An Embedded Upward Flame Spread Model Using 2D Direct Numerial Simulations℗ PDF Download
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Author: Wei Xie
Publisher:
ISBN:
Category :
Languages : en
Pages : 111
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Book Description
A fully coupled 2D fluid-solid direct numerical simulation (DNS) approach is developed to simulate fluid-solid heat and mass transfer processes using Cartesian grids. The solid geometry is identified using level set based embedded interface method. The flow field is described by the 2D Navier Stokes equations using a vorticity-streamfunction approach. First a fluid-solid coupling formulation for the thermal and momentum fields is developed that is robust, computationally efficient and second-order accurate. Solutions for several example problems are presented for flow over stationary and moving cylinders to bench mark the current approach. Heat transfer for an isolated cylinder and two cylinders in series are then examined to explore the Nusselt number dependence on cylinder spacing and unsteady conjugate heat transfer processes. Secondly, the methodology is extended to simulate flame spread over poly(methyl methacrylate) (PMMA) at different angles of inclination. Comparison of simulations and experimental easurements are conducted for flame spread rates. Results show that the heat flux to the preheating region varies considerably in time - contradicting often employed assumptions used in established flame spread theories. Accounting for the time dependent behaviour is essential in accurate predictions of flame spread, however, a universal characterization in terms of easily defined parameters is not found. Alternatively, a reaction progress variable based embedded flame model is developed using mixture fraction, total enthalpy and surface temperature. State maps of the gas-phase properties and surface heat flux are constructed and stored in pre-computed lookup tables. The resulting model provides a computationally efficient and a local formulation to determine the flame heat flux to the surface resulting in excellent agreement to DNS and experiments for predictions of flame spread rate and position of the pyrolysis front.
Author: Wei Xie
Publisher:
ISBN:
Category :
Languages : en
Pages : 111
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Book Description
A fully coupled 2D fluid-solid direct numerical simulation (DNS) approach is developed to simulate fluid-solid heat and mass transfer processes using Cartesian grids. The solid geometry is identified using level set based embedded interface method. The flow field is described by the 2D Navier Stokes equations using a vorticity-streamfunction approach. First a fluid-solid coupling formulation for the thermal and momentum fields is developed that is robust, computationally efficient and second-order accurate. Solutions for several example problems are presented for flow over stationary and moving cylinders to bench mark the current approach. Heat transfer for an isolated cylinder and two cylinders in series are then examined to explore the Nusselt number dependence on cylinder spacing and unsteady conjugate heat transfer processes. Secondly, the methodology is extended to simulate flame spread over poly(methyl methacrylate) (PMMA) at different angles of inclination. Comparison of simulations and experimental easurements are conducted for flame spread rates. Results show that the heat flux to the preheating region varies considerably in time - contradicting often employed assumptions used in established flame spread theories. Accounting for the time dependent behaviour is essential in accurate predictions of flame spread, however, a universal characterization in terms of easily defined parameters is not found. Alternatively, a reaction progress variable based embedded flame model is developed using mixture fraction, total enthalpy and surface temperature. State maps of the gas-phase properties and surface heat flux are constructed and stored in pre-computed lookup tables. The resulting model provides a computationally efficient and a local formulation to determine the flame heat flux to the surface resulting in excellent agreement to DNS and experiments for predictions of flame spread rate and position of the pyrolysis front.
Author: Erik James Stalcup
Publisher:
ISBN:
Category : Aerospace engineering
Languages : en
Pages : 171
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Book Description
Flame spread over solid fuels with simple geometries has been extensively studied in the past, but few have investigated the effects of complex fuel geometry. This study uses numerical modeling to analyze the flame spread and burning of wavy (corrugated) thin solids and the effect of varying the wave amplitude. Sensitivity to gas phase chemical kinetics is also analyzed. Fire Dynamics Simulator is utilized for modeling. The simulations are two-dimensional Direct Numerical Simulations including finite-rate combustion, first-order pyrolysis, and gray gas radiation. Changing the fuel structure configuration has a significant effect on all stages of flame spread. Corrugated samples exhibit flame shrinkage and break-up into flamelets, behavior not seen for flat samples. Increasing the corrugation amplitude increases the flame growth rate, decreases the burnout rate, and can suppress flamelet propagation after shrinkage. Faster kinetics result in slightly faster growth and more surviving flamelets. These results qualitatively agreement with experiments.
