bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct Numerical Simulation of Rotating Cavity Flows Using a Spectral Element-Fourier Method
Authors: Pitz Diogo B. , Chew John W. , Marxen Olaf , Hills Nicholas J.
Published: September 2016

A high-order numerical method is employed to investigate flow in a rotor/stator cavity without heat transfer and buoyant flow in a rotor/rotor cavity. The numerical tool used employs a spectral element discretisation in two dimensions and a Fourier expansion in the remaining direction, which is periodic and corresponds to the azimuthal coordinate in cylindrical coordinates. The spectral element approximation uses a Galerkin method to discretise the governing equations, similarly to a finite element method, but employs high-order polynomials within each element to obtain spectral accuracy. A second-order, semi-implicit, stiffly stable algorithm is used for the time discretisation, and no subgrid modelling is included in the governing equations. Numerical results obtained for the rotor/stator cavity compare favourably with experimental results for Reynolds numbers up to Re1 = 106 in terms of velocities and Reynolds stresses. For the buoyancy-driven flow, the energy equation is coupled to the momentum equations via the Boussinesq approximation, which has been implemented in the code considering two different formulations. Numerical predictions of the Nusselt number obtained using the traditional Boussinesq approximation are considerably higher than available experimental data. Much better agreement is obtained when the extended Boussinesq approximation is employed. It is concluded that the numerical method employed has considerable potential for further investigations of rotating cavity flows.
Full Coverage Film Cooling Performance for Combustor Cooling Manufactured Using DMLS
Authors: Vinton Kyle R. , Nahang-Toudeshki Sara , Wright Lesley M. , Carter Andrew
Published: September 2016

An experimental investigation of effusion film cooling has been completed for cylindrical, simple angle holes (θ = 20°), using a steady state, pressure sensitive paint (PSP) technique. The surface effectiveness measurements were obtained in a low speed wind tunnel where the average blowing ratio (M) was varied from 0.5 to 6. For all cases, the coolant–to–mainstream density ratio was fixed at DR = 1.0. The test surface was manufactured using direct metal laser sintering (DMLS), and was made to replicate full coverage film cooling typically seen for combustor cooling applications. The plate contained 10 staggered rows of film cooling holes, with P/D = 9.8 and S/D = 8.5. At blowing ratios greater than M = 1.0, the downstream film cooling effectiveness is greatly improved by the protection provided from the high momentum jets in the upstream rows. Within the latter-half of the matrix, the effectiveness measured on the surface gradually increased with each passing row. The combination of the holes made a substantial impact downstream, and the effect continued to grow all the way through the end of the plate. With the accumulation of the coolant above the surface, the coolant liftoff was mitigated and enhanced protection was observed on the surface. The DMLS manufacturing technique created surface and hole interior roughness. The hole interior roughness reduced the lateral average film cooling effectiveness by at least 50% when compared to previous investigations.
Rotordynamic Performance of a Shaft With Large Overhung Mass Supported by Foil Bearings
Authors: LaTray Nguyen , Kim Daejong
Published: September 2016

This work presents the theoretical and experimental rotordynamic evaluations of a rotor-air foil bearing system supporting a large overhung mass for high speed application. The proposed system highlights the compact design of a single shaft rotor configuration with turbomachine components arranged on one side of the bearing span. In this work, low speed tests up to 45krpm are performed to measure the lift off speed and to check the bearing manufacturing quality. Rotordynamic performance at high speeds is evaluated both analytically and experimentally. In the analytical approach, simulated imbalance responses are studied using both rigid and flexible shaft models with bearing forces calculated from transient Reynolds equation along with rotor motion. The simulation predicts that the system experiences small synchronous rigid mode vibration at 20krpm and bending mode at 200krpm. A high speed test rig is designed to experimentally evaluate the rotor-air foil bearings system. The high speed tests are operated up to 160krpm. The vibration spectrum indicates that the rotor-air foil bearing system operates under stable conditions. The experimental waterfall plots also show very small sub-synchronous vibrations with frequency locked to the system natural frequency. Overall, this work demonstrates the potential capability of air foil bearings in supporting a shaft with a large overhung mass at high speed.
Liquid CO2 Formation, Impact, and Mitigation at the Inlet to a Supercritical CO2 Compressor
Authors: Poerner Melissa , Musgrove Grant , Beck Griffin
Published: September 2016

