1,721,022 research outputs found
Simulation of DNAPLs migration in saturated porous media by means of Lattice Boltzmann Models
No full textThe aim of the research is to implement a numerical model inspired by the Lattice Boltzmann (LB) theory able to simulate the complex scenario of contamination of DNAPLs (Dense Non Aqueous Phase Liquids) in saturated porous media.
Groundwater contamination is nowadays recognized as a serious environmental problem. Among different polluting substances, the contamination by NAPLs continues to persist as a significant problem in industrialized nations. More specifically, an accidental release at the ground surface of DNAPLs, which include a variety of organic compounds denser than water, can lead to long-term contamination of both the unsaturated and saturated zones.
Once released, the DNAPLs tend to migrate mainly vertically in the subsurface under gravity and capillary forces. Since they are slightly soluble in both water and air, they can exist in the subsurface as a separate and immiscible fluid phases and migrate to significant depths below the water table giving rise to aqueous phase plumes depending on the slow dissolution kinetics with persistence of the source for a long period. Furthermore, during DNAPLS migration in both unsaturated and saturated media, DNAPLs can be trapped by capillary forces in the porous medium (i.e. residual DNAPLS saturation).
Predicting the source localization, often unknown, and the fate of these organic chemicals in the subsurface is challenging. Because of the difficulty in defining the separate phases migration in porous media and the further complication of the soil heterogeneity, a significant effort has been carried out for the investigation of DNAPLs motion and transport by means of experimental tests and image analysis and fate and transport modeling mainly of the dissolved phases. Nevertheless, further effort is requested to model the separate phase motion in porous media.
On the basis of this context, numerical methods can be a support for understanding and investigating the fluids behavior in complex systems such as porous media. Different approaches to modeling flows through porous media can be considered. The classical macro-scale multiphase models however do not explicitly account for many important physical phenomena in which the phases are involved. As an alternative, a mesoscopic model known as Lattice Boltzmann (LB) has been successfully applied to saturated porous media and it has been widely used for its ability to deal with complex geometries as well as its capacity to simulate fluid flow and transport at the pore-scale. The LBM originates from the kinetic theory of gases and represents the microscopic phenomena by means of a statistical (macroscopic) description. The LBM has been shown to recover the conservation laws of mass, momentum and energy and the CFD community agrees that it has reached an high degree of maturity to solve the Navier- Stokes equations for incompressible fluids in single and multiphase environments.
Compared to traditional CFD, the LBMs lead to an easier implementation of multi-phase and multi-component flows and they are applied in many fields such as geologic storage of CO2, Petroleum Engineering or reactive and melting/dissolution phenomena. Even more dealing with multiphase fluids, LBMs provides several advantages compared to the traditional CFD, such as the ability to model the interface dynamics between the different phases and to simply handle the forcing terms involved in the non- wetting phase migration in a porous media e.g. capillary forces, viscous forces and buoyancy forces.
In the thesis, a multiphase multicomponent LB model is therefore presented to simulate DNAPLs migration in saturated porous media. Once described the main DNAPLs physical-chemical properties and the multiphase multicomponent LB theory, the implemented model is validated by means of different analytical solutions. Firstly, the interfacial tension phenomenon is analyzed by means of the bubble coalescence and the spinodal decomposition simulations: a sharp interface of few lattice nodes thickness is present between the two fluid phases in function of the tension parameter. The model has proved to recover the Young-Laplace Law, which correlates the pressure difference (at equilibrium) between the regions inside and outside a bubble of fluid 1 surrounded by the fluid 2, to 1/R, where R is the radius of the bubble. Then, the wettability effects are analyzed and validated by determining the contact angle measure and comparing it with the analytical solution in Huang et al. (2007). Finally, the multiphase Poiseuille flow is simulated and compared to the results obtained from Dou et al. (2012) .Different simulations varying the forces applied to the two fluids are then carried out to investigate the impacts of the drag forces, the viscosity effects and the velocity profiles in the channel.
Regarding the simulations of the DNAPLs migration in saturated porous media, the results obtained aimed to investigate the influence of the gravity, viscous and capillary forces on DNAPLs motion. Specifically, the role of these forces is characterized by means of different dimensionless numbers, which vary during the conducted simulation tests; the simulations results confirm that, once the DNAPL is released, an increase in contaminant amount and a more radial migration of DNAPL occurs in proximity of the release point when the capillary number “Ca” (ratio between the viscosity and the capillary forces) increases. Contrarily, when the Bond number “Bo” (ratio between the buoyancy and the capillary forces) exceeds the “Ca” number, the gravity assumes a “destabilizing” role leading to the formation of gravity driven fingers.
The influence of the hydraulic gradient on DNAPLs migration is also investigated. When a hydraulic gradient is applied, the separate phase shift towards the water flow direction occurs and the pooled DNAPL accumulated above the fine lenses is less. However, the DNAPL phase is also able to move backwards the flow direction when the hydraulic gradient does not let it to exceed the capillary pressure between the pores situated downgradient, explaining the not totally removal of pools over the fine lens even with higher hydraulic gradients. Finally, “trapped DNAPLs” are present: the gravity-driven fingers extend between the pores until they are too long and break. The disconnected mass can be immobilize as blobs and ganglia for the capillary forces.
