bluuebunny/crossref_metadata_2025 huggingface dataset:
Assessment of MULTALL As CFD Code for the Analysis of Tube-Axial Fans
Authors: Danieli Piero , Masi Massimo , Delibra Giovanni , Corsini Alessandro , Lazzaretto Andrea
Published: January 2021

This work deals with the application of the open source CFD code MULTALL to the analysis of tube-axial-fans. The code has been widely validated in the literature for high-speed turbomachine flows but not applied yet to low speed tutbomachines. The aim of this work is to assess the degree of reliability of MULTALL as a tool for simulating the internal flow in industrial axial-flow fan rotors. To this end, the predictions of the steady-state air flow field in the annular sector of a 315mm tube-axial fan obtained by MULTALL 18.3 are compared with those obtained by two state-of-the-art CFD codes and experimental data of the global aerodynamic performance of the fan and the pitch-wise averaged velocity distribution downstream of the rotor. All the steady-state RANS calculations were performed on either fully structured hexahedron or hexa-dominant grids using classical formulations of algebraic turbulence models. The pressure curve and the trend of the aeraulic efficiency in the stable operation range of the fan predicted by MULTALL show very good agreement with both the experimental data and the other CFD results. Although the estimation of the fan efficiency predicted by MULTALL can be noticeably improved by the more sophisticated state-of-the-art CFD codes, the analysis of the velocity distribution at the rotor exit supports the use of MULTALL as a reliable CFD analysis tool for designers of low-speed axial fans.
Load Influence on Hydrostatic Oil Film Journal Bearing Stiffness Characteristics
Authors: Moroz Leonid , Romanenko Leonid , Kochurov Roman , Kashtanov Evgen
Published: January 2021

Hydrostatic bearings are widely used in industry, including aerospace and energy sectors. Hydrodynamic lubrication mechanism has been well studied analytically and experimentally and various types of bearings were developed to provide increasing operating speed, load capacity, stability and efficiency for modern rotating machines. Hydrostatically lubricated bearings have principal difference (in comparison with hydrodynamic bearings) and their characteristics have been an area of continued research.

The goal of this work is to develop a robust algorithm, which can predict hydrodynamical characteristics and dynamic stiffness and damping coefficients of hybrid and hydrostatic bearings with increased accuracy and which can be used for engineering/design purposes. The developed approach is based on Reynold’s equations, where the unknown parameters are the rotor position and fluid pressure in recess pockets. Finite difference method in combination with the successive over-relaxation algorithm is used for a numerical solution of Reynold’s equations. Newton’s method is applied to solve the generated system of equations.

Applying the developed approach, the effect of load influence on the hydrodynamical and the dynamic stiffness characteristics has been studied. Several hydrostatic bearing designs which are based on the published data were considered to compare the results calculated applying the approach with the experimental and theoretical data given in the literature. Performed study shows when journal eccentricity can’t be neglected while simulating hydrostatic bearing characteristics. Simulations also allow for analysis of how different design/geometrical parameters and initial conditions (supply pressure) influence bearing performance characteristics.

The developed approach can be utilized as a practical tool which allows for the prediction of performance characteristics of hydrostatic bearing with increased accuracy.
Multiphase Rotating Detonation Engine
Authors: Dunn Ian , Ahmed Kareem
Published: January 2021

The first experimental evidence of a solid-gas multiphase rotating detonation engine. Coal particles, carbon black with a volatility of 1% and a carbon concentration of 99%, were detonated successfully over many operational parameters. These operational parameters surrounding the various points of investigation are shown in multiple 2-D slices as well as plotted in one 3-D graph to show the effects of varying carbon concentrations. These parameters include: variation in total mass flux injected into the annulus ranging from (≅120–270 kg/(s*m2), variation in hydrogen-air equivalence ratio (0.65–1.0), and finally variation in total concentrations of carbon (0–42.5%). High-speed backend imaging allowed for the analysis of the detonation wave dynamics, where detonation velocities were deduced using Discrete Fourier Transforms. By varying the parameters mentioned above, detonation velocities experienced in the detonation channel allowed for an introduction of an optimal operational point. When carbon was injected into very lean hydrogen-air conditions, the detonation was over-driven, causing fluctuations in the detonation velocities upwards of ∼100 m/s. As carbon concentrations increased further, detonation wave velocities relative to Chapman-Jouguet detonation velocities decreased.
Improvements of Experimental Research of Wet Steam in Turbines Using CFD Simulations
Authors: Kolovratnik Michal , Jun Gukchol
Published: January 2021

The Czech Technical University in Prague (CTU) has been conducting both theoretical and experimental research on wet steam for over 50 years. Part of this research has focused on the development of an instrument for measuring the structure of the liquid phase of wet steam — an optical extinction probe. The measurements of the wet steam structure using our optical extinction probe take place in operative steam turbines. Due to the non-negligible interaction of the probe with the flow field in its vicinity, the wet steam parameters within the probe measuring space change. This probe-flow field interaction (PFFI) negatively affects the accuracy of the measurement of the liquid phase structure.