Cycle efficiency is one of the critical parameters linked to the success of implementing a Supercritical Carbon Dioxide (sCO2) power cycle in a Concentrating Solar Power (CSP) plant application. Ambient conditions often change rapidly during operation, making it imperative that the efficiency of the plant cycle be optimized to obtain the maximum power production when sunlight is available. Past analyses have shown that operating the cycle at the critical point provides the optimum efficiency for dry operation. However, operation at this point is challenging due to the dramatic changes in thermophysical properties of CO2 near the critical point and the risk of the fluid having a two-phase, gas-liquid state. As a result, there is a high likelihood that liquid can form upstream of the primary compressor in the sCO2 power cycle. This paper explores the potential for liquid formation when operating near the critical point and looks at the influence of liquid on the compressor performance. The performance impact is based on industry experience with wet gas compression in power generation and oil and gas applications. Options for mitigating liquid effects are also investigated, such as upstream heating, separation, or compressor internal controls (blade surface gas ejection). The conclusions of the paper focus on the risk, estimated impact on performance, and summary of mitigation techniques for liquid CO2 entering a sCO2 compressor.
Effect of Operating Conditions on the Elasto-Hydrodynamic Performance of Foil Thrust Bearings for Supercritical CO2 Cycles
Authors: Qin Kan , Jahn Ingo H. , Jacobs Peter A.
Published: September 2016

In order to efficiently utilize the abundant solar resources in Australia, the supercritical CO2 cycle is proposed as an alternative to conventional steam power cycles due to high thermal efficiency and compact system layout. To mature the technology readiness of the supercritical CO2 cycle, each part, including turbine, compressor, seals and bearings, needs to be evaluated and possibly re-designed under consideration of the high density working fluid. One key technology is the foil thrust bearing, which is an enabler for high speed operation and oil-free systems. Bearings are at the core of the turbomachinery system and have a significant influence on the performance of the whole system. In this paper, a quasi three-dimensional fluid-structure model, using computational fluid dynamics for the fluid phase is presented to study the elasto-hydrodynamic performance of foil thrust bearings. For the simulation of the gas flows within the thin gap, the computational fluid dynamics solver Eilmer is extended and a new solver is developed to simulate the bump and top foil within foil thrust bearings. These two solvers are linked using a coupling algorithm that maps pressure and deflection at the fluid structure interface. Results are presented for ambient CO2 conditions varying between 0.1 to 4.0MPa and 300 to 400K. It is found that the centrifugal inertia force can play a significant impact on the performance of foil thrust bearings with the highly dense CO2 and that the centrifugal inertia forces create unusual radial velocity profiles. In the ramp region of the foil thrust bearings, they generate an additional inflow close to the rotor inner edge, resulting in a higher peak pressure. Contrary in the flat region, the inertia force creates a rapid mass loss through the bearing outer edge, which reduces pressure in this region. This different flow field alters bearing performance compared to conventional air foil bearings. In addition, the effect of turbulence in load capacity and bearing torque is investigated. This study provides new insight into the flow physics within foil bearings operating with dense gases and for the selection of optimal operating condition to suit foil thrust bearings in supercritical CO2 cycles.
Development of a Multipoint Matching Method for Turbocompound Systems
Authors: Ding Zhanming , Zhuge Weilin , Zhang Yangjun , Yin Yong , Zhang Shuyong
Published: September 2016

Turbocompounding is a promising waste heat recovery technology to improve fuel economy of internal combustion engines and comply with increasingly stringent emission regulations. The performance of a turbocompound engine is significantly influenced by the matching of the turbocharger turbine and the power turbine. Conventionally, the matching of the turbocharger turbine and power turbine is carried out at a single operating point. Single-point matched turbocompound systems tends to have poor performance at off-design operating conditions, which restrained the fuel-saving potential of turbocompound engines under driving cycle conditions.

In the present study, a multipoint matching method for turbocompound systems was developed, which was essentially an optimization process of the swallowing capacities of the turbocharger turbine and the power turbine to achieve best performance at multiple matching points. In order to improve the performance of turbocompound engines under driving cycle conditions, common operating points of a driving cycle were used as matching points in the matching process. Common operating points under a driving cycle were determined by clustering the dynamic profiles of the driving cycle.

A simulation study was carried out to examine the effectiveness of the multipoint matching method. The performance of the multipoint matched turbocompound system was compared against two single-point matched turbocompound systems under stationary operating points and driving cycle conditions respectively. According to the simulation results, the multipoint matched turbocompound system could attain satisfactory BSFC benefit under a wider operating range when compared with the two single-point matched turbocompound systems. The multipoint matched turbocompound engine showed the largest reduction in fuel consumption under driving cycle conditions.
Investigation of Inlet Distortion on the Flutter Stability of a Highly Loaded Transonic Fan Rotor
Authors: Iseni Senad , Micallef Derek , Mailach Ronald
Published: September 2016

