bluuebunny/crossref_metadata_2025 huggingface dataset:
Design and Modeling of a Parallel Continuum Manipulator for Trunk Motion Rehabilitation
Authors: Díaz-Caneja Daniel , Campa Francisco J. , Altuzarra Oscar
Published: January 2021

The objective of the present work is to develop a device for training the trunk balance and motion during the early stage of rehabilitation of patients who have suffered a stroke. It is coupled to a standing frame and is based on a parallel continuum manipulator where a wearable jacket is moved by four flexible limbs actuated by rotary motors, achieving the translation and rotation required in the trunk to perform a given exercise. The flexible limbs act as a natural mechanical filter in such a way that a smooth physiological motion is achieved, and it feels less intimidating to the patient. After measuring the kinematic requirements, a model has been developed to design the system. A prototype has been built and a preliminary experimental validation has been done where the jacket generates translation coupled to a rotation around the anteroposterior, medio-lateral and longitudinal axis. The measurements of the motors torque and the force sensors located in the flexible limbs have been compared with the simulations from the model. The results prove that the prototype can accomplish the motions required for the rehabilitation task, although further work is still required to control the interaction with the patient and improve the performance of the device.
A Comparative Study of Pellet-Based Extrusion Deposition of Short, Long, and Continuous Carbon Fiber-Reinforced Polymer Composites for Large-Scale Additive Manufacturing
Authors: Pappas John M. , Thakur Aditya R. , Leu Ming C. , Dong Xiangyang
Published: January 2021

Pellet-based extrusion deposition of carbon fiber-reinforced composites at high material deposition rates has recently gained much attention due to its applications in large-scale additive manufacturing. The mechanical and physical properties of large-volume components largely depend on their reinforcing fiber length. However, very few studies have been done thus far to have a direct comparison of additively fabricated composites reinforced with different carbon fiber lengths. In this study, a new additive manufacturing (AM) approach to fabricate long fiber-reinforced polymer (LFRP) was first proposed. A pellet-based extrusion deposition method was implemented, which directly used thermoplastic pellets and continuous fiber tows as feedstock materials. Discontinuous long carbon fibers, with an average fiber length of 20.1 mm, were successfully incorporated into printed LFRP samples. The printed LFRP samples were compared with short fiber-reinforced polymer (SFRP) and continuous fiber-reinforced polymer (CFRP) counterparts through mechanical tests and microstructural analyses. The carbon fiber dispersion, distribution of carbon fiber length and orientation, and fiber wetting were studied. As expected, a steady increase in flexural strength was observed with increasing fiber length. The carbon fibers were highly oriented along the printing direction. A more uniformly distributed discontinuous fiber reinforcement was found within printed SFRP and LFRP samples. Due to decreased fiber impregnation time and lowered impregnation rate, the printed CFRP samples showed a lower degree of impregnation and worse fiber wetting conditions. The feasibility of the proposed AM methods was further demonstrated by fabricating large-volume components with complex geometries.
Topological Optimization of Under-Platform Dampers With Moving Morphable Components and Global Optimization Algorithm for Nonlinear Frequency Response
Authors: Denimal E. , El Haddad F. , Wong C. , Salles L.
Published: January 2021

To limit the risk of high cycle fatigue, underplatform dampers (UDPs) are traditionally used in aircraft engines to control the level of vibration. Many studies demonstrate the impact of the geometry of the damper on its efficiency, thus the consideration of topological optimization (TO) to find the best layout of the damper seems natural. Because of the nonlinear behavior of the structure due to the friction contact interface, classical methods of TO are not usable. This study proposes to optimize the layout of an UDP to reduce the level of nonlinear vibrations computed with the multiharmonic balance method (MHBM). The approach of TO employed is based on the moving morphable components (MMC) framework together with the Kriging and the efficient global optimization algorithm to solve the optimization problem. The results show that the level of vibration of the structure can be reduced to 30% and allow for the identification of different efficient geometries.
A Smaller Bearing Ratio, as a Surface Texture Measure, Promotes Faster Sliding on Ice
Authors: Jansons Ernests , Lungevičs Jānis , Jerāne Ilze , Gross Kārlis Agris
Published: January 2021

Surfaces only characterized by a roughness Ra or Sa may have a totally different surface texture and include complex patterns such as grooves, dimples, or a mirror-polish. Here, the bearing ratio is proposed as an additional characterization measure to determine the sliding performance of a steel–ice friction pair. Different steel surfaces were produced by milling, shot blasting, and scratching, followed by texture assessment with a stylus type three-dimensional (3D) profilometer. The bearing ratio and other 3D roughness parameters were determined. Tribology experiments involved a 3 m long inclined plane tribometer and the speed measured at four points during the sliding experiment. Correlation between the steel sliding speed and the bearing ratio was observed under two different regimes: at warmer conditions and at colder conditions. Experiment 1 depicting warmer conditions exhibited a relative humidity of 64%, an air temperature of −2 °C, and an ice temperature of −9 °C. Experiment 2 for colder conditions showed a relative humidity of 78%, an air temperature of 1 °C, and an ice temperature of −4 °C. The sliding speed correlated with the bearing ratio in these two conditions showing −0.91 and −0.96, respectively. A strong correlation between the sliding speed and the bearing ratio shows the value of the bearing ratio as an additional surface characteristic for considering larger surface features.
Nonlinear Wave Loads on Offshore Wind Turbines: Extreme Statistics and Fatigue
Authors: Zhang Yu , Sclavounos Paul D.
Published: January 2021