This paper presents partial results of our research into the interaction between the optical probe and the surrounding flow field. Particularly, it is the result of CFD simulations of wet steam (WS) flow in the low-pressure section of a 1000 MW nuclear plant steam turbine, in which the probe has been used repeatedly. In the simulations we consider, non-equilibrium condensation allows for the observation of the formation and development of the liquid phase within the turbine. The influence of PFFI on the liquid phase structure is evaluated by a coefficient called the Probe Influence Factor (PIF). In this work, the PIF values are presented for 3 varying traversing positions of the probe along the L-1 stage turbine blade. The use of the PIF to analyse the experimental measurement results is also discussed.

The second part of the paper deals with the possibility of modifying the shape of the probe measuring head. Based on detailed analysis of the CFD simulations of PFFI, modifying the shape of the probe is proposed to reduce this interaction. The benefit of this change is evaluated using CFD simulations. Comparisons between the PIF coefficients of the original and modified optical probes indicate that modifying the shape may reduce the PFFI influence on experimental measurements.
Parametric Study on Micro-Roughness Shapes for Enhanced Jet Impingement Heat Transfer
Authors: Devakottai Ramaswamy , Madhavan Srivatsan , Singh Prashant , Ekkad Srinath V.
Published: January 2021

Development of efficient cooling technologies are imperative to support the constant push for higher turbine inlet temperatures to achieve increased overall turbine efficiency. High-pressure stage turbine blades are subjected to hostile environment involving high temperature turbulent flow exiting from the combustor section. The blade leading edge is subjected to flow stagnation and hence requires special attention in terms of both, internal and external cooling. This study is focused on improving the internal side heat transfer coefficient by installing novel micro-roughness elements on the target wall. The study is based on Singh, Prashant, et al. “Effect of micro-roughness shapes on jet impingement heat transfer and fin-effectiveness.” International Journal of Heat and Mass Transfer 132 (2019): 80–95, where different micro-roughness shapes were investigated experimentally and numerically. The authors proposed that the novel concentric-cylinder shaped roughened geometry exhibited highest fin-effectiveness. Present study reports the effect of three micro-roughness shapes, viz. cylindrical, cubic and concentric cylinder. Conjugate heat transfer study was performed, and the heat transfer performance was reported in the form of local Nusselt number and globally averaged fin-effectiveness. An array jet configuration of 5 × 5 jets with a jet-to-jet spacing of X/Djet = Y/Djet = 3 and jet-to-target plate spacing of Z/Djet = 1 was maintained for jet-diameter based Reynolds number (ReDjet) ranging from 3,000 to 12,000. Investigation on the effect of pin-fin shapes shows that the concentric-shaped micro pin-fin element had the highest fin-effectiveness of 2.45 at ReDjet = 12,000. Further, pin-fin optimization studies were performed for the concentric cylinder pin-fin shape, where the effect of pin-fin height and the effect of internal to external diameter ratio was studied. The pin-fin effectiveness increased with increase in height and diameter ratio, and a maximum fin effectiveness was observed for maximum pin-fin height.
An Additively Manufactured Four Sensor Fast Response Aerodynamic Probe
Authors: Chasoglou Alexandros , Tsirikoglou Panagiotis , Kalfas Anestis , Abhari Reza
Published: January 2021

The current work describes the design, development and testing of a miniature fast response aerodynamic probe (FRAP) with 4 sensors (4S), able to perform measurements in unsteady three-dimensional flow field. Moreover, the calibration and first results with the newly developed probe is also provided. The miniature FRAP-4S demonstrates a 3 mm tip diameter, which represents a 25% reduction in diameter size, in comparison to a first generation FRAP-4S, without any loss in terms of measurement bandwidth. The 3 mm outer casing of the probe is additively manufactured with a high-precision binder jetting technique. In terms of aerodynamic performance, the probe demonstrates high angular sensitivity up to at least ± 18° incidence angle in both directions.

