Center Stack Research Articles

Modelling and performance enhancement of the fluid coupling interface for hydraulic pressure energy harvesting by superposition of the static mean pressure and pressure fluctuations

The monitoring and control of hydraulic systems in an autonomous and battery-free manner is receiving increasing attention to improve both safety and performance. In this regard, the conversion of pressure ripples within a hydraulic system into electric energy using hydraulic pressure energy harvesting (HPEH) is attractive due to the high energy intensity associated with dynamic pressure fluctuations. In this paper, a new theoretical model of the fluid to mechanical interface is established based on a central piezoelectric stack subject to a superimposed dynamic pressure fluctuation and a mean static pressure. A lumped-parameter model of the electromechanical coupling system is employed to study the overall harvesting performance of the system, and the force–deflection behavior of a circular edge-clamped plate with a lumped mass and a superimposed excitation is determined. The influence of static pressure on harvesting performance is explored in detail, where the mean static pressure introduces a softening nonlinearity to the harvester which decreases the power output and harvesting bandwidth. To reduce the negative impact of static pressure on power output, an optimized structure based on a quasi-zero stiffness (QZS) disc spring is proposed. The nonlinear restoring force of the circular plate interface on a central piezoelectric stack and disc spring is determined and the electromechanical coupling equations of the quasi-zero stiffness hydraulic pressure energy harvesting (QZS-HPEH) structure is established. Our theoretical analysis shows that the power output is improved using the novel QZS-HPEH structure, in particular at the high static loads presented in hydraulic systems. Experimental validation is performed, where good agreement is observed between model results and experimental measurements. The proposed model and experimental validation provide important new insights into the optimization of power output and application of HPEH devices in practical hydraulic systems.

Effect of defective cells on the temperature distribution of a proton exchange membrane fuel cell stack

Proton exchange membrane fuel cell (PEMFC) stacks may encounter performance degradation or even damage during long-term operations, while the influence of defective cells on the stack performance still remains inadequately understood. In this study, the effect of defective cells on the stack temperature distribution was experimentally investigated. Two defective cells of different defect degrees were prepared by controlling hydrogen starvation time and quantitively characterized by the polarization curves, electrochemical impedance spectroscopy (EIS), and galvanostatic charge method (GCM) tests. Results show that the presence of defective cells reduces the stack performance and the temperature distribution uniformity. The increment of stack temperature and decrement of stack temperature uniformity is more prominent (with 0.9 °C and 0.31 °C at 0.8 A cm−2, respectively) when the defective cell is located at central stack. The peak sub-cell regional temperature rise can even reach 2.9 °C. For defective cells with a relatively low degree of defect, the reverse current density is high, thus leading to an increased heat generation with the average sub-cell regional temperature increment of 0.5 °C and causing adjacent normal cells to operate at higher temperature. For defective cells with an excessive degree of defect, polarization reverse occurs at low current densities, resulting in significant voltage losses and reduced performance of the stack. The present results are insightful for developing thermal management strategies for the long-term operation of PEMFC stacks.

Study on the influence of the hydrogen–oxygen stoichiometric ratio on the power performance improvement in a large-scale PEMFC stack

This study aims to determine the optimized hydrogen–oxygen stoichiometric ratio for power performance improvement in a large-scale proton exchange membrane fuel cell (PEMFC) stack by analyzing the characteristics of internal heat and mass distributions. The power performance of a large-scale PEMFC stack consisting of the designed stack cells with the counter flow directions of hydrogen and oxygen and a coolant flow channel was experimentally evaluated. Temperatures in the central stack cell were monitored. The coupled numerical simulation model was built to analyze the power performance as well as heat and mass distribution characteristics inside a stack cell. It was found that the greatest power performance of the PEMFC stack occurred at a specific hydrogen–oxygen stoichiometric ratio, and this performance had good uniformities of the temperature and water mass fraction distributions at the proton exchange membrane (PEM). The distribution characteristics of the hydrogen and oxygen mass fractions at the interfaces between the gas diffusion layer (GDL) and the catalytic layer (CL) due to the diffusion effect were analyzed. The mechanism of enhancing power performance from an electrochemical reaction was revealed by the greater gas distribution uniformities. Furthermore, a parameter of energy conversion ratio was employed to quantify the hydrogen utilization degree.