The development is presented of an analytical model for the prediction of the stochastic nonlinear wave loads on the support structure of bottom-mounted and floating offshore wind turbines. Explicit expressions are derived for the time-domain and frequency-domain nonlinear exciting forces in a seastate with a significant wave height comparable to the diameter of the support structure based on the fluid impulse theory. The higher-order moments of the nonlinear load are evaluated from simulated force records, and the derivation of analytical expressions for the nonlinear load statistics for their efficient use in design is addressed.
Numerical Investigation of Effects of Blockage, Inclination Angle, and Hole-Size on Film Cooling Effectiveness at Concave Surface
Authors: Wang Fu-qiang , Pu Jian , Wang Jian-hua , Xia Wei-dong
Published: January 2021

Film-hole can be often blocked by thermal-barrier coatings (TBCs) spraying, resulting in the variations of aerodynamic and thermal performances of film cooling. In this study, a numerical study of the blockage effect on the film cooling effectiveness of inclined cylindrical-holes was carried out on a concave surface to simulate the airfoil pressure side. Three typical blowing ratios (BRs) of 0.5, 1.0, and 1.5 were chosen at an engine-similar density ratio (DR) of 2.0. Two common inclination angles of 30 deg and 45 deg were designed. The blockage ratios were adjusted from 0 to 20%. The results indicated the blockage could enhance the penetration of film cooling flow to the mainstream. Thus, the averaged effectiveness and coolant coverage area were reduced. Moreover, the pressure loss inside of the hole was increased. With the increase of BR, the decrement of film cooling effectiveness caused by blockage rapidly increased. At BR = 1.5, the decrement could be acquired up to 70% for a blockage ratio of 20%. The decrement of film cooling effectiveness caused by blockage was nearly nonsensitive to the injection angle; however, the larger angle could generate the higher increment of pressure loss caused by blockage. A new design method for the couple scheme of film cooling and TBC was proposed, i.e., increasing the inlet diameter according to the blockage ratio before TBC spraying. In comparison with the original unblocked-hole, the enlarged blocked-hole not only kept the nearly same area-averaged effectiveness but also reduced slightly the pressure loss inside of the hole. Unfortunately, application of enlarged blocked-hole at large BR could lead to a more obvious reduction of effectiveness near hole-exit, in comparison with the original common-hole.
Investigation of Coupling Guard Temperature Rise Using Advanced CFD Analysis
Authors: Samale Anil P. , El Ghazzani El Mostafa
Published: January 2021

Increasingly, the turbomachinery industry is facing issues related to high temperatures and oil misting inside coupling guards. This leads to machinery downtimes and loss of revenue. Many in the turbomachinery industry are investing significant time and effort to reduce heat generation inside coupling guards. The present computational fluid dynamics (CFD) investigation of Air+Oil flow and heat transfer in the coupling guard was conducted to investigate these issues. In prior investigations, the emphasis has been mostly placed on predicting temperature by modeling pure airflow in the coupling guard. This paper presents CFD simulations completed under two conditions: 1) with pure AIR -in a Single phase and 2) a multi-phase fluid combining Air+Oil.

The coupling guard geometry is cylindrical in nature. Given the symmetry in the geometry, a 90-degree sector was modeled, allowing significantly smaller meshes and lower run times without compromising quality. In this paper, the modeled geometry includes both Fluid + Solid domains that do not contain an inlet or outlet. This work explains the CFD methodology for multiphase and conjugate heat transfer analysis.

The objective of this paper is to predict the temperature on the coupling guard considering Air + Oil flow and heat transfer. The domain evaluated consists of several stationary and rotating components. The CFD domain used for the conjugate heat transfer analysis consists of fluid between the rotating shaft and the stationary cover. The solid domain includes the coupling guard cover. The commercially available software ANSYS CFX 18.1 was used. The mesh was created using Tetrahedral/Prism elements. Both steady-state and unsteady CFD analyses were completed. As noted, multiphase CFD analyses were carried out using Air and Oil VG68 as a homogenous type of mixture.