To evaluate the measurement accuracy of the newly developed FRAP-4S, measurements are performed at the Laboratory for Energy Conversion (LEC) in both a round axisymmetric jet and an one-and-a-half stage, unshrouded and highly-loaded axial turbine configuration. Turbulence measurements performed with the miniature FRAP-4S are compared against hot-wire studies in round free-jets found in the literature. Good agreement in both trends but also absolute values is demonstrated. Moreover, the performance of the probe is compared against traditional instrumentation developed at LEC, namely miniature pneumatic and FRAP-2S probes. The results indicate that the FRAP-4S, despite its larger size in comparison to the other probes tested, can resolve the main flow patterns, while the highest deviations occur in the presence of highly skewed and sheared flows. Furthermore, the additively manufactured probe was proven to be robust after more than 50 hours of testing in representative turbine environment configuration. Finally, it should be highlighted that the newly developed FRAP reduces measurement time by a factor of three in comparison to FRAP-2S, which directly translates to reduced development time and thus cost, during turbomachinery development phase.
Experimental Evaluation of Dynamic Characteristics of Circular Arc Spring Dampers for Rotating Machinery
Authors: Takeuchi Ryota , Ishimaru Hidetsugu , Yamashita Hideaki , Yabui Shota , Inoue Tsuyoshi
Published: January 2021

This study presents an experimental evaluation of the dynamic characteristics of the circular arc spring damper (CASD), which attenuates the vibration of rotating machinery. The CASD is a novel type of fluid film damper that has two or more arc-shaped beams and dual slits (radial clearances) formed by wire electric discharge machining (WEDM). A major advantage of CASDs over conventional Squeeze Film Dampers (SFDs) with squirrel-cage springs is their compactness and low weight. The concept of CASDs was originally proposed by Kanki et al. in 2005. However, the basic characteristics of this type of damper, including the influence of the geometries, added mass coefficients, and cross-coupled terms of dynamic coefficients have not been investigated. To clarify these characteristics, a series of excitation tests was conducted on three types of CASDs to identify their dynamic coefficients. The features of the newly developed test rig and experimental results are presented in this study. Tested dampers have the same outer diameter (160 [mm]) and damper width (44 [mm]) but different arc patterns (2-arc or 4-arc) and radial clearances (about 0.3 or 1.2 [mm]). All dampers were tested in both open-end and end-sealed configuration. The influence of the end-seal clearances (about 0.1 to 0.4 [mm]) was also examined. The following results were obtained: (1) 4-arc type CASDs have greater damping than that of 2-arc types; (2) CASDs have a considerable amount of added mass coefficients, especially in the end-sealed condition; (3) Smaller end-seal clearances make the damping and added-mass coefficients significantly larger; (4) A large level of damping can be produced with dashpot configuration (large radial clearances and small end-seal clearances), though the added-mass effect becomes more prominent; (5) Cross-coupled terms are very small in centered / small amplitude motion. These new findings can be utilized for the design and application of CASDs to real rotating machineries.
Optimization of Turbine Squealer Tip Cooling Design by Combining Shaping and Flow Injection
Authors: Duan P. H. , He L.
Published: January 2021

In this study, a turbine squealer tip is optimized by a multi-objective genetic algorithm (MOGA) with varying the squealer heights and the tip cooling configurations. The three objectives selected are the aerodynamic efficiency, the film cooling effectiveness and the surface fluid temperature variance. The multi-scale methodology is implemented to reduce the computational cost and to skip the meshing of cooling holes. Two optimization approaches are compared: a) a conventional method that optimizes an uncooled shape first and then the cooling configuration sequentially, and b) a method that optimize shaping and cooling concurrently. The concurrent method is found to obtain a heat transfer performance that is not achieved by the conventional optimization. Moreover, by adding the cooling, the performance ranking of the uncooled blades in terms of the aerodynamic efficiency is changed. These observations are due to the strong interaction between the coolant and the tip leakage flow. They indicate that the coolant injected at the tip is not passive as expected in the conventional film cooling designs. By altering the tip leakage flow structure, the coolant can reduce the tip leakage loss, which contradicts the conventional wisdom that the added coolant should always lead to extra losses due to the extra mixing. More detailed observations of the flow field indicate that the influence of the squealer height towards the aerodynamic efficiency is caused by two competing effects: the blockage effect to reduce the tip leakage mass flow rate and the sudden expansion loss effect to generate additional losses. The heat transfer performance can be significantly influenced by increasing the squealer height because of the trapped coolant in the cavity.
A Novel Modular Tool for the Simulation of Gas Turbine-Based Cogeneration Systems
Authors: Carcasci Carlo , Lubello Pietro , Brogelli Riccardo , Snickars Carlo , Vettori Gregorio
Published: January 2021

The effort in the adoption of energy efficient technologies and the attempt at reducing CO2 emissions has brought a new wave in the diffusion of distributed generation and a renewed interested in CHP systems, especially in the case of industrial cogeneration applications. It is therefore of primary importance for prime movers ‘producers — as Baker Hughes — to develop fast and reliable simulation methods that are able to estimate the overall performances of the proposed plants in order to help the customer assessing the feasibility of the investment.

