bluuebunny/crossref_metadata_2025 huggingface dataset:
Unsteady Flow Simulation of Leading-Edge Separation in Thin-Airfoil Cascade: A Modification to Turbulence Model
Authors: Tateishi Atsushi , Watanabe Toshinori , Himeno Takehiro , Inoue Chihiro
Published: August 2015

Leading edge separation of thin airfoil cascade in subsonic flow at large angle of incidence was simulated by implicit large eddy simulation (ILES) and Reynolds averaged Navier-Stokes (RANS) simulations with various turbulence models. In the ILES simulations with fine grids, the time-averaged surface pressure qualitatively agreed with the experimental data. The RANS and ILES simulations on the coarse mesh failed to capture a peak of pressure near the leading edge. From spectrum analysis, it was observed that the flow-field was turbulent in the separation bubble. In the failed RANS simulations, the separation bubble was much longer and the turbulence energy near the leading edge was much lower than those in the ILES results. The development of lambda-shaped vortex structures and their sudden weakening near the reattachment point was observed in the unsteady simulations. Two possible modifications to existing turbulence models in RANS simulations were proposed based on the comparison of turbulence energy between the ILES and RANS results. It is shown that these modifications improve the bubble length and Cp distributions of RANS simulations, though further validation and modeling are needed for the application to realistic cases.
Heat Transfer Enhancement and Pressure Loss Characteristics of Zig-Zag Channel With Dimples and Protrusions
Authors: Siw Sin Chien , Chyu Minking K. , Alvin Mary Anne
Published: August 2015

This paper described a detailed experimental study to explore an internal cooling passage that mimic a “zig-zag” pattern. There are four passages connected by 110° turning angle in a periodic fashion, hence the name. Experiments are performed in a scaled-up test channel with a cross-section of 63.5mm by 25.4mm, corresponding to the aspect ratio of 2.5:1. Compared to the conventional straight internal cooling passages, the zig-zag channel with several turns will generate additional secondary vortices while providing longer flow path that allows coolant to remove much more heat load prior to discharge into the hot mainstream. Surface features, (1) dimples, and (2) protrusions are added to the zig-zag channel to further enhance the heat transfer, while contributed to larger wetted area. The experiment utilizes the well-established transient liquid crystal technique to determine the local heat transfer coefficient distribution of the entire zig-zag channel. Protrusions exhibit higher heat transfer enhancement than that of dimples. However, both designs proved to be inferior compared to the rib-turbulators. Pressure loss in these test channels is approximately twofold higher than that of straight smooth test channel due to the presence of turns; but the pressure loss is lower than the zig-zag channel with rib-turbulators. The result revealed that one advantage of having either protrusions or dimples as these surface elements will resulted in gradual and more uniform increment of heat transfer throughout the entire channel compared to previous test cases.
Computation and Monitoring of the Deviations of Gas Turbine Unmeasured Parameters
Authors: Zarate Luis Angel Miro , Loboda Igor
Published: August 2015

One of the principle purposes of gas turbine diagnostics is the estimation and monitoring of important unmeasured quantities such as engine thrust, shaft power, and engine component efficiencies. There are simple methods that allow computing the unmeasured parameters using measured variables and gas turbine thermodynamics. However, these parameters are not good diagnostic indices because they strongly depend on engine operating conditions but in a less degree are influenced by engine degradation and faults.

In the case of measured gas path variables, deviations between measurements and an engine steady state baseline were found to be good indicators of engine health. In this paper, the deviation computation and monitoring are extended to the unmeasured parameters. To verify this idea, the deviations of compressor and turbine efficiencies as well as a high pressure turbine inlet temperature are examined. Deviation computations were performed at steady states for both baseline and faulty engine conditions using a nonlinear thermodynamic model and real data. These computational experiments validate the utility of the deviations of unmeasured variables for gas turbine monitoring and diagnostics.

The thermodynamic model is used in this paper only to generate data, and the proposed algorithm for computing the deviations of unmeasured parameter can be considered to be a data-driven technique. This is why the algorithm is not affected by inaccuracies of a physics-based model, is not exigent to computer resources, and can be used in on-line monitoring systems.
An Integrated Approach Using Condition Monitoring and Modeling to Investigate Wind Turbine Gearbox Design
Authors: Sheng Shuangwen , Guo Yi
Published: August 2015

