Numerical Modelling of Thermal Dispersion and Viscous Effects in High-Conductivity Porous Materials

Transport in porous media has many applications in the sciences and engineering, including filtration, packed bed reactors, groundwater flows, and more recently, enhanced heat transfer. In heat transfer applications, modem graphitic foams have shown great potential due to their high effective thermal conductivity and large internal surface area. Analysis of flow and heat transfer in porous media is typically conducted using volume-averaged equations for the macroscopic flow and thermal fields, rather than directly simulating the pore-level flow. The derivation of the volume-averaged momentum and energy equations, however, introduce new unknowns as it becomes necessary to decompose the velocity and temperature fields into the sum of their volume-average and a spatial deviation. The spatial deviation terms are recast in terms of closure functions, which map the volume-averaged fields onto the pore-level deviations. As a result, the deviation tenns may be recast in terms of the resolved, volume-averaged fields with coefficients defined by the closure functions. The goal of the present work is to examine the details of the pore-level flow in high- conductivity graphitic foams with a spherical void stmcture in order to produce a closed, volume-averaged model for flow and heat transfer in such materials. Due to the high- conductivity of the medium considered here, local thennal non-equilibrium between the fluid and solid phases is assumed to exist. The approach taken in this work is to solve for the flow and closure function fields in an idealized spherical-void-phase porous geometry using a 3-dimensional, unstructured, finite-volume CFD code. Integration of the closure function fields for the thermal closure problem provides results for the thermal dispersion conductivity, modified convecting velocity, and interfacial heat transfer. For the hydrody namic closure problem, the integration of the closure functions yields the permeability of the medium, as well as an additional form drag term. Results are presented for a range of Reynolds numbers at two porosities. The thermal dispersion conductivity for the idealized graphitic foam considered herein is found to behave quite differently than aluminum foams. The modification to the convecting velocity, which is a result of the pore-level flow fields, was found to have a significant effect on the convection term in the volume-averaged momentum equations. This result is quite interesting considering that this term is nearly always neglected in volume-averaged models. Results are also presented for the interfacial Nusselt number. In terms of the hydrodynamic model, the permeability of the medium is found in addition to the additional form drag term. The form drag term, which accounts for non-Darcian effects, is found to vary non-linearly with Reynolds number, resulting in the need for a cubic velocity term in the volume-averaged momentum equations

Numerical Modelling of Transport in Complex Porous Media: Metal Foams to the Human Lung

Transport in porous media has many practical applications in science and engineering. This work focuses on the development of numerical methods for analyzing porous media flows and uses two major applications, metal foams and the human lung, to demonstrate the capabilities of the methods. Both of these systems involve complex pore geometries and typically involve porous domains of complex shape. Such geometric complexities make the characterization of the relevant effective properties of the porous medium as well as the solution of the governing equations in conjugate fluid-porous domains challenging. In porous domains, there are typically too many individual pores to consider transport processes directly; instead the governing equations are volume-averaged to obtain a new sets of governing equations describing the conservation laws in a bulk sense. There are, however, unknown pore-level terms remaining in the volume-averaged equations that must be characterized using effective properties that account for the effects of processes at the pore level. Once closed, the volume-averaged equations can be solved numerically, however currently available numerical methods for conjugate domains do not perform well at fluid-porous interfaces when using unstructured grids. In light of the preceding discussion, the goals of this work are: (i) to develop a finite-volume-based numerical method for solving fluid flow and non-equilibrium heat transfer problems in conjugate fluid-porous domains that is compatible with general unstructured grids, (ii) to characterize the relevant flow and thermal properties of an idealized graphite foam, (iii) to determine the permeability of an alveolated duct, which is considered as a representative element of the respiratory region of the human lung, and (iv) to conduct simulations of airflow in the human lung using a novel fluid-porous description of the domain. Results show that the numerical method that has been developed for conjugate fluid-porous systems is able to maintain accuracy on all grid types, flow directions, and flow speeds considered. This work also introduces a comprehensive set of correlations for the effective properties of graphite foam, which will be useful for studying the performance of devices incorporating this new material. In order to model air flow in the lung as a porous medium, the permeability of an alveolated duct is obtained using direct pore-level simulations. Finally, simulations of air flow in the lung are presented which use a novel fluid-porous approach wherein the upper airways are considered as a pure fluid region and the smaller airways and alveoli are considered as a porous domain