Design and Development of 0.3-T Toroidal Field Coil System for Small-Sized MT-II Spherical Tokamak

The achievement of a high toroidal magnetic field in a small spherical tokamak is challenging because of the small bore area in the central cylinder of the vacuum vessel. In this paper, we present a toroidal field coil of 0.3 T at the center of the MT-II tokamak. It has been designed, developed, and tested for installation at Pakistan Tokamak Plasma Research Institute (PTPRI). The coil is made of highly pure oxygen-free copper. It has a cross-sectional area of 10 × 15 mm2 (150 mm2) for the flow of an approximately 20-kA current to produce a 0.33 T toroidal magnetic field at the center of the tokamak. Mechanical support for the central stack of the inner legs is provided by a twisted grooved nylon cylinder to control the torque and attractive forces. The repulsive force density between the joints of the outer and inner legs is balanced by nuts and bolts along with an insulated ring of Teflon and an isolated metallic clamp from both ends. This compressive force also reduces connection resistance. The simulated currents and magnetic field are confirmed from the experimental results as well.

Multitasking While Driving: How Drivers Self-Regulate Their Interaction with In-Vehicle Touchscreens in Automated Driving

Driver assistance systems are designed to increase comfort and safety by automating parts of the driving task. At the same time, modern in-vehicle information systems with large touchscreens provide the driver with numerous options for entertainment, information, or communication, and are a potential source of distraction. However, little is known about how driving automation affects how drivers interact with the center stack touchscreen, i.e., how drivers self-regulate their behavior in response to different levels of driving automation. To investigate this, we apply multilevel models to a real-world driving dataset consisting of 31,378 sequences. Our results show significant differences in drivers’ interaction and glance behavior in response to different levels of driving automation, vehicle speed, and road curvature. During automated driving, drivers perform more interactions per touchscreen sequence and increase the time spent looking at the center stack touchscreen. Specifically, at higher levels of driving automation (level 2), the mean glance duration toward the center stack touchscreen increases by 36% and the mean number of interactions per sequence increases by 17% compared to manual driving. Furthermore, partially automated driving has a strong impact on the use of more complex UI elements (e.g., maps) and touch gestures (e.g., multitouch). We also show that the effect of driving automation on drivers’ self-regulation is greater than that of vehicle speed and road curvature. The derived knowledge can inform the design and evaluation of touch-based infotainment systems and the development of context-aware driver monitoring systems.

Analysis and design of the central stack for the SMART tokamak

The SMall Aspect Ratio Tokamak (SMART) is a new spherical machine that is currently under construction at the University of Seville aimed at exploring negative vs positive triangularity prospects in Spherical Tokamaks (ST). The operation of SMART will cover three phases, with toroidal fields Bϕ≤ 1 T, inductive plasma currents up to Ip= 500 kA and a pulse length up to 500 ms, for a plasma with R = 0.4 m, a = 0.25 m and a wide range of shaping configurations (aspect ratio, 1.4 < R/a < 3, elongation, κ≤ 3, and average triangularity, -0.6 ≤δ≤ 0.6). The magnet system of the tokamak is composed by 12 Toroidal Field Coils (TFC), 8 Poloidal Field Coils (PFC) and a Central Solenoid (CS). With such operating conditions, the design of the central stack, usually a critical part in spherical tokamaks due to space limitations, presents notable challenges. The current SMART central stack has been designed to operate up to phase 2 and it comprises the inner legs of the TFC, surrounded by the CS, two supporting rings, a central pole and a pedestal. To achieve the plasma parameters of this phase (Bϕ=0.4 T with inductive Ipup to 200 kA), the high currents required, combined with the low aspect-ratio of the machine lead to high forces on the conductors that represent an engineering challenge. The loads expected in the central stack are a centring force up to 1.5 MN and a twisting torque up to 7.4 kNm. This work describes the design of the central stack and its mechanical validation with a multiphysics finite element assessment. Using a combined electromagnetic and mechanical assessment, it is shown that the SMART central stack will meet the physics requirements in phase 2.

