Effect of pollutant source location on air pollutant dispersion around a high-rise building

This article investigates the dispersion of airborne pollutants emitted from different locations near a high-rise building. A Computational Fluid Dynamics (CFD) model for simulating the wind flow field and the pollutant dispersion was developed and validated by wind tunnel data. Then the spreading of the pollutant emitted from different locations to a rectangular-shaped high-rise residential (HRR) building was numerically studied. The pollutant source location was set in a wide range of the position angle and distance between the source and the building. It was found that the pollutant concentration on the building decreases with an increase in the emission distance whereas the effect of the position angle is more complicated. Interestingly, there is a critical range of the position angle from which the emitted pollutants will not spread to the building in a significant way. The effect of the source location was linked to the wind flow field around the building, particularly with several major flows. The vertical distributions of the pollutant concentration on different faces were also investigated, and it was found that these are more affected by the vertical flow near each face. Finally, a mathematical model was developed to evaluate the pollutant concentration as a function of the emission distance and position angle. These findings are helpful to the understanding of the dispersion of airborne pollutants around high-rise buildings and the related hazard management in urban design.     

Numerical simulation of pollutant dispersion in street canyons: Geometric and thermal effects

Numerical investigations on pollutant dispersion in street canyons with emission sources located near the ground level are performed in the present work. Pollutant dispersion problems in urban areas are usually studied considering the street canyon model, which consists of long streets laterally confined by buildings. Significant changes can be observed in wind flow patterns and pollutant concentration fields when thermal and geometric effects are considered. Thus, the objective of this study is to investigate numerically the wind flow and pollutant dispersion for the following cases: (a) a two-dimensional street canyon model considering three different aspect ratios and four different wall heating configurations; (b) a flow domain with two immersed buildings arranged in two distinct configurations; (c) a three-dimensional urban area model composed of a building set and street intersections. Expected flow structures were obtained inside the canyon when different aspect ratios and wall heating configurations were considered. Flow phenomena such as separation/reattachment were observed when two-buildings models were analyzed. Finally, three-dimensional flow structures, with some characteristic that are not observed in two-dimensional models, affecting the pollutant removal, were simulated in the last case, highlighting the relevance of model dimensionality. The wind flow and pollutant dispersion are investigated using a numerical model based on the finite element formulation utilized by some of the authors of this work, which is extended here to deal with problems of heat and mass transport in the urban micro-scale. Turbulence is reproduced using Large Eddy Simulation (LES) and thermal effects on the momentum equations are considered as a buoyancy force, according to Boussinesq approximation.     

Smoke flow phenomena and turbulence characteristics of tunnel fires

Gravity currents are similar in behavior with smoke flows. This work aims to provide evidence justifying the use of gravity current approach to model smoke flows downstream of the fire source. The turbulence solver available in almost all commercial CFD codes solves RANS for the flow field. To find out how well the nature of smoke flow be accurately modeled using RANS that is widely used for incompressible flows. The feasibility of using both Reynolds- and Favre-averaging schemes was numerically compared and examined in this paper. In this work, numerical simulations of a fire occurred in a 400-m longitudinally ventilated tunnel have been successfully performed using FDS version 4. Large eddy simulation is employed in this study. Although the ranges of fire size and ventilation velocity vary respectively from 0 MW to 100 MW and 0 m/s to 10 m/s, this paper focuses on the general flow and temperature fields and the turbulence characteristics. Furthermore, the turbulence kinetic energy levels of the flow in the tunnel at several locations were investigated. Since the flow field is generally induced by mechanical ventilation and combustion, the main contribution to the turbulence kinetic energy comes from its longitudinal, vertical, or their combination.     

Numerical simulation of methane partial oxidation in the burner and combustion chamber of autothermal reformer

This study aims to model the methane partial oxidation process in the burner and combustion chamber of autothermal reactor. The numerical simulation based on this model offers a powerful tool that can assist in reactor design and optimization and scale up of the process saving expensive pilot work. The steady-state governing equations were solved using the SIMPLE algorithm and the effect of turbulence on the mean flow field was accounted for using the RNG k–ε model. A two-step reaction mechanism was used for the gas combustion with CO as the intermediate species. The reaction rates were modeled using an Eddy-Dissipation Model. In terms of the geometrical model, a 3D model for burner was developed while an axis-symmetric model for the combustion chamber was implemented to reduce the computational costs. The model formulated was validated against a currently operating autothermal reactor and then has been used to investigate different aspects of these reactors. Results show that effect of oxygen to methane ratio is more than that of feed temperature. It is demonstrated that a 60% increase in O₂/CH₄ ratio causes a 15.4% decrease and 42.7% increase in H₂/CO ratio and methane conversion, respectively. In contrast, a 60% increase in feed temperature does not have a significant effect on the process.     