The aim of this paper is to introduce a newly developed tool for the simulation of design and off-design conditions of gas turbine-based cogeneration systems, where heat is recovered through a Heat Recovery Steam Generator. The tool is meant to be driven by a small amount of inputs, making it possible to define sharp strategies for the design of such plants since the early phases of interaction with customers.

Design points have been varied to assess their influence on the off-design working points of the plant and on the overall performances, which have been considered through the parameters of global efficiency and Primary Energy Saving.

Results have highlighted the importance of considering the off-design effects in the simulation of CHP systems, both from a correct evaluation of plant’s critical operating parameters and from a correct estimation of plant’s performances.
Experimental Study on Thermal Efficiency of a Multi-Jet Anti-Icing System With Piccolo Tube
Authors: Bu Xueqin , Huang Ping , Sun Haoyang , Lin Guiping
Published: January 2021

An experimental study was conducted to investigate the jet impingement thermal efficiency of a typical model for a multijet anti-icing system under different experimental conditions such as air mass flow rate at the jet exit, piccolo tube position and circumferential angle of jet holes. The experimental results showed that jet mass flow rate, velocity at the jet exits (whether choking conditions were established at the jet exits or not) and piccolo tube-to-leading edge distance within the cavity had significant influence on thermal efficiency of the anti-icing system. The heat flux on the external surface of leading edge, diameter of jet holes and circumferential angle of jet holes had a measurable but small effect on thermal efficiency of the anti-icing system.
Accuracy and Sensitivity Analysis of Aerodynamic Performance Prediction Models for Transonic Axial-Flow Compressors
Authors: Zhang Xiawen , Ju Yaping , Zhang Chuhua
Published: January 2021

The mean-line method, as the corner stone of preliminary aerodynamic design of axial-flow compressors, is heavily dependent on the accuracy and robustness of empirical prediction models, mainly the deviation models and loss models. A large number of such models have been developed, however, a comprehensive evaluation of their prediction capabilities was lacked yet. To carry out the accuracy and sensitivity analysis of these prediction models developed in the both academic and industry communities, we here developed a one-dimensional mean-line method which implements several widely used deviation and loss models. Then, the developed mean-line method was applied to predict the speed-lines of aerodynamic performance for three representative transonic axial-flow compressors, i.e., NASA Rotor 35, NASA Stage 35 and NASA 74A first front three-stage. The parallel coordinates method was particularly adopted to effectively perform the sensitivity analysis of totally 2448 combinations of deviation and loss models through a heuristic comparison of the model predictions with the available experimental data. The accuracy analysis indicates that, by using the best model combinations, the prediction error of peak efficiency point is generally kept below 2% whereas that of surge margin varies significantly from 3.03% to 18.93%. However, the most accurate model combination is dependent on the compressor type and rotational speed. The sensitivity analysis shows that the prediction robustness is remarkably influenced by the deviation model accounting for axial velocity ratio effect, the design shock loss model and the off-design total loss model. This work provides the design engineers with prediction model selection, and the model developers with prediction improvement direction for axial-flow compressors.
Flame Transfer Functions and Dynamics of a Closely Confined Premixed Bluff Body Stabilised Flame With Swirl
Authors: Nygård Håkon T. , Worth Nicholas A.
Published: January 2021

The Flame Transfer Function (FTF) and flame dynamics of a single, highly swirled, closely confined, premixed flame is studied over a wide range of operating conditions at a fixed perturbation level at the dump plane. The equivalence ratio and bulk velocity are varied in order to examine the important ratio of flame height to velocity in scaling the flame response function. The enclosure geometry is kept constant, and therefore due to the close confinement and varying flame height, strong flame-wall interactions are present for some operating conditions. The effect of these interactions on the FTF due to changes in the “relative” or “effective confinement” of the flame can therefore be studied. When the equivalence ratio is sufficiently high, and therefore the effective confinement sufficiently small, modulations, or dips, in the gain and phase of the FTF are observed, due to interference of the perturbations created at the swirler and at the dump plane. Due to the small length scales and relatively high velocities in the current configuration, the dip is at a high frequency and spans a wide range of frequencies compared to similar studies which have previously identified similar phenomena. It is also observed that when the equivalence ratio was decreased, increasing the effective confinement, a critical point is reached where the modulations are suppressed. This is linked to a temporal shift in the heat release rate at the downstream location where the flame impinges on the combustion chamber walls during the oscillation cycle. The shift causes a decrease in the expected level of interference, demonstrating that the effective confinement is an important parameter to consider for the nature of the FTF response. Additionally, a Distributed Time Lag (DTL) model with two distinct time lags, capturing the swirler perturbations and the perturbations at the inlet, is successfully applied to the FTFs. The model provides a simple way to accurately capture the two dominant time scales in the problem without the need of prior knowledge of the cause of the perturbations, and a simple expression to recreate the complex valued FTF. In addition the model also provides insight into the time scales of the problem, demonstrating in the current work that time scales recovered from the DTL analysis are offset from simple Strouhal number scaling, due to the effects of increasing effective confinement.
A Proposal for Passive Wall Treatment Applied in High Performance Axial Flow Compressors
Authors: Díaz Rubén Bruno , Tomita Jesuino Takachi , Bringhenti Cleverson , Elizio de Paula Francisco Carlos , Lindquist Whitacker Luiz Henrique
Published: January 2021