Highlights from the Detroit SID Vehicle Display Symposium and Exposition

Highlights from the Detroit SID Vehicle Display Symposium and Exposition

Evaluation of driver distraction from in-vehicle information systems: A simulator study of interaction modes and secondary tasks classes on eight production cars

Currently, various modalities for in-vehicle information systems have gained popularity. However, the secondary tasks of an in-vehicle information system (IVIS) may result in driver distraction while driving. This study aimed to evaluate the effects of classified secondary tasks performed in four modalities on driving performance and visual distraction. A driving simulator capable of connecting mass-produced cars to a virtual driving scenario was proposed. Ten secondary tasks were classified based on complexity and difficulty into three classes (basic, medium, and advanced). Thereafter, these tasks were tested using four modalities: knobs and buttons on the center stack, touch screen, steering wheel (SW) buttons, and voice control. For dependent measures, the standard deviation of speed, the standard deviation of lane position, fixation counts, and average fixation duration were collected. Consequently, significant primary effects of modality and task class were found in terms of each dependent measure. The analysis revealed that SW buttons and voice control were more suitable for basic tasks. Moreover, voice control was best for tasks in the medium class. In the advanced class, the voice control and touch screens outperformed the knobs and buttons. However, considering the efficiency and full interaction process, voice control is unsuitable for all advanced secondary tasks. Hence, cross-mode interaction was recommended for advanced class tasks with complicated input and feedback, owing to the trade-off between different interaction modes. The results of this study are significant for selecting the interaction modes and task design of IVIS for enhancing driving safety.

Determination of best operation parameters of electron cyclotron wall conditioning in KSTAR

Dependence of EC injection power (0.6 and 1.4 MW), EC injection width (0.1–1.0 s) and prefilled He pressure ( 2 . 8 × 1 0 − 5 ∼ 2 . 2 × 1 0 − 3 mbar) on D 2 outgas efficiency is investigated at a toroidal field of 2.4 T in a poloidal trapped particle configuration with an EC wave frequency of 140 GHz (X-mode) and a second harmonic resonance layer near the machine center (1.73 m). With increasing EC injection power, D 2 outgas increases clearly. The dependence of the EC power injection is the most pronounced between the scanned parameters. It is found that an optimum prefilled He pressure for D 2 outgas exists under the condition of a constant EC injection power. It is observed that both high EC injection power and the optimum prefilled He pressure enhance the ion flux to the center stack, suggesting that the D 2 outgas originates from the center stack. • Parameter scan (prefilled He pressure, EC pulse width, EC power) was performed to determine best ECWC parameters in KSTAR. Within this experiment. • In terms of D 2 removal efficiency, optimum He pressure existed, higher EC injection power was better but EC injection length was less effective. • It was suggested that D 2 outgassing originated from the center stack. • The reason why ECWC at highest He pressure with very small D 2 outgassing resulted in successful burn-through was still unclear.

Conceptual Design and Analysis of Prototype Center Stack for Spherical Tokamak based Technologies Development

Conceptual Design and Analysis of Prototype Center Stack for Spherical Tokamak based Technologies Development

Overview of coordinated spherical tokamak research in Japan

Spherical tokamak (ST) research in Japan has produced many innovative results: (i) plasma start-up to I p > 70 kA was achieved by electron cyclotron wave (ECW) with N ∥ = 0.75, while electron heating to T e > 500 eV was achieved with N ∥ = 0.26 on QUEST. (ii) The radiofrequency (RF)-induced transport model was combined with the x-ray emission model, and extended magnetohydrodynamics equilibrium with kinetic electrons was developed to interpret fast-electron-dominated lower hybrid wave sustained plasmas on TST-2. (iii) Density as high as 30 times the cutoff density was achieved by electron Berstein wave current drive combined with electron beam injection on LATE. (iv) Multiple plasmoids formed by tearing instability in the elongated current sheet were observed, and flux closure and ion heating by plasmoid-mediated fast magnetic reconnection were observed on HIST. (v) Optimization of ECW-assisted inductive start-up with a vertical field with positive decay index was performed on TST-2. (vi) Stabilization of the vertical displacement event by a set of upper and lower helical field coils was demonstrated on TOKASTAR-2. (vii) A 6 h discharge was achieved by cool-down of the center stack cover on QUEST, where the plasma duration limit was consistent with the wall saturation time estimated by modeling. (viii) Extension of ion heating by plasma merging was achieved on TS-3U, TS-4U, UTST, MAST, and ST40.