Classification of Structural Complexity for Mine Ventilation Networks

The mine ventilation system is most important and technical measure for ensuring safety production in mines. The structural complexity of a mine ventilation network can directly affect the safety and reliability of the underground mining system. Quantitatively justifying the degree of complexity can contribute to providing a deeper understanding of the essential characteristics of a network. However, so far, there is no such a model which is able to simply, practically, reasonably, and quantitatively determine or compare the structural complexity of different ventilation networks. In this article, by analyzing some typical parameters of a mine ventilation network, we conclude that there is a linear functional relationship among five key parameters (number of ventilation network branches, number of nodes, number of independent circuits, number of independent paths, and number of diagonal branches). Correlation analyses for the main parameters of ventilation networks are conducted based on SPSS. Based on these findings, a new evaluation model for the structural complexity of ventilation network (which is represented by C) has been proposed. By combining SPSS classification analyses results with the characteristics of mine ventilation networks, standards for the complexity classification of mine ventilation systems are put forward. Using the developed model, we carried out analyses and comparisons for the structural complexity of ventilation networks for typical mines. Case demonstrations show that the classification results correspond to the actual situations. © 2014 Wiley Periodicals, Inc. Complexity 21: 21–34, 2015     

Dispersion of mass by two-dimensional homogeneous and incompressible Çinlar flows

We consider the dispersion of passive tracers in stationary, homogeneous and incompressible Çinlar flows on the plane. The associated velocity field is generated by the superposition of eddies of various size, arrival time and location which form a Poisson point process. Our focus is on the dispersion of a tracer cloud, which is measured through the variance of its centroid and the mean of the dispersion tensor. We also study single particle dispersion and particle pair separations in conjunction with dispersion. Monte Carlo simulations of all these measures first establish the relation of the dispersion to the parameters of the flow model. Second, the physical predictions on the behavior of these measures with respect to time as well as their relationship to each other are confirmed.     

中国煤层气的产业化进程与发展建议

煤层气产业在中国能源建设中具有重要战略地位。中国煤层气勘探开发历经矿井瓦斯抽排利用、现代煤层气理论技术的引进发展与成功应用、商业化开发试验及产业化发展（初期）4个阶段。由于在煤层气地质基础理论、勘探技术方法、开发工艺技术、集输利用与管理制度等方面的发展创新、系统集成,我国已有能力大规模发展煤层气产业,这在我国能源发展战略上是非常难得的历史机遇。当前,为了加快煤层气产业的发展,必须加强统筹规划,发挥多种积极性;加强煤层气地质研究、提高对区域勘探的指导与目标评价水平;开展高效低成本开发工艺技术攻关;发展煤层气产品集输和加工利用;继续跟踪国外煤层气前沿理论技术,加强技术交流与合作研究。     

Bacterial methane oxidation in landfill cover layers - a coupled FE multiphase description

Landfill gas is composed of methane (CH₄) and (CO₂) at a ratio of about (60% – 40%), whereby the impact of methane on the greenhouse effect is about 25 times higher than that of carbon dioxide. Bacterial methane oxidation, taking place in the landfill cover layer, helps to reduce the climate active emissions from landfill sites. This contribution presents a theoretical and numerical approach to model the coupled processes of bacterial methane oxidation. An isothermal biphasic model based on the Theory of Porous Media (TPM) and Mixture Theory is introduced as well as the coupled finite element (FE) calculation concept. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

New stochastic model for dispersion in heterogeneous porous media: 2. Gaussian plume transmission across stepwise velocity fluctuations

The stochastic solute dispersion model studied in the previous article, can be applied to more realistic velocity variations by approximating them as piecewise constant. This requires treatment by a boundary value formulation, which raises problems connected with entropy considerations. A method is developed to deal with these by the introduction of a specially designed compensator function into the boundary value probability integral for calculating solute concentration. Applying this even for a single velocity step yields an intractable integration, but a suitable approximation is constructed that allows it to be evaluated in analytical form. The result is that a Gaussian solute plume impinging on a velocity step is transmitted as a modulated and compressed or dilated quasi-Gaussian. Plume dispersion is encapsulated in an enhancement factor F that multiplies the diffusive, linear time, dispersion. F is also time dependent; at the time of step penetration it equals kinematical dilation, but anneals away non-linearly so that a length scale can be established over which downstream effects of a velocity step on the dispersion extends.     