Detailed flow field characteristics (total and static temperature, pressure, streamlines colored by Mach number, etc.) and CFD-predicted temperatures are compared between the Pure Air and Air+Oil simulations. Also, available measurements were used to validate the CFD results. Temperature results obtained from the CFD simulations were found to be in good agreement with the experimental data. The CFD modeling approach developed can be used to design coupling guards with lower surface temperatures.
Temperature-Dependent Fatigue Damage Evolution of Fiber-Reinforced Ceramic-Matrix Composites
Authors: Longbiao Li
Published: January 2021

In this paper, the temperature-dependent fatigue damage evolution of fiber-reinforced ceramic-matrix composites (CMCs) is investigated. The fatigue loading/unloading constitutive model considering the effect of temperature is developed based on the damage mechanisms of matrix cracking, interface debonding, and repeated sliding between the fiber and the matrix. The relationships between the fatigue loading/unloading hysteresis loops, testing temperature, applied cycle number, peak stress, and fiber/matrix interface debonding and sliding are established. The evolution of fatigue loading/unloading hysteresis loops, interface debonding and sliding length with applied cycle number is analyzed. The effects of temperature, peak stress level, applied cycle number, interface shear stress, and interface debonding energy on the fatigue damage evolution are discussed based on the developed temperature-dependent fatigue loading/unloading constitutive model. The experimental fatigue damage evolution of SiC/SiC composite at 600°C, 800°C, and 1000°C in inert atmosphere, 1000°C in air and in steam atmosphere, and 1300°C in air atmosphere are predicted. The interface shear stress of SiC/SiC composite decreases with temperature, and the degradation rate of interface shear stress increases with temperature.
Influence of Inflow Distributions on Swirling Flow in Rotor-Stator Cavity With Centripetal and Centrifugal Superposed Flow
Authors: Shi Yu , Ding Shuiting , Qiu Tian
Published: January 2021

The disk cavity of the rotor-stator system is one of important parts of the aero-engine air system. The rotating speed of airflow determines the relative velocity between the airflow and the rotating assembly, so the maintaining characteristic for swirling flow of the rotor-stator cavity will affect the windage of the rotating assembly in the cavity, and the exporting characteristic for swirling flow of the rotor-stator cavity will affect the windage of the rotating assembly in the downstream chamber. Dual-inlet rotor-stator disk cavity is a typical structure of aero-engine. But there are few reports about the flow structure in the dual-inlet rotor-stator disk cavity in the open literature. In this paper, the influence of inflow distributions on maintaining and exporting characteristics for swirling flow was investigated by numerical simulation, aiming at the interaction between the two inflows. The SST turbulence model was well validated against published experimental results. The simulation results show that there are two flow regions in the rotor-stator cavity from the perspective of meridian plane, and there is a rotating vortex in each flow region. In order to quantify the rotational capacity of the airflow, the angular momentum coefficient was defined. Under different inflow distribution ratios, the sizes of the dominant regions of the two vortices in the cavity get varied, which results in the variation of maintaining and exporting characteristics for swirling flow. It was also found that inflow distributions have the same effect on maintaining and exporting characteristics for swirling flow under different rotational Reynolds numbers and different throughflow coefficients, but the capacity of maintaining and exporting swirling flow are different.
Dynamic Characteristics of a Squeeze Film Damped Rotor System Considering Instantaneous Static Eccentricity in Maneuvering Flight
Authors: Chen Xi , Gan Xiaohua , Jiang Shuyun , Ren Guangming
Published: January 2021

An on-board rotor-bearing system operating at high speed is inevitably subjected to additional inertial forces and parametric excitations caused by aircraft maneuvering flights. The differential equations of motion for a squeeze film damped rotor system moving with the aircraft during maneuvering flight are derived based on Lagrange’s principle. Transient characteristics of the rotor system considering instantaneous static eccentricity of journal in turning maneuver are calculated by Newmark-HHT integration method. The effects of forward speed, radius of curvature, and elastic support stiffness on transient responses are discussed subsequently.

The results indicate that when the aircraft conducts a maneuvering flight, the whirl orbit of journal deviates from the center of the damper, and the deviation direction is determined by the centrifugal acceleration of aircraft and the additional gyroscopic moment. The journal performs a nonsynchronous whirl around the instantaneous static eccentricity. Its magnitude is related to the additional maneuvering loads and the stiffness of elastic support. Increasing forward speed or decreasing maneuvering radius, the rotor vibration will enter earlier into or withdraw later from the relatively large eccentric condition. The stiffness of elastic support has a great impact on transient characteristics of rotor-bearing system during maneuvering flight. Overall, using finite element modeling combined with mechanism analysis, a flexible and efficient approach is proposed to predict transient responses of engine rotor systems during aircraft maneuvering flights.
Detecting Nonsynchronous Vibration in Transonic Fans Using Machine Learning Techniques
Authors: Lu Yaozhi , Vahdati Mehdi
Published: January 2021

Due to manufacturing tolerance and deterioration during operation, different blades in a fan assembly exhibit geometric variability. This leads to asymmetry which will be amplified in the running geometry by centrifugal and aerodynamic loads. This study investigates a phenomenon known as Alternate Passage Divergence (APD), where the blade untwist creates an alternating pattern in passage geometry and stagger angle around the circumference. After the formation of alternating tip stagger pattern, APD’s unsteady effect, APD-induced Non-Synchronous Vibration (APD-NSV, abbreviated as NSV), can cause the blades from one group to switch to the other creating a travelling wave pattern around the circumference. Thus, it can potentially lead to high cycle fatigue issues. More importantly, this phenomenon occurs close to, or at, peak efficiency conditions and can significantly reduce overall efficiency. Therefore, it is vital to attenuate the NSV behaviour. The random nature of mis-staggering patterns complicate the evolution of NSV significantly. Thus, machine learning techniques are used to analyse mis-stagger patterns to identify patterns that can lead to NSV and thus help avoid it. Numerical results from 113 numerical cases (1.6 million CPU hours) are used to train and test the classifier. From the results, two parameters contributing to NSV behaviour have been identified with one of them enhancing the understanding found in the previous study.
Aerodynamic Effects of an Incoming Vortex on Turbines With Different Tip Geometries
Authors: Zhou Kai , Zhou Chao
Published: January 2021

