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The Science of Air Pollution
Published in Daniel T. Rogers, Environmental Compliance Handbook, 2023
Contaminant transport in the air environment is dominated by three physical transport mechanisms (USEPA 1996a; USGS 2006a): advection/convection, molecular diffusion, and dispersion. Advection is the horizontal transport of any property of the atmosphere and water. Common examples are the transfer of heat by wind and sediment transport within a flowing stream. Convection is the vertical advection of air, water, or other fluid as a result of thermal differences. We introduced the concept of convection in Chapter 2 as the driving force behind plate tectonics. Molecular diffusion is the movement of a chemical from an area of high concentration to an area of lower concentration due to the random motion of the chemical molecules. Dispersion (also referred to as hydrodynamic dispersion) is the tendency for pollutants to spread out from the path of the expected advective flow (USGS 2006a). Occasionally, the effects of diffusion and dispersion are treated together, but for the purposes of this book, we treat them separately. The rate of advective transport of a pollutant is often expressed in terms of flux density. Flux density is the mass of a chemical transported across an imaginary surface of a unit area per unit of time. Equation 3.2 shows this relationship (Hemond and Fechner-Levy 2000), which is independent of the media involved (soil, surface water, groundwater, or the atmosphere).
Site Assessment and Remedial Investigations
Published in Cristiane Q. Surbeck, Jeff Kuo, Site Assessment and Remediation for Environmental Engineers, 2021
Cristiane Q. Surbeck, Jeff Kuo
The dispersion term in Eq. 4.16 accounts for both the molecular diffusion and hydraulic dispersion. The molecular diffusion, strictly speaking, is due to the concentration gradient (i.e., the concentration difference). The COC diffuses away from the higher concentration zone, and this can occur even when the fluid is not moving. The hydraulic dispersion here is mainly caused by flow in porous media. It results from (i) the velocity variation within a pore, (ii) the different pore geometries, (iii) the divergence of flow lines around the soil grains in the porous media, and (iv) the aquifer heterogeneity (U.S. EPA 1989).
Contaminant Fate and Transport
Published in Daniel T. Rogers, Urban Watersheds, 2020
Pollutant transport in the environment is dominated by three physical transport mechanisms (USEPA 1996a, 2006a): advection/convection, molecular diffusion, and dispersion. Advection is the horizontal transport of any property of the atmosphere and water. Common examples are the transfer of heat by wind and sediment transport within a flowing stream. Convection is the vertical advection of air, water, or other fluid as a result of thermal differences. We introduced the concept of convection in Chapter 2 as the driving force behind plate tectonics. Molecular diffusion is the movement of a chemical from an area of higher concentration to an area of low concentration due to the random motion of the chemical molecules. Dispersion (also referred to as hydrodynamic dispersion) is the tendency for pollutants to spread out from the path of the expected advective flow (USGS 2006a). Occasionally, the effects of diffusion and dispersion are treated together, but for the purposes of this book we treat them separately.
Efficacy of applying discontinuous boundary condition on the heat transfer and entropy generation through a slip microchannel equipped with nanofluid
Published in Engineering Applications of Computational Fluid Mechanics, 2022
Suhong Liu, Dariush Bahrami, Rasool Kalbasi, Mehdi Jahangiri, Ye Lu, Xuelan Yang, Shahab S. Band, Kwok-Wing Chau, Amir Mosavi
From a physical point of view, in layout A, after the fluid passes through the first hot zone, the temperature of the fluid in the area close to the wall, increases. For the adiabatic zone, no extra thermal energy is added to the fluid molecules. Therefore, hot molecules have the opportunity to transfer energy to colder molecules through molecular collisions along with molecular diffusion. Therefore, in the adiabatic zones, the temperature distribution becomes more uniform and therefore the molecules near the upper wall become cooled. As the fluid approaches the second hot zone, these cold molecules can receive more heat from the hot wall because they have cooled in the previous zone (which was adiabatic). Therefore, as the number of hot zones increases, more heat is expected to transfer between the wall and the fluid.
Flame propagation in the mixtures of O2/N2 oxidizer with fluorinated propene refrigerants (CH2CFCF3, CHFCHCF3, CH2CHCF3)
Published in Combustion Science and Technology, 2021
V.I. Babushok, D.R. Burgess, M.J. Hegetschweiler, G.T. Linteris
The Chemkin set of programs (Kee et al., 1991; Kee, Rupley, Miller, 1989, 1990) and the open-source software package Cantera (Goodwin, Moffat, Speth, 2016) were used for combustion equilibrium calculations and for laminar flame modeling in mixtures of fluoropropenes with air of various oxygen volume fractions XO2,ox. For burning velocity calculations, the equations of mass, species, and energy conservation are solved numerically for the initial gas compositions, temperature (298 K), and pressure (101 kPa) corresponding to those in the experiments. The solution assumes isobaric, adiabatic, steady, planar, one-dimensional, laminar flow and neglects radiation and the Dufour effect, but includes thermal diffusion. Molecular diffusion is modeled with the multi-component transport equations. The boundary conditions, corresponding to a freely-propagating flame, are specified inlet mass fraction fluxes, velocity, and temperature (298 K), and vanishing gradients downstream from the flame.
A review on groundwater contaminant transport and remediation
Published in ISH Journal of Hydraulic Engineering, 2020
P. K. Sharma, Muskan Mayank, C. S. P. Ojha, S. K. Shukla
Dispersion is caused by both microscopic and macroscopic effects. Mechanical dispersion on a microscopic scale is a result of deviation of velocity on a microscale from the average groundwater velocity (Anderson 1984). These velocity variation arise because water in the center of a pore space travels faster than the water near the wall. Dispersion is also governed by spatial variability of groundwater velocity in porous media caused by the heterogeneity of hydraulic properties of the porous medium (Bear 1979). Hence, diversion flow paths around individual grains of porous material cause variations in average velocity among different pore spaces. These two factors create mechanical dispersion on microscopic scale. Molecular diffusion occurs as species move from higher to lower concentrations. Thus, microscopic dispersion includes the effects of mechanical dispersion and molecular diffusion.