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Trapped magnetic field distribution above two magnetized bulk superconductors close to each other
No full textBulk large-grain superconductors can be used as high-field permanent magnets. Although the properties of such individual trapped field magnets are well documented, much less is known concerning their behaviour when two are brought together. In this work, the interaction between two cylindrical bulk YBa2Cu3O7 (YBCO) superconductors is described. Two sets of experiments were carried out. The first involved the simultaneous magnetization of two bulk superconductors placed a short distance apart. Here, the applied magnetic field was aligned parallel to the c-axis of one bulk, while the other was oriented with its c-axis offset . For a centre-to-centre distance equal to twice the sample height, the presence of the second sample is found not to alter the current distribution inside the first. Consequently, the contribution of both samples simply sums, thus increasing the magnetic flux density between them. In the second set of experiments, the translational approach of the superconductors with parallel c-axes was investigated. The following configurations were considered: (i) face to face approach (with anti-parallel trapped field orientation) and (ii) sideways approach (with parallel trapped field orientation). An irreversible decrease of the trapped field was measured on separation . Repeated approach cycles showed that the irreversible loss of trapped field is largest for the first approach
The influence of wake chopping on wet-steam turbine modelling
No full textThe formation of water droplets within condensing steam turbines is a complex process that occurs at supersaturated, nonequilibrium conditions and is influenced by the unsteady segmentation of blade wakes by successive blade rows. This is often referred to as 'wake chopping', and its effect on the condensation process is the subject of this paper. The practical significance is that thermodynamic 'wetness losses' (which constitute a major fraction of the overall loss) are strongly affected by droplet size. Likewise, droplet deposition and the various ensuing two-phase phenomena (such as film migration and coarse-water formation) also depend on the spectrum of droplet sizes in the primary fog. The majority of wake-chopping models presented in the literature adopt a stochastic approach, whereby large numbers of fluid particles are tracked through (some representation of) the turbine flowfield, assigning a random number at each successive blade row to represent the particle's pitchwise location, and hence its level of dissipation. This study contributes to the existing literature by adding: (a) a comprehensive study of the sensitivity to key model parameters (e.g., blade wake shape and wake decay rate); (b) an assessment of the impact of circumferential pressure variations; (c) a study of the implications for wetness losses and (d) a study of the implications for deposition rates
Thermodynamic and experimental evaluation of a cloud chamber for ultrafine particle detection
No full textParticle sensing based on condensational growth has long been the basis for robust nanoparticle measurement. Increasingly cloud chamber devices offer the potential for low-cost and portable measurement when operated semi-continuously with relatively small system volumes. Models based on isentropic and isenthalpic expansion are derived to predict the time evolution of temperature, saturation ratio, particle growth, and resultant light extinction in cloud chambers. A laboratory cloud chamber is fabricated and experiments using NaCl aerosol particles as the condensation nucleus are conducted to verify the models. The isentropic model, suggests that the temperature drops 0.6 ℃ within 40 ms, and accordingly, the saturation ratio reaches 1.04. For an aerosol with lognormal distribution, the predicted geometric mean diameter grows more than 5 times while the distribution narrows due to ∝1/dp growth in the continuum regime. The performance of the cloud chamber agrees with the system physics and reference instruments, with relative error in measured extinction coefficient and signal intensities of ±5%. Detailed error propagation shows that the measured number concentrations agree well with reference instruments and the underlying theory. The lower limit of detection (∼4 × 106 cm−3) for the device is suitable for fire detection and emissions characterization
From Collisions to Bundles: An Adaptive Coarse-Grained Model for the Aggregation of High-Aspect-Ratio Carbon Nanotubes
No full textWe present an adaptive mesoscale model for carbon nanotube (CNT) systems. In our model, CNTs are represented as a chain of nodes connected by tensile and torsion springs to describe stretching and bending of the chain, with intermolecular interactions being calculated by using a mesoscopic Lennard-Jones potential. Computational adaptivity was achieved by dynamically adjusting node spacing and number during the simulation to optimize the number of simulated particles and lower computational effort. Adaptive simulations were up to 5 times faster than nonadaptive ones while quantitatively preserving all system dynamics. In particular, the model enables the study of the time scale of CNT bundling that leads to the formation of dilute CNT networks, so-called aerogels. These aerogels constitute the first step in the direct spinning of CNT fibers from chemical vapor deposition synthesis. Understanding the factors governing CNT bundling and network formation is key to controlling CNT fiber microstructure and therefore optimizing their properties. Using the model, we simulated the bundling dynamics of two CNTs with an initial point contact at varying angles for CNT lengths of up to 10 μm. We find that bundling times are an increasing function of initial collision angle and follow a power law with increasing CNT length, ranging from 10-1 to 103 ns. We postulate that when the bundling time becomes of the same order as the CNT bundle collision time, the aerogel will form
Mechanism of droplet-formation in a supersonic microfluidic spray device
No full textSpray drying is an approach employed in automotive, food, and pharmaceutical industries as a robust and cost efficient liquid atomization technique offering direct control over droplet dimensions. The majority of commercially available spray nozzles are designed for large throughput spray drying applications or uniform surface coating, but microfluidic nebulizers have recently been developed as small scale alternatives. Here, we explore the physical parameters that define the droplet size and formation under supersonic flow conditions commonly found in microfluidic spray drying systems. We examined the spray nozzle operation using high speed imaging and laser scattering measurements, which allowed us to describe the spray regimes and droplet size distributions. It was determined that by using this spray nozzle device, droplets with diameters of 4-8 μm could be generated. Moreover, we show that the supersonic de Laval nozzle model can be used to predict the average droplet size. Our approach can be used as a platform for interfacing fluid microprocessing with gas phase detection and characterization
