http://hdl.handle.net/1842/1154 Sat, 25 May 2013 00:02:43 GMT 2013-05-25T00:02:43Z http://www.era.lib.ed.ac.uk:80/retrieve/16799/BRE_New Image.JPG http://hdl.handle.net/1842/1154 http://hdl.handle.net/1842/6511 Title: Uncertainty and complexity in pyrolysis modelling Authors: Bal, Nicolas Abstract: The use of numerical tools in fire safety engineering became usual nowadays and this tendency is expected to increase with the evolution of performance based design. Despite the constant development of fire modelling tools, the current state of the art is still not capable of predicting accurately solid ignition, flame spread or fire growth rate from first principles. The condensed phase, which plays an important role in these phenomena, has been a large research area since few decades, resulting in an improvement of its global understanding and in the development of numerical pyrolysis models including a large number of physical and chemical mechanisms. This growth of complexity in the models has been justified by the implicit assumption that models with a higher number of mechanisms should be more accurate. However, as direct consequence, the number of parameters required to perform a simulation increased significantly. The problem is when the uncertainty in the input parameters accumulates in the model output beyond a certain level. The global error induced by the parameters uncertainty balances the improvements obtained with the incorporation of new mechanisms, leading to the existence of an optimum of model complexity.While one of the first modelling tasks is to select the appropriate model to represent a physical phenomenon, this step is often subjective, and detailed justifications of the inclusion or exclusion of the different mechanisms are infrequent. The issue of how determining the most beneficial level of model complexity is becoming a major concern and this work presents a methodology to estimate the affordable level of complexity for polymer pyrolysis modelling prior ignition. The study is performed using PolyMethylMethAcrylate (PMMA) which is a reference material in fire dynamics due to the large number of studies available on its pyrolysis behaviour. The methodology employed is based on a combination of sensitivity and uncertainty analyses.In the first chapter, the minimum level of complexity required to explain the delay times to ignition of black PMMA samples at high heat flux levels is obtained by exploring one by one the effect on the condensed phase of several mechanisms. It is found that the experimental results cannot be explained without considering the in-depth radiation absorption mechanism. In the second chapter, a large literature review of the variability associated with the main parameters encountered in pyrolysis models is performed in order to establish the current level of confidence associated with the predictions using simple uncertainty analyses. In the third chapter, a detailed analysis of the governing parameters (parametric sensitivity) is performed on the model obtained in chapter 1 to predict the delay time to ignition. Using the ranges obtained in chapter 2 for the input parameters, a detailed uncertainty analysis is performed revealing a large spread of the numerical predictions outside the experimental uncertainty. While several parameters, including the attenuation coefficient (from the in-depth radiation absorption mechanism), present large sensitivity, only a few are responsible for the large spread observed. The parameter uncertainty is shown as the limiting step in the prediction of solid ignition. In the fourth chapter, a new methodology is developed in order to investigate the predominant mechanisms for the prediction of the transient pyrolysis behaviour of clear PMMA (no ignition). This approach, which corresponds to a mechanism sensitivity, consists of applying step-by-step assumptions to the most complex model used in the literature to model non-charring polymer pyrolysis behaviour. This study reveals the relatively high importance of the heat transfer mechanisms, including the process of in-depth radiation. In the fifth chapter, an investigation of the uncertainty related to the calibration of pyrolysis models by inverse modelling is performed using several levels of model complexity. Inverse modelling couples the experimental data to the model equations and this dependency is often ignored. Varying the model complexity, this study reveals the presence of compensation effects between the different mechanisms. The phenomenon grows in importance with model complexity leading to unrealistic values for the calibrated parameters.From the performed sensitivity and uncertainty analyses, the mechanism of in-depth absorption appeared critical for some applications. In the sixth chapter, an experimental investigation on specific conditions impacting the sensitivity of this mechanism shows its large dependency on the heat source emission wavelength when comparing the two heat sources of the most used pyrolysis test apparatuses in fire safety engineering. More fundamental investigations presented in the seventh chapter enabled to quantify this dependency that needs to be considered for modelling or