and the fuel-oil system (FOS) for assessing the cooling capability of the FOS for the more electric era of rotorcrafts. The framework consists of a helicopter model, propulsion system models, both conventional and hybrid-electric, and a FOS model. The test case is a twin-engine medium (TEM) helicopter flying a representative Passenger Air Transport (PAT) mission.

The conventional power plant heat loads are calculated and the cooling capacity of the FOS is quantified for different operating conditions. Having established the baseline, three different Power Management Strategies (PMS) are considered and the integrated simulation framework is utilized for evaluating FOS temperatures. The results highlight the limitations of existing rotorcraft FOS to cope with the high values of thermal loads associated with hybridization for the cases examined. Hence, new ideas and embodiments should be identified and assessed. The case of exploiting the fuel tank as a heat sink is investigated and the results indicate that recirculating fuel to the fuel tank can enhance the cooling capacity of conventional FOS.
Analysis of Transonic Bladerows With Non-Uniform Geometry Using Spectral Method
Authors: Wang Feng , di Mare Luca
Published: January 2021

Turbomachinery blade rows can have non-uniform geometries due to design intent, manufacture errors or wear. When predictions are sought for the effect of such non-uniformities, it is generally the case that whole assembly calculations are needed. A spectral method is used in this paper to approximate the flow fields of the whole assembly but with significantly less computation cost. The method projects the flow perturbations due to the geometry non-uniformity in an assembly in Fourier space, and only one passage is required to compute the flow perturbations corresponding to a certain wave-number of geometry variation. The performance of this method on transonic blade rows is demonstrated on a modern fan assembly. Low engine order and high engine order geometry non-uniformity (e.g. “saw-tooth” pattern) are examined. The non-linear coupling between the flow perturbations and the passage-averaged flow field is also demonstrated. Pressure variations on the blade surface and the potential flow field upstream of the leading edge from the proposed spectral method and the direct whole assembly solutions are compared. Good agreement is observed on both quasi-3D and full 3D cases. A lumped approach to compute deterministic fluxes is also proposed to further reduce the computational cost of the spectral method. The spectral method is formulated in such a way that it can be easily implemented into an existing harmonic flow solver by adding an extra source term, and can be potentially used as an efficient tool for aeromechanical and aeroacoustics design of turbomachinery blade rows.
Numerical Investigation of a Centrifugal Compressor for Supercritical CO2 Cycles
Authors: Karaefe Renan Emre , Post Pascal , Sembritzky Marwick , Schramm Andreas , di Mare Francesca , Kunick Matthias , Gampe Uwe
Published: January 2021

In this work, the performance characteristics and the flow field of a centrifugal compressor operating with supercritical CO2 are investigated by means of three-dimensional CFD. The considered geometry is based on main dimensions of the centrifugal compressor installed in the supercritical CO2 compression test-loop operated by Sandia National Laboratories. All numerical simulations are performed with a recently developed in-house hybrid CPU/GPU compressible CFD solver. Thermodynamic properties are computed through an efficient and accurate tabulation technique, the Spline-Based Table Look-Up Method (SBTL), particularly optimised for the applied density-based solution procedure. Numerical results are compared with available experimental data and accuracy as well as potentials in computational speedup of the solution method in combination with the SBTL are evaluated in the context of supercritical CO2 turbomachinery.
Numerical Investigation of a Partially Loaded Supersonic ORC Turbine Stage
Authors: Ziaja Karl , Post Pascal , Sembritzky Marwick , Schramm Andreas , Willers Ole , Kunte Harald , Seume Jörg , di Mare Francesca
Published: January 2021

The Organic Rankine Cycle (ORC) represents an emerging technology aimed at exploiting lower temperature heat sources, like waste heat in industrial processes or exhaust heat in combustion engines. One key aspect of this technology is an efficient and economical operation at part load, typically realized by a partial admission control, which is challenging to predict numerically. Full annulus computation can only be avoided applying empirical partial admission loss models to conventional full-admission computations.

