Numerical Investigation into the effects of obstacles on heavy gas dispersions in the atomsphere

Computational fluid dynamics (CFD) approach is applied to investigate heavy gas dispersion in the atmosphere, under the action of wind. Because of the effect of buoyancy, steady double peaks of the heavy gas concentrations in the downstream area are observed from the numerical results. The double peaks of the concentrations are a special pattern of heavy gas dispersion, which cannot be found in the neutral gas dispersions. Four types of obstacles are placed behind the leakage source to study the influences of these obstacles to the heavy gas dispersions. The numerical results show the detailed shapes and other contents of the heavy clouds under the obstacles

A finite element dynamical nonlinear subscale approximation for the low Mach number flow equations

In this work we propose a variational multiscale finite element approximation of thermally coupled low speed flows. The physical model is described by the low Mach number equations, which are obtained as a limit of the compressible Navier–Stokes equations in the small Mach number regime. In contrast to the commonly used Boussinesq approximation, this model permits to take volumetric deformation into account. Although the former is more general than the latter, both systems have similar mathematical structure and their numerical approximation can suffer from the same type of instabilities. We propose a stabilized finite element approximation based on the variational multiscale method, in which a decomposition of the approximating space into a coarse scale resolvable part and a fine scale subgrid part is performed. Modeling the subscale and taking its effect on the coarse scale problem into account results in a stable formulation. The quality of the final approximation (accuracy, efficiency) depends on the particular model. The distinctive features of our approach are to consider the subscales as transient and to keep the scale splitting in all the nonlinear terms. The first ingredient permits to obtain an improved time discretization scheme (higher accuracy, better stability, no restrictions on the time step size). The second ingredient permits to prove global conservation properties. It also allows us to approach the problem of dealing with thermal turbulence from a strictly numerical point of view. Numerical tests show that nonlinear and dynamic subscales give more accurate solutions than classical stabilized methods

A combined Finite Volumes -Finite Elements method for a low-Mach model

International audienceIn this paper, we develop a combined Finite Volumes - Finite Elements method based on a time splitting to simulate some low-Mach flows. The mass conservation equation is solved by a Vertex-Based Finite Volume scheme using a $\tau$-limiter. The momentum equation associated with the compressibility constraint is solved by a Finite Element projection scheme. The originality of the approach is twofold. First, the state equation linking the temperature, the density and the thermodynamic pressure is imposed implicitly. Second, the proposed combined scheme preserves the constant states, in the same way as a similar one previously developed for the variable density Navier-Stokes system. Some numerical tests are performed to exhibit the efficiency of the scheme. On the one hand, academic tests illustrate the ability of the scheme in term of convergence rates in time and space. On the other hand, our results are compared to some of the literature by simulating a transient injection flow as well as a natural convection flow in a cavity

An implicit finite element solution of thermal flows at low Mach number

Thermal flows at low Mach numbers are a basic problem in combustion, environmental pollution prediction and atmospheric physics areas. Most of the existing schemes for solving this problem treat convection explicitly, which confines time step width due to the CFL condition. In this paper, based on the pseudo residual-free bubble approach [F. Brezzi, L.P. Franca, T.J.R. Hughes, A. Russo, b=∫g, Methods Appl. Mech. Eng. 145 (1997) 329–339; T.J.R. Hughes, Multiscale phenomena: Green’s functions, the Dirichlet-to-Neumann formulation, subgrid scale models, bubbles and the origins of stabilised methods, Method. Appl. Mech. Eng. 127 (1995) 387–401], we introduce an implicit finite element scheme for the thermal flow problem. We firstly give a low Mach number asymptotics of compressible Navier–Stokes equations for the thermal flows and then derive the numerical scheme for them in detail. Three representative case studies are used to investigate and to test the numerical performances of the proposed scheme

A numerical investigation into natural ventilation of double skin façades and the improvement of energy efficiency in high rise buildings