It can be concluded that the development of a model that reproduces the motion of immiscible fluids starting from the microscale- mesoscopic scale is of particular interest and importance. On one hand, it can be useful to investigate the DNAPLs scenario of contamination as well as the reactions and physical microscale processes that significantly influence their macroscale behavior; on the other hand, it is worth to underline its potential use as a tool to support any projects of environmental remediation, in which the identification of the source of contamination and the prediction migration dynamics of contaminants in porous media are essential in selecting the best remediation technology to apply. Although further research and investigation are needed to simulate the DNAPLs migration paths in real contaminated sites, the LB method can provide good results in such a field. The model can therefore be applied to investigate the migration behavior of these separate phases in saturated porous media. However, further effort has to be spent in order to provide a tool able to simulate greater contaminated domains: the potential use of upscaling techniques, without losing the intrinsic microscopic information provided by the LB model, can be suggested
Back-Diffusion phenomena from low permeability layers of aquifers: laboratory investigations and numerical model
Full text linkThe aim of this thesis is to study the redistribution process of contaminants from low to high-permeability zones of aquifers in order to improve groundwater remediation. When the plume emitted by the primary source of contamination encounters low permeability layers of aquifer generates a concentration gradient between the high and low aquifer permeability zones. A molecular diffusive flux hence is induced causing pollutant accumulation in the low permeable layers (Forward-Diffusion). After the primary source of contamination is reduced, the pollutant concentration in the transmissive zone decreases and the concentration gradient reverses. Previously stored contaminant is released by the so-called “Back-Diffusion” process, which causes a long plume tail. Thus, contaminated low permeability zones of aquifer represent long-term sources of contamination, today considered as one of the main obstacles to effective groundwater remediation. The Back-Diffusion phenomenon was reproduced at laboratory scale reconstructing an aquifer characterized by a high-permeability layer and three different low-permeability zones. The measurement of contaminant fluxes released from the low permeability lenses by image analysis technique permitted to evaluate the influence of the lenses granulometry on the pollutant redistribution process. The results show that the lower the average grain dimension of the formation is, the higher the flux is and the longer the time for which the values of the released concentration can constitute a source of contamination. It is known the large use of pumping technologies to restore groundwater, so the efficiency of the traditional pumping technology to remediate contaminated low permeability zones of aquifer was investigated by laboratories tests. The remediation time of the three lenses were calculated under different groundwater velocities using the diffusive flux values obtained by image analysis. In this way, a relation between “groundwater velocity” and “remediation time” was determined for each lens. This relation shows the low efficiency of the traditional pumping system to restore contaminated low permeability zones of aquifers, demonstrating the need to use alternative technologies. The Groundwater Circulation Well (GCW) system was investigated as an alternative remediation technology. The GCW system creates vertical groundwater circulation cells by drawing groundwater through a one screened section of a multiscreen well and discharging it through another screened section. The suitability of this technology to restore contaminated low permeability layers was investigated by laboratory test. The collected data were used to calibrate a model created to simulate the Back-Diffusion process and to evaluate the effect of pumping technologies on the depletion time of that process. Results show that GCW develops a hydraulic gradient inside the aquifer, which supports the diffusive flow into the lenses and increases the concentration gradient at the interface between low and high permeability zones of aquifers. The efficiency of the GCW system appears dependent on the position of the low permeability zones, but it seems always higher than traditional pumping technology one
PRELIMINARY TRIAL APPLICATION OF BIOLOGICAL DESULFONATION IN PIG FARMS’ ANAEROBIC DIGESTORS
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Effects of maintenance scheduling criteria on the local impact of a MSW incinerator
No full textA sensitivity analysis of the effects of the operating hours scheduling of MSW incinerators is not common in the literature. To this concern, the present paper wants to give a contribution to the understanding of these effects in order to add a new point of view beyond the analyses based on overall balances. The selected case study concerns a plant proposed for a region in the North East of Italy. A characteristics of the selected case study is the long period of non-operation of each RMSW treatment line (3 months per year). A critical analysis of the effects of maintenance stop fractionation, of maintenance stop lasting on yearly basis, of maintenance stop seasonal scheduling, also at device level is presented. Considering the extreme scenarios of seasonal stop, a reduction of up to 20% around could be obtained. In case of more usual periods of stop for maintenance this value would be lower
Batchsize e criteri topologici: un approccio combinato per l'ottimizzazione in sicurezza
No full textSustainable biofuels for rural areas of developing countries: Brasilian case study
No full textThe objective of the application is to describe a project for sustainable development, proposed in Brazil regarding energy production and economic self-sufficiency, using as a raw material, the residual oil and other biomass available in this country. Additionally, the potential of biofuel in Brasil is also discussed
Improving environmental sustainability in developing and emerging countries by upgrading solid waste management techniques
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Processi e tecnologie impiantistiche per il controllo degli odori negli impianti di trattamento delle acque di scarico
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