This article aims at assessing the reliability of such a loss model under real-gas and supersonic conditions as a first step towards knowledge-based improved loss models.

Three different operating points of an 18.3 kW ORC turbine working with an ethanol-water mixture with two open stator passages (2 × 36°) are considered. Full annulus CFD computations are compared to experimental data and results of simulations in a conventional, full admission, periodic 72°-sector model with application of a 1D partial admission loss model. The experimentally obtained mass flow rate and efficiency are matched overall within their measurements accuracy. By highest inlet total pressure, the computed efficiency deviates about 4 % from the experiments. Predictions of efficiency based on the full admission and loss model correction deviate from full annulus computations less than 1 %. These findings suggest that the used empirical correlations for partial admission losses can provide acceptable results in the configuration under investigation.
Retracted: “Turbocharger Radial Turbine Response to Pulse Shape Under Realistic Operating Conditions” [ASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition, Volume 2E: Turbomachinery, Virtual, Online, September 21–25, 2020, Conference Sponsors: International Gas Turbine Institute, ISBN: 978-0-7918-8410-2, Copyright © 2020 by ASME. Paper No. GT2020-15237, V02ET41A025; 14 pages; doi: 10.1115/GT2020-15237]
Authors: ASME
Published: January 2021

This paper was removed from publication at the author’s request. February 18, 2021.

Copyright © 2021 by ASME
Computational Cost Reduction of MIMO Controllers for Active Magnetic Bearing Systems
Authors: Sahinkaya Alican , Hawkins Larry , Sawicki Jerzy T.
Published: January 2021

Generalized MIMO controllers such as H∞ and μ have not been widely adopted in the magnetic bearing industry, partially due to high computational cost relative to simpler SISO schemes. Computational cost is important to industrial magnetic bearing vendors as their controller hardware is based on embedded processors that have limited bandwidth. Studies to mitigate the problem of high order controllers show the limit of the existing methods in order reduction while still maintaining satisfying robust performance. A novel method is proposed to reduce the computational cost of robust controllers by identifying the bounds in their dynamic response, such that an implementation of a controller within those bounds results in the robust performance. The bounds are used to develop two computational cost reduction schemes for controller implementation, i.e., 1) identifying a dual-rate implementation of a single-rate controller which uniformly reduces the computational cost via interlacing technique, and 2) redesign of a controller by identifying its negligible dynamics based on the identified bounds in the controllers’ dynamic response. The results of both approaches are demonstrated on two active magnetic bearing (AMB) systems, a model of a 300 kW turbine generator with permanent magnet biased AMBs and an experimental high-speed AMB machining spindle. μ-synthesis controllers are designed for both systems and the proposed method and schemes are applied accordingly. The comparison of standard implementation of the synthesized controllers and the proposed new implementations is presented. The results demonstrate considerable reduction in the computational cost in terms of required number of multiply-accumulate (MAC) operations.
Comparison of Endwall Loss Reduction Techniques in a High-Lift Turbine Passage
Authors: Marks Christopher R. , Fletcher Nathan , Sondergaard Rolf
Published: January 2021

The development of techniques that reduce the losses in the endwall region is an important area of research as it supports an increase in the turbine design space through the use of higher lift blade designs while maintaining high efficiency. Several active and passive shape contouring methods that reduce losses generated by the secondary flow have been developed and investigated on a high-lift front-loaded LP turbine research profile in a low-speed linear cascade configuration. This paper summarizes and compares alterations to the three-dimensional secondary flow field by the application of three different techniques: blade profile contouring, optimized endwall shape contouring, and localized low mass coefficient jets. Each method was applied to identical research blade profiles and compared in the same linear cascade wind tunnel resulting in a unique perspective into the loss reduction mechanisms associated with each technique. The design strategy will be discussed along with a detailed description of changes to the secondary flow field using in- and out-of-passage total pressure loss measurements and high-speed stereoscopic particle image velocimetry. The key findings include the loss reduction mechanisms associated with each approach and the manipulation of key endwall flow structures such as the passage vortex and a strong suction surface corner separation.
Generalized Model for the Approximation of Coupled Acousto-Mechanical Natural Frequencies in High-Pressure Centrifugal Compressors
Authors: Heinrich Christoph Rocky , Kühhorn Arnold , Steff Klaus , Petry Nico
Published: January 2021