A Numerical Approach for Determining the Resistance of Fine Mesh Filters

Characterizing the resistance of mesh filters, in terms of the pressure drop as a function of flow velocity, is an important part of modeling any filtration process. Most commonly, filters are characterized experimentally, which can be costly and time consuming. This motivates the need for a generalized numerical approach for characterizing the resistance of mesh filters based on the flow through a representative segment of the filter. There is uncertainty, however, in the correct specification of boundary conditions such that the numerical results for flow through the small segment match the overall behaviour of the filter. In this work, an experimentally validated numerical approach is developed by examining the velocity and turbulence intensity experienced across the filter. It has been shown that the flow resistance results are not sensitive to the turbulence intensity, but depend greatly on the imposed flow velocity. Specifying the peak velocity as the boundary condition in the filter simulations resulted in a good match with experiments, while using the bulk velocity was not able to reproduce the experimental results.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Derivatives of spin dynamics simulations

We report analytical equations for the derivatives of spin dynamics simulations with respect to pulse sequence and spin system parameters. The methods described are significantly faster, more accurate and more reliable than the finite difference approximations typically employed. The resulting derivatives may be used in fitting, optimization, performance evaluation and stability analysis of spin dynamics simulations and experiments. Keywords: NMR, EPR, simulation, analytical derivatives, optimal control, spin chemistry, radical pair.Comment: Accepted by The Journal of Chemical Physic

Community Surveillance of Omicron in Ontario: Wastewater-based Epidemiology Comes of Age

Wastewater-based surveillance of SARS-CoV-2 RNA has been implemented at building, neighbourhood, and city levels throughout the world. Implementation strategies and analysis methods differ, but they all aim to provide rapid and reliable information about community COVID-19 health states. A viable and sustainable SARS-CoV-2 surveillance network must not only provide reliable and timely information about COVID-19 trends, but also provide for scalability as well as accurate detection of known or unknown emerging variants. Emergence of the SARS-CoV-2 variant of concern Omicron in late Fall 2021 presented an excellent opportunity to benchmark individual and aggregated data outputs of the Ontario Wastewater Surveillance Initiative in Canada; this public health-integrated surveillance network monitors wastewaters from over 10 million people across major population centres of the province. We demonstrate that this coordinated approach provides excellent situational awareness, comparing favourably with traditional clinical surveillance measures. Thus, aggregated datasets compiled from multiple wastewater-based surveillance nodes can provide sufficient sensitivity (i.e., early indication of increasing and decreasing incidence of SARS-CoV-2) and specificity (i.e., allele frequency estimation of emerging variants) with which to make informed public health decisions at regional- and state-levels.Ontario Ministry of the Environment, Conservation and Parks|| Genome Canada and Ontario Genomics (OGI-209)||NSERC (ALLRP 555041-20 to C.O.)||Ontario Clean Water Agenc

Bubble Growth in Supersaturated Liquids

Bubble formation and dissolution have a wide range of industrial applications, from the production of beverages to foam manufacturing processes. The rate at which the bubble expands or contracts has a significant effect on these processes. In the current work, the hydrodynamics of an isolated bubble expanding due to mass transfer in a pool of supersaturated gas–liquid solution is investigated. The complete scalar transportation equation (advection–diffusion) is solved numerically. It is observed that the present model accurately predicted bubble growth when compared with existing approximated models and experiments. The effect of gas–liquid solution parameters such as inertia, viscosity, surface tension, diffusion coefficient, system pressure, and solubility of the gas has been investigated. It is found that the surface tension and inertia have a very minimal effect during the bubble expansion. However, it is observed that the viscosity, system pressure, diffusion, and solubility have a considerable effect on bubble growth