experimental analyses. The impact of the heat source on the radiation absorption (depth and magnitude) is shown to be predictable thanks to the detailed measurements of the attenuation coefficient of PMMA and the emissive power of the heat sources. The global uncertainty associated with the input parameters, extracted either from independent studies or by inverse modelling, appears as a limiting step in the improvement of pyrolysis modelling when a high level of complexity is implemented. A combination of numerical (sensitivity and uncertainty) analyses and experimental studies is required before increasing the level of complexity of a pyrolysis model. Mon, 01 Oct 2012 00:00:00 GMT http://hdl.handle.net/1842/6511 2012-10-01T00:00:00Z http://hdl.handle.net/1842/6406 Title: Application of fire calorimetry to understand factors affecting flammability of cellulosic material: Pine needles, tree leaves and chipboard Authors: Jervis X, Freddy Abstract: Calorimetry, the science of measuring heat from chemical reactions and physical changes, is one to the most valuable tools fire safety engineering have at their disposal. Calorimetric devices such as the cone calorimeter and the fire propagation apparatus (FPA) give us the means to evaluate and understand how different materials burn at a small scale. Due to fire being affected by many different environmental factors, these devices help us to isolate and examine how each factor affects fire as a whole and be able to apply this knowledge to tools that can be used at larger scales. This thesis reports various pieces of work on different calorimetric studies done on cellulosic material used in today’s natural and built environment. All experimental tests herein are done using the FPA, the state of the art calorimeter for fire safety studies. The experimental techniques presented here show how invaluable calorimetry is in giving us key insights on the combustion dynamics of fire related processes. The thesis is presented in manuscript style. Each chapter is a stand alone research work intended for publication with the exception of the first and last chapter; intended to introduce these and their relevance to the science and the last to summarize on overall findings and recommended improvements. Chapter 2 presents a study on the burning of live and dead pine needles. Pine forests present a relatively high flammability risk comprised in great part by pine needles. Different moisture content, flow conditions and their interrelationship is studied on the different parameters affecting the combustion processes. Overall, the results show that fire physics and chemistry vary with fuel and flow conditions and that moisture content is not the only difference between live and dead fuels but that the needle bed physiochemical mechanisms matter as well. This is the first time calorimetry data is presented on the burning of live and dead pine needles. Chapter 3 complements chapter 2 with an added in-depth analysis on the effect of different pine needle species, fuel load and imposed heat insult. Interrelationship between these variables is shown to have a strong effect on the overall combustion process. Fuel load is shown to be an essential condition to know as it gives a direct indication on the intensity of the fire. Flow is shown to have a varied effect depending on the fuel load, it can either aid or be detrimental to the overall combustion process especially relating to ignition times. Chapter 4 is a study on the effect of leaf morphology to flammability of different natural fuels. This study is a direct extension of the work presented in the paper Belcher et al (2010) in Nature Geoscience. Representative natural fuel samples from the Triassic/Jurassic Boundary, a time period of great importance because it marked a time of major environmental changes, are used to evaluate fire activity as a whole during this time period. The study shows that smaller leaf area and larger surface area to volume ratio show a strong correlation to an increase in flammability of these fuels. The research presents new insight into how leaf morphology can be used as a tool to assess the effect of fire activity around the globe and how closely vegetation is linked to this. Chapter 5 presents a study on flammability of chipboard. Wood being an inhomogeneous, non-isotropic material presents researchers with a complex problem due to its burning behavior. Wood has been a preferred construction material since far back and is widely used in construction today. Different oxygen levels, heat insults, material thicknesses and densities and the interrelationship between these variables are assessed to observe the effect on the flammability of chipboard. Density and thickness is shown to have little effect on the overall burning dynamics with thermally thick samples apart from the increased fuel content. Oxygen levels and imposed heat insults, however, show a wide range of effects and the interrelationship proves to be quite important during the combustion process. The research outlines how char formation is affected by the different variables and how important this process becomes along the overall combustion process. Calorimetric studies are presented that illustrate the use of these devices to study the effect of varying environmental conditions and the importance of their interrelationships on both natural and built environment fuels. The works highlight the importance of first establishing the dynamics of the combustion process in order to be able to extract combustion parameters that are needed for modeling fires better in both wildland and built environments. Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/1842/6406 2012-01-01T00:00:00Z http://hdl.handle.net/1842/5588 Title: Thermal buckling of metal oil tanks subject to an adjacent fire Authors: Liu, Ying Abstract: Fire is one of the main hazards associated with storage tanks containing flammable liquids. These tanks are usually closely spaced and in large groups, so where a petroleum fire occurs, adjacent tanks are susceptible to damage leading to further development of the fire. The structural behaviour such as thermal stability and failure modes of the tanks under such fire scenario are very important to the safety design and assessment of oil depots. However, no previous studies on this problem are known to the best knowledge of the author. This thesis presents a systematic exploration of the potential thermal and structural behaviours of an oil tank when one of its neighbour tanks is on fire. Under such scenario, the oil tanks are found to easily buckle under rather moderate temperature rises. The causes of such buckling failures are the reduced modulus of steel at elevated temperatures, coupled with thermally-induced stresses due to the restraint of thermal expansion. Since the temperatures reached in such structures can be several hundred Centigrade degrees, any restraint to thermal expansion can lead to the development of compressive stresses. The high susceptibility of thin shell structures to elastic buckling under low compressive stresses means that this type of failure can be easily provoked. The main objectives of this thesis were to reveal the thermal distribution patterns developed in an oil tank under the heating from an adjacent tank fire, to understand the underlying mechanism responsible for the buckling of tank structure, and to explore the influences of various thermal and geometrical parameters on the buckling temperature of the tanks. The study began with analytical solutions for stresses and deformations in a partially filled roofless cylindrical tank under an idealised axisymmetrical heating regime involving thermal discontinuity at the liquid level. The results demonstrate that large compressive circumferential membrane stresses occur near the bottom boundary for an empty tank and near the liquid level for a partially-filled tank. Heat transfer analysis was conducted to explore the temperature distribution developed in the tank when the fire reaches a steady state. Parameters and assumptions used in the adopted pool fire model were carefully examined. The results show that a rather non-uniform distribution of temperature is developed in the tank especially around the tank circumference. A simple model was then proposed to describe the temperature distribution based on the numerical heat transfer analysis. The accuracy of the proposed temperature distribution model for predicting the structure behaviour was evaluated by comparing its predictions with those using directly the temperature distribution obtained from the numerical heat transfer analysis. Extensive geometric and material nonlinear analyses were carried out to capture the buckling behaviour of the tank using both the proposed temperature distribution and that from heat transfer analysis. It was found large vertical compressive membrane stresses are induced in the tank, causing buckling. The influence of fire diameter, location, liquid filling level and tank geometry were investigated. Sat, 01 Jan 2011 00:00:00 GMT http://hdl.handle.net/1842/5588 2011-01-01T00:00:00Z http://hdl.handle.net/1842/5587 Title: Smouldering and self-sustaining reactions in solids: an experimental approach Authors: Hadden, Rory Abstract: Smouldering combustion governs the burning of many materials in the built and natural environments. Smouldering is flameless, heterogeneous combustion which occurs when oxygen reacts with the surface of a solid fuel. Understanding the conditions which will result in the ignition and smouldering of a porous fuel is important and the phenomena involved are complex and coupled, involving heat and mass transfer, and chemical kinetics. This thesis reports experimental studies of the ignition, spread, suppression and emissions from reactions in porous media. Similar experimental techniques are shown in this thesis to be applicable when studying a wide range of solids which undergo self-sustaining reactions. This thesis is presented in a manuscript style. Each chapter takes the form of an independent paper which has been prepared for journal publication and as such, each chapter can stand on its own as a piece of research. A final chapter summarizes the findings and conclusions and suggests further areas of research. Chapter 1 presents a study of self-sustaining decomposition of ammonium nitrate containing inorganic