A coupled multi-component approach for bacterial methane oxidation in landfill cover layers

Methane (CH₄), which has a 25 times higher global warming potential than carbon dioxide (CO₂), can be oxidated by methanotrophic bacteria into carbon dioxide and water. The biological oxidation of methane can be considered in the passive aftercare phase of landfills in order to reduce climate-damaging methane emissions. Methanotrophic bacteria are situated within the landfill cover layer and convert the harmful methane emissions arising from the degradation of organic waste to the less harmful carbon dioxide. Hence, the passive aftercare of landfills in terms of methane oxidation layers is an efficient method to reduce contributions to the greenhouse effect. To model the coupled processes during phase transition from methane to carbon dioxide, the well-known Theory of Porous Media (TPM) combined with the Mixture Theory has been used in order to develop a multi-component Finite Element calculation concept, see [1, 3]. The thermodynamic consistent model analyzes the relevant gas productions of methane, carbon dioxide and oxygen. The model also accounts for the driving phenomena of production, diffusion and advection. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Application of fuzzy control to a road tunnel ventilation system

This paper deals with the serious problems of ventilation system in a large road tunnel. Higher visibility and lower concentration of carbon monoxide are the key issues concerning the ventilation system. Prior to designing the fuzzy control model, a configuration layout of the ventilation system including sensing, control and traffic prediction as well is conceptually constructed. Based on the layout that offers assignments of sensors and control elements, a fuzzy logic control model is developed. Membership functions of sensor errors and control increments are physically submitted in order to set up the fuzzy logic rules. Timing and spacing filtering in terms of weighting approaches is employed in the fuzzy logic rules. A dynamic equation describing the concentration of air pollution is also given so as to cooperate with the fuzzy logic rules and to play roles in the computer simulation. The result of computer simulation involving five cases indicates that a multi-level scheme is able to solve the engineering problems.     

On unsteady dispersion flow in porous media

The generalising dispersion equations of flow through porous media have been investigated. The Laplace transform has been applied to obtain the solution to dispersion problem as a result of adsorption. The generalised closed form solution for dispersion has been presented and the different types of variations in concentration have been graphically discussed. When the steady state occurs, the concentration becomes constant but for small value of time (say 0.5) the concentration tends to zero as distance increases.     

Axial dispersion in pressure perturbed flow through an annular pipe oscillating around its axis

A method of moment is employed to study the axial dispersion of passive tracer molecules released in an unsteady pressure-driven flow through an annular pipe which is oscillating around its longitudinal axis. The flow unsteadiness is caused by the oscillation of the tube around its axis as well as by a periodic pressure gradient. A finite difference implicit scheme is adopted to solve the Aris integral moment equations arising from the unsteady convective-diffusion equation for all time periods. The main objective is to study the nature of the dispersion coeffcient and mean concentration distribution under the sole as well as combined oscillation of the two driving forces. The behaviour of the dispersion coeffcient due to the variation of the aspect ratio, the absorption parameter for purely periodic flow has been examined and the sound response from dispersion coeffcient is found with the variation of these parameters in the sole presence of pressure pulsation. There is a remarkable difference in the behavior of the dispersion coeffcient depending on whether the ratio of two frequencies arising from the oscillations of the tube and the pressure gradient possesses a proper fraction or not. Oscillation of the tube produces much more dispersion than the pulsation of the pressure gradient and their combined effect leads to a further increase in dispersion. Tube oscillation shows a stronger effect on the dispersion coeffcient than the pressure pulsation though the effect of physical parameters are pronounced in the presence of pressure pulsation. The effect of the frequency parameter on the axial distribution of mean concentration is insensible when the oscillation of the annular tube is the only forcing. However this effect is much noticeable under the combined action of both forcing and much more effective under the sole influence of pressure pulsation.     

Axial dispersion in pressure perturbed flow through an annular pipe oscillating around its axis

A method of moment is employed to study the axial dispersion of passive tracer molecules released in an unsteady pressure-driven flow through an annular pipe which is oscillating around its longitudinal axis. The flow unsteadiness is caused by the oscillation of the tube around its axis as well as by a periodic pressure gradient. A finite difference implicit scheme is adopted to solve the Aris integral moment equations arising from the unsteady convective-diffusion equation for all time periods. The main objective is to study the nature of the dispersion coeffcient and mean concentration distribution under the sole as well as combined oscillation of the two driving forces. The behaviour of the dispersion coeffcient due to the variation of the aspect ratio, the absorption parameter for purely periodic flow has been examined and the sound response from dispersion coeffcient is found with the variation of these parameters in the sole presence of pressure pulsation. There is a remarkable difference in the behavior of the dispersion coeffcient depending on whether the ratio of two frequencies arising from the oscillations of the tube and the pressure gradient possesses a proper fraction or not. Oscillation of the tube produces much more dispersion than the pulsation of the pressure gradient and their combined effect leads to a further increase in dispersion. Tube oscillation shows a stronger effect on the dispersion coeffcient than the pressure pulsation though the effect of physical parameters are pronounced in the presence of pressure pulsation. The effect of the frequency parameter on the axial distribution of mean concentration is insensible when the oscillation of the annular tube is the only forcing. However this effect is much noticeable under the combined action of both forcing and much more effective under the sole influence of pressure pulsation.     