Vibration-based condition monitoring (CM) of geared utility-scale turbine drivetrains has been used by the wind industry to help improve operation and maintenance (O&M) practices, increase turbine availability, and reduce O&M cost. This study is a new endeavor that integrates the vibration-based CM technique with wind turbine gearbox modeling to investigate various gearbox design options. A team of researchers performed vibration-based CM measurements on a damaged wind turbine gearbox with a classic configuration, (i.e., one planetary stage and two parallel stages). We observed that the acceleration amplitudes around the first-order sidebands of the intermediate stage gear set meshing frequency were much lower than that measured at the high-speed gear set, and similar difference was also observed in a healthy gearbox. One factor for a reduction at the intermediate stage gear set is hypothesized to be the soft sun-spline configuration in the test gearbox. To evaluate this hypothesis, a multibody dynamic model of the healthy test gearbox was first developed and validated. Relative percent difference of the first-order sidebands — of the high-speed and intermediate stage gear-meshing frequencies — in the soft and the rigid sun spline configurations were compared. The results verified that the soft sun-spline configuration can reduce the sidebands of the intermediate stage gear set and also the locating bearing loads. The study demonstrates that combining vibration-based CM with appropriate modeling can provide insights for evaluating different wind turbine gearbox design options.
Numerical Analysis on Axial Compressor Stage Performance With Vortex Generators
Authors: Agarwal Ruchika , Narayanan Sridharan R. , Goswami Shraman N. , Srinivasan Balamurugan
Published: August 2015

The performance of axial flow compressor stage can be improved by minimizing the effects of secondary flow and by avoiding flow separation. At higher blade loading, interaction of tip secondary flow and separated flow on blade surface is more near the tip of the rotor. This results in stall and hence decreases compressor performance. A previous study [1] was carried out to improve the performance of a rotating row of blades with the help of Vortex Generators (VGs) and considerable effects were observed. The current investigation is carried out to find out the effect of Vortex Generator (VG) on the performance of a compressor stage. NASA Rotor 37 with NASA Stator 37 (stage) is used as a test case for the current numerical investigation. VGs are placed at different chord wise as well as span wise locations. A mesh sensitivity study has been done so that simulation result is mesh independent. The results of numerical simulation with different geometrical forms and locations of VGs are presented in this paper. The investigation includes a description of the secondary flow effect and separation zone in compressor stage based on numerical as well as experimental results of NASA Rotor 37 with Stator 37 (without VG). It is also observed that the shape and location of the VG impacts the end wall cross flow and flow deflection [1], which result in enhanced stall range.
Near-Hole Thermal Field Measurements for Round Compound Angle Film Cooling Holes Fed by Cross-Flow
Authors: McClintic John W. , Wilkes Ellen K. , Bogard David G. , Dees Jason E. , Laskowski Gregory M. , Briggs Robert
Published: August 2015

The effectiveness of film cooling from short cooling holes, scaled to engine conditions, has been shown to be dependent on the nature of the internal coolant feed. A common method of supplying coolant to film cooling holes in engine components is through an internal cross-flow, which causes skewed effectiveness profiles on the surface of film cooled parts. For round axial holes, this effect causes coolant jets to more effectively spread across the surface. Additionally, for compound angle round holes, the direction of the cross-flow relative to the direction of injection has a substantial effect on film cooling effectiveness. A cross-flow directed counter to the span-wise direction of coolant injection has previously been shown to cause greater lateral jet spreading than cross-flow directed in-line with the span-wise injection direction. To better understand the phenomena responsible for the improved coolant spreading, two-dimensional thermal field profiles were measured downstream of compound angle film cooling holes fed by an internal cross-flow. A smooth-walled rectangular channel was used to produce an internal cross-flow in both a counter and in-line flow direction. Thermal field cross-section data was collected at three stream-wise locations: 0.7, 3.4, and 8.8 diameters downstream of the holes. Blowing ratios of 0.75 and 1.00 were studied at a density ratio of 1.5. Experiments were performed in a low speed recirculating wind tunnel at high mainstream turbulence with a thick approach boundary layer relative to the film cooling holes. It was found that the improved lateral spreading observed in the coolant jets fed by a counter cross-flow occurred due to the formation of a bulge on the downstream side of the jet.
A LCF Life Assessment Method for Steam Turbine Long Blade Based on Elastoplastic Analysis and Local Strain Approach
Authors: Yu Deqi , He Aping , Yang Hong , Yang Jiandao , Cheng Kai , Zhou Daiwei , Wang Gongyi , Gu Luyin
Published: August 2015

Cyclic loads applied to a structure can develop local cyclic plasticity deformation, lead to fatigue damage and fracture at the high-stress regions, which can be assessed through a local strain approach. Each cycle of start-operation-stop steam turbine, making the low cycle fatigue (LCF) load of long blade, results in damage to the long blade, and the fatigue fracture occurs when the damage accumulated to its critical value. To evaluate the fatigue life, the experimental data illustrating the cyclic behavior of a material under simple loading condition must be gathered, and also a suitable local stress-strain range calculation approach needs to be chosen to represent the accurate material behavior under loadings. With the consideration of the difference between the specimen and actual blade, the influential factors, such as mean stress, geometry effect, blade surface quality, and water erosion, on the fatigue life should be investigated when using the cyclic fatigue data of specimen to predict fatigue life of actual blade.