fertilizer. This is of importance to the shipping industry which transports these materials in large quantities. Upon exposure to a heat source, ammonium nitrate may undergo exothermic decomposition which can propagate through the material, posing safety and economic threats. This reaction does not involve oxygenbased chemistry, but has many similarities to the propagation of a smoulder front in a porous material. Small-scale experiments to investigate the self-sustaining decomposition (SSD) behaviour of NPK (nitrogen, potassium, phosphorous) 16.16.16 fertilizer were undertaken. Experiments showed that this material will undergo self-sustaining decomposition and are used to formulate a reaction framework. Findings were applied to the events that occurred aboard the Ostedijk in 2007. Chapter 2 is a study of smoulder in polyurethane foam to study the relationship between sample size, critical heat flux and spread rate. Smouldering fires are the leading cause of residential fire deaths in developed countries and polyurethane foam is ubiquitous in the modern world. The critical heat flux for ignition was found to decrease with increasing sample size and the spread rate was found to be a function of the sample size, smoulder propagation depth and the applied heat flux. This is the first time that results on the effect of sample size on smouldering have been reported in the literature and these can be used to aid the extrapolation of small-scale flammability testing results to large scale scenarios. Chapter 3 presents an experimental investigation into the ignition of porous fuels by hot particles. This is related to the problem of spotting ember ignition in wildland fires which is a major, but poorly understood, spread mechanism. The process of spotting occurs in wildland fires when fire-lofted embers or hot particles land downwind, leading to ignition of new, discrete fires. The work studies the ignition of a fuel as a function of ember size and temperature. Metal particles are used as a proxy for burning embers and powdered cellulose to represent the forest fuel. Relationships between the size and temperature of the particle required for flaming and smouldering ignitions are found. These results are used to assess the ability of hot-spot ignition theory to determine the particle size–temperature relationship required for ignition of a cellulose fuel bed. Chapter 4 is an investigation into the suppression of smouldering coal. Subsurface coal fires are a significant global problem with fires in China alone estimated to consume up to 200 million tons of coal per year. As global demand for coal increases, accidental fires are a waste of a useful energy resource as well as a source of pollution and greenhouse gases. The results are the first attempt reported in the literature to study the suppression of these fires under controlled laboratory conditions. The ignition, spread and suppression of subsurface coal fires were studied using small-scale laboratory experiments. Time to ignition was seen to depend on particle size with small and large particles resulting in long times to ignition, while medium sized particles resulted in the shortest time to ignition. The maximum temperature, spread rate and mass lost were found to be independent of particle size above a critical particle size. The effectiveness of three systems for delivery of a suppression agent were assessed – direct injection, shower and spray. The effect of particle size on the water required for extinguishing using a spray was found to be weak. Chapter 5 presents an experimental investigation of the smouldering behaviour of peat. This is of particular interest in understanding the impact of smouldering fires on the earth system. The longer burn durations and different combustion dynamics of smouldering compared to flaming means that they have been shown to consume large amounts of biomass in, and contribute significantly to the emissions from, natural fires occurring in peatlands. The dynamics of smouldering peat in shallow, strong fronts was studied in the Fire Propagation Apparatus and a smoulder reaction framework with two burning regimes is presented. The first regime is peat smouldering and was found to be controlled by the applied heat flux and the second regime corresponded to char smouldering and was more sensitive to the flow of oxidizer. Chapter 6 complements Chapter 5 with an analysis of the CO and CO2 emissions for smouldering and flaming peat. This data can be used with large-scale measurement techniques to improve emission estimates. The emissions are found to be dependent of the burning regime and the type of combustion with flaming resulting in higher fluxes of CO2 and lower fluxes of CO compared to peat smouldering. Char smouldering resulted in the highest yields of CO and CO2. The large majority of emissions (85% of CO2 and 97% of CO) are released during the smoulder phase of the reaction. This highlights the differences in the chemical processes occurring under these two modes of combustion. Chapter 7 summarizes the research undertaken in this thesis and presents possible further work. Sat, 01 Jan 2011 00:00:00 GMT http://hdl.handle.net/1842/5587 2011-01-01T00:00:00Z