The fundamental mechanisms of blade flutter in modern aircraft engines are very complex. Flutter is a self-excited aeroelastic instability phenomenon which can finally cause material fatigue and, in the worst case, leads to blade failure within a very short time. The risk of flutter has to be considered during the design process and it is necessary to avoid that safety risk. The aeroelastic stability has to be ensured over the whole operating range especially near operating limits or typical flutter boundaries, like at stall or choke conditions. Topic of this paper are inlet distortions, which can have an additional influence on the flutter stability of the fan and the first compressor stages of jet engines. For this purpose a sinusoidal steady total pressure inlet distortion was defined. The influence of this inlet distortion on the flow field and the flutter stability of a highly loaded transonic fan rotor (NASA rotor 67) is investigated. The static deflection of the manufactured blade was considered using an accurate mesh morphing algorithm to update the fan performance characteristic considering the deformed blade structure. The fan rotor interacts with the upstream distorted flow which leads to different blade loading between the adjacent blades. A decoupled flutter stability analysis using the three-dimensional viscous flow solver TBLOCK and the open-source software package CalculiX for pre-stressed modal analyses is carried out. The flutter stability analyses with TBLOCK are performed using the so-called energy method which was introduced by Carta. In order to predict the flutter stability under clean inflow conditions, two different formulations, the Influence Coefficient Method (ICM) and Traveling Wave Mode (TWM) formulation, are taken into account, whereas both formulations are compared to each other. The influence coefficients were directly calculated from the TWM formulation to determine the required number of passages for the ICM. It can be seen that the stability curves obtained with the ICM are in a good agreement to the TWM-method. The use of ICM reduces substantially the number of unsteady CFD calculations because of the fact that only one unsteady CFD calculation is needed to reconstruct the stability curve for each eigenmode and operating point. The effect of inlet distortion on flutter stability is investigated applying the TWM formulation only. Indeed, it was established that such flow disturbances have also for specific blades, considering the operating point, eigenmode and nodal diameter a destabilising impact on their aeroelastic behavior and can cause flutter, which is mostly determined by the time-averaged stability parameter. Just in the same manner a positive effect was observed for certain blades in the blade row.
Rotordynamic Computational and Experimental Characterization of a Convergent Honeycomb Seal Tested With Negative Pre-Swirl, High Pressure With Static Eccentricity and Angular Misalignment
Authors: Vannini Giuseppe , Mazzali Carlo , Underbakke Harald
Published: September 2016

Hole-pattern or honeycomb seals have been commonly used for many years in the Oil & Gas industry as damper seals for turbomachinery. The main motivation has been to introduce additional damping to improve the shaft rotordynamic stability operating under high pressure conditions.

Experience has shown that the dynamic and even static characteristics of those seals are very sensitive to the operating clearance profile as well as the installation tolerances. Rotordynamic stability is related not only to the seal effective damping but to the effective stiffness as well. In fact, for this kind of seal, the effective stiffness can be high enough to alter the rotor system’s natural frequency. The seal stiffness is strictly related to the tapering contour: if the clearance profile changes from divergent to convergent, the effective stiffness may change from a strong negative to a strong positive magnitude, thus avoiding the rotor natural frequency drop as it is detrimental for the stability. Unfortunately the effective damping is reduced at the same time but this can be improved using proper devices to keep the pre-swirl low or even negative (e.g. swirl brakes, shunt holes).

This paper presents the results from an extended test campaign performed in a high-speed rotor test rig equipped with active magnetic bearings working under high pressure (14krpm, 200bar gas inlet pressure), with the aim to validate the rotordynamic characteristics of a negative pre-swirl, convergent honeycomb seal and demonstrate its ability to effectively act as a gas bearing as well as a seal.

The test plan included variations of inlet pressure, differential pressure (given the same inlet pressure) as well as rotational speed in order to fully validate the seal behaviour. This kind of test was performed in a “dynamic mode”, exciting the spinning test rotor through a pair of AMBs along linear orbits.

Additionally the impact of the seal to rotor static eccentricity and the seal to rotor angular misalignment were both experimentally investigated and compared to relevant CFD simulations. This kind of test was performed in a “static mode”, imposing through the AMBs the required eccentricity / angular misalignment and then measuring the forces needed to keep the rotor in the original position. “Dynamic mode” test was also performed in order to check the impact of the seal static eccentricity on its dynamic behaviour.

Finally the test results were compared with predictions from a state of the art bulk flow code in order to check the predictability level for future design applications.
Quasi 2D Flow-Adaptive Algorithm for Pneumatic Probe Measurements
Authors: Bartsch Christian , Hölle Magnus , Jeschke Peter , Metzler Timo
Published: September 2016

The subject of this paper is a flow-adaptive measurement grid algorithm developed for 1D and 2D flow field surveys with pneumatic probes in turbomachinery flows. The algorithm automatically determines the distribution and the amount of measurement points needed for an approximation of the pressure distribution within a predefined accuracy.