In this study, a new local stress-strain range approach is introduced based on elastoplastic finite element analysis and Neuber rule. And also a modified strain-life fatigue model is used by considering leading causes of fatigue and also the cumulative damage rule is set up to predict the LCF life of the steam turbine long blade. It is found that the assessment method proposed in this study is capable of predicting the LCF life of steam turbine long blade.
Numerical Modelling of Savonius Wind Turbine With Downstream Baffle
Authors: El Baz Ahmed M. , Mahmoud Nabil A. , Hamed Ashraf M. , El Kholy Ahmed M.
Published: August 2015

The application of small power wind turbines has been widely spreading over the last decade. The vertical axis wind turbine type is one class which is very attractive in this respect. However, due to its lower performance, it has not been extensively used in such application. The objective of the present paper is to investigate methods of Savonius turbine performance improvement. It is suggested to use a downstream baffle to achieve this goal. The baffle position, length and inclination angle are optimized using 2D Computational Fluid Dynamics model. The model is initially validated by comparison to experimental data reported for the Savonius turbine without baffle. The results show that 40–50% improvement in turbine performance can be obtained by using the optimum baffle design, compared to the turbine performance without baffle. The baffle effect is analyzed and results show that its major role is to reduce the pressure on the backside of the advancing blade. This effect enhances the positive driving torque on the advancing blade of the Savonius turbine and thus increases its power coefficient near the optimum tip speed ratio.
Design Optimization of Gas Foil Thrust Bearings for Maximum Load Capacity
Authors: Kim Tae Ho , Lee Tae Won
Published: August 2015

Improvement of the load capacity of gas foil thrust bearings (GFTBs) is important to broadening their application in oil-free microturbomachinery (<250 kW) with high power density. Although GFTBs have the significant advantage of low friction without the use of lubrication systems compared to oil film thrust bearings, their inherently low load capacity has limited their application. The aim of the present study was to develop a design guideline for increasing the load capacity of GFTBs. The Reynolds equation for an isothermal isoviscous ideal gas was used to calculate the gas film pressure. To predict the ultimate load capacity of the GFTB, the pressure was averaged in the radial direction of the gas flow field used to deflect the foil structure. The load capacity, film pressure profile, and film thickness profile were predicted for a GFTB with an outer radius of 55 mm, inner radius of 30 mm, and eight foils each of arc length 45°. The predictions showed that the load capacity of the GFTB increased with increasing rotor speed and decreasing minimum film thickness, and was always lower than the analytically determined limit value for infinite rotor speed (obtained by simple algebraic equations). A parametric study in which the ramp extent (or inclined angle) was increased from 5° to 40°, and the ramp height from 0 to 0.320 mm, revealed that the GFTB had an optimal ramp extent of ∼22.5° and ramp height of ∼0.030 mm for maximum load capacity. Interestingly, the optimal values were also valid for a rigid-surface bearing. The predicted load capacities for a ramp extent of ∼22.5° and increasing ramp height from 0.030 to 0.320 mm were compared with experimental data obtained from a previous work. The predictions for a ramp height of 0.155 mm were in good agreement with the experimental data for all three test GFTBs with outer radii of 45, 50, and 55 mm, respectively. In addition, this paper shows that the predicted drag torque increases linearly with increasing rotor speed and decreasing minimum film thickness, and nonlinearly with decreasing ramp height. The drag torque significantly increased only for ramp heights below the optimal value. The predictions imply that the optimal ramp height improves the load capacity of the GFTB with little change in the drag torque.
Investigation of Axial Spacing and Effect of Interface Location in a 1.5 Stage Transonic Axial Compressor
Authors: Choi Jaeho , Choi Taewoo
Published: August 2015

An axial compressor that is used for gas turbines has to meet axial length constraints as well as performance requirements. Therefore, the axial spacing between the stator and the rotor is one of the important design variables in the compressor design. Thus, a 3-dimensional fluid analysis is usually conducted to predict the performance of the compressor and to determine axial spacing in the preliminary design phase. The length of the inlet computational domain, which is influenced by shock waves from the transonic rotor, affects the compressor performance in the computational fluid analysis. This paper deals with a transonic axial compressor stage which consists of an inlet guide vane, a rotor and a stator which are designed as part of a high pressure compressor in a gas turbine. The purpose of this paper is to investigate the axial spacing between an IGV and a rotor, and effects of interface location in the spacing on the performance prediction of the axial compressor with the 3-dimensional fluid analysis. A multi-stage transonic axial compressor with an inlet guide vane was designed. Three dimensional Reynolds-averaged Navier-Stokes equation was used for the numerical analysis and the k-ω SST turbulence model was used to analyze the fluid flow in the transonic axial compressor. Numerical analyses were carried out at three interface locations in the axial spacing between the IGV and the rotor for two axial spacings in order to investigate the effect of interface location limiting the upstream flow of shock waves by the transonic rotor. The results show that the flow characteristics upstream of the rotor depend on the interface location. The detailed results on the performance and the flow fields are shown and discussed in the paper.
Verification of Performance and Operating Characteristics of High Pressure Gas Injection Centrifugal Compressor
Authors: Yanagihara Kazutoshi , Miura Haruo , Nishida Hideo , Tanaka Kenji , Takahashi Naohiko , Yagi Manabu , Naruse Tomohiro , Ridens Brandon L.
Published: August 2015

