Experimental Study of the Effect of Fuel Thickness on Opposed Flow Flame Spread Over PMMA PDF Download
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Category : Electronic books
Languages : en
Pages : 69
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The research presented below intends to investigate the role of Poly methyl methacrylate (PMMA) fuel thickness on the spread rate of a downward spreading flame, the thermal radiation being emitted by the flame, and to compare results for both microgravity and normal gravity. To simplify the complex problem of flame spread over solid fuels, the concept of the thermal regime is used to find a constant spread rate for a given fuel thickness. In the thermal regime the opposed flow velocity is high enough to neglect losses due to radiation from the flame but still small enough to not affect the flame through finite rate kinetics. The microgravity results were performed on the International Space Station in the Bass-II Microgravity Science Glove box. This 7.62 cm square duct allows the variation of opposed flow velocity while holding pressure, oxygen and nitrogen constant during each run. The runs are recorded using a digital video camera for spread rate analysis and thermal radiation is read using a radiometer. For normal gravity, SDSU's Flame Stabilizer was used to acquire the downward spread rate from video analysis and thermal radiation is read by a radiometer developed here at SDSU. With the use of a Matlab image analysis code, the videos are analyzed to obtain the spread rate for each fuel thickness. When compared, these results show good experimental agreement for spread rate and thermal radiation. These results, along with known thermodynamic properties and scaling analysis are used to refine the de Ris-Delichatsios formula for the thermal regime. With very few examples of the de Ris-Delichatsios formula being matched to experimental results it is hard to define where the thin regime ends and where the thick regime starts. The refined formula is applied to both the thin and thick regimes to show approximately where the transition lies between the two and compared to experimental results. This transition zone in both microgravity and normal gravity is of great interest for researchers trying to predict the behavior of flame spread both here on earth and in space aboard the International Space Station.
Author:
Publisher:
ISBN:
Category : Electronic books
Languages : en
Pages : 69
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Book Description
The research presented below intends to investigate the role of Poly methyl methacrylate (PMMA) fuel thickness on the spread rate of a downward spreading flame, the thermal radiation being emitted by the flame, and to compare results for both microgravity and normal gravity. To simplify the complex problem of flame spread over solid fuels, the concept of the thermal regime is used to find a constant spread rate for a given fuel thickness. In the thermal regime the opposed flow velocity is high enough to neglect losses due to radiation from the flame but still small enough to not affect the flame through finite rate kinetics. The microgravity results were performed on the International Space Station in the Bass-II Microgravity Science Glove box. This 7.62 cm square duct allows the variation of opposed flow velocity while holding pressure, oxygen and nitrogen constant during each run. The runs are recorded using a digital video camera for spread rate analysis and thermal radiation is read using a radiometer. For normal gravity, SDSU's Flame Stabilizer was used to acquire the downward spread rate from video analysis and thermal radiation is read by a radiometer developed here at SDSU. With the use of a Matlab image analysis code, the videos are analyzed to obtain the spread rate for each fuel thickness. When compared, these results show good experimental agreement for spread rate and thermal radiation. These results, along with known thermodynamic properties and scaling analysis are used to refine the de Ris-Delichatsios formula for the thermal regime. With very few examples of the de Ris-Delichatsios formula being matched to experimental results it is hard to define where the thin regime ends and where the thick regime starts. The refined formula is applied to both the thin and thick regimes to show approximately where the transition lies between the two and compared to experimental results. This transition zone in both microgravity and normal gravity is of great interest for researchers trying to predict the behavior of flame spread both here on earth and in space aboard the International Space Station.
Author: Sarzina Hossain
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages : 203
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Book Description
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:
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ISBN:
Category : Electronic books
Languages : en
Pages : 70
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The purpose of this research is to investigate how oxygen concentration, opposed flow velocity and thickness of a thin PMMA fuel affect the flame spread rate and flame extinction in microgravity. The flame spread rate increases with an increase in oxygen concentration. The critical oxygen level, which is the minimum concentration for a flame to spread, is inversely related to the fuel thickness. For fuel thickness above and below a critical thickness, the flame spread rate increases and decreases with a decrease in fuel thickness, respectively. Also, an unexpected extinction is discovered. The critical fuel thickness is inversely related to the opposed flow velocity. The flame spread rate decreases when the opposed flow velocity decreases. Unexpected extinction is discovered when oxygen level is low and opposed flow is absent or weak. The simulation results are consistent with the available experimental results obtained by NASA. For a quiescent environment in microgravity, the critical oxygen level increases with the fuel thickness while the critical oxygen level decreases with the fuel thickness for environments with an opposed flow. The research on how a flame extinguishes reveals that the flame temperature in the anomaly region is lower than the flame temperature in the normal region. A flame extinguishes when the percentage surface radiation loss, which is the ratio of the surface radiation loss to heat generated from combustion, is higher than 45% with an opposed flow and 48% in quiescent environment.