The algorithm is based on transient traverses, conducted back and forth in the circumferential direction. The dynamic response of the pressure-measuring system is disregarded during the traverses, which serve to detect changes in the pressure field. In contrast to previous investigations by the authors, a correction of the dynamic response is applied by deconvolving the transient measurement data using the information embedded in both transient measurements. In consequence, the performance of the algorithm is — to a large extent — independent of the transient traversing speed and the geometry of the pressure-measuring system. Insertion and removal strategies are incorporated in order to reduce measurement points and increase robustness towards differing flow field conditions. By approximation of the pressure distribution, the flow-adaptive measurement data is suited for the application of post-processing corrections without any constraints. The performance of the algorithm is demonstrated for 2D flow field surveys with a pneumatic 5-hole probe in an annular cascade wind tunnel. Compared to conventional techniques for data sampling, e.g., uniform measurement grids, the measurement grid points are automatically adjusted so that a consistent resolution of the flow features is achieved within the measurement domain. Furthermore, the application of the algorithm shows a significant reduction in the number of measurement points. Compared to the measurement duration based on uniform grids, the duration is reduced by at least 7%, while maintaining a high accuracy of the measurement.

The purpose of this paper is to demonstrate the performance of measurement grids adapted to local flow field conditions. Consequently, valuable measurement time can be saved without a loss in quality of the data obtained.
Calculation of Enthalpy and Entropy From Experimentally Measurable Quantities
Authors: Coogan Shane , Brun Klaus , Simons Sarah , Ridens Brandon , Kurz Rainer
Published: September 2016

In the design and testing of gas compressors, the correct determination of the thermodynamic properties of the gas, such as enthalpy, entropy, and specific volume from pressure, temperature, and composition, plays an important role. Due to the wide range of conditions encountered, pressure, specific volume and temperature (p-v-T) equations of state (EOS) are used to determine the isentropic or polytropic efficiency, the work input, and capacity of a compressor configuration. However, accurate equations of state may be lacking for some more complicated gas mixtures. Experimentally determined thermodynamic state information is more accurate and is needed to validate, correct, or supplant existing equations of state.

The methodology for calculating enthalpy and entropy from experimental data is presented including the full step-by-step derivation from first principles. The thermodynamic relations or calculus properties used in each step of the derivation are clearly identified to provide traceability and encourage verification. Results are presented in three forms to match all possible tabulation formats of temperature, pressure, and volume data. Calculation methods for other useful but unmeasured thermodynamic properties such as the specific heat at constant pressure and the ratio of specific heats are also given. The methodology is demonstrated in two examples. The first is a verification case where REFPROP, an equation of state software, is used to generate input data, and the enthalpy and entropy values calculated from the input data are shown to match those given directly by REFPROP. The second is a practical demonstration where the methodology is used with actual experimental data.
Potential of Retrofit Passive Flow Control for Small Horizontal Axis Wind Turbines
Authors: Holst D. , Pechlivanoglou G. , Wegner F. , Nayeri C. N. , Paschereit C. O.
Published: September 2016

The present paper analyzes the effect of passive flow control (PFC) with respect to the retrofitting on small horizontal axis wind turbines (sHAWT). We conducted extensive wind tunnel studies on an high performance low Reynolds airfoil using different PFC elements, i.e. vortex generators (VGs) and Gurney flaps. QBlade, an open source Blade Element Momentum (BEM) code, is used to study the retrofitting potential of a simulated small wind turbine. The turbine design is presented and discussed. The simulations include the data and polars gained from the experiments and give further insight into the effects of PFC on sHAWT. Therefore several different blades were simulated using several variations of VG positions. This paper discusses their influence on the turbine performance. The authors focus especially on the start-up performance as well as achieving increased power output at lower wind speeds. The vortex generators reduce the risk of laminar separation and enhance the lift in some configurations by more than 40% at low Reynolds numbers.
Conjugate Heat Transfer Analysis for Gas Turbine Film-Cooled Blade
Authors: Ho Kuo-San , Liu Jong S. , Elliott Thomas , Aguilar Bruno
Published: September 2016

This paper presents results of a study of the conjugate heat transfer (CHT) to calculate the metal temperature for a film-cooled gas turbine blade. ANSYS CFX14.0 code was selected as the computational fluid dynamic (CFD) tool to perform the CHT simulation. The two-equation SST turbulence model with automatic wall treatment was employed. The main flow inlet and exit boundary conditions were deduced from a multi-blade-row CFD code, Fine/Turbo by NUMECA.

A core engine test operated at the maximum power condition. Thermocouples were used to validate the blade metal temperature calculations.

The blade temperature comparison between test data and CHT predictions was in good agreement except at the suction side near the leading edge region. The pressure, temperature and Mach number distributions for blade internal and external flows were presented and examined. The streamline contours of the film flows on the pressure side and suction side were plotted and used to visualize the cooling effectiveness.