The development process of a 70-MPa high pressure compressor for oil and gas applications is presented in this paper. Great attention was paid to any relevant technical challenges due to the high design pressures, such as the material selection, deformation casing structure, stability of the rotordynamics, and the aerodynamic performance. Among the technical considerations, some technical findings for the operational tests are proposed in this paper as well.
Influence of Temperature Distribution on Tighten Force in Tie-Bolt Fastened Rotor With Considering Thermal Contact Conductance
Authors: Peng He , Xu Ning , Liu Zhansheng
Published: August 2015

Tighten force has much influence on tie-bolt fastened rotor dynamics. Temperature distribution in tie-bolt fastened rotor results in thermal expansion of rotor and rods. The difference of thermal expansion between rotor and rods causes the variation of bolt load. With considering the thermal contact conductance, the thermal model of tie-bolt fastened rotor was established by finite element method and the axial temperature distribution was obtained. The influences of surface roughness, nominal contact pressure and axial position of contact on axial temperature distribution were analysed. Based on temperature distribution in the tie-bolt fastened rotor, the variation of tighten force was investigated. Results show that nominal contact pressure, surface roughness and axial contact arrange have different influences on the variation of tighten force with temperature.
Creep in Interlaminar Shear of an Oxide/Oxide Ceramic Matrix Composite at Elevated Temperature
Authors: Ruggles-Wrenn M. B. , Hilburn S. R.
Published: August 2015

Creep behavior in interlaminar shear of an oxide-oxide ceramic composite was evaluated at 1100°C in laboratory air and in steam environment. The composite (N720/AS) consists of a porous aluminosilicate matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The interlaminar shear properties were measured. The interlaminar shear strength (ILSS) was determined as 7.6 MPa. The creep behavior was examined for interlaminar shear stresses in the 2–6 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air and in steam. Tertiary creep was noted in tests performed at 6 MPa. Creep run-out defined as 100 h at creep stress was not achieved in any of the tests. Larger creep strains and higher creep strain rates were produced in steam. However, the presence of steam had a beneficial effect on creep lifetimes. Composite microstructure, as well as damage and failure mechanisms were investigated.
Foreign Object Damage in an Oxide/Oxide CMC Subjected to Tensile Preloading
Authors: Faucett D. Calvin , Kedir Nesredin , Choi Sung R.
Published: August 2015

Foreign object damage (FOD) phenomenon of an N720/alumina oxide/oxide ceramic matrix composite (CMC) was determined previously using 1.59 mm-diameter hardened steel ball projectiles using impact velocities ranging from 150 to 350 m/s at a normal incidence angle. Target specimens were impacted under tensile preloading with three different levels of load factors of 0, 30, and 50%. Difference in impact damage between no-preload and pre-load was significant particularly at 350 m/s with the highest load factor of 50%. A fracture mechanics approach was used to predict post-impact strength as a function of load factor and was assessed its applicability using the experimental data.
Design of an Air Dynamometer for Direct Connection to the SunShot SCO2 Turbine Expander Shaft
Authors: Hoopes Kevin , Moore J. Jeffrey , Cich Stefan , Kalra Chiranjeev
Published: August 2015

In conjunction with the SunShot program to develop a supercritical CO2 turbine expander, an air dynamometer was developed to absorb the power of the SCO2 turbine during development testing. The dynamometer is unique in that it is directly mounted to the shaft of the turbine expander replacing the associated compressor coupling during the test campaign. It is able to replicate both the load of the compressor as well as the rotordynamic qualities of the compressor coupling all without the need for additional bearings, casing, or larger skid. The dynamometer consists of a single-stage centrifugal air compressor ingesting ambient air. To maximize turn-down and minimize weight, the impeller diameter and flow coefficient were optimized along with the design of the loop throttle valves. The detailed aerodynamic, rotordynamic, and mechanical design for the dynamometer is presented. The aerodynamic methodology to minimize unsteady forces transmitted to the turbine will also be reviewed.
Stabilizing Hydrodynamic Bearing Oil Whip With μ-Synthesis Control of an Active Magnetic Bearing
Authors: Pesch Alexander H. , Sawicki Jerzy T.
Published: August 2015

Oil whip is a self-excited vibration in a hydrodynamic bearing which occurs when the rotation speed is above approximately twice the first natural frequency. Because of this, the oil whip phenomenon limits the operational speed of a rotor system on hydrodynamic bearings. Below the oil whip threshold, the related phenomenon of oil whirl can cause large vibrations at frequencies below half the rotation speed.

A method is presented for stabilizing oil whip and oil whirl in a hydrodynamic bearing with an active magnetic bearing (AMB). The AMB controller is designed with μ-synthesis model-based robust control utilizing the Bently-Muszynska fluid film bearing model, which predicts the unstable phenomena. Therefore, the resulting AMB controller stabilizes the natural instability in the hydrodynamic bearing. Rotor speed is taken into account by use of a parametric uncertainty such that the method is robust to changes in running speed.