Experimental and numerical methods were used to investigate the aerodynamic effects of a near-casing streamwise incoming vortex flow on the tip leakage flow of different tip geometries in an unshrouded high pressure turbine. A flat tip, a cavity tip and a suction-side winglet tip were investigated with quasi-steady method first. A swirl generator was used to produce the incoming vortex in a linear cascade. In the flat tip case, the incoming vortex interacts with the tip leakage flow and the two vortices gradually mix together. The tip leakage loss is reduced due to the streamwise momentum supplement within the tip leakage vortex core. For the cavity tip, the tip leakage vortex appears at a location relatively downstream in the blade passage compared to the flat tip and no evident vortex interaction is observed. The incoming vortex causes extra viscous dissipation within the blade passage and increases the aerodynamic loss for the cavity tip. For the winglet tip, the extension of the suction side winglet tends to push the incoming vortex and the tip leakage vortex move and mix together, thus reducing the loss. Then the effects of periodic unsteady vortex transportations were investigated by conducting unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations. The incoming vortex is stretched as it transports downstream. The unsteady incoming vortex is easier to interact with the tip leakage vortex for the winglet tip. As a result, the winglet tip is the most efficient tip design with unsteady incoming flow among the three tips, and achieves 3.7% reduction of mixed-out loss coefficient compared to the flat tip, larger than 2.8% reduction in the uniform inlet condition. The detailed loss mechanism is discussed in the paper.
Joint Wavelet Transform and Time Synchronous Averaging Source Separation Method and its Application on Aero-Engine
Authors: Cong Jiqing , Jing Jianping , Chen Changmin , Dai Zezeng , Cheng Jianhua
Published: January 2021

The reliability and safety of aero-engine are often the decisive factors for the safe and reliable flight of commercial aircraft. Hence, the vibration source location and fault diagnosis of aero-engine are of prime importance to detect faults and carry out fast and effective maintenance in time. However, the vibration signals collected by the sensors arranged on the casing of the aero-engine are generally the mixed signals of the main vibration sources inside the engine, and the components are extremely complicated. Therefore, the vibration source identification is a big challenge for a fault diagnosis and health management of the engine. In order to separate the key vibration sources of rotating machinery such as aero-engine, a Joint Wavelet Transform and Time Synchronous Averaging based algorithm (JWTS) is proposed in this paper. Based on the fact that the fundamental frequency and its harmonic and sub-harmonic components are generally included in the vibration spectrum of shaft fault signal of rotating machinery, wavelet transform and time synchronous averaging algorithm are combined to extract them. The algorithm completes separating the main vibration sources with three steps. First, the source number and fundamental frequency of each source are estimated using the wavelet transform. Second, every source is extracted from each observed signal by the time synchronous averaging method. Time synchronous averaging method can effectively extract a signal of cycle and harmonic rotor components and can suppress noise. Third, the optimal estimation of each source is determined according to signal’s 2-norm. Since the extracted source with a larger energy is closer to the real source, and signal’s 2-norm is a good indicator of the signal energy. Hence, the key vibration sources related to rotary speeds of the engine are obtained separately. The method is verified by synthetic mixed signals first. Three periodic signals of different frequencies are used to simulate the vibration sources of the aeroengine. The fundamental, harmonic and sub-harmonic components of them, as well as Gaussian white noise, are randomly mixed. The results show that the JWTS algorithm can estimate the number of the main sources and can extract each source effectively. Then the method is demonstrated using vibration signals of a real aero-engine. The results indicate that the proposed JWTS method has extracted and located the main sources within the aero-engine, including sources from the low-pressure rotor, high-pressure rotor, combustion chamber and accessory. Therefore, the proposed method provides a new fault diagnosis technology for rotating machinery, especially for a real aero-engine.
Single Nodal Diameter Excitation of Turbine Blades: Experimental and Theoretical Study
Authors: Hoffmann Thomas , Panning-von Scheidt Lars , Wallaschek Jörg
Published: January 2021