Dispersion models for annular climbing film flow

The mathematical theory of dispersion in annular climbing film flow is developed. Starting with dispersion in a uniform film, the theory is extended to incorporate successively the effects of a viscous sublayer, disturbance waves and interchange of material with entrained droplet. These effects are considered independently but their combined influence on the overall dispersion characteristics of the system is shown to be capable of analysis in terms of an interchange dispersion model (IDM). A solution method for this interchange model is given which may be used to obtain values for the dispersion parameter, P_f, and an ion fractionation coefficient, _f, by non-linear regression on experimental concentration distributions. Values for the dispersion parameter so obtained can be used to give an induction of the viscous layer thickness as well as other film characteristics.     

Evolution of phase transitions in methane hydrate

We consider a simplified model of methane hydrates which we cast as a nonlinear evolution problem. For its well-posedness we extend the existing theory to cover the case in which the problem involves a measurable family of graphs. We represent the nonlinearity as a subgradient and prove a useful comparison principle, thus optimal regularity results follow. For the numerical solution we apply a fully implicit scheme without regularization and use the semismooth Newton algorithm for a solver, and the graph is realized as a complementarity constraint (CC). The algorithm is very robust and we extend it to define an easy and superlinearly convergent fully implicit scheme for the Stefan problem and other multivalued examples.     

Modelling of tunnel fires and experimental validation

The relevant scaling parameters for physical modelling of tunnel fires in an experimental test channel are identified. These parameters can be used to correlate the experimental data related to the critical ventilation velocity. Critical ventilation prevents backlayering of smoke during a tunnel fire. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multi-objective-oriented removal of airborne pollutants from a slot-ventilated enclosure subjected to mechanical and multi component buoyancy flows

Slot-ventilated enclosure flows, jointly driven by two component buoyancy forces and forced ventilation, extensively exist in the industrial and civil environment. Inverse fluid flow solutions of the multi-objective oriented removal of airborne pollutants from the slot ventilated enclosure, simultaneously subjected to mechanical forces and multi-component buoyancy forces, are conducted in the present work. A simplified conjugate gradient methodology has been implemented to provide effective convection removal of contaminants, under different flow regimes. In particular, direct and inverse convection problems are subsequently solved in detail. For the direct convection problem, aiding and opposing multi-component buoyancy effects are included to study the convective heat and species transport mechanism. For the inverse optimization problem, a single-objective optimization is firstly implemented, which also provides input parameters for the multi-objective optimization. Following that, a multi-objective function is set up by combining the two single objectives involving the spatial average concentration and the mean radius of diffusion. Multi-objective optimization is then implemented depending on the conjugate gradient procedures. Both the single and multi-objectives could be achieved reasonably through positioning of local heating sources and the free vent outlet. Our solution methodology will be useful for improving room pollutant removal and developing efficient ventilation strategies.     

Large eddy simulation of coherent structures in rectangular methane non-premixed flame

Large eddy simulation of a three-dimensional spatially developing transitional free methane non-premixed flame is performed. The solver of the governing equations is based upon a projection method. The Smagorinsky model is utilized for the turbulent subgrid scale terms. A global reaction mechanism is applied for the simulation of methane/air combustion. Simulation results clearly illustrate the coherent structure of the rectangular non-premixed flame, consisting of three distinct zones in the near field. Periodic characteristics of the coherent structures in the rectangular non-premixed flame are discussed. The predicted structure of the flame is in good agreement with the experimental results. Distributions of species concentrations across the flame surfaces are illustrated and typical flame structures in the far field are analyzed. Local mass fraction analysis and flow visualization indicate that the black spots of the flames are due to strong entrainment of oxygen into the central jet by streamwise vortices, and breaking up of the flame is caused by an enormous amount of entrainment of streamwise vortices as well as stretching of spanwise vortices at the bottom of the flame.     

Über die Äquivalenz von Wärme-, Kraft- und Massenquellen

Summary The steady, nonviscous flow in fields with sources of energy, force and mass is investigated under the assumption of small perturbations. The different sources are equivalent in such a way that it is possible to change the sources without changing certain values of the flow field if one does not change certain combinations of the sources. In the same way it is possible to obtain an unchanged drag-coefficient by changing the sources.Herrn Professor Dr.K. Oswatitsch möchte ich für zahlreiche Ratschläge vielmals danken.