The proposed method is demonstrated on an experimental hydrodynamic bearing test rig. Details of the test rig and implementation of the AMB controller design are presented. Waterfall plots for the controlled and uncontrolled system are presented which demonstrate the improved stability limit.
Exploring the Impact of Elevated Turbine Blade Cooling Effectiveness and Turbine Material Temperatures on Gas Turbine Engine Performance and Cost
Authors: Byerley Aaron R. , Rolling August J.
Published: August 2015

Since the 1950’s, the turbine inlet temperatures of gas turbine engines have been steadily increasing as engine designers have sought to increase engine thrust-to-weight and reduce fuel consumption. In turbojets and low-bypass turbofan engines, increasing the turbine inlet temperature boosts specific thrust, which in some cases can support supersonic flight without the use of an afterburner. In high-bypass gas turbine engines, increasing the turbine inlet temperature makes possible higher bypass ratios and overall pressure ratios, both of which reduce specific fuel consumption. Increased turbine inlet temperatures, without sacrificing blade life, have been made possible through advances in blade cooling effectiveness and high-temperature turbine blade materials. Investigating the impact of higher turbine inlet temperatures and the corresponding cooling air flow rates on specific thrust, specific fuel consumption, and engine development cost is the subject of this paper. A physics-based cooling effectiveness correlation is presented for linking turbine inlet temperature to cooling flow fraction. Two cases are considered: 1) a low-bypass, mixed-exhaust, non-afterburning turbofan engine intended to support supercruising at Mach 1.5 and 2) a high-bypass, unmixed-exhaust turbofan engine intended to support highly efficient, long range flight at Mach 0.8. For each of these two cases, both baseline and enhanced cooling effectiveness values as well as both baseline and elevated turbine blade material temperatures are considered. Comparing these cases will ensure that students taking courses in preliminary engine design understand why huge research investments continue to be made in turbine blade cooling and advanced, high-temperature turbine blade material development.
Experimental and Computational Study of Novel Tip Leakage Mitigation Methods for High Pressure Turbine Blades
Authors: Ibrahim Mounir B. , Volino Ralph J.
Published: August 2015

This paper presents computational and experimental study of a possible approach to reduce tip leakage losses. The study was conducted on the EEE (Energy Efficient Engine) HPT (High Pressure Turbine) rotor tip geometry. The CFD was done utilizing the commercial numerical solver ANSYS FLUENT. The experimental work was conducted in a low speed wind tunnel with linear cascade at the USNA (for Re = 30,000) and the NASA Transonic Turbine Blade Cascade facility at the NASA John H. Glenn Research Center for Re of 85,000 to 683,000 at two isentropic exit Mach numbers of 0.74 and 0.34 were reported. The overall scope of this study is to investigate how the tip leakage and overall blade losses are affected by injection from the tip surface at the camber line, and the jet blowing ratio. The results identify areas where future investigation can be explored in order to achieve higher performance of the high pressure turbines.
Study and Analysis of a Power Turbine Preliminary Design for a Small Turboshaft
Authors: Bringhenti C. , Tomita J. T. , Cavalca D. F. , Monteiro V. G. , da Silva O. F. R.
Published: August 2015

This work describes the continuous study that is being done in a small gas turbine that can be used for power generation purposes. Previous studies were conducted aiming to develop a gas generator able to be used in both applications, as a turbojet or as a turboshaft. The gas generator was designed, manufactured and is still under test. The thermodynamic cycle calculation was evaluated as a project-based class, hence, a power turbine was specified and its requirements were determined. The outlet conditions from the gas generator were used to perform the preliminary size of the power turbine. At this phase, the students must use 1D design models considering loss modeling to improve the machine design prediction. The meanline technique was used and the calculations at leading and trailing edges were extrapolated from hub-to-tip, using vortex design methods. With the airfoil stacking for each blade row was possible to determine the 3D geometry of the single stage axial flow turbine. This geometry was assembled in a CAD software to start the mesh generation procedure. After this step, a commercial CFD software was used to calculate the continuity, momentum and energy equations from fluid mechanics. The flow was considered fully turbulent and the two-equation SST turbulence model was set to determine the flow eddy viscosity. The results from preliminary design and 3D techniques were compared and evaluated to complete the first round of the design phase. In this work, experiences from the project-based class on turbomachinery design are described together with the challenges and difficulties that appeared during the project.
Influence of Design Parameters on the Performance of a Multistage Centrifugal Compressor for Supercritical Carbon Dioxide Applications
Authors: Monge B. , Sánchez D. , Savill M. , Pilidis P. , Sánchez T.
Published: August 2015

The development of the supercritical Carbon Dioxide power cycle has relied on parallel tracks along which theoretical and experimental works have successfully complemented each other in the last few years. Following this approach, intensive work on the development of critical components has enabled the demonstration of the technology in small-scale test loops. The next step in the roadmap is scaling-up the technology in order to bridge the gap to commercialisation. To this aim, not only is it necessary to demonstrate that the cycle works, but it is also mandatory to rise component (and system) efficiencies to levels comparable with competing technologies. In this process, assessing the impact of the main design parameters on the efficiency of turbomachinery is deemed crucial.