In order to evaluate the influence of the turbulence model, the thermal results of four additional turbulence models (SA, RNG, K-ε, and SST with transition control) were compared to the test data. The SST model is suggested to be the appropriate turbulence model for the film-cooled blade temperature calculation in this study.
Dependence of Film Cooling Effectiveness on 3D Printed Cooling Holes
Authors: Aghasi Paul , Gutmark Ephraim , Munday David
Published: September 2016

Film Cooling Effectiveness is closely dependent on the geometry of the hole emitting the cooling film. These holes are sometimes quite expensive to machine by traditional methods so 3D printed test pieces have the potential to greatly reduce the cost of film cooling experiments. What is unknown is the degree to which parameters like layer resolution and the choice among 3D printing technologies influence the results of a film cooling test. A new flat-plate film cooling facility employing oxygen sensitive paint (OSP) verified by gas sampling and the mass transfer analogy and measurements both by gas sampling and OSP is verified by comparing measurements by both gas sampling and OSP. The same facility is then used to characterize the film cooling effectiveness of a diffuser shaped film cooling hole geometry. These diffuser holes are then produced by a variety of additive manufacturing technologies with different build layer thicknesses. Technologies used include Fused Deposition Modeling (FDM), Stereo Lithography Apparatus (SLA) and PolyJet with build layer thicknesses ranging from 0.001D (25 μm) to 0.12D (300 μm). These are compared with an aluminum coupon manufactured by traditional machining methods. The objective is to determine if cheaper manufacturing techniques afford usable and reliable results. Tests are carried out at mainstream flow Mach number of 0.30 and blowing ratios (BRs) from 1.0 to 3.5. The coolant gas used is CO2 yielding a density ratio of 1.5. Surface quality is characterized by an Optical Microscope that measures surface roughness. Test coupons with rougher surface topology generally showed delayed blow off and higher film cooling effectiveness at high blowing ratios compared to the geometries with lower measured surface roughness. At the present scale, none of the additively manufactured parts consistently matched the traditionally machined part, indicating that caution should be exercised in employing additively manufactured test pieces in film cooling work.
Shock Structure, Mode Shape, and Geometric Considerations for Low-Pressure Turbine Flutter Suppression
Authors: Waite Joshua J. , Kielb Robert E.
Published: September 2016

Modern design principles are pushing for lighter aircraft engines, and higher tip Mach numbers in steam turbines. Consequently, the loading and shock structures seen on aft stages of low-pressure turbines are evolving at a fast pace. Since flutter is a high risk event in these low reduced frequency blades, physically understanding and predicting how new operating conditions affect aeroelastic behavior proves crucially important. Previous numerical studies on the Standard Configuration 4 LPT concluded loading, and in particular the shock structure, strongly impacted the aerodynamic damping as a function of mode shape. This paper presents a deeper investigation into the physical explanation behind the interdependency of loading, mode shape, reduced frequency, and LPT geometry. Serving as computational test rigs, four unique quasi-2D LPT configurations are exhaustively analyzed using validated linearized unsteady CFD. Three primary contributors to this mode-dependent flutter phenomenon are shown to be the passage shocks, blade geometry, and interblade phase angle. Further, the sensitivity of critical reduced frequency as functions of bending mode and loading are divided into four zonal regions exhibiting qualitatively similar behavior. It is shown in some zones, depending on airfoil geometry and shock strength, that loading will always either increase or decrease stability monotonically. For pitching modes, the shock strength, phase, and location with respect to the rigid body center of torsion prove to play a key part in determining stability. Understanding these shock effects generalized to all LPT geometries yield new design considerations useful for suppressing flutter.
Minimising Clearance Consumption: A Key Factor for the Design of Blades Robust to Rotor/Stator Interactions?
Authors: Batailly Alain , Millecamps Antoine
Published: September 2016

The recent development of a numerical strategy dedicated to the simulation of rotor/stator interactions stemming from structural contacts in modern aircraft engines led to the first optimization of a high-pressure compressor blade profile accounting for criteria related to non-linear contact simulations. This optimization procedure revealed very significant improvements in terms of amplitudes of vibration but failed to identify key design parameters. Satisfying numerical results were obtained by a minor modification of a combination of many design parameters. Based on this observation, this contribution intends to shed a new light on this previous redesign operation focusing on one key quantity: the clearance consumption. This quantity is presented in the first section. In the second section, results of the redesign operation are recalled before the presentation of original results, featuring detailed interaction maps in the frequency domain, on which focuses the third section of the article. Finally, the blade profiles are extensively compared based on their specific clearance consumption and presented results suggest that this quantity may be key in discriminating acceptable from unacceptable blade profiles with respect to their vibratory behaviour when structural contacts occur.
Numerical Investigation of Impingement Heat Transfer on Concave- and Convex-Dimpled Plate With Different Dimple Arrangements
Authors: Zhang Feng , Wang Xinjun , Li Jun , Tan Rui , Wei Dongliang
Published: September 2016