Numerical simulations were carried out with the purpose of investigating the effect of applying circumferential grooves at axial compressor casing passive wall treatment to enhance the stall margin and change the tip leakage flow. The tip leakage flow is pointed out as one of the main contributors to stall inception in axial compressors. Hence, it is of major importance to treat appropriately the flow in this region. Circumferential grooves have shown a good performance in enhancing the stall margin in previous researches by changing the flow path in the tip clearance region. In this work, a passive wall treatment with four circumferential grooves was applied in the transonic axial compressor NASA Rotor 37. Its effect on the axial compressor performance and the flow in the tip clearance region was analyzed and set against the results attained for the smooth wall case. A 2.63% increase in the operational range of the axial compressor running at 100%N, was achieved, when compared with the original smooth wall casing configuration. The grooves installed at compressor casing, causes an increase in the flow entropy generation due to the high viscous effects in this gap region, between the rotor tip surface and casing with grooves. These viscous effects cause a drop in the turbomachine efficiency. For the grooves configurations used in this work, an efficiency drop of 0.7% was observed, compared with the original smooth wall. All the simulations were performed based on 3D turbulent flow calculations using Reynolds Averaged Navier-Stokes equations, and the flow eddy viscosity was determined using the two-equation SST turbulence model. The details of the grooves geometrical dimensions and its implementation are described in the paper.
Hydrogen Enrichment Impact on Gas Turbine Combustion Characteristics
Authors: Mohammad Bassam S. , McManus Keith , Brand Anthony , Elkady Ahmed M. , Cuppoletti Daniel
Published: January 2021

The current research provides the impact of hydrogen enrichment on gas turbine combustion characteristics. The uniqueness of this study is it isolates the hydrogen effects while minimizing the impact of other parameters that are known to influence combustion characteristics. Experiments are carried out under high operating pressure and a wide range of firing temperatures that extend from the Lean Blow Out (LBO) limit to beyond J class firing temperature. Aerodynamic effects are isolated by using a perforated plate burner to provide a simple flame structure. The study is conducted under perfectly premixed conditions to exclude the mixing effects from the problem under investigation. Air flow, residence time, pressure and temperature are all held constant to enable back to back comparison. Hydrogen enrichment is varied from 0 to 25 percent by volume while holding the combustor exit temperature constant. No cooling air or effusion air is used in the combustion zone to ensure that there is no impact on the problem under investigation and to focus the study on kinetics effects and flame shape variation. NOx, CO emissions, LBO limits as well as flame luminosity are reported. Oxygen and carbon dioxide are measured at the combustor exit and used to ensure test integrity and for confirmation of the exit temperature. A reactor network model is used to mimic the experimental work and study sensitivity. The effective residence time in the model is varied slightly to mimic the slight change observed in flame length with hydrogen addition. This basic research provides a key resolution to the contradictory results that are typically reported in the literature for the impact of hydrogen on NOx emissions.
On the Prediction of Rotating Stall in an Industrial Gas Turbine Compressor
Authors: Krishnababu Senthil K.
Published: January 2021

An investigation is presented into the computation of rotating stall in an industrial gas turbine compressor using a hybrid whole annulus and single passage computational domain. The objective of this investigation is to demonstrate the use of large-scale unsteady computations with quicker turn-around times in the design cycle to develop and evaluate several variable guide vane schedules and/or bleed settings. This means that subsequent engine test campaign could be carried out with significantly lower test matrix size in terms of the number of variable guide vane schedules and/or the handling bleed settings thus reducing the overall development time and cost. Rotating stall that was measured and characterised during a previous compressor rig test (Krishnababu, et al. [1]) were successfully predicted by large-scale unsteady computations using TurboStream. The predicted number of stall cells and their speed agreed closely with the test data. The methodology validated was applied to predict and mitigate the rotating stall in the development of a compressor for a new gas turbine engine. Using this approach, it was possible to define bleed control system that ensured stall free operation.
On the Formulation of Nonreflecting Boundary Conditions for Turbomachinery Configurations: Part II — Application and Analysis
Authors: Wolfrum Nina , Bechlars Patrick , Beck Maximilian , Frey Christian , Schlüß Daniel
Published: January 2021

The flow in turbomachinery components is complex due to the relative motion of rotating and non-rotating elements. A proper design and prediction of physical phenomena requires reliable CFD tools. One important aspect is the incorporation of sophisticated algorithms at the boundaries of the computational domain.