Validating simulation results of vibrating turbine blades relies on measurements of realistic or academic cyclic structures on special test rigs. In real operation the blades are excited mainly by aerodynamic forces. For measurements of blade vibration on special test rigs, the excitation should be well known. It is desirable to use excitation spectra that consist of only a few engine order excitations. Especially for nonlinear systems, unwanted excitation orders can possibly lead to nonlinear effects which may interfere with the measurement. To separate different engine orders, an innovative electromagnetic excitation device was developed at the institution to overcome the aforementioned problems. The excitation force spectrum is controlled by a variable air gap over the circumference between device and blade. Any desired engine order excitation can be realized. Additionally, by varying the devices coil current in a harmonic fashion, frequency sweeps at constant speed can be performed. In this paper an extensive study of the excitation force spectrum of the device is conducted. Therefore, theoretical investigations of the expectable spectrum are given under simultaneous variation of air gap geometry and excitation current. These predictions are then validated by experiments featuring a small, academic bladed disk. The vibrations of the blades are measured. The device promises to create well predictable and controllable excitation force spectra which will improve the validation strategy in particular of non-linear simulation tools for the prediction of turbine blade vibrations.
Assessment of the Loss Map of a Centrifugal Compressor’s External Volute
Authors: Ceyrowsky Thomas , Hildebrandt Andre , Heinrich Martin , Schwarze Rüdiger
Published: January 2021

A volute’s loss coefficient is highly sensitive to Mach number, circumferential velocity and flow rate at volute inlet. In case of a backswept impeller, these parameters are coupled to each other. An increased flowrate leads to a steeper absolute flow angle at impeller exit and hence to a decrease of circumferential velocity. The absolute Mach number is also altered. Therefore, in order to investigate the effects of flowrate and flow angle separately, one would have to vary the diffuser width together with the flowrate, keeping the flow angle constant. This corresponds to coupling the volute with aerodynamically similar impellers, designed for higher and lower flowrates. Since this is elaborate, there is no adequate study available in open literature, assessing a volute’s global loss map. In this work, a new numerical approach for the prediction of a volute’s representative loss map is presented: The volute is calculated by means of steady CFD as a standalone component. The inlet boundary conditions are carefully selected by means of 1D and applied together with different diffuser widths. This allows for separate investigation of the impacts of flow angle, flow rate and Mach number. Validation against full stage CFD confirms the applicability of the standalone model. The results exhibit that minimum losses do not necessarily occur at the theoretical matching point but either when the volute is smaller or bigger, depending on the inlet flow angle. Investigations of the loss mechanisms at different operating conditions provide useful guidelines for volute design. Finally, the validity of these study’s findings for volutes with different geometrical features is examined by comparison with experimental data as well as with fullstage CFD.
Ignition Behavior of Oxy-Methane With High Fuel Loading and CO2 Dilution in a Shock Tube
Authors: Laich Andrew R. , Baker Jessica , Ninnemann Erik , Sigler Clayton , Naumann Clemens , Braun-Unkhoff Marina , Vasu Subith S.
Published: January 2021

Ignition delay times were measured for methane/O2 mixtures in a high dilution environment of either CO2 or N2 using a shock tube facility. Experiments were performed between 1044 K and 1356 K at pressures near 16 ± 2 atm. Test mixtures had an equivalence ratio of 1.0 with 16.67% CH4, 33.33% O2, and 50% diluent. Ignition delay times were measured using OH* emission and pressure time-histories. Data were compared to the predictions of two literature kinetic mechanisms (ARAMCO MECH 2.0 and GRI Mech 3.0). Most experiments showed inhomogeneous (mild) ignition which was deduced from five time-of-arrival pressure transducers placed along the driven section of the shock tube. Further analysis included determination of blast wave velocities and locations away from the end wall of initial detonations. Blast velocities were 60–80% of CJ-Detonation calculations. A narrow high temperature region within the range was identified as showing homogenous (strong) ignition which showed generally good agreement with model predictions. Model comparisons with mild ignition cases should not be used to further refine kinetic mechanisms, though at these conditions, insight was gained into various ignition behavior. To the best of our knowledge, we present first shock tube data during ignition of high fuel loading CH4/O2 mixtures diluted with CO2 and N2.
Numerical Study on the Characteristics of Lean Direct Injection Combustor With Elevated Fuel Temperatures
Authors: Lu Jinghe , Liu Xiao , Li Shuying , Liu Enhui , Zhang Zhihao , You Binchuan
Published: January 2021

With the development of high performance gas turbine engines, the temperature before turbine is rising and it presents a serious challenge to existing thermal management. It is very attractive to use fuel as the cooling medium for gas turbine engines. For this purpose, the effects of fuel temperature on combustion characteristics are urgently needed to be understood. In this work, the characteristics of lean direct injection (LDI) combustor is simulated by changing the physical properties of fuel with different temperatures. The predictions of gas phase and droplet velocity, droplet diameter are compared well with the experiment data. The numerical results show that as fuel temperature rises, the droplet evaporation rate and mixing efficiency of fuel and air in non-reacting case is improved significantly, the spray angle, concentration and distribution profile of fuel in reacting case are enlarged as well. When fuel temperature is raised from 350K to 550K, the peak value of droplet evaporation rate at the vicinity of nozzle is increased by 26.7 times, the uniformity index downstream of the primary recirculation zone (PRZ) is increased by 2.57%, the axial length and maximum negative axial velocity of PRZ are reduced by 13% and 21%. The average temperature and NO emission at combustor outlet are increased by 1.99% and 48.15%, the mass fraction of CO is decreased by 5.45%. Besides, the number, diameter, and distribution space of droplets are decreased sharply. The formation of premixed flame and propagation of high-temperature region are promoted, the flame front is changed from a conical shape to a recessed shape. The combustion efficiency can be improved by increasing fuel temperature. The present study is expected to provide insightful information for understanding characteristics of LDI combustor with elevated fuel temperatures.
Effect of Fuel Stage Proportion on Flame Position in an Internally-Staged Combustor
Authors: Zhao Tiezheng , Liu Xiao , Zheng Hongtao , Zhang Zhihao , Yang Jialong , Li Zhiming
Published: January 2021