The present numerical study is conducted to investigate the flow and heat transfer characteristics for impingement cooling on concave or convex dimpled plate with four different dimple arrangements. The investigation of the impingement cooling on the flat plate is also conducted to serve as a contrast and these results are compared with experimental measurements to verify the computational method. Dimples studied here are placed, relative to impingement holes, in either spanwise shifted, in staggered, in in-line, or in streamwise shifted arrangements. The flow structure, pressure loss and heat transfer characteristics of the concave and convex dimpled plate of four different dimple arrangements have been obtained and compared with flat plate for the Reynolds number range of 15000 to 35000. The results show that compared with flat plate, the added concave or convex dimples only causes a negligible increase in the pressure loss, and the pressure loss is insensitive to concave or convex dimple arrangement patterns. In addition, compared with flat plate, both spanwise shifted and staggered concave dimple arrangements show better heat transfer performance, while in-line concave dimple arrangement show worse results. Besides that, the heat transfer performance for streamwise shifted concave dimple arrangement is the worst. Furthermore, compared with flat plate, all convex dimple arrangements studied here show better heat transfer performance.
Influence of Impeller Fairing Cone Geometry on Cavitating Flow Behavior in a Cryogenic Liquid Turbine Expander
Authors: Wang Ke , Sun Jinju , Song Peng , Huo Changjiang
Published: September 2016

A single stage cryogenic liquid turbine expander is developed as a replacement for traditional Joule-Thomson valves used in the large-scale internal compression air-separation unit for the purpose of energy saving. Similar to the conventional hydraulic turbine, detrimental swirling and cavitation flow is also encountered at turbine expander impeller exit and its successive diffuser tube, but due to significant thermodynamic effect of cryogenic fluid flow, it is much more complicated than the conventional hydraulic turbine.

In the present study, cavitating flow mechanism of the turbine expander is investigated first with a combination of the homogenous multiphase mixture model and the Rayleigh-Plesset model, where the former treats liquid and gas as a continuum mixture and the latter depicts the bubble dynamics. Then sensitivity study is conducted for the impeller fairing cone geometry on suppression of cavitating flow. The following are demonstrated: with a use of the fairing cone, flow behavior near and downstream the impeller exit is significantly improved, where the low static pressure region is reduced and the local temperature rise decreases, subsequently the cavitating flow is effectively suppressed. The cavitating flow is sensitive to a tuning of the fairing cone geometry, and an optimal design of the cone geometry is essential.
Numerical Study of Cavitating Flow in Two-Phase LNG Expander
Authors: Song Peng , Sun Jinju , Li Kaiqiang , Wang Ke , Huo Changjiang
Published: September 2016

LNG expander is developed and used as a replacement of a J-T valve in liquefaction process of natural gas to reduce significantly the energy consumption in the LNG plant. Similar to conventional hydraulic turbines, the unexpected cavitation also occurs in the LNG expander. In the present study, cavitating flow in two-phase LNG expander is investigated. With the justified Rayleigh-Plesset cavitation model, cavitating flow characteristics is investigated for the LNG expander in the entire stage environment including an annular bend, nozzle ring, and radial inflow impeller. On the basis of cavitating flow analysis, a coaxial rotating exducer is developed and fitted downstream to the impeller, so as to reduce the cavitation in impeller and subsequently prevent impeller damage.

The following are demonstrated: (1) without exducer, significant cavitating flow is encountered at the impeller trailing edge and also in half streamline-wise region, and they are resulted from the viscous dissipation and flow separation; (3) with exducer, the impeller cavitation has diminished entirely but it has occurred in the successive exducer; (3) a use of exducer enhances the energy conversion capability of the rotors, but reduces the overall temperature drop and efficiency of the expander; (4) the design optimization of exducer is required to suppress the exducer cavitation, which also needs to be incorporated with the impeller design to achieve a better match between rotor/stator, so as to maximize the design optimization benefits.
An Experimental Study of the Aerodynamic Forcing Function in a 1.5 Steam Turbine Stage
Authors: Gatti Giacomo , Gaetani Paolo , Paradiso Berardo , Dossena Vincenzo , Arcangeli Lorenzo , Maceli Nicola , Bellucci Juri
Published: September 2016

The usual ways to measure the aerodynamic forcing function are complex and expensive. The aim of this work is to evaluate the forces acting on the blades using a relatively simpler experimental methodology based on a time-resolved pressure measurement at the rotor discharge. Upstream of the rotor, a steady three holes probe has been used.

The post processing procedures are described in detail, including the application of a phase-locked average and of an extension algorithm with phase-lag. The algorithm for the computation of the force components is presented, along with the underlying assumptions.

In order to interpret the results, a preliminary description of the flow field, both upstream and downstream of the rotor, is provided. This gives an insight of the most relevant features that affect the computation of the forces.

