789 research outputs found
Heat transfer in microwave heating
Heat transfer is considered as one of the most critical issues for design and implement of large-scale microwave heating systems, in which improvement of the microwave absorption of materials and suppression of uneven temperature distribution are the two main objectives. The present work focuses on the analysis of heat transfer in microwave heating for achieving highly efficient microwave assisted steelmaking through the investigations on the following aspects: (1) characterization of microwave dissipation using the derived equations, (2) quantification of magnetic loss, (3) determination of microwave absorption properties of materials, (4) modeling of microwave propagation, (5) simulation of heat transfer, and (6) improvement of microwave absorption and heating uniformity.
Microwave heating is attributed to the heat generation in materials, which depends on the microwave dissipation. To theoretically characterize microwave heating, simplified equations for determining the transverse electromagnetic mode (TEM) power penetration depth, microwave field attenuation length, and half-power depth of microwaves in materials having both magnetic and dielectric responses were derived. It was followed by developing a simplified equation for quantifying magnetic loss in materials under microwave irradiation to demonstrate the importance of magnetic loss in microwave heating. The permittivity and permeability measurements of various materials, namely, hematite, magnetite concentrate, wüstite, and coal were performed. Microwave loss calculations for these materials were carried out. It is suggested that magnetic loss can play a major role in the heating of magnetic dielectrics.
Microwave propagation in various media was predicted using the finite-difference time-domain method. For lossy magnetic dielectrics, the dissipation of microwaves in the medium is ascribed to the decay of both electric and magnetic fields. The heat transfer process in microwave heating of magnetite, which is a typical magnetic dielectric, was simulated by using an explicit finite-difference approach. It is demonstrated that the heat generation due to microwave irradiation dominates the initial temperature rise in the heating and the heat radiation heavily affects the temperature distribution, giving rise to a hot spot in the predicted temperature profile. Microwave heating at 915 MHz exhibits better heating homogeneity than that at 2450 MHz due to larger microwave penetration depth. To minimize/avoid temperature nonuniformity during microwave heating the optimization of object dimension should be considered.
The calculated reflection loss over the temperature range of heating is found to be useful for obtaining a rapid optimization of absorber dimension, which increases microwave absorption and achieves relatively uniform heating. To further improve the heating effectiveness, a function for evaluating absorber impedance matching in microwave heating was proposed. It is found that the maximum absorption is associated with perfect impedance matching, which can be achieved by either selecting a reasonable sample dimension or modifying the microwave parameters of the sample
Instance-weighted Central Similarity for Multi-label Image Retrieval
Deep hashing has been widely applied to large-scale image retrieval by
encoding high-dimensional data points into binary codes for efficient
retrieval. Compared with pairwise/triplet similarity based hash learning,
central similarity based hashing can more efficiently capture the global data
distribution. For multi-label image retrieval, however, previous methods only
use multiple hash centers with equal weights to generate one centroid as the
learning target, which ignores the relationship between the weights of hash
centers and the proportion of instance regions in the image. To address the
above issue, we propose a two-step alternative optimization approach,
Instance-weighted Central Similarity (ICS), to automatically learn the center
weight corresponding to a hash code. Firstly, we apply the maximum entropy
regularizer to prevent one hash center from dominating the loss function, and
compute the center weights via projection gradient descent. Secondly, we update
neural network parameters by standard back-propagation with fixed center
weights. More importantly, the learned center weights can well reflect the
proportion of foreground instances in the image. Our method achieves the
state-of-the-art performance on the image retrieval benchmarks, and especially
improves the mAP by 1.6%-6.4% on the MS COCO dataset.Comment: 10 pages, 6 figure
Upstream swimming and Taylor dispersion of active Brownian particles
Locomotion of self-propelled particles such as motile bacteria or phoretic swimmers often takes place in the presence of applied flows and confining boundaries. Interactions of these active swimmers with the flow environment are important for the understanding of many biological processes, including infection by motile bacteria and the formation of biofilms. Recent experimental and theoretical works have shown that active particles in a Poiseuille flow exhibit interesting dynamics including accumulation at the wall and upstream swimming. Compared to the well-studied Taylor dispersion of passive Brownian particles, a theoretical understanding of the transport of active Brownian particles (ABPs) in a pressure-driven flow is relatively less developed. In this paper, employing a small wave-number expansion of the Smoluchowski equation describing the particle distribution, we explicitly derive an effective advection-diffusion equation for the cross-sectional average of the particle number density in Fourier space. We characterize the average drift (specifically upstream swimming) and effective longitudinal dispersion coefficient of active particles in relation to the flow speed, the intrinsic swimming speed of the active particles, their Brownian diffusion, and the degree of confinement. In contrast to passive Brownian particles, both the average drift and the longitudinal dispersivity of ABPs exhibit a nonmonotonic variation as a function of the flow speed. In particular, the dispersion of ABPs includes the classical shear-enhanced (Taylor) dispersion and an active contribution called the swim diffusivity. In the absence of translational diffusion, the classical Taylor dispersion is absent and we observe a giant longitudinal dispersion in the strong flow limit. Our continuum theory is corroborated by a direct Brownian dynamics simulation of the Langevin equations governing the motion of each ABP
Transport and Microrheology of Active Colloids
Active colloids are micron-sized particles that self-propel through viscous fluids by converting energy extracted from their environment into mechanical motion. The origin or mechanism of their locomotion can be either biological or synthetic ranging from motile bacteria to artificial phoretic particles. Owing to their ability to self-propel, active colloids are out of thermodynamic equilibrium and exhibit interesting macroscopic or collective dynamics. In particular, active colloids exhibit accumulation at confining boundaries, upstream swimming in Poiseuille flow, and a reduced or negative apparent shear viscosity. My work has been focused on a theoretical and computational understanding of the dynamics of active colloids under the influence of confinement and external fluid flows, which are ubiquitous in biological processes. I consider the transport of active colloids in channel flows, the microrheology of active colloids, and lastly I propose and study a vesicle propulsion system based on the learned principles.