For inviscid, one-dimensional and two-dimensional Euler-flows there exist analytical solutions for the formulation of a boundary condition. Realistic applications, however, are viscous and consist of a complex three-dimensional character. Nevertheless, the analytical 2D nonreflecting boundary conditions are commonly used in CFD codes for their high computational efficiency and numerical robustness.

The application becomes more challenging when the boundaries are close to geometrical features such as blades and vanes. In practical applications, the position of the boundaries is dictated by geometrical constraints and hence the proximity to the blading cannot always be avoided. The interaction of rotating and non-rotating geometrical features in a turbomachine produces complex flow patterns that propagate in the form of acoustic, vorticity and entropy waves. A boundary condition must be implemented in such a way that waves can propagate undisturbed out of the computational domain. Any reflection may unphysically affect the solution within the computational domain which is especially harmful to sensitive values such as unsteady aeroelastic quantities. But also steady-state computations may suffer from errors produced by reflective boundary conditions.

The following paper is the second of two papers on the formulation of unsteady boundary conditions based on a two-dimensional analytical approach. The first part of this paper [6] explains how to extend 2D nonreflecting boundary conditions to real 3D annular domains by applying them in certain conical rotational surfaces. Two different formulations are discussed referring to the orientation of said rotational surfaces. In the first case the surfaces are oriented perpendicular to the boundary panel. In the second case the surfaces are aligned with the circumferentially averaged meridional flow velocity.

In the present paper a thorough analysis of the two different approaches will be given. Both formulations of the boundary algorithm are validated on the basis of several elementary model flows. The behavior is analyzed for various unsteady wave patterns of different propagation directions with respect to the boundary. It will be shown that the alignment of the rotational surfaces with the meridional flow has a beneficial effect on the reflective behavior for the majority of the investigated flow conditions.

The boundary conditions are then tested on realistic turbomachinery components in order to analyze their applicability on complex flows.
Numerical Analysis of the Stability and Operation of an Axial Compressor Connected to an Array of Pulsed Detonation Combustors
Authors: Dittmar Louise , Stathopoulos Panagiotis
Published: January 2021

Conventional gas turbines are a very mature technology and performance improvements are becoming increasingly difficult and costly to achieve. Pressure Gain Combustion has emerged as a promising technology in this respect, due to the higher thermal efficiency of the respective ideal gas turbine cycles. Up to date, the majority of studies in the field has concentrated on the connection of the turbine to various types of pressure gain combustion systems. The current work focusses on the connection of an array of pulsed detonation tubes at the outlet of an axial compressor. An ideal 0-D plenum is assumed between the compressor outlet and the pulsed detonation tubes’ inlet. The tubes are simulated as time-dependent mass sinks in this plenum. A solution of the time-dependent mass conservation equations delivers the pressure variation in this ideal plenum for various combinations of its size, the tube number, the tube frequency, their firing pattern and their closing time. This pressure variation is subsequently used as a boundary condition in a numeric model of a compressor that can resolve very fast changes of its operational condition. The model is based on the solution of the time-dependent 1-D Euler equations with source terms for the blade forces and the work exchange with the fluid along the compressor stages. These source terms are computed from the operational map of the compressor. The influence of the aforementioned five parameters (number of tubes, frequency, closing time, firing pattern and plenum size) on the compressor operational stability and the flow in its stages is studied in detail. The aim is to benchmark the design of a plenum that results in the lowest possible compressor outlet pressure fluctuation without becoming too large. At the same time, the pressure fluctuations and their propagation along the compressor stages are analyzed.
The Effects of Reynolds Number on the Stall and Pre-Stall Behaviour of Compact Axial Compressors
Authors: Hutchings Jack , Hall Cesare
Published: January 2021

Compact axial compression systems are of interest to the domestic appliance industry. The associated low Reynolds number leads to high losses compared to large-scale compressors due to a transitional flow field with large regions of separation. This paper investigates how Reynolds number variations affect the three-dimensional and unsteady flow field in a compact compressor both pre-stall and in stall. An experimental study has been conducted using a scaled-up singlestage axial compressor across a Reynolds number range of 104 to 105. Steady and unsteady casing static pressure measurements, along with rotor upstream and downstream unsteady velocity measurements, have been used to observe the rotor flow field. As the Reynolds number is reduced below a critical value, 60,000 in the case of the compressor studied, the pressure rise coefficient of the compressor decreases. The exact value of the critical Reynolds number is expected to vary with the compressor geometry. This fall off in performance corresponds to an increase in the compressor rotor secondary flows. Prior to stall, a broadband hump at around 50% of the blade passing frequency is present in the near-field casing static pressure spectra. At Reynolds numbers below the critical value, multiple equally spaced peaks also appear around the peak of the broadband hump. The spacing of these peaks has been found to be exactly equal to the measured stall cell speed once rotating stall is established. When operating in stall, the stall cell is found to increase in size and slow down as Reynolds number decreases. The measured spectra and observed flow structures show that disturbances exist prior to stall at frequencies consistent with the frequencies within stall. The size and shape of the stall cells that form are related to the extent of the three-dimensional flow field present prior to stall. Below a critical value, all of these flow features are highly sensitive to Reynolds number.
Application and Verification of an Engineering Approach to Assess Notch Support for Low Cycle Fatigue Loadings
Authors: Garnadt Florian , Kontermann Christian , Almstedt Henning , Oechsner Matthias
Published: January 2021

