bluuebunny/crossref_metadata_2025 huggingface dataset:
Analytical Heat Transfer Correlation for a Multistage Steam Turbine in Warm-Keeping Operation With Air
Authors: Toebben Dennis , Hellmig Adrian , Luczynski Piotr , Wirsum Manfred , Mohr Wolfgang F. D. , Helbig Klaus
Published: July 2018

Due to the growing share of volatile renewable power generation, conventional power plants with a high flexibility are required. This leads to high thermal stresses inside the heavy components which reduces the lifetime. To improve the ability for fast start-ups, information about the metal temperature inside the rotor and the casing are crucial. Thus, an efficient calculation approach is required which enables the prediction of the temperature distribution in a whole multistage steam turbine. Considerable improvements of the computing power and numerical simulation tools today allow detailed investigations of the heat transfer and the flow phenomena by conjugate-heat-transfer (CHT) simulations. However, these simulations are still restricted to smaller geometries mostly by the number of elements. This leads to coarser numerical meshes for larger geometries, and thus, to a reduced accuracy. A highly accurate three-dimensional-CHT simulation of a whole multistage steam turbine can only be conducted with huge computational expense. Therefore, a simplified calculation approach is required. Heat transfer correlations are a commonly used tool for the calculation of the heat exchange between fluid and solid. Heat transfer correlations for steam turbines have been developed in a multitude of investigations. However, these investigations were based on design or to some extent on part-load operations with steam as the working fluid. The present paper deals with the theoretical investigation of steam turbine warm-keeping operation with hot air. This operation is totally different from the conventional operation conditions, due to a different working fluid with low mass flow rates and a slow rotation. Based on quasi-steady transient multistage CHT simulations, an analytical heat transfer correlation has been developed, since the commonly known calculation approaches from the literature are not suitable for this case. The presented heat transfer correlations describe the convective heat transfer separately at vane and blade as well as the seal surfaces. The correlations are based on a CHT model of three repetitive steam turbine stages. The simulations show a similar behavior of the Nusselt-number in consecutive stages. Hence, the developed area related heat transfer correlations are independent of the position of the stage. Finally, the correlations are implemented into a solid body finite element model and compared to the fluid-dynamic simulations.
Parametric Studies of Mechanical Power Loss for Helical Gear Pair Using a Thermal Elastohydrodynamic Lubrication Model
Authors: Liu Mingyong , Xu Peidong , Yan Chunai
Published: July 2018

In this study, a comprehensive mechanical efficiency model based on the thermal elastohydrodynamic lubrication (TEHL) is developed for a helical gear pair. The tribological performance of the helical gear pair is evaluated in terms of the average film thickness, friction coefficient, mechanical power loss, mechanical efficiency, etc. The influence of basic design parameters, working conditions, thermal effect, and surface roughness are studied under various transmission ratios. Results show that the contribution of thermal effect on the tribological performance is remarkable. Meanwhile, the rolling power loss constitutes an important portion of the total mechanical power loss, especially around the meshing position where the pitch point is located in the middle of contact line and the full elastohydrodynamic lubrication (EHL) state with the friction coefficient less than 0.005. The proper increase of normal pressure angle and number of tooth can improve the tribological performance. The influence of helix angle on the mechanical efficiency is less significant. A positive addendum modification coefficient for pinion and a negative addendum modification coefficient for wheel are good for improving the mechanical efficiency. The results provide the tribological guidance for design of a helical gear pair in engineering.
Dynamics and Stability of Turn-Milling Operations With Varying Time Delay in Discrete Time Domain
Authors: Comak Alptunc , Altintas Yusuf
Published: July 2018

Turn-milling machines are widely used in industry because of their multifunctional capabilities in producing complex parts in one setup. Both milling cutter and workpiece rotate simultaneously while the machine travels in three Cartesian directions leading to five axis kinematics with complex chip generation mechanism. This paper presents a general mathematical model to predict the chip thickness, cutting force, and chatter stability of turn milling operations. The dynamic chip thickness is modeled by considering the rigid body motion, relative vibrations between the tool and workpiece, and cutter-workpiece engagement geometry. The dynamics of the process are governed by delayed differential equations by time periodic coefficients with a time varying delay contributed by two simultaneously rotating spindles and kinematics of the machine. The stability of the system has been solved in semidiscrete time domain as a function of depth of cut, feed, tool spindle speed, and workpiece speed. The stability model has been experimentally verified in turn milling of Aluminum alloy cut with a helical cylindrical end mill.
Theoretical Prediction of Sheet Metal Wrinkling Based on the Potential Function Analysis
Authors: Zhao Yixi , Wan Xumin , Gao Leitao , Kong Qingshuai , Yu Zhongqi
Published: July 2018