Buildings consume a large amount of energy, around 40% of global energy use. Under keeping comfortable environments for building occupants, reduction of buildings’ energy use is significant and also challenging. Passive techniques, such as natural ventilation, are promoted in certain climates to provide low energy cooling and ventilation. However, controlling natural ventilation in an effective manner to maintain occupant comfort can be a difficult task, particularly during warm periods. One of the passive techniques is carefully designing building façade, e.g., ‘double-skin faҫade’, one of the best options in managing the interaction between the outdoor and internal spaces. Double-skin façade (DSF) building is one of the energy conservation opportunities available through recent intelligent buildings. Not only does the façade constitute the architectural aesthetics of the building, but it is also of great importance due to its impact on energy performance and interior function. Therefore, the development of innovative façade technology continues to be one of the most active research areas for the built environment. In this work, an investigation into the optimal application of a double-skin façade (DSF) for high-rise buildings is presented using computational fluid dynamics (CFD) approaches. The work firstly reviewed state-of-the-art research, technologies and applications for double-skin façades. Based on the review, the author then proposed some new and innovative forms of double-skin faҫade which are particularly applicable to high-rise buildings. These façades offer natural ventilations for tall office buildings. The forces driving the ventilations, i.e., buoyancies, are produced from the solar energy. As CFD is applied, the effects of the wind and buoyancy are then investigated separately or in combination. The overall objectives of the investigations are to determine whether the magnitude of airflow rates and the desired flow pattern through openings can be achieved over a range of specified conditions. Potential conditions where the design goals may not be ensured are identified. It is supposed that a seasonal control could be developed to provide the optimum desired flow pattern, sufficient flow rates for ventilated cooling and uniform airflow rates across floors. Segmented and non-segmented DSF cavity patterns with ventilated double façades are adopted as the main building configurations for coping with the potential magnitude of wind at high levels. The ducts between cavities are designed to control the natural ventilations in tall office buildings. Steady state condition approaches are adopted for investigating these cases. The results show that segmentation has tends to create relatively uniform air pressure, airflow and temperature at various elevations within the building, and therefore has the best performance. In order to quantitatively assess the performance of the proposed double-skin faҫades, various CFD models were developed. These models are involved in turbulence calculations with kappa-epsilon model heat transfer. Various validations of the CFD models show that the models are able to produce precise results. Ultimately, the CFD, CFX5 codes were applied to estimate and investigate the performance of the proposed DSFs and produce the optimal application of double-skin façades for high-rise buildings

Investigation into the Mechanisms and Consequences of Explosions of Premixed Gaseous Combustibles with Detailed Chemical Kinetics

Detonation is a self-sustaining combustion wave with a rapid reaction process and a propagation speed. It is a central topic in combustion and serves a significant role in the theory and the application of combustion. The volatility of petroleum products and crude oil in the downstream and upstream sectors of the oil and gas industry constitutes a high degree of fire explosion risks and disasters thereby leading to losses of over 528 lives, more than 1,289 persons injured, over 1,280 nearby homes burnt, and numerous workshops destroyed, large quantities of barrels of crude oil spilled into the environment and billion dollar projects burnt down. The aim of this project is to explore the effects and influences of chemical kinetics and geometric configurations to the wave behaviours using computational fluid dynamics (CFD) approach. Mathematical models and numerical methods will be employed in solving the problems in this research work. This project report focused on numerical investigation of indirect initiation of detonation using direct numerical simulations (DNS). In this simulation, the chemical combustion reactions are ignited in a shock tube and then the processes of transition of deflagration to detonation (DDT) were explored. The DNS database provides a source to investigate the influences and effects of chemical kinetics of explosions on hydrogen-oxygen and propane oxygen combustion reaction processes were explored. For this work, the CFD programme employed is an Adaptive Mesh Refinement in object-oriented C++ (AMROC) tools which can be executed in parallel processes to obtain an accurate DNS database on the chemical kinetics of the elements. From the simulation results, the influences, and the effects of chemical kinetics of explosions on hydrogen-oxygen and propane oxygen combustion reaction combustion reactions were investigated; and slow flame (called laminar flow), fast flame, DDT and Detonation data were obtained. When the concentration was low, the reaction rate was very slow, no DDT and no detonation were achieved, but when the concentration was high/large, the reaction rate was very fast, and thus DDT and detonation would be formed and consequently explosion occurred. Exploring the influence of free radical H on flame propagation, it was found that in each case study, as the concentration of the reacting species increases, the flame speed increases for each propagation for certain limited duration. The results showed that as the flame moves through more volume, more fuel is thereby being burnt and so, less free radical, H around that is being burnt. Moreover, in this research work, the influences and effects of geometric configurations on explosion of hydrogen-oxygen and propane -oxygen mixtures using numerical simulation method were equally investigated. Hence, when vent is created in the tube, DDT will occur and consequently detonation is achieved, and vent explosion took place. Moreover, for closed end tube such as that of case study with one Block, the block constituted an artificial obstacle, hence, FD, DDT, and detonation were formed, and explosion would consequently occur. Therefore, the main significance of this work showed that chemical kinetics and geometric configurations have influences and effects on explosion of hydrogen-oxygen and propane -oxygen reaction mixtures

Nonlinear subgrid finite element models for low Mach number flows coupled with radiative heat transfer