The present work is a follow-up to others presented by the authors in previous years where preliminary analysis on centrifugal compressor design combining tools of different levels of fidelity were used. Nevertheless, whilst these presented guidelines to design the main compressor successfully, this new piece of research presents how the design space of the unit is affected by the characteristics of the working fluid. A review of past research is first presented to evidence that the design space is largely influenced by the particular behaviour of the working fluid close to the critical point. Then, design maps are presented for different operating conditions (cycle heat balance), showing that their shapes change substantially depending on compressor inlet pressure and temperature. Also, a comparison of these maps confirms that the design regions enabling high efficiency can be substantially reduced depending on the inlet/outlet thermodynamic states. Finally, conclusions are drawn regarding optimal intervals for the main design parameters involved in the process.
Prediction of Sand Deposition in a Two-Pass Internal Cooling Duct
Authors: Singh Sukhjinder , Tafti Danesh
Published: August 2015

Sand transport and deposition is investigated in a two pass internal cooling ribbed geometry at near engine conditions. LES calculations are performed for bulk Reynolds number of 25,000 to calculate flow field and heat transfer. Constant wall temperature boundary condition is used to investigate the effect of temperature on particle deposition. Three different wall temperatures of 950 °C, 1000 °C and 1050 °C are considered. Particle sizes in range 0.5–25 microns are considered. A new deposition model which accounts for particle composition, temperature, impact velocity and angle and material properties of particle and surface is developed and applied. Calculated impingement and deposition patterns are discussed for different exposed surfaces in the two pass geometry. The highest particle impingement and deposition is observed in the bend region and first quarter of the second pass. Significant deposition is observed in the two pass geometry for all three wall temperatures considered. Particle impingement and hence deposition is dominated by larger particles except in the downstream half of the bend region. In total, approximately 38%, 59% and 67% of the injected particles deposit in the two passes, for the three wall temperatures of 950 °C, 1000 °C and 1050 °C, respectively. While particle impingement is highest for wall temperature of 950 °C, higher deposition is observed for 1000 °C and 1050 °C cases. Deposition increases significantly with wall temperature. For 1000 °C, roughly 12% of the impacting particles deposit. For 1050 °C, approximately 23% of the particles deposit on impact. For all the three cases, the second pass experiences higher deposition compared to the first pass due to higher turbulence and direct impingement.
Development and Assessment of Combustion Models for Gas Turbine and Aero-Engine Applications
Authors: Anker Jan E. , Wunsch Dirk , Romagnosi Luigi , Claramunt Kilian , Hirsch Charles
Published: August 2015

The classical flamelet method, the new Flamelet Generated Manifolds method (FGM), and the hybrid BML/flamelet approach are assessed in the context of the Reynolds-averaged Navier-Stokes (RANS) equations on a large range of configurations for both gaseous and spray flames. The conceptual differences, advantages, and shortcomings of the models are discussed in detail both from a theoretical and a practical point of view.

In order to assess the models under gas turbine like conditions, the reactive flow in TU Darmstadt’s Generic Gas Turbine (GGT), DLR Stuttgart’s PRECCINSTA burner, and a premixed industrial combustor are computed. The computational results are compared to available measurement data and are used to discuss the strengths and the weaknesses of each of the aforementioned combustion models.

In the current study it is shown that the hybrid BML/flamelet method globally performs well, but that it can be difficult to obtain a burning solution with this method, especially when the combustion process is operated close to the flammability limits. While the flamelet method is very robust, it is outperformed by the FGM method even for purely non-premixed configurations. It is demonstrated that the FGM approach can be used for the whole range of combustion modes, from non-premixed over to premixed combustion processes. Since the model did not lead to any difficulties with attaining a burning solution, and is computationally as efficient as the flamelet approach, the authors recommend the usage of this model over the other models investigated.
High Cycle Fatigue of Ti-6Al-4V Alloy in Simulated Steam Environment
Authors: Li Yu-Jia , Mei Lin-Bo , Xuan Fu-Zhen
Published: August 2015

Fatigue life and reliability are the critical problems for long blades design due to complicated stress state, wet steam and aggressive environment. In this report, the effects of stress ratio, surface properties, steam, and sodium-chloride (NaCl) aqueous environments on the fatigue strength and fracture mechanisms of Ti-6Al-4V alloy have been investigated. Results indicate that residual compressive stress decreases and vanishes finally with increasing stress ratio. Compared to fatigue crack originating from surface for annealed specimens, the fatigue crack initiation sites are located in the interior of the specimen due to the effect of residual stress when low stress ratios are present. Fatigue experiments have been performed in saturated steam with low oxygen content at 100°C and NaCl aqueous at 80 °C. Results indicate that, for 0.1 stress ratio loading conditions, steam environment demonstrates the most serious effect on the endurance limit with 12.3% reduction of fatigue strength. NaCl aqueous leads to the 9.6% drop in fatigue strength corresponding to 107 cycles of design life. For all corrosion environments, cracks originated from the surface and no corrosion pits were observed.
An Adjoint-Based Multi-Point Optimization Method for Robust Turbomachinery Design
Authors: Walther Benjamin , Nadarajah Siva
Published: August 2015

