Computational fluid dynamics modelling of dissolved oxygen in oxidation ditches.

This research aims to reveal new knowledge about the factors that affect the hydrodynamics, dissolved oxygen (DO) and aeration performance of a wastewater oxidation ditch. The literature is reviewed on the Computational Fluid Dynamics (CFD) modelling of wastewater aeration tanks. This study develops a CFD model of an aerated oxidation ditch, by taking into account two-phase gas-liquid flow, inter- phase oxygen mass transfer and dissolved oxygen. The main contributions to knowledge are the effect of bubble size distribution (BSD) and biochemical oxygen demand (BOD) distribution on the DO distribution. Species transport modelling predicts the BOD and DO distribution in the ditch. De-oxygenation of local dissolved oxygen by BOD is modelled by an oxygen sink that depends on the local BOD concentration. This is a novel approach to flow modelling for the prediction of the DO distribution. The local BOD concentration in the ditch may depend on either the local DO concentration or the local residence time. The numerical residence time distribution (RTD), heterogeneous flow pattern and DO distribution indicate that the flow behaviour in the ditch is non-ideal. Dissolved oxygen is affected by BOD distribution, bubble size, BSD, mechanical surface aeration and temperature. There is good agreement between the numerical simulation and both the observation of flow pattern and the measurement of mean DO. The BSD predicts a mean bubble size of around 2 mm, which is also the bubble size that best agrees with the measurements of DO. This study identifies that the BOD distribution and the BSD are key parameters that affect the DO distribution and improve the accuracy of the agreement with experimental data. In decreasing order of aeration performance are the air membrane diffuser, Fuch air jet aerator, Kessener brush surface aerator and Maguire hydro-jet aerator

Numerical Optimization of Cylinder Flow Structure of CO 2 Laser

Abstract: The cylinder flows structure is important part in the transverse-flow CO 2 laser, which affects the internal gas flow, and influences the flow kinetic energy loss. To study structural optimization to reduce the flow kinetic energy loss is a key concern of designers. In this study, the dynamic mesh model is used to simulate the rotation of cross-flow fan, the calculation process shows that it is easy to understand how to set parameter and treat mesh. Through optimization analysis on six structure parameters of the cylinder flow, we obtain the results that some of the parameters significantly influence the mixture gas velocity distribution in the discharge domain, but some other parameters of the flow field less affected velocity distribution of the discharge domain. The variation trends of the velocity distribution is also inconsistent which impacted by variation of each parameter. Through comprehensive analysis, the results of the optimization are: "1 = 14; "2 = 67.9; "3 = 73.1; "4 = 9.5; "5 = 65.6; "6 = 41.7. By validation calculations for the optimized structure of cylinder flow, we obtain the results that the flow state of discharge domain in the optimized structure is better than that of the previous structure

New Advances of Cavitation Instabilities

Cavitation refers to the formation of vapor cavities in a liquid when the local pressure becomes lower than the saturation pressure. In many hydraulic applications, cavitation is considered as a non-desirable phenomenon, as far as it may cause performance degradation, vibration problems, enhance broad-band noise-emission, and eventually trigger erosion. In this Special Issue, recent findings about cavitation instabilities are reported. More precisely, the dynamics of cavitation sheets are explored at very low Reynolds numbers in laminar flows, and in microscale applications. Both experimental and numerical approach are used. For the latter, original methods are assessed, such as smooth particles hydrodynamics or detached eddy simulations coupled to a compressible approach

Treatment of Landfill Waste, Leachate and Landfill Gas: Modelling/Simulation and Experimental Studies

Landfilling has been relegated to containing waste and hoping for minimal environmental impact. However, landfills produce harmful leachate and landfill gas that require treatment. To speed up the landfill biodegradation process, aerating the landfill to promote aerobic biodegradation has been implemented successfully. However, the conversion from a traditional anaerobic landfill to an aerobic landfill is to this point, not well researched. A 3-dimensional dynamic mathematical model was developed that depicts the conversion of a landfill from an anaerobic to an aerobic operation. The results of the model (CO2 volume fraction and temperature), agreed with data from published work. The model solved for the liquid and gaseous pressures/velocities, gas composition, anaerobic/aerobic biomass concentrations and temperature; all were solved with respect to space and time. Landfill leachate requires treatment before release and landfill gas requires purification (removal of CO2) before it can be used as a fuel. A hybrid sorption (absorption and adsorption/ion exchange) system was developed to treat leachate and purify landfill gas in the same column. The absorption results showed that leachate could remove more carbon dioxide from the landfill gas than pure water, due to its slight basicity. The adsorption/ion exchange results showed that lead could be removed from model leachate but not below Ontario discharge guidelines with the length of the column used (50-55 cm zeolite bed height)