Looking at HMI Concepts for Highly Automated Vehicles: Permanent vs. Context-Adaptive Information Presentation

Abstract To facilitate the usage and expected benefits of higher-level automated vehicles, passengers’ distrust and safety concerns should be reduced through increasing system transparency (ST) by providing driving-related information. We therefore examined the effects of ST on passengers’ gaze behavior during driving, trust in automated driving and evaluation of different human-machine interface (HMI) concepts. In a driving simulator, 50 participants experienced three identical highly automated drives under three HMI conditions: no HMI (only conventional speedometer), context-adaptive HMI (all system information only available in more complex situations) or permanent HMI (all system information permanently available). Compared to driving without HMI, the introduction of the two HMIs resulted in significantly higher usage of the center stack display (i.e. gazes towards the HMIs), which was accompanied by significantly higher trust ratings. The considerable differences in information availability provided by the context-adaptive versus permanent HMI did not reflect in similarly considerable differences regarding the passengers’ gaze behavior or accompanied trust ratings. Additionally, user experience evaluations expressed preferences for the context-adaptive HMI. Hence, the permanent HMI did not seem to create benefits over the context-adaptive HMI, supporting the usage of more economical, context-adaptive HMIs in higher-level automated vehicles.

Self-calibrated driver gaze estimation via gaze pattern learning

Driver’s eye gaze is regarded as an important clue for evaluating the driver’s awareness in the Advanced Driver Assistance Systems and the Automated Driving Systems. Existing driver gaze estimation methods have poor performance on uncalibrated drivers or camera views due to the tedious calibration process. To solve this problem, a knowledge-based solution based on domain prior of typical driver gaze patterns is proposed to realize self-calibration for real driving in naturalistic driving scenarios, which implicitly select the representative time samples of the forward-view gaze zone, the left-side mirror, the right-side mirror, and the rear-view mirror, the speedometer, and the center stack as calibration points. Supporting by the gaze pattern learning algorithm, the proposed method can extract the relevant driver status features gradually and update the estimation parameters periodically. Experimental results demonstrate that the auto-calibrated gaze estimation method can achieve automatic gaze calibration for gaze tracking during in-the-wild on-road driving, and does not require any cooperation from the driver.

Control of electron acceleration process during merging start-up of spherical tokamak

The axial plasma merging method is one of the start-up schemes to form spherical tokamak (ST) plasma without use of a center-solenoid coil. It involves a magnetic reconnection process in the presence of high toroidal magnetic field parallel to the reconnection electric field and has the potential to accelerate electrons directly along the magnetic field lines. In the paper, the role of boundary conditions of the reconnection downstream region was investigated in the UTST experiment. Self-generated axial electric field in the reconnection downstream region was observed to grow up to 20–30 times as large as the toroidal electric field where the MHD condition holds, but was largely suppressed inside the area where the magnetic field lines are contacted to the inboard limiter conductors equipped on the center stack. This suppression effect was attributed to the short circuit connection of the magnetic field lines to hinder the axial charge separation and then a significant part of the parallel electric field remained. These experimental results suggest that the merging start-up method has the capability to enhance electron acceleration efficiency by magnetic reconnection and may have the potential to produce high electron temperature ST plasma applicable for additional heating and current drive techniques.

Analytic Model and Optimization of Transmission-Line Impedance Transformers for Tens of MA-Class Pulsed Power Accelerator

The monolithic radial transmission line (MRTL) is a key component in tens of MA-class pulsed power accelerators, which connects the multiparallel modules of the prime pulsed power sources to the central vacuum insulator stack and magnetically insulated transmission line (stack-MITL system). The choice of electrical parameters of MRTL is essential for the entire accelerator design, which would bring great influences on the peak output current and the power couple efficiency. In Phys. Rev. Special Topics-Accel. Beam s, 2015, 18(10): 110401, some semianalytical formulae have been proposed to optimize the MRTL impedance that maximizes the peak current or peak power on the stack-MITL system. However, the formulae are only valid under several assumptions that are thought to be unpractical. In this article, an analytic model of the accelerator is developed to optimize the electrical parameters of MRTL. Analytical expressions of the output voltage and current at the terminal of MRTL are deduced. By simplifying the analytic model, mathematical expressions of the optimal MRTL output impedance that maximizes the peak current or peak power delivered from the prime power sources to the stack-MITL system are obtained. The factors that contribute to the optimal impedance are analyzed comprehensively with short-circuit load and Z-pinch load. Conclusions are beneficial to design the pulsed power accelerator in the future.