This paper introduces a multi-point design capability to discrete adjoint-based aerodynamic shape optimization for multi-stage turbomachines. The developed optimization framework allows to improve a compressor or turbine design not only for a certain operating point, but enables the inclusion of additional off-design operation points, therefore guaranteeing a robust design and annihilating the risk of improving the configuration for a specific design point while deteriorating the overall operability of the turbomachine. To keep the computational cost to a minimum, at every design cycle the flow and adjoint solutions are first calculated and stored for each operating point. This approach ensures that the subsequent finite-difference approximation of the residual sensitivity with respect to the design variables is obtained at a cost nearly independent of the number of investigated operating points. The objective function gradient is then assembled as a weighted sum of the sensitivities calculated for the different operating points. The developed multi-point optimization method is applied to a single-stage transonic compressor and both the back pressure and the rotor wheel speed are varied to investigate the use of adjoint-based design methods to efficiently explore robust turbomachinery designs.
Global and Local Hydrodynamic Stability Analysis as a Tool for Combustor Dynamics Modeling
Authors: Paredes Pedro , Theofilis Vassilis , Terhaar Steffen , Oberleithner Kilian , Paschereit Christian Oliver
Published: August 2015

Coherent flow structures in shear flows are generated by instabilities intrinsic to the hydrodynamic field. In a combustion environment, these structures may interact with the flame and cause unsteady heat release rate fluctuations. Prediction and modeling of these structures is thereby highly wanted for thermo-acoustic prediction models. In this work we apply hydrodynamic linear stability analysis to the time-averaged flow field of swirl-stabilized combustors obtained from experiments. Recent fundamental investigations have shown that the linear eigenmodes of the mean flow accurately represent the growth and saturation of the coherent structures. In this work biglobal and local stability analysis is applied to the reacting flow in an industry-relevant combustion system. Both the local and the biglobal analysis accurately predicts the onset and structure of a self-excited global instability that is known in the combustion community as a precessing vortex core (PVC). However, only the global analysis accurately predicts a globally stable flow field for the case without the oscillation, while the local analysis wrongly predicts an unstable global growth rate. The predicted spatial distribution of the amplitude functions using both analysis agree very well to the experimentally identified global mode. The presented tools are considered as very promising for the understanding of the PVC and physics based flow control.
Resistive switching characteristics of integrated polycrystalline hafnium oxide based one transistor and one resistor devices fabricated by atomic vapor deposition methods
Authors: Kim Hee-Dong , Crupi Felice , Lukosius Mindaugas , Trusch Andreas , Walczyk Christian , Wenger Christian
Published: August 2015

In this work, the bipolar resistive switching (RS) characteristics of polycrystalline hafnium oxide are studied in embedded 1 transistor-1 resistor (1T1R) resistive random access memory (RRAM) devices. The HfO2 films with varied thickness from 15 to 25 nm were grown by the atomic vapor deposition method at 400 °C. The HfO2 films are polycrystalline, as evaluated by applying atomic force microscopy and x-ray diffraction. In addition, current–voltage characteristics of the 1T1R devices were investigated. The forming voltages of 15 nm thick HfO2 films are about 4 V and increase gradually with increasing film thickness. A reproducible resistance switching behavior was observed with a high resistance ratio of ∼20 during DC cycling of 100 times. The observed SET and RESET voltages are about 1.2 and 1.6 V, respectively, indicating that the 1T1R RRAM device can be operated below 2 V. The current–voltage characteristics are discussed in the frame of the quantum point contact model.
Evaluation of an empirical monitor output estimation in carbon ion radiotherapy
Authors: Matsumura Akihiko , Yusa Ken , Kanai Tatsuaki , Mizota Manabu , Ohno Tatsuya , Nakano Takashi
Published: August 2015

Purpose:

A conventional broad beam method is applied to carbon ion radiotherapy at Gunma University Heavy Ion Medical Center. According to this method, accelerated carbon ions are scattered by various beam line devices to form 3D dose distribution. The physical dose per monitor unit (d/MU) at the isocenter, therefore, depends on beam line parameters and should be calibrated by a measurement in clinical practice. This study aims to develop a calculation algorithm for d/MU using beam line parameters.

Methods:

Two major factors, the range shifter dependence and the field aperture effect, are measured via PinPoint chamber in a water phantom, which is an identical setup as that used for monitor calibration in clinical practice. An empirical monitor calibration method based on measurement results is developed using a simple algorithm utilizing a linear function and a double Gaussian pencil beam distribution to express the range shifter dependence and the field aperture effect.