Dynamic Modeling and Environmental Analysis of Hydrokinetic Energy Extraction

The world is facing an imminent energy supply crisis. Our well-being is linked to the energy supply, and energy is in high demand in both the developed and the developing world. In order to sustain our energy supply, it is necessary to advance renewable technologies. Despite this urgency, however, it is paramount to consider the larger environmental effects associated with using renewable resources.Hydropower, in the past, has been seen as a viable resource to examine, given that its basics of mechanical to electrical energy conversion seem to have little effect on the environment. Discrete analysis of dams and in-stream diversion set-ups has shown otherwise, though. Modifications to river flows and changes in temperature (from increased and decreased flows) cause adverse effects to fish and other marine life because of changes in their adaptive habitat.Recent research has focused on kinetic energy extraction in river flows, which may prove to be more sustainable, as this type of extraction does not involve a large reservoir or large flow modification. The field of hydrokinetic energy extraction is immature; little is known about the devices' performance in the river environment, and their risk of impingement, fouling, and suspension of sediments. Governing principles of hydrokinetic energy extraction are presented, along with a two-dimensional computational fluid dynamics (CFD) model of the system. Power extraction methods are compared, and verification and validation of the CFD model through mesh sensitivity and experimental data are presented. A 0.0506 average mesh skew and 0.2m/s velocity convergence was obtained within the mesh sensitivity analysis. In comparing particle image velocimetry (PIV) data with the CFD model, a 0.0155m offset and 20\% error were present. However, including a volume of fluid (VOF) model within the CFD model produced a 5\% error improvement and gave a 0.0124m offset. These are improvements over the current state of the art, where visual comparisons are common. Three-dimensional CFD models of a submerged water wheel, Savonius turbine, squirrel cage Darrieus turbine, and Gorlov Darrieus turbine are also presented; however, they are non-VOF CFD models.Using the results of the CFD models, preliminary predictions could be made of the environmental impact of hydrokinetic turbines with respect to fish swimming patterns. Additionally, a life cycle assessment (LCA) was conducted for hydrokinetic energy extraction (HEE), which gives insight into the total system environmental impact. HEE has been seen as a potentially ``benign' form of renewable hydropower. This work provides a benchmark for initial measurement of HEE environmental impacts, since negative outcomes have been present with previously-assumed benign renewable hydropower. A Gorlov system was used to represent a HEE system. LCA was utilized to compare the environmental impacts of HEE with small hydropotential (HPP) power, coal, natural gas and nuclear power. Environmental Protection Agency (EPA) criteria air emissions were quantified and compared over the life cycle of the systems. Life cycle air emissions were used in combination with the TRACI impact assessment tool to compare the systems. The Gorlov system was found to have the lowest life cycle impact with a system lifetime comparison, and compared closely with small HPP.Finally, various issues connected to the implementation of hydrokinetic power generation were discussed. Policy development and sediment movement were investigated in more detail. Additionally, two applications of this technology were explored: in-situ river health monitoring and remote energy generation

A critical review on latest innovations and future challenges of electrochemical technology for the abatement of organics in water

Updated water directives and ambitious targets like the United Nations’ Sustainable Development Goals (SDGs) have emerged in the last decade to tackle water scarcity and contamination. Although numerous strategies have been developed to remove water pollutants, it is still necessary to enhance their effectiveness against toxic and biorefractory organic molecules. Comprehensive reviews have highlighted the appealing features of the electrochemical technologies, but much progress has been made in recent years. In this timely review, a critical discussion on latest innovations and perspectives of the most promising electrochemical tools for wastewater treatment is presented. The work describes the performance of electrocatalytic anodes for direct electrochemical oxidation, the oxidation mediated by electrogenerated active chlorine, the electrocatalytic reduction as well as coupled approaches for synchronous anodic and cathodic processes combined with homogeneous and heterogeneous catalysis. The last section is devoted to the assessment of scale-up issues and the increase in the technology readiness level

Characterization of the mechanism of action of spin-filters for animal cell perfusion cultures

