Application of Validated Radiation Model in Flame Spread Rate Over Solid Fuels PDF Download
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Category : Electronic books
Languages : en
Pages : 79
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In this thesis the radiative effects of opposed flow flames spreading over solid fuels are discussed as well as the coupling of a radiation and CFD program. The coupled programs are used to show the radiative heat transfer mechanisms and how they affect the flame globally. A radiation program is used to calculate radiation properties of the flame such as the heat flux distribution, net heat flow, and mean Plank absorptivity constant for a particular flame. The radiation program imports the temperature fields from a CFD program. Trends in the mean Plank absorptivity constant with varying ambient conditions are analyzed and an application of the radiation program to simulate a physical radiometer is demonstrated for a test case. The CFD program can import radiation results to help improve the accuracy of the simulation. A script was written to automate the update process to produce more accurate results for flame simulations. Flux distributions, stability and relative error are analyzed to show the coupled programs are producing results within an acceptable error. Trends in error and stability are discussed and stable regions with low enough error are determined. The coupled programs are used to gather data on flame spread rate and find differences in flame structure and properties of neglecting certain radiation mechanisms. No radiation included produced the hottest fastest moving flame, while no gas to surface radiation produced the coolest flame. Including the gas to surface radiation produced a slightly hotter faster moving flame. This trend was studied across different opposed flow velocities and sample widths. The radiative heat fluxes are analyzed for the cases as well. All the flame simulations in this thesis were run for a microgravity, 21% oxygen, and PMMA fuel.
Author:
Publisher:
ISBN:
Category : Electronic books
Languages : en
Pages : 79
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Book Description
In this thesis the radiative effects of opposed flow flames spreading over solid fuels are discussed as well as the coupling of a radiation and CFD program. The coupled programs are used to show the radiative heat transfer mechanisms and how they affect the flame globally. A radiation program is used to calculate radiation properties of the flame such as the heat flux distribution, net heat flow, and mean Plank absorptivity constant for a particular flame. The radiation program imports the temperature fields from a CFD program. Trends in the mean Plank absorptivity constant with varying ambient conditions are analyzed and an application of the radiation program to simulate a physical radiometer is demonstrated for a test case. The CFD program can import radiation results to help improve the accuracy of the simulation. A script was written to automate the update process to produce more accurate results for flame simulations. Flux distributions, stability and relative error are analyzed to show the coupled programs are producing results within an acceptable error. Trends in error and stability are discussed and stable regions with low enough error are determined. The coupled programs are used to gather data on flame spread rate and find differences in flame structure and properties of neglecting certain radiation mechanisms. No radiation included produced the hottest fastest moving flame, while no gas to surface radiation produced the coolest flame. Including the gas to surface radiation produced a slightly hotter faster moving flame. This trend was studied across different opposed flow velocities and sample widths. The radiative heat fluxes are analyzed for the cases as well. All the flame simulations in this thesis were run for a microgravity, 21% oxygen, and PMMA fuel.
Author: Linton Kaneki Honda
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ISBN:
Category :
Languages : en
Pages : 300
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Author:
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ISBN:
Category : Electronic books
Languages : en
Pages : 86
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The purpose of this thesis presented is to analyze flame spread over thermally thin solid fuels in three regimes of flame spread process; radiative, thermal, and kinetic regimes. The analyses have been performed using a comprehensive two dimensional computational fluid dynamics (CFD) model written in Fortran language developed by Bhattacharjee. Flame spread over thermally thin fuels in quiescent and opposing flow microgravity environments is investigated. An extinction study is performed with different computational domain sizes for a set of fuel thicknesses to understand the effect of domain size on the extinction velocities in the radiative and kinetic regimes. The effect of development length boundary layer is studied in both radiative and kinetic regimes. It is found that flame spread rate, flame size, flame temperature, blow-off and radiative extinction velocities depend on the development length and the boundary layer created by the opposing flow. A correlation between the extinction development length and opposed flow velocity is established. Flame spread over open cell phenolic foam is investigated in detail in a quiescent microgravity environment. The critical fuel thickness is found at different oxygen concentrations and compared to those for PMMA. Pressure, oxygen concentration, and radiation studies are also performed to analyze the flame spread over foam. To understand the effect of radiation on flame spread, the CFD model is coupled with two different radiation models in a microgravity environment. The first radiation model includes gas to surface conduction, gas to environment radiation loss, gas to surface feedback radiation, and surface to environment radiation loss. The second model only excludes gas to surface radiation feedback. The results obtained using these two models are compared with the CFD results; one with radiation completely neglected, and one with only gas to surface radiation feedback neglected. Flame spread in downward configuration is also studied using the radiation models in a quiescent normal gravity environment. The radiation effects, fuel width effect, and kinetic effects are analyzed for different fuel thicknesses.