The wrinkling research in sheet metal forming process has always been one of the most common hot topics. There are many methods to predict the sheet metal wrinkling while it is still difficult to accurately predict the initiation of wrinkling. The variational study of the potential function can be used to analyze the sheet metal wrinkling and acquire the stable energy criterion. In this paper, the sheet metal wrinkling mechanisms are explained in detail, and a wrinkling prediction model is proposed based on derivation and the potential function analysis during sheet metal forming processes. Meanwhile, the finite element (FE) simulation and experimental results of Yoshida buckling test (YBT) are used to verify the accuracy of the theoretical wrinkling prediction model. And the wrinkling prediction model has also applied to analyze the conventional spinning forming process, and the critical moment of flange wrinkling had been accurately predicted.
Stochastic Modeling and Analysis of Spindle Power During Hard Milling With a Focus on Tool Wear
Authors: Wang Xingtao , Williams Robert E. , Sealy Michael P. , Rao Prahalad K. , Guo Yuebin
Published: July 2018

The rapid development of modern science and technology brings with it a high demand for manufacturing quality. The surface integrity of a machined part is a critical factor which needs to be considered in the selection of the appropriate machining processes. Surface integrity is also tightly linked with tool wear. Tool wear is one of the most significant and necessary parameters to be considered for machining sustainability. By monitoring and predicting tool wear, it is possible to improve sustainability by reducing the scrap rate due to poor surface integrity. In this work, data-dependent systems (DDS), a stochastic modeling and analysis technique, was applied to study the power of spindle motor during a hard milling operation. The objective was to correlate the spindle power to tool wear conditions using DDS analysis. The spindle power was monitored, and the time series trends were decomposed to study the frequency variation with different severities of tool wear conditions and processing parameters. Analysis of variance (ANOVA) was also used to determine factors significant to the power by a spindle motor. Experiments indicate that low-level frequency of spindle power is correlated with the amount of tool wear, cutting speed, and feed per tooth. The results suggest that effective tool wear monitoring may be achieved by focusing on low-level frequencies highlighted by DDS methodology.
Algorithm to Reduce Leading and Lagging in Conformal Direct-Print
Authors: Vatani Morteza , Alkadi Faez , Choi Jae-Won
Published: July 2018

A novel additive manufacturing algorithm was developed to increase the consistency of three-dimensional (3D) printed curvilinear or conformal patterns on freeform surfaces. The algorithm dynamically and locally compensates the nozzle location with respect to the pattern geometry, motion direction, and topology of the substrate to minimize lagging or leading during conformal printing. The printing algorithm was implemented in an existing 3D printing system that consists of an extrusion-based dispensing module and an XYZ-stage. A dispensing head is fixed on a Z-axis and moves vertically, while the substrate is installed on an XY-stage and moves in the x–y plane. The printing algorithm approximates the printed pattern using nonuniform rational B-spline (NURBS) curves translated directly from a 3D model. Results showed that the proposed printing algorithm increases the consistency in the width of the printed patterns. It is envisioned that the proposed algorithm can facilitate nonplanar 3D printing using common and commercially available Cartesian-type 3D printing systems.
Impact Analysis of Pocket Damper Seal Geometry Variations on Leakage Performance and Rotordynamic Force Coefficients Using Computational Fluid Dynamics
Authors: Griebel Clemens
Published: July 2018

In this paper, different notch and partition wall arrangements of a fully partitioned pocket damper seal (FPDS) are investigated using computational fluid dynamics (CFD). The CFD model is derived for a baseline FPDS design reflecting the full sealing configuration with a structured mesh. Steady-state simulations are performed for eccentric rotor position and different operational parameters. The results are validated using experimental cavity pressure measurements. In transient computations, rotor whirl is modeled as a circular motion around an initial eccentricity using a moving mesh technique. Different whirl frequencies are computed to account for the frequency-dependent behavior of damper seals. The stiffness and damping coefficients are evaluated from the impedances in the frequency domain using a fast Fourier transform. The validated model is then transferred to varying designs. In addition to the baseline design, six different notch arrangements with constant clearance ratio were modeled. Moreover, two partition wall design variations were studied based on manufacturability considerations. Predicted leakage as well as frequency-dependent stiffness and damping coefficients are presented and the impact of geometry variations on these parameters is discussed. The results suggest that a single centered notch is favorable and indicate considerably higher effective damping for a design with staggered partition walls. A rounded partition wall design with significantly eased manufacturing reveals good performance.
A Simplified Transient Hardening Formulation for Modeling Stress–Strain Response Under Multiaxial Nonproportional Cyclic Loading
Authors: Gates Nicholas R. , Fatemi Ali
Published: July 2018

Accurate estimation of material stress–strain response is essential to many fatigue life analyses. In cases where variable amplitude loading conditions exist, the ability to account for transient material deformation behavior can be particularly important due to the potential for periodic overloads and/or changes in the degree of nonproportional stressing. However, cyclic plasticity models capable of accounting for these complex effects often require the determination of a large number of material constants. Therefore, an Armstrong–Frederick–Chaboche style plasticity model, which was simplified in a previous study, was extended in the current study to account for the effects of both general cyclic and nonproportional hardening using a minimal number of material constants. The model was then evaluated for its ability to predict stress–strain response under complex multiaxial loading conditions by using experimental data generated for 2024-T3 aluminum alloy, including a number of cyclic incremental step tests. The model was found to predict transient material response within a fairly high overall level of accuracy for each loading history investigated.
Effects of Nonlinear Modal Interactions on the Thermoacoustic Stability of Annular Combustors
Authors: Orchini Alessandro , Mensah Georg A. , Moeck Jonas P.
Published: July 2018