A generalized Taylor dispersion theory is developed to study the transport of active colloids in channel flows. I show that the often-observed upstream swimming can be explained by the biased upstream reorientation due to the flow vorticity. The longitudinal dispersion of active colloids includes the classical shear-enhanced dispersion and an active swim diffusivity. Their coupling results in a non-monotonic variation of the dispersivity as a function of the flow speed. To understand the effect of particle shape on the transport of active colloids, a simulation algorithm is developed that is able to faithfully resolve the inelastic collision between an ellipsoidal particle and the channel walls. I show that the collision-induced rotation for active ellipsoids can suppress upstream swimming. I then investigate the particle-tracking microrheology of active colloids. I show that active colloids exhibit a swim-thinning microrheology and a negative microviscosity can be observed when certain hydrodynamic effects are considered. I show that the traditional constant-velocity probe model is not suitable for the quantification of fluctuations in the suspension. To resolve this difficulty, a generalized microrheology model that closely mimics the experimental setup is developed. I conclude by proposing a microscale propulsion system in which active colloids are encapsulated in a vesicle with a semi-permeable membrane that allows water to pass through. By maintaining an asymmetric number density distribution, I show that the vesicle can self-propel through the surrounding viscous fluid.</p
Activity-induced propulsion of a vesicle
Modern biomedical applications such as targeted drug delivery require a
delivery system capable of enhanced transport beyond that of passive Brownian
diffusion. In this work an osmotic mechanism for the propulsion of a vesicle
immersed in a viscous fluid is proposed. By maintaining a steady-state solute
gradient inside the vesicle, a seepage flow of the solvent (e.g., water) across
the semipermeable membrane is generated which in turn propels the vesicle. We
develop a theoretical model for this vesicle-solute system in which the seepage
flow is described by a Darcy flow. Using the reciprocal theorem for Stokes flow
it is shown that the seepage velocity at the exterior surface of the vesicle
generates a thrust force which is balanced by the hydrodynamic drag such that
there is no net force on the vesicle. We characterize the motility of the
vesicle in relation to the concentration distribution of the solute confined
inside the vesicle. Any osmotic solute is able to propel the vesicle so long as
a concentration gradient is present. In the present work, we propose active
Brownian particles (ABPs) as a solute. To maintain a symmetry-breaking
concentration gradient, we consider ABPs with spatially varying swim speed and
ABPs with constant properties but under the influence of an orienting field. In
particular, it is shown that at high activity the vesicle velocity is
, where is the swim pressure just
outside the thin accumulation boundary layer on the interior vesicle surface,
is the unit normal vector of the vesicle boundary,
is the membrane permeability, is the viscosity of the solvent, and
is the membrane thickness
Dynamic Local Aggregation Network with Adaptive Clusterer for Anomaly Detection
Existing methods for anomaly detection based on memory-augmented autoencoder
(AE) have the following drawbacks: (1) Establishing a memory bank requires
additional memory space. (2) The fixed number of prototypes from subjective
assumptions ignores the data feature differences and diversity. To overcome
these drawbacks, we introduce DLAN-AC, a Dynamic Local Aggregation Network with
Adaptive Clusterer, for anomaly detection. First, The proposed DLAN can
automatically learn and aggregate high-level features from the AE to obtain
more representative prototypes, while freeing up extra memory space. Second,
The proposed AC can adaptively cluster video data to derive initial prototypes
with prior information. In addition, we also propose a dynamic redundant
clustering strategy (DRCS) to enable DLAN for automatically eliminating feature
clusters that do not contribute to the construction of prototypes. Extensive
experiments on benchmarks demonstrate that DLAN-AC outperforms most existing
methods, validating the effectiveness of our method. Our code is publicly
available at https://github.com/Beyond-Zw/DLAN-AC.Comment: Accepted by ECCV 202
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