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Protocolli sostenibili di seed priming per potenziare la performance di germinazione della lattuga (Lactuca sativa L.)
No full textMicroscale modeling of zerovalent iron micro and nanoparticles transport in porous media
No full textA new definition of a correlation equation for single collector efficiency
No full textThe transport and deposition of colloidal particles in porous media are important phenomena involved in many environmental and engineering problems as, for instance, the use of micro- and nanoscale zerovalent iron, a promising reagent in the field of groundwater remediation [1]. Particle transport and deposition in the proximity of injection or pumping wells and in porous media in general may also be relevant in other fields of chemical and petroleum engineering.
Mathematical models able to predict particles transport and deposition in porous media are often needed in order to design field applications. The basic concept of these models is the single collector efficiency η, which predicts particles deposition onto a single grain of a complex porous medium in terms of probability that an approaching particle would be retained on a solid grain. Many different approaches and equations exist in the literature, however most of them are valid only under specific conditions (eg. specific range of flow rate, particle size, etc.), and predict, for certain parametric conditions, efficiency values exceeding unity, which is, for an efficiency concept, a contradiction [2][3].
The objectives of this study are to analyze the causes of the failure of the existing models in predicting the deposition rate in certain conditions and to modify the definition of collector efficiency in order to have a more general equation.
The definition of collector efficiency, first proposed by Yao at al. [4], is based on the particles deposition onto a spherical grain (the collector) in an infinite domain. It is defined as the ration between the flux of particles that deposit on the grain and the total amount of particles that could reach the collector by advective flux from an area equal to the projection of the spherical grain itself.
In the present work Yao’s model has been implemented by COMSOL Multiphysics and solved with an Eulerian approach; particles deposition simulations were run. From these results a new definition of η is proposed, considering all the flux that potentially reach the collector. A new equation, valid in a broader range of parameters (eg. low Pe number, big particle size, etc.), has been formulated starting from the numerical results
Normalization and extension of the single-collector efficiency correlation equation for predicting transport of (nano)particles
No full textThe colloidal transport and deposition are important phenomena involved in many engineering problems. In the environmental engineering field the use of micro- and nano-scale zerovalent iron (M-NZVI) is one of the most promising technologies for groundwater remediation. Colloid deposition is normally studied from a micro scale point of view and the results are then implemented in macro scale models that are used to design field-scale applications. The single collector efficiency concept predicts particles deposition onto a single grain of a complex porous medium in terms of probability that an approaching particle would be retained on the solid grain. In literature, many different approaches and equations exist to predict it, but most of them fail under specific conditions (e.g. very small or very big particle size and very low fluid velocity) because they predict efficiency values exceeding unity. By analysing particle fluxes and deposition mechanisms and performing a mass balance on the entire domain, the traditional definition of efficiency was reformulated and a new definition was proposed. This new formulation is valid for a wide range of parameters and it never predicts efficiency values exceeding unity because of its formulation. Moreover a new traditional single collector contact efficiency correlation equation was formulated starting from the fitting of the mass flow on the collector. This equation shows that there are important deposition terms, neglected by previous correlation equations present in literature, but that are essential to extend the definition of single collector contact efficiency to a wider range of parameters (e.g. very small Peclet numbers
Normalization and extension of single-collector efficiency correlation equation
Full text linkThe colloidal transport and deposition are important phenomena involved in many engineering problems. In the environmental engineering field the use of micro- and nano-scale zerovalent iron (M-NZVI) is one of the most promising technologies for groundwater remediation. Colloid deposition is normally studied from a micro scale point of view and the results are then implemented in macro scale models that are used to design field-scale applications. The single collector efficiency concept predicts particles deposition onto a single grain of a complex porous medium in terms of probability that an approaching particle would be retained on the solid grain. In literature, many different approaches and equations exist to predict it, but most of them fail under specific conditions (e.g. very small or very big particle size and very low fluid velocity) because they predict efficiency values exceeding unity. By analysing particle fluxes and deposition mechanisms and performing a mass balance on the entire domain, the traditional definition of efficiency was reformulated and a novel total flux normalized correlation equation is proposed for predicting single-collector efficiency under a broad range of parameters. It has been formulated starting from a combination of Eulerian and Lagrangian numerical simulations, performed under Smoluchowski- Levich conditions, in a geometry which consists of a sphere enveloped by a control volume. In order to guarantee the independence of each term, the correlation equation is derived through a rigorous hierarchical parameter estimation process, accounting for single and mutual interacting transport mechanisms. The correlation equation provides efficiency values lower than one over a wide range of parameters and is valid both for point and finite-size particles. A reduced form is also proposed by elimination of the less relevant term
A Normalized and Extended Correlation Equation for Predicting Single-Collector Efficiency in Physicochemical Filtration in Saturated Porous Media