In this theoretical and numerical analysis, a low-order network model is used to investigate a thermoacoustic system with discrete rotational symmetry. Its geometry resembles that of the MICCA combustor (Laboratoire EM2C, CentraleSupelec); the flame describing function (FDF) employed in the analysis is that of a single-burner configuration and is taken from experimental data reported in the literature. We show how most of the dynamical features observed in the MICCA experiment, including the so-called slanted mode, can be predicted within this framework, when the interaction between a longitudinal and an azimuthal thermoacoustic mode is considered. We show how these solutions relate to the symmetries contained in the equations that model the system. We also discuss how considering situations in which two modes are linearly unstable compromises the applicability of stability criteria available in the literature.
Cyclogram-Based Joint Symmetry Assessment After Utilization of a Foot Drop Stimulator During Post-stroke Hemiplegic Gait
Authors: Pilkar Rakesh , Ramanujam Arvind , Chervin Kathleen , Forrest Gail F. , Nolan Karen J.
Published: July 2018

In the absence of standardized symmetry assessments, quantifying symmetry based on the kinematic evolution of lower extremity joints can elucidate gait irregularities. The objective was to develop a novel cyclogram-based symmetry (CBS) method to quantify lower extremity joints' symmetry and assess the effect of six-month utilization of foot drop stimulator (FDS) on CBS of the lower limbs during hemiplegic gait poststroke. Twenty-four participants (13 stroke and 11 healthy controls (HC)) performed ten walking trials at a free cadence on level ground. Symmetry values were computed using geometric properties of bilateral cyclograms obtained from normalized sagittal ankle, knee, and hip kinematics. CBS and traditional temporospatial symmetry values were compared between the two groups using independent sample t-test. The effect of FDS utilization on the symmetry was assessed by a paired sample t-test computed at baseline and six-month follow up. The CBS method successfully showed that the HC group was significantly more symmetrical at the ankle (p = 0.001), knee (p = 0.001), and hip (p < 0.005) compared with the stroke group. The stroke group showed significant increment in the hip symmetry with FDS at the baseline but did not show any significant CBS changes at follow up. Pearson correlations revealed that hip and knee CBS had a significant influence on the overall walking speed. The CBS method presents a unique approach to calculate the symmetry based on the kinematics of lower extremities during gait.
Passive Prosthetic Foot Shape and Size Optimization Using Lower Leg Trajectory Error
Authors: Olesnavage Kathryn M. , Prost Victor , Johnson William Brett , Winter Amos G.
Published: July 2018

A method is presented to optimize the shape and size of a passive, energy-storing prosthetic foot using the lower leg trajectory error (LLTE) as the design objective. The LLTE is defined as the root-mean-square error between the lower leg trajectory calculated for a given prosthetic foot's deformed shape under typical ground reaction forces (GRFs), and a target physiological lower leg trajectory obtained from published gait data for able-bodied walking. Using the LLTE as a design objective creates a quantitative connection between the mechanical design of a prosthetic foot (stiffness and geometry) and its anticipated biomechanical performance. The authors' prior work has shown that feet with optimized, low LLTE values can accurately replicate physiological kinematics and kinetics. The size and shape of a single-part compliant prosthetic foot made out of nylon 6/6 were optimized for minimum LLTE using a wide Bezier curve to describe its geometry, with constraints to produce only shapes that could fit within a physiological foot's geometric envelope. Given its single part architecture, the foot could be cost effectively manufactured with injection molding, extrusion, or three-dimensional printing. Load testing of the foot showed that its maximum deflection was within 0.3 cm (9%) of finite element analysis (FEA) predictions, ensuring the constitutive behavior was accurately characterized. Prototypes were tested on six below-knee amputees in India—the target users for this technology—to obtain qualitative feedback, which was overall positive and confirmed the foot is ready for extended field trials.
Experimental Analysis of the Impact of Nanoinclusions and Surfactants on the Viscosity of Paraffin-Based Energy Storage Materials
Authors: Weigand Rebecca , Hess Kieran , Fleischer Amy S.
Published: July 2018

Phase change materials (PCMs) are commonly used in many applications, including the transient thermal management of electronics. For many systems, paraffin-based PCMs are used with suspended nanoinclusions to increase their effective thermal conductivity. The addition of these materials can have a positive impact on thermal conductivity, but can also increase the viscosity in the liquid phase. In this paper, the impact of different nanoinclusions and surfactants on the dynamic viscosity of a common paraffin wax PCM is quantified in order to determine their suitability for thermal energy storage applications. The effect of the nanoparticles on the viscosity is found to be a function of the nanoparticle type with multiwalled carbon nanotubes (MWCNT) yielding the greatest increase in viscosity. The addition of both nanoparticle and surfactant to the base PCM is found to affect the viscosity even when the loading levels of the nanoparticles or surfactant alone are not enough to affect the viscosity, thus the combination must be carefully considered in any heat transfer application.
Characterization of Heat Transfer by Overlapped-Quadruple Counter Tapes
Authors: Eiamsa-Ard S. , Samravysin P.
Published: July 2018