Author: Matthew D. King
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ISBN:
Category : Combustion
Languages : en
Pages : 340
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This dissertation is an investigation into the effects of natural convection on the combustion process of a spreading flame in a gravitational environment. The flame is spreading into an opposing flow of oxidizer over a solid fuel. This is approached as a steady state problem with coordinates fixed at the tip of the flame. This investigation incorporates the use of experimental data, numerical simulations and a simplified approach to develop a better understanding of combustion. The focus of the material presented can be separated in two components: First, a well validated forced flow numerical model is used to evaluate flame structure for the natural convection configuration. A simplified approach is developed and compared to the numerical model for flame structure and flame spread rates in chapters 2 and 3. Critical parameters controlling flame spread such as pressure, fuel thickness, oxygen concentration, and strength of gravitational field are widely varied. In the thermal regime, where this simplified approach applies, comparisons between experimental data, numerical solutions and simplified approach predictions are excellent. The numerical model is also compared to experimental data outside the thermal regime including a prediction of the regression rate of the solid fuel and gas phase characteristics. Second, a hybrid two-color pyrometry technique is developed and used to analyze flame structure for experiments in a microgravity environment. Images of flame intensity are calibrated and converted into temperature profiles for various opposed flow velocities and oxygen concentration. Numerical simulations are used to demonstrate various approximate techniques and their accuracies. The experimental images are used in conjunction with the numerical simulation to determine the temperature profiles and the partial pressure of carbon dioxide. Techniques are discussed on how to improve the results for future experiments by modifying the filter bandwidth selections. Through a greater understanding of the physics and controlling mechanisms for flame spread, the ability to control fire and the establishment of comprehensive guidelines for fire safety will be realized. This dissertation is another step toward that goal.
Author: Jennifer L. Rhatigan
Publisher:
ISBN:
Category : Dynamics
Languages : en
Pages : 158
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This is the first attempt to analyze both radiation and detailed kinetics on the burning and extinction of a solid fuel in a stagnation-point diffusion flame. We present a detailed and comparatively accurate computational model of a solid fuel flame along with a quantitative study of the kinetics mechanism, radiation interactions, and the extinction limits of the flame. A detailed kinetics model for the burning of solid trioxane (a trimer of formaldehyde) is coupled with a narrowband radiation model, with carbon dioxide, carbon monoxide, and water vapor as the gas-phase participating media. The solution of the solid trioxane diffusion flame over the flammable regime is presented in some detail, as this is the first solution of a heterogeneous trioxane flame. We identify high-temperature and low-temperature reaction paths for the heterogeneous trioxane flame. We then compare the adiabatic solution to solutions that include surface radiation only and gas-phase and surface radiation using surface model.
Author: Makoto Endo
Publisher:
ISBN:
Category : Aerospace engineering
Languages : en
Pages : 0
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Flame spread over solid fuel presents distinctive characteristics in reduced gravity, especially when the forced flow velocity is low. The lack of buoyancy allows a blue, dim flame to sustain where the induced velocity would otherwise blow it off. At such low velocities, a quenching limit exists where the soot content is low and the effect of radiative heat loss becomes important. The objective of this study is to establish a high fidelity numerical model to simulate the growth and extinction of flame on solid fuels in a reduced gravity environment. The great importance of the spectral dependency of the gas phase absorption and emission were discovered through the model development and therefore, Statistical Narrow-Band Correlated-k (SNB-CK) spectral model was implemented. The model is applied to an experimental configuration from the recent space experiment, Burning And Suppression of Solids (BASS) project conducted aboard the International Space Station. A poly(methyl methacrylate) (PMMA) sphere (initial diameter of 2cm) was placed in a small wind tunnel (7.6cm x 7.6cm x 17cm) within the Microgravity Science Glovebox where flow speed and oxygen concentration were varied. Data analysis of the BASS experiment is also an important aspect of this research, especially because this is the first space experiment that used thermally thick spherical samples. In addition to the parameters influencing the flammability of thin solids, the degree of interior heat-up becomes an important parameter for thick solids. For spherical samples, not only is the degree of internal heating constantly changing, but also the existence of stagnation point, shoulder, and wake regions resulting in a different local flow pattern, hence a different flame-solid interaction. Parametric studies using the numerical model were performed against (1) chemical reaction parameters, (2) forced flow velocity, (3) oxygen concentration and (4) amount of preheating (bulk temperature of the solid fuel). Flame Spread Rate (FSR) was used to evaluate the transient effect and maximum flame temperature, standoff distance and radiative loss ratio were used to evaluate the spontaneous response of the gas phase to understand the overall response of the burning solid fuel. After evaluating the individual effect of each parameter, the efficacy of each parameter was compared. Selected results of this research are:[1]Experimental data from BASS and numerical simulation both showed that within the time periodbetween ignition until the flame tip reaches the shoulder of the sample, the flame length and timehave almost a linear relation.