Qualification of NSTX-U Inner TF Bundle Using Hi-Fidelity Models

The National Spherical Torus eXperiment Upgrade (NSTX-U) Recovery Project provides a national facility for the study of plasma confinement, heating, and current drive in a low-aspect-ratio, spherical torus configuration. For the NSTX-U Recovery Project, key system capabilities must be achieved in order to meet operational goals. The inner toroidal field (TF) bundle is part of the center stack with 36 pie-shaped wedges (TF inner legs) bonded into a cylindrical bundle. The inner bundle is connected to outer legs via flex straps, and an ohmic heating (OH) coil is wound around the inner TF bundle, with a layer of aquapour in between acting as a winding surface between TF-OH coils. The TF bundle for the Recovery Project underwent two expert reviews to gain an improved understanding of issues identified from the extent of condition and the magnet design verification and validation review; this was to ensure confidence in the ability of the TF-OH center stack assembly to meet physics requirements for operations. A hi-fidelity, finite element model was developed with appropriate detail for multiphysics analysis of the bundle and to provide a baseline model to use as a standard against which the allowable stresses were compared. The model was also used to understand mechanical behavior and to assess implications of hypothetical delamination to the machine operation. Normal tensile stress at the bundle stub was the result of a nonuniform resistive current distribution where a relatively cold region was formed when the TF coil was pulsed. The worst case occurred at the end of the pulse when the maximum temperature gradient was established at the bundle ends. The hot spot was in compression, and the cold region was in tension. The compressive stress, which had a maximum at the bundle stub, enhanced the insulation shear capability. The in-plane torsional shear stress peaked at the end corner of the inner leg outer diameter at end of flat top. The insulation tensile and shear stresses were extracted from the hi-fidelity model using orthotropic material properties derived from first principles, and these theoretical data were verified via material tests. Load path probes were used to quantify the redistribution of torsional loads between the bundle and the external load path for a hypothetical extreme delamination condition. The extent of possible delamination was limited to regions within the radius of the flex straps. There is no significant effect on the bundle inner leg copper stress. The differential movement between the flex straps in the hypothetical extreme delamination condition is far less than the toroidal gaps between them, so there is no risk of closing these electrical gaps between the bundle turns. There is thus no impact on machine operation or even hypothetical delamination to an extreme delamination beyond the simulation-predicted delamination boundary.

Center Stack Casing Bakeout BUS Design for the NSTX-Upgrade Fusion Device

The National Spherical Torus eXperiment (NSTX) has undergone a major upgrade to NSTX-U at the Princeton Plasma Physics Laboratory. NSTX-U will double the toroidal field, plasma current, and neutral beam injection heating power, as well as significantly increase the pulse duration. The plasma-facing components (PFCs) in the NSTX-U vacuum vessel are mainly graphite, which has a total surface area of about 41 m2. To achieve high vacuum and reduce impurity from PFCs during operation, it is important to bake the graphite parts and remove most of the moisture absorbed by graphite during installation. Typical bakeout for NSTX-U lasts about 3 to 4 weeks. The NSTX-U inner vacuum vessel, i.e., the center stack casing, will be heated to about 450°C by passing 8 KA direct current through it during bakeout. The design of the bakeout bus directly attached to the casing flanges at vessel top and bottom are covered in detail in this paper. At the vessel top, the water-cooled bus terminal is subject to high thermal growth (about 18 mm in the vertical direction and 3 mm in the radial direction). At the vessel bottom, the bakeout bus must withstand 120 KA of halo current during disruption, as well as dislocation from thermal growth. This paper covers the design to address all these challenges. A machined CuCrZr terminal with internal water-cooling channels was used to prevent any brazing work in high stress areas. Detailed analysis will also be covered to show that the proposed design can satisfy thermal, structural, and fatigue requirements during bakeout and operation.