This paper deals with the influence of overlapped-quadruple twisted tape (OQT) and typical quadruple twisted tape (QT) on the enhancing heat transfer rate in a heat exchanger tube. The effects of tube orientation (co-tapes and counter tapes), tape arrangement (cross and parallel arrangements), and overlapped twist ratio (yo/y = 0.625, 0.75, and 0.875) were investigated. The experiments were performed in the turbulent flow regime (5000 ≤ Re ≤ 20,000) under uniform wall heat flux boundary condition, using air as the testing fluid. The experimental results indicates that all OQTs consistently yield higher Nusselt number and thermal performance than QTs. The tapes in cross arrangement show better heat transfer enhancement and thermal performance than the ones in parallel arrangement. Heat transfer, friction loss, and thermal performance increase with decreasing overlapped twist ratio. The maximum thermal enhancement factor (TEF) of 1.58 is achieved by the use of overlapped-quadruple counter tapes in cross arrangement (CC-OQTs) with yo/y = 0.625 at the lowest Reynolds number of 5000.
Network Analysis of Design Automation Literature
Authors: Guo Tinghao , Xu Jiarui , Sun Yue , Dong Yilin , Davis Neal , Allison James T.
Published: July 2018

In this paper, we present the results of a study of citation and co-authorship networks for articles published at the ASME Design Automation Conference (DAC) during the years 2002–2015. Two topic-modeling methods are presented for studying the DAC literature: A frequency-based model was developed to explore DAC topic distribution and evolution, as well as citation analysis for each core topic. Correlation analysis and association-rule mining were used to discover relationships between topics. A new unsupervised learning algorithm, propagation mergence (PM), was created to address identified shortcomings of existing methods and applied to study the existing DAC citation network. Influential articles and important article clusters were identified and effective visualizations created. We also investigated the DAC co-authorship network by identifying key authors and showing that the network structure exhibits small-world-network properties. The resulting insights, obtained by the both the proposed and existing methods, may be beneficial to the engineering design research community, especially with respect to determining future research directions and possible actions for improvement. The data set used here is limited; expanding to include additional relevant conference proceedings and journal articles in the future would offer a more complete understanding of the engineering design research literature.
Numerical Simulation of JCO-E Pipe Manufacturing Process and Its Effect on the External Pressure Capacity of the Pipe1
Authors: Antoniou Konstantinos , Chatzopoulou Giannoula , Karamanos Spyros A. , Tazedakis Athanasios , Palagas Christos , Dourdounis Efthimios
Published: July 2018

Large-diameter thick-walled steel pipes during their installation in deep-water are subjected to external pressure, which may trigger structural instability due to pipe ovalization, with detrimental effects. The resistance of offshore pipes against this instability is affected by local geometric deviations and residual stresses, introduced by the line pipe manufacturing process. In the present paper, the JCO-E pipe manufacturing process, a commonly adopted process for producing large-diameter pipes of significant thickness, is examined. The study examines the effect of JCO-E line pipe manufacturing process on the external pressure resistance of offshore pipes, candidates for deepwater applications using nonlinear finite element simulation tools. The cold bending induced by the JCO forming process as well as the subsequent welding and expansion (E) operations are simulated rigorously. Subsequently, the application of external pressure is modeled until structural instability (collapse) is detected. Both the JCO-E manufacturing process and the external pressure response of the pipe, are modeled using a two-dimensional (2D) generalized plane strain model, together with a coupled thermo-mechanical model for simulating the welding process.
Clearance Effects on Rotordynamic Performance of a Long Smooth Seal With Two-Phase, Mainly-Air Mixtures
Authors: Zhang Min , Childs Dara W. , Tran Dung L. , Shrestha Hari
Published: July 2018

This paper experimentally studies the effects of changing radial clearance Cr on the performance of a long (length-to-diameter ratio L/D = 0.65) smooth seal under mainly-air (wet-gas) conditions. The test fluid is a mixture of air and silicone oil. Tests are conducted with Cr = 0.188, 0.163, and 0.140 mm, inlet pressure Pi = 62.1 bars, exit pressure Pe = 31 bars, inlet liquid volume fraction LVF = 0%, 2%, 5%, and 8%, and shaft speed ω = 10, 15, and 20 krpm. The seal's complex dynamic stiffness coefficients Hij are measured. The real parts of Hij cannot be fitted by frequency-independent stiffness and virtual-mass coefficients. Therefore, frequency-dependent direct KΩ and cross-coupled kΩ stiffness coefficients are used. The imaginary parts of direct Hij produce frequency-independent direct damping C. Test results show that, for all pure- and mainly-air conditions, decreasing Cr decreases (as expected) the leakage mass flow rate m˙. Under mainly-air conditions, decreasing Cr decreases KΩ. This outcome is contrary to the test results at pure-air conditions, where KΩ increases as Cr decreases. Since an unstable centrifugal compressor rotor may precess at approximately 0.5ω, the effective damping Ceff at about 0.5ω is used as an indicator of the impact a seal would have on its associated compressor. For pure-air conditions, when Ω ≈ 0.5ω, decreasing Cr increases Ceff and makes the seal more stabilizing. This trend continues after the oil is added. A bulk-flow model developed by San Andrés (2011, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503) produces predictions to compare with test results. m˙ predictions correlate with measurements. Under pure-air conditions, the model correctly predicts the effects of changing Cr on KΩ and the Ceff value near 0.5ω. After the oil is added, as Cr decreases, predicted KΩ increases while measured KΩ decreases. Also, for mainly-air cases and Ω ≈ 0.5ω, decreasing Cr does not discernibly change predicted Ceff but increases the measured value.
Stably Stratified Wall-Bounded Turbulence
Authors: Zonta Francesco , Soldati Alfredo
Published: July 2018