[2]Decreasing forced flow velocity increases the radiative loss ratio whereas decreasing oxygen molefraction decreases the radiative loss ratio. This finding must be considered in the effort to replicatethe behavior of flame spread over thick solid fuels in microgravity on earth.[3]Although the standoff distance will increase when the forced flow velocity is decreased as well aswhen the oxygen mole fraction is decreased, the forced flow velocity has a much stronger effect onthe standoff distance than the oxygen mole fraction.[4]Unlike the previous two comparisons, the effect of forced flow velocity and oxygen mole fraction onthe maximum flame temperature was at similar level, reduction of either parameter would result inlowering the maximum flame temperature.[5]The effect of preheating on the flame spread rate becomes stronger when either the oxygen flowrate or forced flow velocity becomes larger. Depending on which element is more important, we candistinguish oxygen flow rate driven flame spread from preheating driven flame spread. Findings of this research are being utilized in the design of the upcoming space experiment, Growth and Extinction Limits of solid fuel (GEL) project. This research is supported by the National Aeronautics and Space Administration (NASA). This work made use of the High Performance Computing Resource in the Core Facility for Advanced Research Computing at Case Western Reserve University and the Ohio Supercomputer Center.
Author: Sarzina Hossain
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages : 203
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The opposed flow flame spread over flat solid fuels is of fundamental importance to the field of fire safety. Several features of opposed flow flame spread are experimentally, numerically and analytically investigated.Thermally thick slab of PolyMethylMethAcrylate (PMMA) was used to study the effects of opposed flow velocity (8-58 cm/s) and fuel thickness (6.6, 12.1 and 24.5 mm). The experiments were conducted with a Narrow Channel Apparatus (NCA) at Michigan State University (MSU). The flame spread rate results show that the maximum flame spread occurs at a lower flow velocity for relatively thicker fuel. The peak flame spread rate for 6.6 mm, 12.1 mm and 24.5 mm occurs at 18.5 cm/s, 12.1 cm/s and 10.3 cm/s, respectively. Several flame spread regimes: thermal, chemical and regressive burning are identified from the results. Flame spread regimes are usually depend on the opposed flow velocity. However, the flame spread rate for newly found regressive burning regime is independent of flow velocities. Visual observation of the flame indicates that the flame intensity augments with flow velocity for all thicknesses of PMMA. The comparison between NCA data and legacy data for similar material (PMMA) and thickness (12.1 mm) demonstrated excellent agreement, subject to the extension of the numerical and theoretical analysis to include relevant features of the flame spread stretch rate theory. The results also demonstrated the effectiveness of the stretch rate theory for markedly different experimental configurations. Although thick slab is used to perform tests, complete burn out of the samples for thickness 6.6 and 12.1 mm are observed at high opposed flow velocities (30 ℗ł 5 cm/s and higher). On contrary, the thickest sample (24.5 mm) did not go through complete burning. This indicates the nature of surface regression and its impact on flame spread rate.Based on the results, it can be emphasized that the factors controlling the flame front advancement involves both flame spread and surface regression. So, the burnt samples at different opposed flow velocities of 24.5 mm thickness from flame spread study is measured for surface regression depth experimentally. A semi-empirical correlation is developed to relate the flame spread and regression and to determine the mass loss rate from the burnt fuel surface. Mass loss rate is also a key aspect of characterizing the flammability of materials. Results show that the power law dependency of mass loss rate changes with opposed flow velocity. A comparison of power law exponents of current results and results from literature are made. Results demonstrate that the power law dependency at flow velocity 8.2, 10.3 and 12 cm/s is -0.5 which show excellent agreement with legacy work.Next, another study is conducted on the post-flame-spread 24.5 mm PMMA sample, burnt at opposed flow velocity 15 cm/s. Visual observation of post-burn sample shows the formation of significant number of internal bubbles. Three samples of similar thickness burnt at similar condition were investigated for bubble count and size. Results indicate higher and smaller bubble presence near the leading edge of the flame compared to the trailing edge side. Comparison of bubble size distribution with several distribution function demonstrates that the bubble size shows good agreement with Log-normal distribution function.Finally, the transient regression rate has been investigated analytically and numerically. The effect of external heat flux simulating flame heat flux is analyzed for PMMA considering it as an ideal-vaporizing solid. Results indicate a strong dependency of heat flux on material regression for a time duration. After a certain time period, the regression rate became insensitive to heat flux change. A scale analysis is performed to compare the analytical-numerical regression rate results with experimental surface regression depth. The predicted regression followed a similar pattern as the experimental surface regression.