To study the effect of fuel stage proportion on flame position and combustion characteristics of the internally-staged combustor, a detailed numerical investigation is performed in the present paper. The prediction method of flame position is established by analyzing the variations of the distribution of intermediate components and the turbulent flame speed. Meanwhile, the flame position is simulated to verify the accuracy of the prediction method. It is demonstrated that the flame position prediction model established in this paper can accurately predict the flame position under different fuel stage proportions. On this basis, special attention is paid to analyze the variation of velocity field, temperature field, distribution of intermediate components and emissions under different fuel stage proportions. As the proportion of pilot fuel stage increases slightly, the mass fraction of fuel at the combustor dome increases. In addition, the combustion characteristics change significantly with the increase in the proportion of pilot stage fuels. The flame moves downstream and the high temperature area increases as the proportion of pilot fuel increases. In particular, when the proportion of pilot stage reaches 3%, the highest flame temperature is generated due to the most concentrated reaction area, resulting in the largest emission of NOx. At the same time, due to the most complete reaction, the minimum CO emission is produced. When the proportion of pilot fuel stage reaches 1%, the NOx emission is the lowest, and the highest CO emission is generated due to the incomplete reaction.
Effects of Attack Angle and Relative Thickness of Novel Wing-Shaped Turbulators on Turbulent Hydrothermal Performance in a Two-Pass Square Channel
Authors: Liou Tong-Miin , Chen Chieh-Chu , Wang Chun-Sheng
Published: January 2021

This work aims to combine the effects of the near wall and core flow disturbance by proposing novel wing-shaped tabulators. The new tabulators are fabricated with the fused deposition modeling (3D printing) technology. To explore their effects on detailed flow fields, local temperature distributions, and pressure drops in a two-pass square channel, Particle Image Velocimetry (PIV), Infrared Thermography (IR camera), and pressure transducer measurements are performed. The tabulator pitch, clearance, and truncation gap ratio based on the channel hydraulic diameter of 45.5 mm are respectively fixed at 0.7, 0.25 and 0.06. Varied parameters include tabulator attack angle (α = 10°, 15°, 20°, and 30°), maximum thickness to chord line ratio (t/C = 0.08, 0.13, 0.16, 0.20, and 0.23), and bulk Reynolds number (Re = 5,000–20,000). From the experimental results and flow parameters analyzed, the dimensionless spanwise-averaged mean transverse velocity and cross-sectionally averaged vorticity magnitude are identified to be the most relevant ones to spanwise-averaged local Nusselt number ratio in the first and second pass. Among all examined cases and previous data with Fanning friction factor ratio (f¯/fo) less than 50, the case with α = 20° and t/C = 0.20 attains the highest thermal performance factor and overall Nusselt number ratio (Nu¯/Nuo) up to 1.68 and 5.36, respectively. Furthermore, empirical correlations of Nu¯/Nuo and f¯/fo versus α, t/C, and Re are proposed.
Quantification of Low-Cycle Fatigue Life for a Gas Turbine Compressor Vane Carrier Under Varying Operating Conditions
Authors: Han Zixi , Jiang Zixian , Ehrt Sophie , Li Mian
Published: January 2021

The design of a gas turbine compressor vane carrier (CVC) should meet mechanical integrity requirements on, among others, low-cycle fatigue (LCF). The number of cycles to the LCF failure is the result of cyclic mechanical and thermal strain effects caused by operating conditions on the components. The conventional LCF assessment is usually based on the assumption on standard operating cycles — supplemented by the consideration of predefined extreme operations and safety factors to compensate a potential underestimate on the LCF damage caused by multiple reasons such as non-standard operating cycles. However, real operating cycles can vary significantly from those standard ones considered in the conventional methods. The conventional prediction of LCF life can be very different from real cases, due to the included safety margins.