Stably stratified wall-bounded turbulence is commonly encountered in many industrial and environmental processes. The interaction between turbulence and stratification induces remarkable modifications on the entire flow field, which in turn influence the overall transfer rates of mass, momentum, and heat. Although a vast proportion of the parameter range of wall-bounded stably stratified turbulence is still unexplored (in particular when stratification is strong), numerical simulations and experiments have recently developed a fairly robust picture of the flow structure, also providing essential ground for addressing more complex problems of paramount technological, environmental and geophysical importance. In this paper, we review models used to describe the influence of stratification on turbulence, as well as numerical and experimental methods and flow configurations for studying the resulting dynamics. Conclusions with a view on current open issues will be also provided.
A Broad Comparison of Solar Photovoltaic and Thermal Technologies for Industrial Heating Applications
Authors: Bany Mousa Osama M. , Taylor Robert A.
Published: July 2018

Solar harvesting designs aim to optimize energy output per unit area. When it comes to choosing between rooftop technologies for generating heat and/or electricity from the sun, though, the literature has favored qualitative arguments over quantitative comparisons. In this paper, an agnostic perspective will be used to evaluate several solar collector designs—thermal, photovoltaic (PV), and hybrid (PV/T) systems—which can result in medium temperature heat for industry rooftops. Using annual trnsys simulations in several characteristic global locations, it was found that a maximum solar contribution (for all selected locations) of 79.1% can be achieved for a sterilization process with a solar thermal (ST) system as compared to 40.6% for a PV system. A 43.2%solar contribution can be obtained with a thermally coupled PV/T, while an uncoupled PV/T beam splitting collector can achieve 84.2%. Lastly, PV and ST were compared in a side-by-side configuration, indicating that this scenario is also feasible since it provides a solar contribution of 75.2%. It was found that the location's direct normal incident (DNI) and global horizontal irradiation (GHI) are the dominant factors in determining the best technology for industrial heating applications. Overall, this paper is significant in that it introduces a comparative simulation strategy to analyze a wide variety of solar technologies for global industrial heat applications.
Double Modulation Pyrometry Applied to Radiatively Heated Surfaces With Dynamic Optical Properties
Authors: Potamias Dimitrios , Alxneit Ivo , Koepf Erik , Wokaun Alexander
Published: July 2018

The accuracy of radiometric temperature measurement in radiatively heated environments is severely limited by the combined effects of intense reflected radiation and unknown, dynamically changing emissivity, which induces two correlated and variable error terms. While the recently demonstrated double modulation pyrometry (DMP) eliminates the contribution of reflected radiation, it still suffers from the shortcomings of single-waveband pyrometry: it requires knowledge of the emissivity to retrieve the true temperature from the thermal signal. Here, we demonstrate an improvement of DMP incorporating the in situ measurement of reflectance. The method is implemented at Paul Scherrer Institute (PSI) in its 50 kW high-flux solar simulator and used to measure the temperature of ceramic foams (SiSiC, ZrO2, and Al2O3) during fast heat-up. The enhancement allows DMP to determine the true temperature despite a dynamically changing emissivity and to identify well-documented signature changes in ZrO2 and Al2O3. The method also allows us to study the two dominant error sources by separately tracking the evolution of two error components during heat-up. Furthermore, we obtain measurements from a solar receiver, where the cavity reflection error limits measurement accuracy. DMP can be used as an accurate radiometric thermometer in the adverse conditions of concentrated radiation, and as a diagnostic tool to characterize materials with dynamic optical properties. Its simple design and ability to correct for both errors makes it a useful tool not only in solar simulators but also in concentrated solar facilities.
Erosion in Gas-Turbine Grade Ceramic Matrix Composites
Authors: Kedir N. , Gong C. , Sanchez L. , Presby M. J. , Kane S. , Faucett D. C. , Choi S. R.
Published: July 2018