Leveraging Mobility Flows from Location Technology Platforms to Test Crime Pattern Theory in Large Cities
No full textCrime has been previously explained by social characteristics of the residential population and, as stipulated by crime pat- tern theory, might also be linked to human movements of non- residential visitors. Yet a full empirical validation of the latter is lacking. The prime reason is that prior studies are limited to aggregated statistics of human visitors rather than mobility flows and, because of that, neglect the temporal dynamics of individual human movements. As a remedy, we provide the first work which studies the ability of granular human mo- bility in describing and predicting crime concentrations at an hourly scale. For this purpose, we propose the use of data from location technology platforms. This type of data allows us to trace individual transitions and, therefore, we succeed in distinguishing different mobility flows that (i) are incom- ing or outgoing from a neighborhood, (ii) remain within it, or (iii) refer to transitions where people only pass through the neighborhood. Our evaluation infers mobility flows by lever- aging an anonymized dataset from Foursquare that includes almost 14.8 million consecutive check-ins in three major U.S. cities. According to our empirical results, mobility flows are significantly and positively linked to crime. These findings advance our theoretical understanding, as they provide con- firmatory evidence for crime pattern theory. Furthermore, our novel use of digital location services data proves to be an effective tool for crime forecasting. It also offers unprece- dented granularity when studying the connection between hu- man mobility and crime
Going with the Flow: Tunable Flow-Induced Polymer Mechanochemistry
No full textMechanical forces can drive chemical transformations in polymers, directing reactions along otherwise inaccessible pathways, providing exciting possibilities for developing smart, responsive materials. The state-of-the-art test for solution-based polymer mechanochemistry development is ultrasonication. However, this does not accurately model the forces that will be applied during device fabrication using processes such as 3D printing or spray coating. Here, a step is taken toward predictably translating mechanochemistry from molecular design to manufacturing by demonstrating a highly controlled nozzle flow setup in which the shear forces being delivered are precisely tuned. The results show that solvent viscosity, fluid strain rate, and the nature of the breaking bond can be individually studied. Importantly, it is shown that the influence of each is different to that suggested by ultrasonication (altered quantity of chain breakage and critical polymer chain length). Significant development is presented in the understanding of polymer bond breakage during manufacturing flows to help guide design of active components that trigger on demand. Using an anthracene-based mechanophore, the triggering of a fluorescence turn-on is demonstrated through careful selection of the flow parameters. This work opens the avenue for programmed chemical transformations during inline manufacturing processes leading to tunable, heterogeneous final products from a single source material
Ultra-Sensitive Force Transduction in Weakly Coupled Resonators
No full textThis paper, for the first time, reports on a transducer configuration employing weakly coupled resonators demonstrating significant modal overlap. Existing multimode/mode-localized sensors conventionally operate in a regime where modal overlap is negligible; however, theoretically, the sensitivity of such systems significantly increases as the coupling strength between resonators is reduced, moving the system to a regime described by modal overlap. In this work, operation in the modal overlap regime is made possible with consequent benefits in input-referred resolution and stability. Specifically, a coupled resonator microsystem subject to electrostatic stiffness perturbations shows a sensitivity enhancement of 33682 and an input-referred amplitude ratio stability of 28.2ppb, demonstrating a ∼3x improvement in resolution compared to the resonant frequency shift technique
Impact of Fluid Substitution on the Performance of an Axial Compressor Blade Cascade Working with Supercritical Carbon Dioxide
No full textRecent research on turbomachinery design and analysis for supercritical carbon dioxide (sCO2) power cycles has relied on computational fluid dynamics. This has produced a large number of works whose approach is mostly case-specific, rather than of general application to sCO2 turbomachinery design. As opposed to such approach, this work explores the aerodynamic performance of compressor blade cascades operating on air and supercritical CO2 with the main objective to evaluate the usual aerodynamic parameters of the cascade for variable boundary conditions and geometries, enabling "full"or "partial"similarity. The results present both the global performance of the cascades and certain features of the local flow (trailing edge and wake). The discussion also highlights the mechanical limitations of the analysis (forces exerted on the blades), which is the main restriction for applying similarity laws to extrapolate the experience gained through decades of work on air turbomachinery to the new working fluid. This approach is a step toward the understanding and appropriate formulation of a multi-objective optimization problem for the design of such turbomachinery components where sCO2 is used as the operating fluid. With this objective, the paper aims to identify and analyze what would be expected if a common description of such computational design problems similar to those where air is the working fluid were used
Sympathetic quantisation - A new approach to hologram quantisation
No full textSpatial light modulators can typically only modulate the phase or the amplitude of an incident wavefront, with only a limited number of discrete values available. This is often accounted for in computer-generated holography algorithms by setting hologram pixel values to the nearest achievable value during what is known as quantisation. Sympathetic quantisation is an alternative to this nearest-neighbour approach that takes into account the underlying diffraction relationships in order to obtain a significantly improved post-quantisation performance. The concept of sympathetic quantisation is introduced in this paper and a simple implementation, soft sympathetic quantisation, is presented which is shown to improve mean squared error and structural similarity index error metrics by 50% for the considered case of single-transform algorithms