Finally, the analysis of the results is presented. These are first described and then compared with overall section-average results (torque-sensor), and with the results from 3D unsteady simulations (integral of pressure over the blade surface) in order to assess the accuracy of the method. Both the experimental and the numerical results are also compared for two different operating conditions with increasing stage load.
Brush Seal Frictional Heat Generation: Test Rig Design and Validation Under Steam Environment
Authors: Raben M. , Friedrichs J. , Flegler J.
Published: September 2016

Sealing technology is a key feature to improve efficiency of steam turbines for both new power stations and modernization projects. One of the most powerful sealing alternatives for reducing parasitic leakages in the blade path of a turbine as well as in shaft sealing areas is the use of brush seals, which are also widely used in gas turbines and turbo compressors. The advantage of brush seals over other sealing concepts is based on the narrow gap that is formed between the brush seal bristle tips and the mating rotor surface together with its radial adaptivity. While the narrow gap between the bristle tips and the rotor leads to a strongly decreased flow through the seal compared with conventional turbomachinery seals, it is important to be aware of the tight gap that can be bridged by relative motion between the rotor and the brush seal, leading to a contact of the bristles and the rotor surface. Besides abrasive wear occurrence, the friction between the bristles and the rotor leads to heat generation which can be detrimental to turbine operation due to thermal effects, leading to rotor bending connected to increasing shaft vibrations. In order to investigate the frictional heat generation of brush seals, different investigation concepts have been introduced through the past years. To broaden the knowledge about frictional heat generation and to make it applicable for steam turbine applications, a new testing setup was designed for the steam test rig of the Institute of Jet Propulsion and Turbomachinery - TU Braunschweig, Germany, enabling temperature measurements in the rotor body under stationary and transient operation in steam by using rotor-integrated thermocouples. Within this paper, the development of the instrumented new rotor design and all relevant parts of the new testing setup is shown along with the testing ability by means of the validation of the test rig concept and the achieved measurement accuracy. First results prove that the new system can be used to investigate frictional heat generation of brush seals under conditions relevant for steam turbine shaft seals.
Numerical Investigation of Tip Clearance Flow Induced Flutter in an Axial Research Compressor
Authors: Möller Daniel , Jüngst Maximilian , Holzinger Felix , Brandstetter Christoph , Schiffer Heinz-Peter , Leichtfuß Sebastian
Published: September 2016

A flutter phenomenon was observed in a 1.5-stage configuration at the Darmstadt transonic compressor. This phenomenon is investigated numerically for different compressor speeds. The flutter occurs for the second eigenmode of the rotor blades and is caused by tip clearance flow which is able to pass through multiple rotor gaps at highly throttled operating points. The vibration pattern during flutter is accompanied by a pressure fluctuation pattern of the tip clearance flow which is interacting with the blade motion causing the aeroelastic instability. The velocity of the tip clearance flow fluctuation is about 50% of the blade tip speed for simulation and experiment and also matches the mean convective velocity inside the rotor gap. This is consistent for all compressor speeds. From this investigations, general guidelines are drawn which can be applied at an early stage during compressor design to evaluate the susceptibility to this kind of blade vibration.
Hot Gas Ingress Estimation Methods Validation in GT Stator-Rotor Cavities by Means of Extensive Field Measurements
Authors: D’Alessandro A. , Bavassano F. , Marré Brunenghi M. , Patrone S. , Pirotta M. , Mantero M. , Santolini B.
Published: September 2016

Flow and heat exchange in rotor-stator turbine cavities are very complex and it can be difficult to isolate contributions of normal heat and work exchange with the stator (and rotor) from the contribution coming from the possible hot gas ingestion from the main-flow, even for a well-instrumented machine with temperature probes. A simple method of detecting hot gas ingestion in stator-rotor turbine cavities is herein presented, this method is based on the analysis of differential temperatures and their time trends, acquired within several test campaigns recently carried out on three Gas Turbines (GTs).
Fault Diagnosis on an Aircraft Engine Model Equipped With Self-Sensing Piezoelectric Actuators
Authors: Ambur Ramakrishnan , Zhao Xiaonan , Rinderknecht Stephan
Published: September 2016

Piezoelectric actuators provide an active solution for vibration control in aircraft engines compared to the state-of-the-art squeeze film dampers. The property of piezoelectric materials enable them to be used as sensors and actuators simultaneously. This self-sensing property of the actuator is analyzed in this paper for its ability to detect unbalance faults, which are common in rotor systems. In this paper two different actuator configurations are studied for its ability to diagnose unbalance faults in an aircraft engine. Three parameters of unbalances such as its magnitude, its position in the circumferential and axial direction in a rotor are estimated through simulations. Finally a suitable position to achieve a better fault diagnosis is identified.
Low Cycle Fatigue of 9-12% Cr Steel at Elevated Temperature: Cyclic Deformation Behavior and Life Assessment
Authors: Zhao Peng , Xuan Fu-Zhen , Wu De-Long
Published: September 2016