Erosion behavior of a large number of gas-turbine grade ceramic matrix composites (CMCs) was assessed using fine to medium grain garnet erodents at velocities of 200 and 300 m/s at ambient temperature. The CMCs used in the current work were comprised of nine different SiC/SiCs, one SiC/C, one C/SiC, one SiC/MAS, and one oxide/oxide. Erosion damage was quantified with respect to erosion rate and the damage morphology was assessed via scanning electron microscopy (SEM) and optical microscopy in conjunction with three-dimensional (3D) image mapping. The CMCs response to erosion appeared to be very complicated due to their architectural complexity, multiple material constituents, and presence of pores. Effects of architecture, material constituents, density, matrix hardness, and elastic modulus of the CMCs were taken into account and correlated to overall erosion behavior. The erosion of monolithic ceramics such as silicon carbide and silicon nitrides was also examined to gain a better understanding of the governing damage mechanisms for the CMC material systems used in this work.
A Critical Analysis on Low-Order Simulation Models for Darrieus Vawts: How Much Do They Pertain to the Real Flow?
Authors: Bianchini Alessandro , Balduzzi Francesco , Ferrara Giovanni , Persico Giacomo , Dossena Vincenzo , Ferrari Lorenzo
Published: July 2018

To improve the efficiency of Darrieus wind turbines, which still lacks from that of horizontal-axis rotors, computational fluid dynamics (CFD) techniques are now extensively applied, since they only provide a detailed and comprehensive flow representation. Their computational cost makes them, however, still prohibitive for routine application in the industrial context, which still makes large use of low-order simulation models like the blade element momentum (BEM) theory. These models have been shown to provide relatively accurate estimations of the overall turbine performance; conversely, the description of the flow field suffers from the strong approximations introduced in the modeling of the flow physics. In this study, the effectiveness of the simplified BEM approach was critically benchmarked against a comprehensive description of the flow field past the rotating blades coming from the combination of a two-dimensional (2D) unsteady CFD model and experimental wind tunnel tests; for both data sets, the overall performance and the wake characteristics on the midplane of a small-scale H-shaped Darrieus turbine were available. Upon examination of the flow field, the validity of the ubiquitous use of induction factors is discussed, together with the resulting velocity profiles upstream and downstream the rotor. Particular attention is paid on the actual flow conditions (i.e., incidence angle and relative speed) experienced by the airfoils in motion at different azimuthal angles, for which a new procedure for the postprocessing of CFD data is here proposed. Based on this model, the actual lift and drag coefficients produced by the airfoils in motion are analyzed and discussed, with particular focus on dynamic stall. The analysis highlights the main critical issues and flaws of the low-order BEM approach, but also sheds new light on the physical reasons why the overall performance prediction of these models is often acceptable for a first-design analysis.
Aerothermodynamics and Exergy Analysis in Radial Turbine With Heat Transfer
Authors: Lim Shyang Maw , Dahlkild Anders , Mihaescu Mihai
Published: July 2018

This study was motivated by the difficulties to assess the aerothermodynamic effects of heat transfer on the performance of turbocharger turbine by only looking at the global performance parameters, and by the lack of efforts to quantify the physical mechanisms associated with heat transfer. In this study, we aimed to investigate the sensitivity of performance to heat loss, to quantify the aerothermodynamic mechanisms associated with heat transfer and to study the available energy utilization by a turbocharger turbine. Exergy analysis was performed based on the predicted three-dimensional flow field by detached eddy simulation (DES). Our study showed that at a specified mass flow rate, (1) pressure ratio drop is less sensitive to heat loss as compared to turbine power reduction, (2) turbine power drop due to heat loss is relatively insignificant as compared to the exergy lost via heat transfer and thermal irreversibilities, and (3) a single-stage turbine is not an effective machine to harvest all the available exhaust energy in the system.
Improving Purge Air Cooling Effectiveness by Engineered End-Wall Surface Structures—Part II: Turbine Cascade
Authors: Miao Xin , Zhang Qiang , Atkin Chris , Sun Zhengzhong , Li Yansheng
Published: July 2018

Motivated by the recent advances in additive manufacturing, a novel turbine end-wall aerothermal management method is presented in this two-part paper. The feasibility of enhancing purge air cooling effectiveness through engineered surface structure was experimentally and numerically investigated. The fundamental working mechanism and improved cooling performance for a 90 deg turning duct are presented in Part I. The second part of this paper demonstrates this novel concept in a low-speed linear cascade environment. The performance in three purge air blowing ratios is presented and enhanced cooling effectiveness and net heat flux reduction (NHFR) were observed from experimental data, especially for higher blow ratios. The Computational fluid dynamics (CFD) analysis indicates that the additional surface features are effective in reducing the passage vortex and providing a larger area of coolant coverage without introducing additional aerodynamic loss.
Nonsmooth Thermoelastic Simulations of Blade–Casing Contact Interactions
Authors: Thorin Anders , Guérin Nicolas , Legrand Mathias , Thouverez Fabrice , Almeida Patricio
Published: July 2018