The oil and gas, chemical, and process industries employ centrifugal compressors for a wide range of applications. Due to this, the conditions under which centrifugal compressors have to operate, vary significantly from case to case. Gas pipeline compressors, for example, may feature discharge pressures well over 100 bar. In other fields of application, like gas injection, which is used to enhance oil recovery, this quantity can reach considerably higher values. Here, discharge pressures over 600 bar and gas densities over 300 kg/m3 are not uncommon. During the last several decades, comprehensive research was conducted on the impact of high pressure operating conditions on the vibrational behavior of centrifugal compressor wheels. Nowadays, it is well-known that an increase in gas pressure levels leads to a more pronounced interaction between the side cavities and the impeller, which results in a frequency shift of the acoustic and structural modes.

For the safe operation of compressors, it is necessary to predict these coupled natural frequencies accurately. The state-of-the-art approach to achieve this objective is the finite element method. While this technique provides high-quality results, the simulation of acousto-mechanical systems is still a time-consuming process that incurs high computational costs. Therefore, finite element models are, in this case, not suitable for probabilistic studies, sensitivity analyses, and comprehensive simulations of the full operating range of the compressor. In 2013, Magara proposed a simplified model based on an annular plate between two cylindrical cavities to solve this problem. While this method reduces the required computational effort significantly, its use is limited to platelike impellers.

The authors of the current paper propose a more generalized method to overcome the challenges mentioned above. It uses the uncoupled structural and acoustic modes of the actual impeller and side cavities in a modal superposition to approximate the natural frequencies of the coupled acousto-mechanical system. In this way, the intended design geometries of the impeller and side cavities are considered while maintaining the advantages of Magara’s model regarding the computational effort. In a numerical study, Magara’s method and the generalized model are applied to different systems of increasing complexity. The investigation starts with a simple annular plate in a cylindrical cavity and ends with an actual compressor impeller. At every complexity level, the results of both approaches are compared to a finite element analysis. Moreover, measurement data of a simplified rotor in a cylindrical cavity is used to validate the numerical models. Finally, the paper concludes with a discussion of the limitations and benefits of all employed numerical methods.
Effects of Circumferential Nonuniform Tip Clearance on Flow Field and Performance of a Transonic Turbine
Authors: Zheng Yun , Jin Xiubo , Yang Hui , Gao Qingzhe , Xu Kang
Published: January 2021

The numerical study is performed by means of an in-house CFD code to investigate the effect of circumferential nonuniform tip clearance due to the casing ovalization on flow field and performance of a turbine stage. A method called fast-moving mesh is used to synchronize the non-circular computational domain with the rotation of the rotor row. Four different layouts of the circumferential nonuniform clearance are calculated and evaluated in this paper. The results show that, the circumferential nonuniform clearance could reduce the aerodynamic performance of the turbine. When the circumferential nonuniformity δ reaches 0.4, the aerodynamic efficiency decreases by 0.58 percentage points. Through the analysis of the flow field, it is found that the casing ovalization leads to the difference of the size of the tip clearance in the circumferential direction, and the aerodynamic loss of the position of large tip clearance is greater than that of small tip clearance, which is related to the scale of leakage vortex. In addition, the flow field will become nonuniform in the circumferential direction, especially at the rotor exit, which will adversely affect the downstream flow field.
Impact of Secondary Air Flow on Combustor Emissions
Authors: Mohammad Bassam S. , Volk Brian , McManus Keith
Published: January 2021