Notches according to the design of components often represent the failure location. If a crack is nucleating in the area of a notch, crack growth speed is a function of the stress gradient. The difference in allowable cycles between a notched and an unnotched structure can be quantified by a notch support factor. The approach presented in this paper is based on the determination of the stress intensity factor of a short crack in the vicinity of a notch using FEA. To consider the influence of plasticity induced crack closure the analytical approach by Newman is employed. This relatively easy and straight-forward approach is benchmarked in two ways. First, the results are compared with a series of experimental data produced for notched round bar specimens made of a forged steel. Here, crack growth is measured by using the alternating current potential drop method. Second, crack growth is calculated with a full numerical approach. The results are discussed in detail to investigate the application range of the easy-to-use Engineering Approach to compute notch support factors for real component structures.
Influences of Turbulence Boundary Conditions on RANS and URANS Simulations for an Inter-Turbine Duct
Authors: Firrito Alessio , Bousquet Yannick , Binder Nicolas , Pintat Ludovic
Published: January 2021

In recent years, lot of turbine research is focused on the study and optimization of inter-turbine ducts, an aero-engine component for which the design is becoming more challenging due to the turbofan architecture evolution. Starting from the early design phase, the knowledge of the component performance and outlet flow pattern is crucial in the design of the low pressure turbine. To improve prediction, multi-row unsteady simulations are deployed. Unfortunately, some questions arise in the use of these simulations, among others the knowledge of the turbulent boundary conditions and the contribution of the unsteady simulations to the flow solution. In this paper steady and time resolved RANS simulations of a turning inter-turbine duct are investigated. Particularly, two questions are addressed. The first one is the influence of the turbulent quantities boundary conditions in the case of a k–ω Wilcox turbulence model in the flow field solution. The second one is the contribution of the unsteadiness to the mean flow prediction. It will be shown that the mean flow depends on inlet turbulence only if the turbulence length scale is relatively high; otherwise the flow field is almost turbulence-invariant. For the unsteady simulations, unsteadiness modifies the mean flow solution only with low inlet turbulence.
Aeromechanical Characterization of a Last Stage Steam Blade at Low Load Operation: Part 1 — Experimental Measurements and Data Processing
Authors: Bessone A. , Guida R. , Marrè Brunenghi M. , Patrone S. , Carassale L. , Kubin Z. , Arnone A. , Pinelli L.
Published: January 2021

This paper is the first of a two-part publication that aims to experimentally evaluate, simulate and compare the aerodynamic and mechanical damping for a last stage steam turbine rotor blade at part load operation. Resulting strong off-design partial load regimes expose the last stage moving blade (LSMB) to the possible onset of aero-elastic instabilities, such as stalled and un-stalled flutter. This interaction can lead to asynchronous blade vibrations and then the risk of blade failures for high cycle fatigue. In this framework, it is necessary to develop and validate new tools for extending operating ranges, controlling non-synchronous phenomenon and supporting the design of new flutter resistant LSMB.

To this end, a 3-stage downscaled steam turbine with a snubbered LSMB was designed by Ansaldo Energia and tested in the T10MW test facility of Doosan Skoda Power R&D Department within the FlexTurbine European project. The turbine was operated in a wet steam environment at very low volume flow conditions simulating different part load regimes. The steady flow field throughout the LSMB was characterized and the occurrence of flutter was investigated by inducing the blade resonance through an AC magnet excitation and measuring the overall damping.