In turbomachinery, it is well known that tighter operating clearances improve the efficiency. However, this leads to unwanted potential unilateral and frictional contact occurrences between the rotating (blades) and stationary components (casings) together with attendant thermal excitations. Unilateral contact induces discontinuities in the velocity at impact times, hence the terminology nonsmooth dynamics. Current modeling strategies of rotor–stator interactions are either based on regularizing penalty methods or on explicit time-marching methods derived from Carpenter's forward Lagrange multiplier method. Regularization introduces an artificial time scale in the formulation corresponding to numerical stiffness, which is not desirable. Carpenter's scheme has been successfully applied to turbomachinery industrial models in the sole mechanical framework, but faces serious stability issues when dealing with the additional heat equation. This work overcomes the above issues by using the Moreau–Jean nonsmooth integration scheme within an implicit θ-method. This numerical scheme is based on a mathematically sound description of the contact dynamics by means of measure differential inclusions and enjoys attractive features. The procedure is unconditionally stable opening doors to quick preliminary simulations with time-steps one hundred times larger than with previous algorithms. It can also deal with strongly coupled thermomechanical problems.
An Investigation on the Loss Generation Mechanisms Inside Different Centrifugal Compressor Volutes for Turbochargers
Authors: Tanganelli Andrea , Balduzzi Francesco , Bianchini Alessandro , Cencherle Francesco , De Luca Michele , Marmorini Luca , Ferrara Giovanni
Published: July 2018

In centrifugal compressor design, the volute plays a key role in defining the overall efficiency and operating range of the stage. The flow at the impeller outlet is indeed characterized by a high kinetic energy content, which is first converted to potential energy in the diffuser downstream. The compressed gas is then collected by the volute at the cylindrical outlet section of the diffuser and directed to the intake piping, possibly with a further pressure recovery to enhance the stage performance. Due to the high flow speed at the volute inlet, the capability of ensuring the lowest amount of total pressure loss is pivotal to prevent a detriment of the machine efficiency. Moreover, the flow conditions change when the volute operates far from its design point: at mass flow rates lower than the design one, the flow becomes diffusive, while at higher mass flow rates the fluid is accelerated, thus leading to different loss-generation mechanisms. These phenomena are particularly relevant in turbocharger applications, where the compressor needs to cover a wide functioning range; moreover, in these applications, the definition of the volute shape is often driven also by space limitations imposed by the vehicle layout, leading to a variety of volute types. The present paper reports an analysis on the sources of thermodynamic irreversibilities occurring inside different volutes applied to a centrifugal compressor for turbocharging applications. Three demonstrative geometrical configurations are analyzed by means of three-dimensional (3D) numerical simulations using common boundary conditions to assess the overall volute performance and different loss mechanisms, which are evaluated in terms of the local entropy generation rate. The modification of the loss mechanisms in off-design conditions is also accounted for by investigating different mass flow rates. It is finally shown that the use of the entropy generation rate for the assessment of the irreversibilities is helpful to understand and localize the sources of loss in relation to the various flow structures.
A Class of Symmetrical 3T, 3T-1R, and 3R Mechanisms With Parallel Linear Motion Elements
Authors: Yang Yi , Zhang Wuxiang , Pu Huayan , Peng Yan
Published: July 2018

A kind of kinematic chain with parallel linear motion elements (PLMEs) is proposed and studied in this paper. Based on screw theory, the kinematic screw equations of these linkages are established. The two special categories of PLMEs, with pure translational motion and with pure rotational motion respectively, are identified. The mobilities and the singularities of these kinematic chains are also investigated. By the utilization of these PLMEs, three types of the compound limbs are invented and analyzed. Through assembling these compound limbs in different ways, a class of lower mobility symmetrical 3T, 3T-1R, and 3R mechanisms is synthesized and presented for the first time. The simplified kinematic equations for this class of mechanisms driven by the linear actuators are derived. And the workspaces, singularities, and kinematic performance are addressed. Finally, three typical prototypes with regard to 3T, 3T-1R, and 3R mechanisms are manufactured and experimented to validate the mobility and motion feasibility of these mechanisms.
On the Comprehensive Kinematics Analysis of a Humanoid Parallel Ankle Mechanism
Authors: Zhou Chengxu , Tsagarakis Nikos
Published: July 2018

In this paper, we present a thorough kinematics analysis of a humanoid two degrees-of-freedom (DoF) ankle module based on a parallel kinematics mechanism. Compared with the conventional serial configuration, the parallel kinematics ankle permits the distribution of the torque/power of the actuators to the two DoF of the ankle taking full advantage of available power/torque capacity of the two actuators. However, it complicates the kinematics study in return. In this work, a complete study of a parallel ankle mechanism is performed that permits the full characterization of the ankle module for the purpose of its design study, control, and performance evaluation. Screw theory is employed for mobility analysis to first determine the number and properties of the mechanism's DoFs. Then the inverse kinematics is solved analytically and the Jacobian matrix for describing the velocity relation between the ankle joints and motors is found. Based on these results, the forward kinematics of the parallel mechanism can be numerically computed using the Newton–Raphson method. The workspace of the ankle is also analyzed and the motor limits are decided accordingly. Finally, an experimental demonstration consisting of four tests is carried out to evaluate the proposed methods and ankle module.
Markov/CCMT Dynamic Reliability Analysis of the Main and Startup Feedwater Control System in Nuclear Power Plant
Authors: Li Wentao , Gu Danying , Zhang Henan
Published: July 2018

