1,721,007 research outputs found
The role of shrinkage on food isothermal drying: a moving boundary model
Full text linkA moving-boundary model is proposed for describing food isothermal drying. The key point lies in the introduction of a point wise shrinkage velocity equal and opposite in sign to the water diffusive flux times a shrinkage factor representing the fingerprint of the food material under investigation. The model provides good results in terms of prediction of time evolution of moisture content and sample volume and furnishes an accurate and reliable estimate of water diffusivity at different operating temperatures
An alternative general model for the effective longitudinal diffusion in chromatographic beds filled with ordered porous particles
No full textThe two-zone moment-analysis method for the determination of the dispersion tensor in hierarchical retentive porous media has been adopted to compute and model the effective longitudinal diffusion Deff, or equivalently the B-term band broadening, in chromatographic beds filled with ordered porous particles. On the one hand, this approach offers accurate numerical results for Deff while keeping computational expenses low. On the other hand, it also gives direct insight for the analytical modelling, readily revealings the two main essential quantities (resp. referred to as the mobile-zone and stationary-zone effective diffusion factors γm and γs) that contribute to Deff. Modelling these two main parameters provided us with two new analytical models for Deff: a general one, valid for diluted and concentrated packings and accurate in the whole range of relevant intra-particle diffusion coefficient Dpz, and an approximate one, reliable for diluted packings and accurate also for concentrated packings with low to intermediate values of Dpz. The large advantage of both models is that they do not need any fitting parameter because all the required information is incorporated into the experimentally accessible geometric obstruction factor in the mobile phase originating from the tortuosity of the through-pore space (limiting case of fully solid particles without any retention). These models hence serve as an alternative to the Effective Medium Theory (EMT) models used so far in the literature. To validate the theory, five ordered geometries have been investigated. The accuracy of the general model proposed has been quantified and found to be comparable with that of the 3rd order approximate Torquato model for four geometries, even for macro-porosities close to the close-packing limit. The case of a 2-d triangular array of ellipsoidal particles with different elongations is also investigated to show the general validity and applicability of the models
Generalized reflection method for the stokes equation in confined geometries. Applications to microfluidics and particle transport
Full text linkA generalized reflection method is proposed for determining the hydrodynamic resistance matrix of a rigid particle translating in a confined Stokesian fluid, by reducing its estimate to the solution of two simpler problems: (i) the singular expansion of the hydrodynamic variables due to the particle motion in the free space, represented by the Faxén operator associated with the particle geometry, and (ii) the determination of the Green function associated with the specific channel geometry. This approach extends the Stokeslet approximation to a Faxén approximation that is reliable to ratios of the particles characteristic length l to the characteristic distance d from the walls higher than the existing literature approximations, the error of which is order of O((l/d)4). Given the solution of the two above mentioned subproblems, the proposed method applies to any particle and channel geometry. The application to prolate spheroidal particles translating parallel to a plane is discussed and compared against numerical simulations
A moving boundary model for food isothermal drying and shrinkage. One-dimensional versus two-dimensional approaches
No full textWe investigate and quantify the error in the estimate of water diffusivity resulting from the adoption of one-dimensional (1-d) models to describe the drying process of thin discoidal or long-thin cylindrical samples. Numerical results obtained with the 1-d and the 2-d moving boundary models recently proposed by Adrover, Brasiello, and Ponso (2019a, 2019b) are compared for different sample aspect ratios. The overestimate error of 1-d model predictions of diffusivity falls below 5% for an aspect ratio radius-over-thickness R0/L0 > 10 for discoidal samples while, for cylindrical samples, the aspect ratio height-to-radius L0/R0 must be greater than 15 to get a similar level of accuracy. To overcome this issue we propose a modified 1-d model able to take into account water flux from lateral surfaces (usually neglected in a 1-d approach) thus reducing the overestimate error of water diffusivity below 2% even for low-intermediate values of the aspect ratio. The 1-d modified model is successfully applied to experimental data of convective drying of eggplant and chayote discoidal samples. Practical applications: End consumer is increasingly demanding for ever more standardized and high quality food products. To meet these requirements, food industries are challenged to refine process design and control tools in order to be able to take into account the variability of both food matrices and operating conditions. This objective cannot be reached without the help of mathematical models based on first principles as the one here discussed and applied to food drying. The comparison between one- and two–dimensional approaches and the resulting quantification of errors in the adoption of simpler one-dimensional models have a great practical interest for industrial applications as they provide reliable criteria to obtain good predictions even when the whole set of the needed experimental data are not available or too expensive. The developed method can be easily adopted to predict final food quality (moisture content, sample volume reduction, and deformation) and therefore to implement more refined control systems and to optimize process-operating conditions, thus saving experimental and processing costs
Prediction of Plate Height Curves of Porous-Shell Pillar Array Columns Micro-Pillar Array Columns
No full textWe investigate band broadening in the most widely adopted configuration of micro-pillar array columns ((Formula presented.) PACs)—specifically, a cylindrical pillar array where both the pillar walls and the channel bottom are coated with a thin layer of mesoporous material. The two-zone moment analysis method is adopted to investigate the dispersion properties of (Formula presented.) PACs in a broad range of shell thicknesses, reduced fluid velocities, and retention factors. Three different models of the unit cell, of increasing complexity, have been implemented, namely a two-dimensional model and two different three-dimensional models with and without the retentive bottom layer, the presence of which seems to have a very significant effect on the plate height curves. Model predictions are compared with experimental van Deemter curves for uncoated and coated porous layers, and a robust relationship between the intra-particle (porous-zone) diffusion coefficient (Formula presented.) and the retention factor (Formula presented.) is established
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Construction of approximate inertial manifold by decimation of collocation equations for distributed parameter systems
No full textBrownian sieving effect for boosting the performance of microcapillary hydrodynamic chromatography. Proof of concept
Full text linkMicrocapillary hydrodynamic chromatography (MHDC) is a well-established technique for the size-based separation of suspensions and colloids, where the characteristic size of the dispersed phase ranges from tens of nanometers to micrometers. It is based on hindrance effects which prevent relatively large particles from experiencing the low velocity region near the walls of a pressure-driven laminar flow through an empty microchannel. An improved device design is here proposed, where the relative extent of the low velocity region is made tunable by exploiting a two-channel annular geometry. The geometry is designed so that the core and the annular channel are characterized by different average flow velocities when subject to one and the same pressure drop. The channels communicate through openings of assigned cut-off length, say A. As they move downstream the channel, particles of size bigger than A are confined to the core region, whereas smaller particles can diffuse through the openings and spread throughout the entire cross section, therein attaining a spatially uniform distribution. By using a classical excluded-volume approach for modeling particle transport, we perform Lagrangian-stochastic simulations of particle dynamics and compare the separation performance of the two-channel and the standard (single-channel) MHDC. Results suggest that a quantitative (up to thirtyfold) performance enhancement can be obtained at operating conditions and values of the transport parameters commonly encountered in practical implementations of MHDC. The separation principle can readily be extended to a multistage geometry when the efficient fractionation of an arbitrary size distribution of the suspension is sought
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