The filtration efficiency of a granular bed filter was investigated in the domain where inertial effects are dominant. A filtration model predicting single sphere and total bed efficiencies based on trajectory calculations of dust particles in the voids of the granular bed was developed. The model uses the flow field in a bed of spheres arranged in a body-centered cubic array for which the filtration efficiency was found to be a function of a modified Stokes number defined by Sf' = St X F where the function Fdepends only on the Reynolds number. The value of the function F can be found experimentally or predicted from the Ergun correlation to be F = 1 + 0.0157 Re, The filtration efficiency calculated using the model agrees well with experimental results obtained in beds of dense cubic packing of spheres especially at relatively low Reynolds numbers (Re < 2W) and relatively high Stokes numbers (St' > 0.01). The prediction of the theoretical model also agrees well with experimental data for randomly packed beds at low porosities and low Reynolds numbes. Most theoretical models of filtration in granular beds are based on evaluating the filtration efficiency of a single granule and then integrating the results over the entire bed by assuming that all granules in the bed experience similar filtration conditions and have the same filtration efficiency. The single sphere efficiency is usually determined by calculating the trajectories of aerosol particles flowing toward a single sphere and modifying the gas flow field to account for the porosity of the bed. The major difficulty in using this approach is that the particle trajectories and therefore the filtration efficiency depends strongly on the actual threedimensional flow profile of the gas flowing through the bed. A representative and accurate threedimensional velocity profile for flow in a granular bed is not yet availabe in the literature so that simplified flow models must be used for trajectory calculations with results that do not agree well with experimental data.In the present work an improved version of the Snyder and Stewart (1966) flow model for a dense cubic packing of spheres is used to evaluate the filtration efficiency in a granular bed. The threedimensional flow model enables trajectory calculations to be made for a single sphere as well as for the entire bed. The calculated single sphere and total bed efficiencies are compared to determine the validity of the single sphere efficiency concept. The results for the single sphere and total bed efficiencies are also compared with experimental data obtained in both dense cubic packing of spheres and in randomly packed beds.
CONCLUSIONS AND SIGNIFICANCEA filtration model predicting both single sphere and total bed efficiency was developed for a dense cubic packing using the flow field developed by Snyder and Stewart (1966). The model is based on trajectory calculations of dust particles inside the granular bed and assumes no bouncing of dust particles from the surface of the granules. The filtration efficien...
The objective of this review paper is to survey the state of the art on nature-based solutions (NBS) in the built environment, which can contribute to a circular economy (CE) and counter the negative impacts of urbanization through the provision of ecosystem services. NBS are discussed here at three different levels: (i) green building materials, including biocomposites with plant-based aggregates; (ii) green building systems, employed for the greening of buildings by incorporating vegetation in their envelope; and (iii) green building sites, emphasizing the value of vegetated open spaces and water-sensitive urban design. After introducing the central concepts of NBS and CE as they are manifested in the built environment, we examine the impacts of urban development and the historical use of materials, systems and sites which can offer solutions to these problems. In the central section of the paper we present a series of case studies illustrating the development and implementation of such solutions in recent years. Finally, in a brief critical analysis we look at the ecosystem services and disservices provided by NBS in the built environment, and examine the policy instruments which can be leveraged to promote them in the most effective manner – facilitating the future transition to fully circular cities.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.