Author: Kazunori Harada
Publisher: Springer
ISBN: 9811003769
Category : Technology & Engineering
Languages : en
Pages : 891
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Book Description
This book focuses on topics in the entire spectrum of fire safety science, targeting research in fires, explosions, combustion science, heat transfer, fluid dynamics, risk analysis, structural engineering, and other subjects. The book contributes to a gain in advanced scientific knowledge and presents or advances new ideas in all topics in fire safety science. Two decades ago, the 1st Asia-Oceania Symposium on Fire Science and Technology was held in Hefei, China. Since then, the Asia-Oceania Symposia have grown in size and quality. This book, reflecting that growth, helps readers to understand fire safety technology, design, and methodology in diverse areas including historical buildings, photovoltaic panels, batteries, and electric vehicles.
Author: Eric Michael Doran
Publisher: Stanford University
ISBN:
Category :
Languages : en
Pages : 162
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Book Description
This work develops a computational framework for modeling turbulent combustion in multi-feed systems that can be applied to internal combustion engines with multiple injections. In the first part of this work, the laminar flamelet equations are extended to two dimensions to enable the representation of a three-feed system that can be characterized by two mixture fractions. A coupling between the resulting equations and the turbulent flow field that enables the use of this method in unsteady simulations is then introduced. Models are developed to describe the scalar dissipation rates of each mixture fraction, which are the parameters that determine the influence of turbulent mixing on the flame structure. Furthermore, a new understanding of the function of the joint dissipation rate of both mixture fractions is discussed. Next, the extended flamelet equations are validated using Direct Numerical Simulations (DNS) of multi-stream ignition that employ detailed finite-rate chemistry. The results demonstrate that the ignition of the overall mixture is influenced by heat and mass transfer between the fuel streams and that this interaction is manifested as a front propagation in two-dimensional mixture fraction space. The flamelet model is shown to capture this behavior well and is therefore able to accurately describe the ignition process of each mixture. To provide closure between the flamelet chemistry and the turbulent flow field, information about the joint statistics of the two mixture fractions is required. An investigation of the joint probability density function (PDF) was carried out using DNS of two scalars mixing in stationary isotropic turbulence. It was found that available models for the joint PDF lack the ability to conserve all second-order moments necessary for an adequate description of the mixing field. A new five parameter bivariate beta distribution was therefore developed and shown to describe the joint PDF more accurately throughout the entire mixing time and for a wide range of initial conditions. Finally, the proposed model framework is applied in the simulation of a split-injection diesel engine and compared with experimental results. A range of operating points and different injection strategies are investigated. Comparisons with the experimental pressure traces show that the model is able to predict the ignition delay of each injection and the overall combustion process with good accuracy. These results indicate that the model is applicable to the range of regimes found in diesel combustion.
Author: Songwei Zhang
Publisher:
ISBN:
Category :
Languages : en
Pages : 292
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Book Description
Author: Zhenghua Yan
Publisher:
ISBN:
Category : Combustion
Languages : en
Pages : 60
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Book Description
Theoretical models have been developed to address several important aspects of numerical modeling of turbulent combustion and flame spread. The developed models include a pyrolysis model for charring and non-charring solid materials, a fast narrow band radiation property evaluation model (FASTNB) and a turbulence model for buoyant flow and flame. In the pyrolysis model, a completely new algorithm has been proposed, where a moving dual mesh concept was developed and implemented. With this new concept, it provides proper spatial resolution for both temperature and density and automatically considers the regression of the surface of the non-charring solid material during its pyrolysis. It is simple, very efficient and applicable to both charring and non-charring materials. FASTNB speeds up significantly the evaluation of narrow band spectral radiation properties and thus provides a potential of applying narrow band model in numerical simulations of practical turbulent combustion. The turbulence model was developed to improve the consideration of buoyancy effect on turbulence and turbulent transport. It was found to be simple, promising and numerically stable. It has been tested against both plane and axisymmetric thermal plumes and an axisymmetric buoyant diffusion flame. When compared with the widely used standard buoyancy-modified k-e model, it gives significant improvement on numerical results. These developed models have been fully incorporated into CFD (Computational Fluid Dynamics) code and coupled with other CFD sub-models, including the DT (Discrete Transfer) radiation model, EDC (Eddy Dissipation Concept) combustion model, flamelet combustion model, various soot models and transpired wall function. Comprehensive numerical simulations have been carried out to study soot formation and oxidation in turbulent buoyant diffusion flames, flame heat transfer and flame spread in fires. The gas temperature and velocity, soot volume fraction, wall surface temperature, char depth, radiation and convection heat fluxes, and heat release rate were calculated and compared with experimental measurements. In addition to provide comprehensive data for comparison, experiments on room corner fire growth were undertaken, where the gas temperature, solid fuel surface temperature, radiative heat flux, char depth and heat release rate were all measured.