Author: Hai-Tien Loh
Publisher:
ISBN:
Category : Combustion
Languages : en
Pages : 147
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An experimental study has been performed of the spread of flames over the surface of thick PMMA and thin filter paper sheets in a forced gaseous flow of varied oxygen concentration moving in the direction of flame spread. It is found that the rate of spread of the PMMA pyrolysis front is time independent, linearly dependent on the gas flow velocity and approximately square power dependent on the oxygen concentration of the gas . The experimental data with thin filter paper sheets shows that the flame spread rate is independent of the flow velocity for forced flow conditions and linearly dependent on the oxygen concentration of the flow. In both experiments, it was found that the flame spread rate data can be correlated in terms of parameter deduced from heat transfer considerations only. This indicates that heat transfer from the flame to the condensed fuel is the primary mechanism controlling the spread of flame. Finite rate chemical kinetic effects have apparently a small influence on the flame spread process itself. Analytical and numerical methods were also employed to study theoretically the name spread process over thermally thick fuel and the influence on the flow field behavior in the presence of a flame. It is found that an analytical model based on a quasi-steady analysis and the flame sheet approximation predicts a square power law dependence of the flame spread rate on the flow oxygen concentration and a linear dependence on the flow velocity. The correct and encouraging qualitative descriptions of the flow structure and surface fluxes in the region downstream from the pyrolysis front.
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ISBN:
Category : Electronic books
Languages : en
Pages : 64
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Research on flame spread and mass burning rate is relevant not only for the improvement our understanding of fire but also for the control of unwanted fires. In flame spread studies, fuel samples are generally burnt and the leading edge is tracked by the researcher to determine how far the flame propagates over the duration of the experiment. In burning rate studies, regression of the surface and mass loss is quantitatively measured and recorded to be related to burning rate with constants associated with the field. Spread rate is generally not considered in burning rate experiments, however there are few studies that correlate the two. This thesis proposes a simplified relationship between the laminar flame spread rate and the burning rate. For this thesis, downwards spread flame experiments are performed in ambient conditions for a variety of thicknesses of flat PMMA. In one geometry, ceramic plates are used to minimize heat losses to the sides and permit buoyancy driven opposed flow of ambient gases to two open faces. In the second geometry, samples are burnt while all four sides are exposed to the ambient conditions while the bottom is pinched by ceramic plates to elevate the sample and prevent the entrainment of air caused from being held too close to the floor. Experiments are analyzed using a MATLAB tool called the Flame Analyzer, and compared to previous spread rate experiments. A conservation of mass analysis is performed to relate the spread rate and the burn angle, defined as half of the angle subtended by the thickness length from the tip of the pyrolysis region, to the burning rate. The burning rate, spread rate, and burn angle vary inversely as thickness increases approaching an asymptotic limit. This trend persists in open and closed geometries though the thick limit changes between the two. A brief study with cylindrical samples confirms the same trends as the closed geometry. Finally, a study with a forced counter flow is discussed, although only thin fuel is studied in this configuration.
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:
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ISBN:
Category :
Languages : en
Pages : 62
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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: Elmahadi A. Abulbaida
Publisher:
ISBN:
Category :
Languages : en
Pages : 170
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ABSTRACT: The purpose of this thesis is to experimentally study the downward flame spread rate and the angle of pyrolysis with different shapes of PMMA at different oxygen concentrations. The effect of sample shape is considered. The effect of oxygen concentration on flame spread rate and the angle of pyrolysis were studied. A modified Critical Oxygen Index Apparatus was used to perform these experiments. The samples of PMMA were burned at different oxygen concentrations. A camera was used to capture the images of the burning and ImageJ software was also used to analyze the images. The result of this work shows that the shape of the sample plays an important role in burning time, where the fuel with a large surface area takes a longer time to burn. It also showed that the velocity increases as the oxygen concentration increases, while the angle of pyrolysis decreases as the oxygen concentration increases for all of the sample shapes tested.
Author: Maria R. Hamilton
Publisher:
ISBN:
Category : Flame spread
Languages : en
Pages : 178
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Author:
Publisher:
ISBN:
Category : Combustion
Languages : en
Pages : 546
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