It is a common practice to relate emissions performance of Dry Low Emissions (DLE) combustion systems to the flame temperature that is estimated from the mass flows of air and fuel flowing through the premixer. In many combustion systems, the exit temperature (or turbine nozzle inlet temperature) is quite low and is not a good parameter for estimating combustion emissions. The difference between the combustion flame temperature and exit temperature is mainly due to secondary air dilution. To our knowledge there are no detailed published data that quantify the impact of this temperature difference on combustion emissions. The target of this study is to quantify the impact of secondary air variation on emissions, both globally and locally. High pressure experiments are conducted at H class gas turbine operating conditions using a DLE combustion system. In the context of this DLE system, secondary air refers to cooling and leakage flows because direct air dilution of the combustion gasses is not necessary. This is because the flame stabilized downstream of the premixer is well mixed and fuel-lean. With NOx requirements moving toward single digit (ppm) levels, it becomes essential to accurately quantify the impact of reducing the secondary air percentage on emissions performance. In addition to the need to carefully study the impact of local interaction of the secondary air with the flame. The combustion system is configured with two independently controlled mixers along with a variable secondary air circuit that can change the secondary air fraction from 14 to 8%. Multiple emissions rakes are used at the combustor exit to delineate the interaction and relate it to the flame structure. The system is configured to enable sampling from individual rakes to study local emissions and the rakes can be ganged together to measure the bulk-averaged combustion emissions. This research provides a quantification of the improvement of the NOx margin with a decrease in the secondary air percentage. The study shows that the increase in margin is not a simple re-estimate of the combustor emissions using the NOx design curve due to flame quenching effects. The results also show that the secondary air can be used to improve the NOx emissions via controlling the interaction with the primary flame. The impact is quantified in terms of emissions, acoustics and metal temperatures.
Generation and Development of Klebanoff Streaks in Low-Pressure Turbine Cascade Under Upstream Wakes
Authors: Sun Shuang , Wu Xingshuang , Tan Tianrong , Zuo Canlin , Pan Sirui , Liu Fulin
Published: January 2021

At low Reynolds numbers operating condition, the boundary layer of the high-lift low-pressure turbine (LPT) of aero-engines is prone to separate on the suction surface of the airfoil. The profile losses of the airfoil are largely governed by the size of the separation bubble and the transition process in the boundary layer. However, the wake-induced transition, the natural transition and the instability induced by the Klebanoff streaks complicate the transition process.

The boundary layer on the suction surface of a high-lift LPT was investigated at Re = 50,000 with upstream wakes. The numerical simulation was performed with the CFX software using large eddy simulations (LES), and the experiment was performed on a linear cascade. In this study, the wake is divided into the wake center and the wake tail, the unsteady formation process of the streaks and the wall shear stress caused by the wake are analyzed. A new mechanism of generation and development of Klebanoff Streaks was presented to better understand the effect of the wake on the boundary layer.

Moreover, it was found that after entering the blade passage, the wake center does not contact the blade but causes the wall shear stress of the front part on the suction surface to increase. However, it is not possible to form strong Klebanoff streaks at the leading edge of the blade by shear sheltering effect. Only the wake tail can form Klebanoff streaks when it contacts the blade.
Dynamic Modelling of Small Scale and High Temperature ORC System Using Simulink and CoolProp
Authors: Dhanasegaran Radheesh , Uusitalo Antti , Turunen-Saaresti Teemu
Published: January 2021

In the present work, a dynamic model has been developed for the small-scale high-temperature ORC experimental test rig at the LUT University that utilizes waste heat from a heavy-duty diesel engine exhaust. The experimental facility consists of a high-speed Turbogenerator, heat exchanger components such as recuperator, condenser, and evaporator with a pre-feed pump to boost the working fluid pressure after the condensation process constituting a cycle. The turbogenerator consists of a supersonic radial-inflow turbine, a barske type main-feed pump, and a permanent magnet type generator components connected on a single shaft. Octamethyltrisiloxane (MDM) is the chosen organic working fluid in this cycle. Matlab-Simulink environment along with the open-source thermodynamic and transport database Cool-Prop has been chosen for calculating the thermodynamic properties of the dynamic model. A functional parameter approach has been followed for modeling each block component by predefined input and output parameters, aimed at modeling the performance characteristics with a limited number of inputs for both design and off-design operations of the cycle. The dynamic model is validated with the experimental data in addition to the investigation of exhaust gas mass flow regulation that establishes a control strategy for the dynamic model.
Aerodynamic Retrofit Design for a High Pressure Compressor Using a High Hub/Tip Ratio Mixed-Flow Compressor
Authors: Xiang Hang , Chen Jiang , Cheng Jinxin , Niu Han , Liu Yi , Song Xiancheng
Published: January 2021