Fatigue testing for an important turbine rotor material (X12CrMoWVNbN10-1-1 steel) was carried out over a wide range of strain and stress amplitudes at 873K. Particular attention was paid to the effect of control mode on the cyclic deformation behavior and life assessment at elevated temperature. Two main domains were observed depending both on the strain and stress amplitudes, where the effect of control mode was different. In the micro plastic deformation domain, the cyclic softening is slight and there is no clear difference in fatigue behavior between the stress and strain modes. In the plastic damage regime, stress cycling causes more significant softening or damage than strain cycling. The dependence of damage behavior on the evolution of dislocation substructure was focused. On the other hand, it is not possible to use strain based life model to predict fatigue life with the test results under a different control mode. A unified energy-based model is proposed based upon the deformation mechanism and the experimental results, which can assess the low cycle fatigue life with different control modes. The results obtained in this study could have significant implications in the design of structures.
HCF Component Tests on Full-Scale Low Pressure Steam Turbine End Stage Blades
Authors: Sheng Shilun , Flegler Johan , Becs Balazs Janos , Dankert Michael
Published: September 2016

The design of steam turbine components is driven by high efficiency demands and also requirements for increased operational flexibility due to more renewable energy sources being added to the grid. Therefore, fossil power plants which operate reliably under these conditions must be designed. Robust low pressure (LP) end stage blades are one key factor for modern steam turbine design to meet current and future market requirements.

In operation, LP end stage blades of steam turbines are exposed to complex mechanical load, resulting in stresses mainly due to blade vibration and high centrifugal forces. Design methods accounting for high cycle fatigue (HCF) and low cycle fatigue (LCF) are required for fatigue lifetime calculation. To determine the HCF component strength and to validate the calculation procedure, an HCF component test facility for full-scale LP end stage blades has recently been established at Siemens. Besides the validation of the calculation procedures, the full-scale component tests serve as part of upfront validation to minimize risk for first time implementation of newly developed as well as next generation blades, and to demonstrate operational robustness of the existing fleet.

This paper describes the development and setup of the HCF component test facility for full-scale LP end stage blades at Siemens, the successful execution of HCF component tests with blades of different sizes, surface conditions and materials, and the evaluation of the results. In addition, crack growth and threshold behavior has been investigated in detail. Based on the test results, validation of the corresponding calculation methods has been performed. An outlook on further development of test facilities is provided.
Experimental and Numerical Development of a Dynamic Clearance Seal for Steam Turbine Application
Authors: Messenger Andrew , Williams Richard , Ingram Grant , Hogg Simon , Tibos Stacie , Seaton Jon , Charnley Bernard
Published: September 2016

This paper presents a series of experiments on the Aerostatic Seal, a dynamic clearance seal for steam turbine application first described at the 2015 ASME Turbo Expo (Paper Number GT2015-43471). This dynamic clearance seal moves with rotor excursions and so has the potential to deliver a smaller clearance than traditional seals. The concept is an extension of the retractable seal design which is widely used in existing steam turbines.

The experimental program was carried out in a low cost static test facility using an aerostatic seal design. The seal exhibited a dynamic clearance response and will therefore respond to rotor excursions. 3D CFD was also used to aid the understanding of flow features not captured by the analytical design tool. Adjustments to both the design process and to future seal designs are proposed in the body of the paper.

This paper therefore describes an experimental proof of concept for the aerostatic seal and paves the way for future development in rotating facilities.
Multi-Disciplinary Optimization of a Mixed-Flow Compressor Impeller
Authors: Diener Olaf H. F. , van der Spuy Sybrand J. , von Backström Theodor W. , Hildebrandt Thomas
Published: September 2016

In this paper the procedure and results of the aeromechanical optimization of a mixed-flow compressor impeller to be used in a 600 N micro gas turbine (MGT) are presented. Today’s unmanned aerial vehicles (UAVs) demand high thrust-to-weight ratios and low engine frontal area. This may be achieved using mixed-flow compressors. The initial impeller design was obtained using a 1-D turbomachinery layout tool. A multi-point optimization of the impeller aerodynamic performance was completed, followed by a mechanical optimization to reduce mechanical stresses in the impeller. A coupled aero-mechanical optimization was concluded with the purpose of increasing the choke limit and reducing stresses while conserving aero-performance. Subsequently, a modal analysis of the rotor was conducted to determine its vibrational characteristics. The optimization process was set up and controlled in an integrated environment that includes a 3-D Navier-Stokes flow solver and a 3-D finite element (FE) structural solver. The optimization process is based on the use of a database, an artificial neural network (ANN), a user-defined objective function and a genetic algorithm (GA). The overall optimization process achieved an increase in pressure ratio (total-to-total) of 30.6% compared to the initial design while the efficiency (isentropic total-to-total) was increased by 5% at design conditions. A decrease in the surge margin was experienced, but the final surge margin was still acceptable (12%). The choke limit was increased meaningfully. This was achieved while also decreasing the peak von Mises stress from far above the material yield strength to 30% below the yield limit.