Full text linkThe colloidal transport and deposition are phenomena involved in different engineering problems. In the environmental engineering field the use of micro- and nano-scale zerovalent iron (M-NZVI) is one of the most promising technologies for groundwater remediation. Colloid deposition is normally studied from a micro scale point of view and the results are then implemented in macro scale models that are used to design field-scale applications. The single collector efficiency concept predicts particles deposition onto a single grain of a complex porous medium in terms of probability that an approaching particle would be retained on the solid grain. Different approaches and models are available in literature to predict it, but most of them fail in some particular conditions (e.g. low fluid velocity and/or very small or very big particle dimension) because they predict efficiency values exceeding unity. By analysing particle fluxes and deposition mechanisms and performing a mass balance on the entire domain, the traditional definition of efficiency was reformulated and a novel total flux normalized correlation equation is proposed for predicting single-collector efficiency under a broad range of parameters. The new equation has been formulated starting from a combination of Eulerian and Lagrangian numerical COMSOL Multiphysics® simulations, performed under Smoluchowski-Levich conditions in a geometry which consists of a sphere enveloped by a cylindrical control volume (Figure 1). The normalization of the deposited flux is performed accounting for all of the particles entering into the control volume through all transport mechanisms (not just the upstream convective flux as conventionally done) to provide efficiency values lower than one under any possible combination of transport mechanisms. The particle fluxes onto the collector and through the control volume have been described mathematically as a summation of terms. In order to guarantee the independence of each term, the correlation equation is derived through a rigorous hierarchical parameter estimation process, accounting for single and mutual interacting transport mechanisms. The new correlation equation provides efficiency values lower than one over a wide range of parameters (Figure 2) and it is valid both for point and finite-size particles. Moreover the correlation equation is extended to include porosity dependence and reduced forms are also proposed by elimination of the less relevant terms without losing the main features of the full equatio
An extended and total flux normalized correlation equation for predicting single-collector efficiency
Full text linkIn this study a novel total flux normalized correlation equation is proposed for predicting single-collector efficiency under a broad range of parameters. The correlation equation does not exploit the additivity approach introduced by Yao et al. (1971), but includes mixed terms that account for the mutual interaction of concomitant transport mechanisms (i.e., advection, gravity and Brownian motion) and of finite size of the particles (steric effect). The correlation equation is based on a combination of Eulerian and Lagrangian simulations performed, under Smoluchowski-Levich conditions, in a geometry which consists of a sphere enveloped by a cylindrical control volume. The normalization of the deposited flux is performed accounting for all of the particles entering into the control volume through all transport mechanisms (not just the upstream convective flux as conventionally done) to provide efficiency values lower than one over a wide range of parameters. In order to guarantee the independence of each term, the correlation equation is derived through a rigorous hierarchical parameter estimation process, accounting for single and mutual interacting transport mechanisms. The correlation equation, valid both for point and finite-size particles, is extended to include porosity dependency and it is compared with previous models. Reduced forms are proposed by elimination of the less relevant term
Pore scale simulation of micro and nanoscale zerovalent iron particles transport
No full textIn the field of groundwater remediation, nano and microscale zerovalent iron (NZVI - MZVI) is one of the most promising reagent. It is able to degrade, through redox reactions, recalcitrant and carcinogenic compounds such as perchloroethylene and trichloroethylene. These particles are characterized by a high reactivity and can be injected in the subsurface more easily if compared to the emplacement of granular iron commonly used in permeable reactive barriers (PRBs). Therefore, the characteristics of these particles, their behaviour once injected into the subsoil, their mobility and interactions with the solid matrix of the aquifer must be well known in order to setup an efficient remediation operation. The aim of this study is to analyze the transport of iron micro and nanoparticles and their interaction with the porous media at the microscale, by simulating the capture and deposition of iron particles on the porous matrix. This phenomenon, in fact, is an important mechanism controlling the mobility of colloids in aquifer systems and the effectiveness of the technology. The study was performed by means of numerical simulations of both the flow field and the explicit particles trajectories. The particles transport was simulated with a Lagrangian approach. This gives the possibility to describe accurately the particles trajectories in the flow field and their interaction with the sand grains at the microscale by implementing all the forces acting on them. The basic equation of this approach is the classical Newton's law. The main aim is to derive a correlation (and to compare it with other correlations found in literature for similar systems) between the parameters characterizing the iron particles, the sand grains and the flow field and the capacity of MZVI and NZVI to cover long distance without be captured from the porous media grains. The resulting correlation will be used for macroscale simulation and future application of a real reclamation activit
Two and three dimensional simulation of flow and particle transport in porous media
No full textThe simulation of fluid flow and particle transport in porous media finds important applications in many different fields, ranging from environmental and oil engineering to filtration and industrial chromatography. Different types of approaches exist and are generally classified by the length- and time-scales involved. Real industrial problems require the treatment of the porous medium as a continuum via the definition of porosity and permeability. However, these parameters are very difficult to be determined and therefore a strategy to identify them from pore-scale simulation is investigated here. Two and three dimensional geometries characterized by different degrees of complexity are created and studied using finite-element and finite-volume computational fluid dynamics codes. First the flow and particle transport around spherical grains arranged in a regular lattice is investigated. Then the analysis is extended to irregularly arranged spherical grains (of equal and different size) mimicking a realistic porous medium. Eventually geometries constituted by grains of realistic shapes are also considered. The accurate simulation of these systems require the solution of a number of mathematical and numerical challenges, related to computational geometry, mesh pro- cessing and discretization techniques. Particular attention is devoted to the assessment of two meshing strategies: highly irregular body-fitted meshes versus regular cartesian-based immersed-boundary approac
Microscale Simulation of Nanoparticles Transport in Porous Media for Groundwater Remediation
No full textMicro and Nanoscale zerovalent iron (MZVI and NZVI) is one of the most promising reagent for the remediation of contaminated groundwater; these particles, in fact, can efficiently degrade, through redox reactions, recalcitrant and carcinogenic compounds. The aim of this study is to simulate at the microscale the transport of iron nanoparticles, their interaction with the porous media and their deposition on the aquifer material. The simulations have been carried out with a Langrangian approach implemented in COMSOL Multiphysics 4.2a. The model under study includes the relevant forces acting on the single particles such as drag, Brownian, gravity, Van der Waals and electric double layer force. The simulation results can deliver, thanks to this microscale description, an estimation of the attachement efficiency that can be used for macroscale simulations and compared with the relationships obtained by the clean bed filtration approac
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