Author: Jeffrey S. West
Publisher:
ISBN:
Category : Combustion
Languages : en
Pages : 842
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A detailed numerical model of opposed-flow flame spread over solid fuels is developed. The model is used to study flame spread in three regimes of flame spread; the Thermal, Chemical Kinetic and Near Quiescent Regimes. Simplifying assumptions that have been historically applied to this problem are investigated and their effect on the flame spread rate and flame structure are quantified in each regime. A semi-empirical flame spread formula for thermally thick fuels is developed from knowledge of the dominant simplifying assumptions in this regime. Spread rate predictions compare well to experimental and computed results. This semi-empirical model provides field variables which previous theories are unable to predict. Mechanisms of heat transfer ahead of the flame are studied in each regime. Forward heat transfer though the solid fuel becomes more important in the Chemical Kinetic and Near Quiescent Regimes, a previously unknown result. The rate and path of forward heat transfer is found to depend strongly on simplifying assumptions and the flame anchor location. These results explain the relationship between previous analytical and experimental forward heat transfer results. A dimensionless criterion predicting the fuel thickness at which transition from thermally thick to thermally thin is developed which compares well with experimental and computed results. Finite-rate gas-phase chemical kinetics are found to be the cause of the super-thin regime of flame spread. A formula for the limiting flame spread rate in this regime is developed. Correlation of computed spread rates with the Damkohler number is revisited. Uncertainty in residence time due to uncertainties in characteristic velocity and gas-phase properties is found to be the cause of spread in the correlation. The Damkohler number alone explains variations in many parameters although it alone cannot explain changes in gas-phase activation energy. The boundary between the Near Quiescent and Thermal Regime is quantified using a dimensionless radiation number. A new extinction limit for thick fuels in the Near Quiescent Regime is discovered. Radiative losses cause the flame to grow small and spread so slowly that sufficient oxygen is not available to sustain the flame. Recent experimental results confirm this conclusion.
Author: Ching-Biau Jiang
Publisher:
ISBN:
Category : Flame spread
Languages : en
Pages : 312
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Author: Jeanhyuk Park
Publisher:
ISBN:
Category : Flame spread
Languages : en
Pages : 131
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Book Description
Building fire, Forrest fire, and warehouse compartment fire are some of the most frequently occurring practical fire hazards in modern world. Although these types of hazards seem irrelevant from one another, they have some things in common from the perspective of fire protection engineering, in that they all have a very similar fundamental fuel-gap configuration, or discrete fuel configuration. There has been some studies in the past regarding the subject, yet it is not the most popular in the field. Furthermore, there is even fewer, if not any, numerical analysis done to fires in discrete fuel configuration. Discrete fuel arrangements represent some practical fire hazard situations, such as compartment fires in enclosed vehicles. In this study, an unsteady two-dimensional numerical model (Fire Dynamics Simulator) was used to simulate concurrent flame spreadover paper-like thin solid fuels in discrete configurations in microgravity (0g, where a20cm/s flow is imposed) and in normal gravity (1g). An array of ten 1cm-long fuel segments is uniformly distributed in the flow direction (0g) or in the vertical direction (1g).A hot spot ignition source is applied at the upstream leading edge of the first fuel seg-ment. The separation distance between the fuel segments is a parameter in this study, ranging from 0 (corresponding to a continuous fuel) to 3cm. Using this setup, the spread rate of the flame base and the fuel burning rate were studied. The spread rate in 1g and 0g increases with increasing separation distance. This is due to the gaps in the discrete fuel that force the flame base to jump to the subsequent fuel segment when the upstream segment burns out. On the other hand, the fuel burning rate behaves differently in 1g versus 0g. At a flow velocity of 20 cm/s in 0g, the flame reaches a limiting length and the flame length is approximately the same ( 4cm) for all fuel configurations. Therefore, as the separation distance increases, the preheating length (the fuel area exposed to the flame) decreases, resulting in a smaller burning rate. In 1g, the buoyancy driven flow accelerates as it rises, resulting in a longer flame as the separation distance increases. In all simulated configurations, the flame extends to the last fuel segment before the first fuel segment burns out and the flame spans the entire set of fuel segments. However, flame standoff distance reduces at the gaps between fuel segments, and in some con-figurations, the flame breaks into multiple flamelets. The shorter standoff distance and intense burning at each flamelet base result in a larger total burning rate as the separation distance increases.