The growing demand for high levels of recombinant proteins of medical and pharmaceutical interest stimulates the development of cell bioprocess technology. Spin-filter technology is employed in order to reach high levels of such compounds. The aim of this thesis was to characterize the mechanisms of cell retention, as well as filter fouling during animal cell spin-filter perfusion cultures. A good understanding of these mechanisms would allow a good optimization of spin-filter parameters and culture conditions in order to achieve high cell density cultures at large scale operation and for long-time and thus increase proteins productivity. The first part of the thesis was focused on the study of particle retention as a function of four main parameters: filter pore size, filter rotation speed, perfusion rate and particle concentration, during perfusion simulations of agarose beads of 13 μm in diameter. Bead retention by filters with pore sizes of 13 and 14.5 μm, larger than the mean particle diameter was found to be dependent mainly on the filter rotation velocity and filter pore size. Filter retention followed a saturation dynamics with an initial direct correlation with respect to filter rotation rate. A plateau was reached above a filter tangential velocity of 0.45 m/s and 0.87 m/s for filters with pore size of 13 and 14.5 μm respectively. The lower the filter velocity was, the greater the influence of perfusion rate on bead retention, whereas the retention was slightly improved when the particle concentration was increased. The presence of a draft tube around open spin-filters was observed to lower the retention, with the effect being greater for non-porous than for porous draft tubes. In the second part of this work, a prediction of radial particle migration near the surface of rotating filter was developed. The lift force was demonstrated to be important in the spin-filter system since it contributes to particle removal from the filter surface. Competition between centrifugal sedimentation, lift forces, Stokes drag and perfusion forces were found to be responsible for determining particle motion relative to the filter. At certain filter rotation rates, centrifugation and lift forces are sufficiently high as to balance perfusion flow and result in the movement of particles away from the filter, a situation that experimentally was found to correspond to maximum particle retention. The model also revealed that filter acceleration is the key parameter to be conserved from small to large scale in order to achieve similar retention rates. This hypothesis has been confirmed experimentally. Then spin-filter cell retention was modeled using response surface methodology. A second-order polynomial model was used to predict the effects of the filter pore size, cell concentration, perfusion capacity and filter acceleration on cell retention. The retention rates obtained experimentally during two different spin-filter perfusion cultures of CHO SSF3 agreed with those predicted by the model, indicating the applicability of the model to animal cell perfusion culture. In the last part of this work the study of filter fouling during long-term perfusion simulations with CHO animal cells was investigated. It was demonstrated that at low filter acceleration, below 6.2 m/s2, a high perfusion rate of 25 cm/h induced rapid filter pore, whereas increasing the filter acceleration to 25 m/s2 increased filter longevity eight times, for filters with a pore size of 8.5 μm. Increasing the filter pore size to 14.5 μm improved filter longevity by 84% and revealed less viable and dead cell deposits on filter surface. Ultrasonic technology was used to reduce filter fouling. Filter vibration, induced by a piezo actuator, improved filter longevity by 113% during real CHO perfusion culture. This work allowed a better understanding of the mechanism of action of spin-filters. The cell retention model developed in this study permits to choose the optimal acceleration at which the filter, of a certain pore size has to be operated in order to achieve similar retention rates for small scale as well as for large-scale processes. The ultrasonic technology through the use of piezoactuators was demonstrated to be a powerful technique for the on-line reduction of filter-fouling, during animal cell perfusion cultures

Development and Applications of a Novel Intermittent Solids Feeder for Pyrolysis Reactors

This PhD research addresses the challenge of feeding biomass residues into fluidized bed reactors for pyrolysis, through the development of a novel intermittent solid slug feeder, both for laboratory-scale and large-scale reactors. The new feeder can successfully handle biomass residues that are either too cohesive or thermally sensitive for traditional feeders. To optimize the novel feeder performance, a model for the pulsating solids flow was developed from experimental data collected with ideal slugs, as well as real biomass flow. The model was validated using both a laboratory-scale (\u3c 10 kg/hr) and large-scale feeder (\u3e 250 kg/hr). Several important variables were identified. They include the material flow properties, the pulse gas pressure and volume, and the feeding tube length and material. The goals of this study were to (a) characterize the fundamental dynamic behavior of the biomass slugs in the feeder, (b) maximize the solid-to-gas feeding ratio, and thus minimize energy consumption and cost, (c) minimize the accumulation of “straggler” biomass material in the feeding tube between pulses, and thus prevent biomass heating in the feeding tube, which can induce plugging, and (d) develop and validate a predictive model for the slug velocity at any location in the feeding tube, which can be applied to feeder design for any biomass feedstock. An advantage of the new large-scale feeder technology is that it can handle larger biomass particles than traditional feeder technologies. An issue with large particles is that they require relatively long drying, which must be optimized. A model was therefore developed for drying, which takes shrinkage, and internal and external mass transfer limitations into account. The thesis is supplemented with additional work based on the application of the novel feeder for pyrolysis studies with various biomass residues. The feeder technology made it possible to perform the first ever pyrolysis studies, in industrially-relevant equipment, on pure meat and bone meal residue, and on unmodified and undiluted Kraft lignin. Appendices include a business case-study of the implementation of the technologies developed in this thesis on large-scale pyrolysis and an additional pyrolysis study on tucumã seeds, which utilized the novel feeder