A high-load mixed-flow compressor with an extremely high inlet hub/tip ratio (0.889) is designed and analyzed for replacing the rear stages of a multistage high pressure axial compressor. The effects of blade number, splitter blades and dimensionless geometric parameters on the impeller performance are investigated by an improved loss model. A full-surface parameterization control method is adopted for blade optimizations of the mixed-flow impeller and the tandem stator. As a retrofit design of the multistage axial compressor, an unconventional type of axial-co-mixed-flow combined compressor scheme is proposed and discussed. Further, in order to minimize the axial dimension and maximize the load, this paper also proposed preliminary designs of the twin-stage mixed-flow compressor and the twin-stage counter-rotating mixed-flow compressor respectively equipped with the high hub/tip ratio mixed-flow compressor. The results indicate that the mixed-flow impeller configuration with 42 principal blades and splitter blades with a fifth of principal blade length has the maximum efficiency at design flow rate. Blade height/pitch ratio is a considerable parameter which demonstrates the interaction among hub/tip ratio, aspect ratio and solidity especially for high hub/tip ratio cascade designs. The mixed-flow compressor can greatly improve the load capacity of the high pressure compressor with slight impact on efficiency and surge margin. At low rotate speed, the mixed-flow impeller can maintain relatively high efficiency level and even carry a higher proportion of the load, while the tandem stator limits the overall efficiency improvement. Besides, structures with no return channel of the three unconventional combined compressors are beneficial to the reduction of dimension and cost, which shows the potential application prospects of high hub/tip ratio mixed-flow compressors.
Influence of Blade Pitch and Number of Blades of a Pump Inducer on Single and Two-Phase Flow Performance
Authors: Mansour Michael , Parikh Trupen , Thévenin Dominque
Published: January 2021

This study investigates the influence of various inducer configurations upstream of a pump impeller on the single and two-phase flow performance. Three pitch values (P = 0.151, 0.251, and 0.351 m), as well as three different numbers of blades (N = 2, 3, and 4 blades), were studied, leading to a total of 9 different inducer geometries. The main objective of the present study is to analyze and compare the corresponding performances and the two-phase mixing behavior, which is necessary for improving the two-phase pumping ability. 3D steady-state simulations using the Moving Reference Frame (MRF) approach were applied for single-phase flow, while a transient setup using a moving-mesh approach was employed for two-phase simulations. Turbulence was modeled by the Reynolds Stress Model (RSM), whereas the Volume of Fluid (VOF) method was applied to model air-water interactions. The results show that the increase in the number of blades leads to a high performance drop at overload (high-flow) conditions, but only to a slight performance enhancement at part-load (low-flow) conditions. Additionally, the effective flow range of the inducer corresponding to high efficiency becomes narrower for a higher number of blades. Concerning the inducer pitch, at part-load conditions, a lower pitch is slightly beneficial to smoothly suck the flow and damp the low-flow vortices; employing a high pitch at these conditions results in intensified flow vortices, reducing slightly the performance. On the other hand, the blade pitch is very influential for the performance at optimal and overload conditions, where a lower pitch causes flow blockage, leading to significant performance deterioration and a very limited range of applications. Generally, it was found that a modification of the inducer configuration affects the performance much more at overload compared to part-load conditions. Concerning two-phase mixing performance, the highest pitch provides the best mixing since the inducer is able to effectively churn the two phases. Similarly, an increase in the number of blades amplifies the turbulence between the two phases, thus improving mixing. Overall, a higher inducer pitch and a low to moderate number of inducer blades best ensure high performance, wide working range, and efficient two-phase mixing.