The general description of a fluid flow involves the solution of the compressible Navier-Stokes equations, a very complex problem whose mathematical structure is not well understood. It is widely accepted that these equations provide an accurate description of any problem in fluid mechanics which may present many different nonlinear physical mechanisms. Depending on the physics of the problem under consideration, different simplified models neglecting some physical mechanisms can be derived from asymptotic analysis. On the other hand, radiative heat transfer can strongly interact with convection in high temperature flows, and neglecting its effects may have significant consequences in the overall predictions. Problems as fire scenarios emphasized the need for an evaluation of the effect of radiative heat transfer. This work is directed to strongly thermally coupled low Mach number flows with radiative heat transfer. The complexity of these mathematical problem makes their numerical solution very difficult. Despite the important difference in the treatment of the incompressibility, the low Mach number equations present the same mathematical structure as the incompressible Navier-Stokes equations, in the sense that the mechanical pressure is determined from the mass conservation constraint. Consequently the same type of numerical instabilities can be found, namely, the problem of compatibility conditions between the velocity and pressure finite element spaces, and the instabilities due to convection dominated flows. These instabilities can be avoided by the use of stabilization techniques. Many stabilization techniques used nowadays are based on the variational multiscale method, in which a decomposition of the approximating space into a coarse scale resolvable part and a fine scale subgrid part is performed. The modeling of the subgrid scale and its influence leads to a modified coarse scale problem providing stability. The quality of the final approximation (accuracy, efficiency) depends on the particular model. The extension of these techniques to nonlinear and coupled problems is presented. The distinctive features of our approach are to consider the subscales as transient and to keep the scale splitting in all the nonlinear terms appearing in the finite element equations and in the subgrid scale model. The first ingredient permits to obtain an improved time discretization scheme(higher accuracy, better stability). The second ingredient permits to prove global conservation properties, being also responsible of the higher accuracy of the method. This ingredient is intimately related to the problem of thermal turbulence modeling from a strictly numerical point of view. The capability for the simulation of turbulent flows is a measure of the ability of modeling the effect of the subgrid flow structures over the coarser ones. The performance of the model in predicting the behavior of turbulent flows is demonstrated. The radiation transport equation has been also approximated within the variational multiscale framework, the design and analysis of stabilized finite element methods is presented.La descripción general del movimiento de un flujo implica la solución de las ecuaciones de Navier-Stokes compresibles, un problema de muy compleja estructura matemática. Estas ecuaciones proporcinan una descripción detallada de cualquier problema en mecánica de fluidos, que puede presentar distintos mecanismos no lineales que interactúan entre si. En función de la física del problema que se esté considerando, pueden derivarse modelos simplificados de las ecuaciones de Navier-Stokes mediante analisis dimensional, que ignoran algunos fenómenos físicos. Por otro lado, la transferencia de calor por radiación puede interactuar con el movimiento de un fluido, e ignorar sus efectos puede tener consecuencias importantes en las predicciones del flujo. Problemas donde hay fuego implican la evaluacion del efecto del calor por radiación. El presente trabajo está dirigido a flujos a bajo número de Mach térmicamente acoplados, donde el calor por radiación afecta al flujo. Debido a la complejidad del problema matemático, la solución numérica es muy complicada. A pesar de las diferencia en el tratamiento de la incompresibilidad, las ecuaciones de flujo a bajo número de Mach poseen una estructura matemática similar a la de flujo incompresible, en el sentido que la presión mecánica se determina a partir de la ecuación de conservación de la masa. En consecuencia poseen el mismo tipo de inestabilidades numéricas, que son el problema de condiciones de compatibilidad entre los espacios de elementos finitos de velocidad y presión, y las inestabilidades debidas a flujos con convección dominante. Estas inestabilidades pueden evitarse mediante técnicas de estabilización numérica. Muchos métodos de estabilización utilizados hoy día se basan en el método de multiscalas variacionales, donde el espacio funcional de la solucion se divide en un espacio discreto y resolubre y un espacio infinito de subscalas. El modelado de las subescalas y su influencia modifican el problema discreto proporcionando estabilidad. La calidad de la aproximación numérica final (precisión, eficiencia) depende del modelo particular de subescalas. En este trabajo se extienden estas técnicas de estabilización a problemas no lineales y acoplados. Las características que distinguen a nuestra aproximación son considerar las subsecalas como transitorias y mantener la división de escalas en todos los términos no lineales que aparecen en las ecuaciones de elementros finitos y en las del modelo de subescalas. La primera característica permite obtener mayor precisión y mejor estabilidad en la solución, la segunda característica permite obtener esquemas donde las propiedades se conservan globalmente, y mayor precisión del método. El hecho de mantener la división de escalas en todos los términos no lineales está intimamemte relacionado con el modelado de turbulencia en flujos térmicamente acoplados desde un punto de vista estrictamente numérico. La capacidad de simulación de flujo turbulento es una medida de la habilidad de modelar el efecto de las estructuras de escala fina sobre las estructuras de escala gruesa. Se muestra en esta tesis el desempeño del método para de predecir flujo turbulento. La ecuación de transporte de radiación también se aproxima numéricamente en el marco de multiscala variacional. El diseño y análisis de este método se presenta en detalle en esta tesi