The results presented in this paper indicate that the blade always operates over the flutter stability margin validating this new blade design. In the second part of this work, the mechanical and aerodynamic contribution to the damping will be separated in order to validate the aerodynamic damping prediction of an upgraded CFD tool, already adopted in the design phase of the blade at design point.
Error Considerations for Turbine-Generator Torsional Vibration Monitoring
Authors: Liska Jindrich , Jakl Jan , Kunkel Sven
Published: January 2021

Turbine-generator torsional vibration is linked to electrical events in the power grid by the generator air-gap torque. Modern power systems are subject to gradual transformation by increasing flexibility demands and incorporation of renewable resources. As a result, electrical transient events are getting more frequent and thus torsional vibration is getting more and more attention. Especially in the case of large steam and gas turbines torsional vibration can cause material fatigue and present a hazard for safe machine operation. This paper freely builds on previous work, where a method for torsional vibration evaluation using an incremental encoder measurement was presented, in that it supplements error considerations to this methodology. Measurement errors such as precision of the rotor encoder manufacturing, choice of the proper sensor, its signal to noise ratio and the error of instantaneous velocity computation algorithm are analyzed. The knowledge of these errors is essential for torsional vibration as there is an indirect and relatively complicated path from the measurement to the final torsional vibration results compared to other kinds of vibration. The characteristics of particular errors of the processing chain are validated both on experimental data from a test rig as well as field data measured on turbine-generators in power plants.
Novel Approach for Optical Characterization of Thrust Collar Lubricated Area: Experimental and Numerical Results
Authors: Kerr Thomas , Delgado Adolfo
Published: January 2021

Thrust collars (TCs) are bearing elements used in geared machinery that transmit axial loads from one shaft to another. TCs are primarily used in integrally geared compressors (IGCs), but are also found in gearboxes and marine propulsion applications. TCs are hydrodynamic elements featuring a converging-diverging wedge to generate a pressure field that reacts axial loads. Accurate modeling requires knowledge of the film characteristics such as cavitation, turbulence, and air ingestion, all of which reduce load capacity. Current models in the literature do not include mass-conserving cavitation algorithms or turbulent flow. The following paper introduces a new test rig that optically characterizes the thin film region of a thrust collar. The test rig geometries, speeds, and loads match those typically seen in IGC applications. The test rig utilizes a transparent acrylic window in conjunction with a high-speed camera to obtain high-speed images of the oil film. Images are filtered and averaged to obtain areas of interest in the oil film. Cavitation and turbulence areas are captured for pinion speeds of 2.5, 5, and 7.5 krpm, and axial loads of 0.5, 1, and 1.5 kN. Cavitation occurs in the diverging (upper) region of the TC and appears at pinion speeds over 5,000 rpm, but does not change in shape after that speed. The cavitation is independent of applied load. Turbulence at the inlet region (bottom) occurs at all speeds, but increases to almost 35% of the total area at the highest speed. This paper also presents a finite element (FE) model that includes predictions for the static characteristics of the TC, specifically the cavitation area. The cavitation modeling uses an iterative Elord’s method, which conserves mass. The model predicts a similar cavitation area for all speeds and loads. A computation fluid dynamics (CFD) study predicts a similar cavitation area, and pressure field to the FE model. The CFD model predicts turbulence in the lower region that increases for increasing spin speed, which matches the experimental results. The CFD model tends to underpredict the turbulence area when compared to the experiments. As IGCs move into novel application areas to satisfy new needs, the increase in efficiency and capacity comes at a cost of more load and higher speed requirements on the TCs. This work will help original equipment manufacturers model TCs more accurately to ensure safe and efficient operation.
Performance Degradation due to Fouling and Recovery After Washing in a Multistage Test Compressor
Authors: Casari Nicola , Pinelli Michele , Spina Pier Ruggero , Suman Alessio , Vulpio Alessandro
Published: January 2021

Land-based power units have to fulfill even more high levels of production and reliability. In harsh environments (desert and tropical installations, typically), the power unit ingests high amounts of dust that might deposit inside the compressor. In this paper, the analysis of a multistage compressor performance that operates under sandy and humid conditions has been assessed. The compressor units, that equips the Allison 250 C18 compressor, has been subjected to multiple runs under severe conditions of soil dust ingestion. The compressor has been operated according to subsequent runs and at the end of each run, the performance curve was recorded and the performance losses, in terms of pressure ratio, have been measured during the operations. The characteristic curve of each run is representative of the level of contamination of the unit. Finally, the compressor has been washed and the performance curve, in the recovered conditions, has been recorded. The results show the modification and the downward shift of the characteristic curves which lead to a gradual loss of the compressor performance. The curves realized after dust ingestion have been compared with the recovered curve after online washing. The measurement shows a promising recovery of the performances, even if, the compressor flow path appears affected by localized deposits able to resist to the droplet removal action. Detailed photographic reports of the IGV and the first compressor stages have been included in this analysis. After each run, the IGV, the rotor blade and stator vane of the first stage and the hub and the shroud surfaces have been photographed. The pictures show the deposition patterns on the blades and the compressor surfaces. The comparison of the pictures of the internal surfaces, before and after the washing, highlights the parts that are more critical to clean and needy of attention during offline washing and overhaul.