This work presents a probabilistic method to estimate the distributions of the LCF life under varying operating conditions using operational fleet data. Finite element analysis (FEA) results indicate that the first ramp-up loading in each cycle and the turning time before hot-restart cycles are two predominant contributors to the LCF damage. A surrogate model of LCF damage has been built with regard to these two features to reduce the computational cost of FEA. Miner’s rule is applied to calculate the accumulated LCF damage on the component and then obtain the LCF life. The proposed LCF assessment approach has two special points. First, a new data processing technique inspired by the cumulative sum (CUSUM) control chart is proposed to identify the first ramp-up period of each cycle from noised operational data. Second, the probability mass function of the LCF life for a CVC is estimated using the sequential convolution of the single-cycle damage distribution obtained from operational data. The result from the proposed method shows that the mean value of the LCF life at a critical location of the CVC is significantly larger than the calculated result from the deterministic assessment, and the LCF lives for different gas turbines of the same class are also very different. Finally, to avoid high computational cost of sequential convolution, a quick approximation approach for the probability mass function of the LCF life is given. With the capability of dealing with varying operating conditions and noises in the operational data, the enhanced LCF assessment approach proposed in this work provides a probabilistic reference both for reliability analysis in CVC design, and for predictive maintenance in after-sales service.
Unsteady Simulations of a Trailing-Edge Slot Using Machine-Learnt Turbulence Stress and Heat-Flux Closures
Authors: Lav Chitrarth , Sandberg Richard D.
Published: January 2021

The trailing edge slot is a canonical representation of the pressure-side bleed flow encountered in high pressure turbines. Predicting the flow and temperature downstream of the slot exit remains challenging for RANS and URANS, with both significantly overpredicting the adiabatic wall-effectiveness. This over-prediction is attributable to the incorrect mixing prediction in cases where the vortex shedding is present. In case of RANS the modelling error is rooted in not properly accounting for the shedding scales while in URANS the closures account for the shedding scales twice, once by resolving the shedding and twice with the model for all the scales. Here, we present an approach which models only the stochastic scales that contribute to turbulence while resolving the scales that do not, i.e. scales considered as contributing to deterministic unsteadiness. The model for the stochastic scales is obtained through a data-driven machine learning algorithm, which produces a bespoke turbulence closure model from a high-fidelity dataset. We use the best closure (blowing ratio of 1.26) for the anisotropy obtained in the a priori study of Lav, Philip & Sandberg [A New Data-Driven Turbulence Model Framework for Unsteady Flows Applied to Wall-Jet and Wall-Wake Flows, 2019] and conduct compressible URANS calculations. In the first stage, the energy equation is solved utilising the standard gradient diffusion hypothesis for the heat-flux closure. In the second stage, we develop a bespoke heat-flux closure using the machine-learning approach for the stochastic heat-flux components only. Subsequently, calculations are performed using the machine-learnt closures for the heat-flux and the anisotropy together. Finally, the generalisability of the developed closures is evaluated by testing them on additional blowing ratios of 0.86 and 1.07. The machine-learnt closures developed specifically for URANS calculations show significantly improved predictions for the adiabatic wall-effectiveness across the different cases.
Generation Mechanism of Broadband Whoosh Noise in an Automotive Turbocharger Centrifugal Compressor
Authors: Dehner Rick , Sriganesh Pranav , Selamet Ahmet , Miazgowicz Keith
Published: January 2021

The present study focuses on the acoustics of a turbocharger centrifugal compressor from a spark-ignition (SI) internal combustion (IC) engine for passenger car applications. Whoosh noise is typically the primary concern for this type of compressor, which is loosely characterized by broadband sound elevation in the 4 to 13 kHz range. To identify the generation mechanism of broadband whoosh noise, the present study combines three approaches: three-dimensional (3D) computational fluid dynamics (CFD) predictions, experiments, and modal decomposition of 3D CFD results. CFD predictions include four operating points at the (relatively low) 80 krpm rotational speed, spanning from near choke to the peak pressure ratio, along with an additional point at the peak whoosh noise (from experiments) at 140 krpm. Predicted compressor performance, along with noise in the compressor inlet duct agree reasonably well with the corresponding experimental results.

After establishing the accuracy of predictions, flow structures and time-resolved pressures are closely examined in the vicinity of the main blade leading edge. This reveals the presence of rotating instabilities that may interact with the rotor blades to generate noise. An azimuthal modal decomposition is performed on the predicted pressure field to determine the number of cells and the frequency content of these rotating instabilities.

The strength of the rotating instabilities and the frequency range in which noise is generated as a consequence of the rotor-rotating instability interaction, is found to correspond well with the qualitative trend of the whoosh noise that is measured several duct diameters upstream of the rotor blades. The variation of whoosh frequency range between low (4 to 6 kHz at 80 krpm) and high (4 to 13 kHz at 140 krpm) rotational speeds is interpreted through this analysis. It is also found that the whoosh noise primarily propagates along the duct as acoustic azimuthal modes. Hence, the inlet duct diameter, which governs the cut-off frequency for multi-dimensional acoustic modes, determines the lower frequency bound of the broadband noise.
Performance Improvement of a Supercritical CO2 and Transcritical CO2 Combined Cycle for Offshore Gas Turbine Waste Heat Recovery
Authors: Zhou Aozheng , Ren Xiaodong
Published: January 2021