Results:

The range shifter dependence and the field aperture effect are evaluated to have errors of 0.2% and 0.5%, respectively. The proposed method has successfully estimated d/MU with a difference of less than 1% with respect to the measurement results. Taking the measurement deviation of about 0.3% into account, this result is sufficiently accurate for clinical applications.

Conclusions:

An empirical procedure to estimate d/MU with a simple algorithm is established in this research. This procedure allows them to use the beam time for more treatments, quality assurances, and other research endeavors.
Compressed sensing for longitudinal MRI: An adaptive‐weighted approach
Authors: Weizman Lior , Eldar Yonina C. , Ben Bashat Dafna
Published: August 2015

Purpose:

Repeated brain MRI scans are performed in many clinical scenarios, such as follow up of patients with tumors and therapy response assessment. In this paper, the authors show an approach to utilize former scans of the patient for the acceleration of repeated MRI scans.

Methods:

The proposed approach utilizes the possible similarity of the repeated scans in longitudinal MRI studies. Since similarity is not guaranteed, sampling and reconstruction are adjusted during acquisition to match the actual similarity between the scans. The baseline MR scan is utilized both in the sampling stage, via adaptive sampling, and in the reconstruction stage, with weighted reconstruction. In adaptive sampling, k‐space sampling locations are optimized during acquisition. Weighted reconstruction uses the locations of the nonzero coefficients in the sparse domains as a prior in the recovery process. The approach was tested on 2D and 3D MRI scans of patients with brain tumors.

Results:

The longitudinal adaptive compressed sensing MRI (LACS‐MRI) scheme provides reconstruction quality which outperforms other CS‐based approaches for rapid MRI. Examples are shown on patients with brain tumors and demonstrate improved spatial resolution. Compared with data sampled at the Nyquist rate, LACS‐MRI exhibits signal‐to‐error ratio (SER) of 24.8 dB with undersampling factor of 16.6 in 3D MRI.

Conclusions:

The authors presented an adaptive method for image reconstruction utilizing similarity of scans in longitudinal MRI studies, where possible. The proposed approach can significantly reduce scanning time in many applications that consist of disease follow‐up and monitoring of longitudinal changes in brain MRI.
On the validity of the effective density fluid model as an approximation of a poroelastic sediment layer
Authors: Bonomo Anthony L. , Chotiros Nicholas P. , Isakson Marcia J.
Published: August 2015

The effective density fluid model (EDFM) was developed to approximate the behavior of sediments governed by Biots theory of poroelasticity. Previously, it has been shown that the EDFM predicts reflection coefficients and backscattering strengths that are in close agreement with those of the full Biot model for the case of a homogeneous poroelastic half-space. However, it has not yet been determined to what extent the EDFM can be used in place of the full Biot-Stoll model for other cases. Using the finite element method, the flat-interface reflection and rough-interface backscattering predictions of the Biot-Stoll model and the EDFM are compared for the case of a poroelastic layer overlying an elastic substrate. It is shown that considerable differences between the predictions of the two models can exist when the layer is very thin and has a thickness comparable to the wavelength of the shear wave supported by the layer, with a particularly strong disparity under the conditions of a shear wave resonance. For thicker layers, the predictions of the two models are found to be in closer agreement, approaching nearly exact agreement as the layer thickness increases.
Acoustics of Italian Historical Opera Houses
Authors: Prodi Nicola , Pompoli Roberto , Martellotta Francesco , Sato Shin-ichi
Published: August 2015

Opera houses represent a large group of performance spaces characterized by great complexity and, at the same time, versatility with respect to different usage (from opera to symphonic music and ballet). This kind of building originated in Italy during the 17th century and later spread across the country and then Europe and the rest of the world, slowly evolving into modern theatre shapes. As a consequence of the changes undergone by the interior space, the original acoustic features, which likely influenced many composers, experienced important variations. Thanks to acoustic measurement campaigns inside Italian Historical Opera Houses, promoted by National and Regional Projects, the distinctive features of these spaces were investigated in comparison to modern spaces. In this work, the newly acquired data are merged with data in the literature in order to present and discuss some of the distinctive acoustic features of historical spaces as regards their original function. Moreover, specific issues such as listening in stalls and boxes and the criteria governing the preference judgment of listeners are considered. The concept and the crucial role of the balance between stage and pit sources are also discussed by means of previous literature studies.
A maximum likelihood direction of arrival estimation method for open-sphere microphone arrays in the spherical harmonic domain
Authors: Hu Yuxiang , Lu Jing , Qiu Xiaojun
Published: August 2015

Open-sphere microphone arrays are preferred over rigid-sphere arrays when minimal interaction between array and the measured sound field is required. However, open-sphere arrays suffer from poor robustness at null frequencies of the spherical Bessel function. This letter proposes a maximum likelihood method for direction of arrival estimation in the spherical harmonic domain, which avoids the division of the spherical Bessel function and can be used at arbitrary frequencies. Furthermore, the method can be easily extended to wideband implementation. Simulation and experiment results demonstrate the superiority of the proposed method over the commonly used methods in open-sphere configurations.