The dynamic reliability of the main and startup feedwater control system in nuclear power plant is evaluated by conducting the Markov cell-to-cell mapping technology (Markov/CCMT) methodology. All the equipment failure modes and potential failure states within the system are analyzed. This process illustrates the uncertainty in the physical process of the system. Furthermore, the failure probability and cut-set of the system can be computed to provide a more comprehensive and accurate response to the system characteristics and reflect the two types of interaction within the system. In contrast to the traditional static probability safety assessment, the Markov/CCMT methodology remedies the defect in terms of event sequence setting, control loop, multiple top event competition, uncertainty of the analysis result, as well as the insufficient analysis of human-caused failure. The reliability analysis of the main and startup feedwater control system (FWCS) based on the self-developed Markov/CCMT reliability analysis software verifies the feasibility and engineering application value of the methodology and software.
Finite Element Analysis of the Mechanical Behavior of Multibolted Joints Subjected to Shear Loads
Authors: Fukuoka Toshimichi
Published: July 2018

When subjected to external forces, bolted joints behave in a complex manner especially in the case of the joints being clamped with multiple bolts. Friction type joints are widely used for the joints subjected to shear loads. Bearing type joints, which support the shear loads on the bolt cylindrical surface, are used less frequently, since its mechanical behavior is too complicated to accurately estimate the load capacity. In this study, mechanical behavior of the bearing type multibolted joints subjected to shear loads is analyzed by three-dimensional (3D) FEM. As a result of comprehensive calculations, it has been found that the shear load applied to bearing type joints distributes with a concave shape along the load direction, and a fair amount of the shear load is supported by friction forces as in the case of friction type joints. In addition, a simple method that calculates the shear load distribution using elementary theory of solid mechanics is proposed, which can estimate the shear load distribution with sufficient accuracy especially for the case of small friction coefficient.
Proposal of Failure Mode Map Under Dynamic Loading—Ratcheting and Collapse
Authors: Al Bari Md Abdullah , Sakemi Ryota , Katsura Yamato , Kasahara Naoto
Published: July 2018

Ratcheting, collapse, and fatigue are the probable failure modes which can occur under alternate dynamic loading like seismic loading. The objective of this study is to propose a failure mode map for rectangular beams by determining the conditions of occurrence of the ratcheting and collapse failure modes. The paper considers the analogy between thermal ratcheting and dynamic ratcheting. The nonlinear dynamic finite element method was used to analyze a rectangular beam model for different loading conditions. The results were plotted on a nondimensional primary and secondary stress parameter graph similar to the Bree diagram for thermal ratcheting. The similarity between thermal load and dynamic load was observed. The main difference between thermal and dynamic loading is the effect of the frequency of dynamic loading on the occurrence of ratcheting and collapse. Experimental observations of ratcheting have been obtained and are used for comparison to validate the analytical predictions. From the above results, a failure mode map has been proposed which can evaluate the occurrence conditions of ratcheting and collapse under dynamic loadings.
Mechanism
Analysis of Weakening Reverse Compression Waves in Gas Wave
Refrigerator
Authors: Liu Peiqi , Wu Kehan , Liu Sheng , Tan Wenhu , Zhu Che , Hu Dapeng
Published: July 2018

Pressure oscillating tubes are core components of the gas wave refrigerator. The reverse compression waves will limit refrigeration efficiency by reheat cooling gas. The promotion from traditional rotation gas wave refrigerator with single opened pressure oscillating tubes to the streamlined pressure exchange gas wave refrigerator with double opened pressure oscillating tubes is mainly in terms of this issue. However, through weakened, reverse compression waves are still inevitable. For further development, the concept of a wave attenuator has been proposed, installed at the high-temperature (HT) port. Numerical simulation has been utilized to analyze mechanism of wave attenuator and the practical effect has been proved by experiment research. The conclusions are as follows: due to structure of wave attenuator, the intensity of reverse compression waves has been weakened; the optimal structure of wave attenuator has been obtained; the refrigeration efficiency of the refrigerator has been significantly increased because of wave attenuator in HT port.
High-Fidelity Sensitivity Analysis of Modal Properties of Mistuned Bladed Disks Regarding Material Anisotropy
Authors: Koscso Adam , Dhondt Guido , Petrov E. P.
Published: July 2018

A new method has been developed for sensitivity calculations of modal characteristics of bladed disks made of anisotropic materials. The method allows the determination of the sensitivity of the natural frequencies and mode shapes of mistuned bladed disks with respect to anisotropy angles that define the crystal orientation of the monocrystalline blades using full-scale finite element models. An enhanced method is proposed to provide high accuracy for the sensitivity analysis of mode shapes. An approach has also been developed for transforming the modal sensitivities to coordinate systems (CS) used in industry for description of the blade anisotropy orientations. The capabilities of the developed methods are demonstrated on examples of a single blade and a mistuned realistic bladed disk finite element models. The modal sensitivity of mistuned bladed disks to anisotropic material orientation is thoroughly studied.