Design and Analysis of High Heat Flux Plasma-Facing Components for NSTX Upgrade

The National Spherical Tokamak Experiment (NSTX) upgrade (NSTX-U) requirements lead to enhanced heat loads on plasma-facing components (PFCs) especially in the divertor regions, where normal heat flux density can reach up to 8 MW/m2. For these high heat flux (HHF) regions, the PFCs were redesigned, to use castellations which relieved the thermal stresses associated with high incident heat flux. Another design feature of HHF PFCs is the absence of front face mounting holes which create localized areas of high temperature and thermal stress concentrations. Optimized shaping of the front face of the HHF PFCs eliminates regions where the front face is perpendicular to the heat flux direction during normal helicity operation and spreads the heat load. A unique mechanism was designed to connect PFC tiles to the NSTX-U center stack casing using locking features accessible from the low heat flux regions. Isotropic graphite was selected as the HHF PFC tile material. Initial tile design parameters were accessed using analytical expressions for pulsed heat flux loading. A working prototype of the locking mechanism was created during the initial stages of the design to prove the concept performance. This article presents an overview of the divertor HHF PFC design and the results of the thermal and structural analyses performed using ANSYS software. The results of the analyses cover normal operating conditions and disruptions which impose electromagnetic (EM) loads from eddy and halo currents. The 3-D, transient, nonlinear analyses took into account the temperature-dependent properties of the materials, friction interfaces between the parts, and variable electric properties of the parts and interfaces. The results confirm that the HHF PFC tiles remain within the allowable limits for the loads defined by the NSTX-U Recovery Project. In addition, tolerance stack up analyses were performed to ensure tile performance in the worst possible assembly configuration. The design process was completed successfully, and the NSTX HHF PFC tiles are currently in the production phase.

New paleomagnetic constraints for the large-scale displacement of the Hronic nappe system of the Central Western Carpathians

The thin-skinned Hronic nappe system represents the structurally highest tectonic unit in the Late Cretaceous thrust-stack of the Central Western Carpathians. It mostly comprises a Permian volcano-sedimentary sequence and Triassic carbonate sediments which crop out in different parts of the Central Western Carpathians. We carried out a systematic paleomagnetic study on 24 Permian and 20 Triassic localities geographically distributed over 300 km in W-E direction. Several samples from each locality were drilled and oriented in-situ and specimens cut from them subjected to standard paleomagnetic and magnetic mineralogy experiments. The results were evaluated using principal component analysis, statistical evaluation of the characteristic remanences, and applying inclination-only and tilt tests. We documented the pre-tilting age of remanences for the majority of both the Permian and Triassic age groups. However, the latter was interpreted as remagnetized during the Cretaceous Normal Super-Chron in the course of nappe stacking between 90−80 Ma. The Permian group is exhibiting about 70°, the Triassic about 34° clockwise vertical axis rotations with respect to the present north. There is no indication in our data set for oroclinal bending of the Hronic Unit. We interpret the difference in clockwise rotations (about 36°) between Permian and 90−80 Ma as a clockwise block rotation taking place during major extensional and/or compressive events between stable Europe and Africa. Taking into consideration the well-documented counterclockwise rotation observed for the overstep sequences in the Central Western Carpathians and in the Pieniny Klippen Belt, the remagnetization of the Triassic sediments was closely followed by about 94° clockwise rotation. Research in progress will serve to decide if this large clockwise rotation involved the whole Central Carpathian nappe stack or part of this was due to the thin-skinned nappe emplacement of the Hronic Unit.

Dependence upon charge of the vibrational spectra of small Polycyclic Aromatic Hydrocarbon clusters: the example of pyrene.

Infrared spectra are computed for neutral and cationic clusters of Polycyclic Aromatic Hydrocarbon clusters, namely , using the Density Functional based Tight Binding scheme combined with a Configuration Interaction scheme (DFTB-CI) in the double harmonic approximation. Cross-comparison is carried out with DFT and simple DFTB. Similarly to the monomer cation, the IR spectra of cluster cations are characterized by a depletion of the intensity of the CH stretch modes around 3000 cm-1, with a weak revival for n = 3 and 4. The in-plane CCC modes in the region 1400-2000 cm-1 are enhanced while the CH bending modes in the range 700-1000 cm-1 are significantly weakened with respect to the monomer cation, in particular for n = 2. Finally, soft modes corresponding to diedral fluctuations of the monomers within the central stack of the ion structure, possibly mixed with monomer folding, are also observed in the region 70-120 cm-1.