The supercritical CO2 (S-CO2) and transcritical CO2 (T-CO2) combined cycle is a promising technology for the waste heat recovery of the offshore gas turbine. In this paper, a new system layout is proposed to investigate the thermodynamic and economic performances improvement of the S-CO2-T-CO2 combined cycle. Compared to the original systems, the residual heat of the topping S-CO2 cycle is recovered and extra recuperation is added to the bottoming T-CO2 cycle. Sensitivity analysis of different systems is carried out based on the mathematical model and multi-objective optimization is conducted. The net power out and the net present value (NPV) are chosen as the objective functions of thermodynamic and economic aspects. The results reveal that at the design point, the improved system has more than 8.23% increment on the net power output and 3.55% increment on the NPV. Besides, the optimized net power output of the improved system attains at least 5.16% increment with 3.15% increment for the optimized NPV at the same time. The improved system can be applied in some practical cases from both the thermodynamic and economic views.
Pressurized SOFC System Fuelled by Biogas: Control Approaches and Degradation Impact
Authors: Ferrari Mario L. , Zaccaria Valentina , Kyprianidis Konstantinos
Published: January 2021

This paper shows control approaches for managing a pressurized Solid Oxide Fuel Cell (SOFC) system fuelled by biogas. This is an advanced solution to integrate the high efficiency benefits of a pressurized SOFC with a renewable source. The operative conditions of these analyses are based on the matching with an emulator rig including a T100 machine for tests in cyber-physical mode (a real-time model including components emulated in the rig, operating in parallel with the experimental facility and used to manage some properties in the plant, such as the turbine outlet temperature set-point and the air flow injected in the anodic circuit). The T100 machine is a microturbine able to produce a nominal electric power output of 100 kW. So, the paper presents a real-time model including the fuel cell, the off-gas burner, and the recirculation lines. Although the microturbine components are planned to be evaluated with the hardware devices, the model includes also the T100 expander for machine control reasons, as detailed presented in the devoted section. The simulations shown in this paper regard the assessment of an innovative control tool based on the Model Predictive Control (MPC) technology. This controller and an additional tool based on the coupling of MPC and PID approaches were assessed against the application of Proportional Integral Derivative (PID) controllers. The control targets consider both steady-state (e.g. high efficiency solutions) and dynamic aspects (stress smoothing in the cell). Moreover, different control solutions are presented to operate the system during fuel cell degradation. The results include the system response to load variations, and SOFC voltage decrease. Special attention is devoted to the fuel cell system constraints, such as temperature and time-dependent thermal gradient. Considering the simulations including SOFC degradation, the MPC was able to decrease the thermal stress, but it was not able to compensate the degradation. On the other hand, the tool based on the coupling of the MPC and the PID approaches produced the best results in terms of set-point matching, and SOFC thermal stress containment.
Performance Simulation of Roughness Induced Module Variations of a Jet Propulsion by Using Pseudo Bond Graph Theory
Authors: Göing Jan , Bode Christoph , Friedrichs Jens , Seehausen Hendrik , Herbst Florian , Seume Joerg R.
Published: January 2021

A scientific method is developed to determine the impact of a deteriorated HPC (high-pressure compressor) on the overall performance of a turbofan jet engine. Initially, the HPC performance with roughness variations on vanes and blades is simulated by using 3D CFD (Computational Fluid Dynamics) at different operating points. Afterwards, the overall performance of the full jet engine is computed by the in-house 1D performance tool ASTOR (AircraftEngine Simulation for Transient Operating Research). ASTOR is based on the Pseudo Bond Graph approach to model and connect miscellaneous components with the equations of motion and dynamic volumes. By solving this differential equation system, transient performance is calculated in higher accuracy compared to common Reduced Order Models (ROM). Transient load cases are modelled to analyse the impact of deteriorated HPC on the overall performance. Furthermore, similarities and differences to the steady performance are considered. The maximum deviation of EGT between an engine with a new and deteriorated HPC during a fast acceleration is 32% while it is 27% at steady operation. Also a trade-off between an increasing EGT and a decreasing pressure downstream of the LPT is analysed. Finally, relations between the engine performance, Reynolds-number and roughness are analysed to classify the performance drop due to a HPC with roughness.
Design and Testing of a Hot Gas Blower for Solid Oxide Fuel Cell Applications
Authors: Honkatukia Juha , Jaatinen-Värri Ahti , Turunen-Saaresti Teemu , Grönman Aki , Sallinen Petri , Backman Jari
Published: January 2021

The study focuses on the design and testing of a centrifugal blower to be used for recirculating hot anode off-gas in solid oxide fuel cells (SOFC). SOFCs operate at high temperatures, even at 800 °C and typically, the anode off-gas is led out of the SOFC stack. However, nowadays it is commonly known that the efficiency of the SOFC system can be improved by recirculating the anode off-gas.

Up until recent years, the problem with the recirculation has been that there are no suitable blowers feasible enough to sustain the high temperature of the anode off-gas. In this study, a 500 W centrifugal blower having a design pressure ratio of 1.126 was designed and tested using a 50 kW SOFC application as a basis of the design, but the designs can also be applied to other applications, e.g. recirculation needs of hot air.

The blower was first tested using inlet air at room temperature and then using heated inlet air at about 300 °C. In the design phase, cooling was found one of the main challenges when hot inlet air is used. The results obtained from the test runs were well-matched with the design values.