Author: Chunsang Yoo
Publisher:
ISBN:
Category :
Languages : en
Pages : 352
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Book Description
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 62
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Book Description
The purpose of this thesis is to simulate the downward flame spread over thin fuel (Cellulose and Polymethylmethacrylate) in a natural convection environment. Flame spread over thermally thin fuels in quiescent and opposed-flow environment condition is studied. The study of the flame geometry, size of domain, grid points in x and y directions and boundary conditions are considered. For PMMA fuel comparison of the computational and experimental result for quiescent environment is performed. Effect of fuel half thickness, opposed flow velocity, ambient oxygen concentration and ambient pressure level on the flame spread rate was studied. Comparison of flame spread rate of complete combustion model, equilibrium model and experiments with different half thicknesses for PMMA and cellulose was performed. For cellulose fuel velocity fields and pressure field plots are plotted to understand the flow behavior near the leading edge of the flame. Two dimensional Navier-Stokes equations were implemented in a FORTRAN code which was used for numerical simulation and later on the code is modified. A Matlab code is implemented for plotting the pressure field, temperature field, reaction rate contours, fuel mass fraction and other kind of plots.
Author:
Publisher:
ISBN:
Category : Mechanics, Applied
Languages : en
Pages : 348
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Book Description
Author: Eric Michael Doran
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
This work develops a computational framework for modeling turbulent combustion in multi-feed systems that can be applied to internal combustion engines with multiple injections. In the first part of this work, the laminar flamelet equations are extended to two dimensions to enable the representation of a three-feed system that can be characterized by two mixture fractions. A coupling between the resulting equations and the turbulent flow field that enables the use of this method in unsteady simulations is then introduced. Models are developed to describe the scalar dissipation rates of each mixture fraction, which are the parameters that determine the influence of turbulent mixing on the flame structure. Furthermore, a new understanding of the function of the joint dissipation rate of both mixture fractions is discussed. Next, the extended flamelet equations are validated using Direct Numerical Simulations (DNS) of multi-stream ignition that employ detailed finite-rate chemistry. The results demonstrate that the ignition of the overall mixture is influenced by heat and mass transfer between the fuel streams and that this interaction is manifested as a front propagation in two-dimensional mixture fraction space. The flamelet model is shown to capture this behavior well and is therefore able to accurately describe the ignition process of each mixture. To provide closure between the flamelet chemistry and the turbulent flow field, information about the joint statistics of the two mixture fractions is required. An investigation of the joint probability density function (PDF) was carried out using DNS of two scalars mixing in stationary isotropic turbulence. It was found that available models for the joint PDF lack the ability to conserve all second-order moments necessary for an adequate description of the mixing field. A new five parameter bivariate beta distribution was therefore developed and shown to describe the joint PDF more accurately throughout the entire mixing time and for a wide range of initial conditions. Finally, the proposed model framework is applied in the simulation of a split-injection diesel engine and compared with experimental results. A range of operating points and different injection strategies are investigated. Comparisons with the experimental pressure traces show that the model is able to predict the ignition delay of each injection and the overall combustion process with good accuracy. These results indicate that the model is applicable to the range of regimes found in diesel combustion.
