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.

In many practical combustion devices, including those used in gas turbine engines for aircraft and power generation, a high energy spark kernel is necessary to reliably ignite the turbulently flowing flammable gases. Complicating matters, the spark kernel is sometimes generated in a region where a non-flammable mixture is present, or where there is no fuel at all. This requires the spark kernel to travel to a flammable region before rapid combustion can begin in non-premixed or stratified flows. This transit time allows for chemical reactions to take place within the kernel as well as mixing with surrounding gases. Despite these demanding conditions, the majority of research in ignition has been for low energy sparks and premixed conditions, not resembling those found in many combustion devices. Similarly, there is little work addressing this issue of spark kernel evolution in the non-premixed flowing environment, and none available that control the time allowed for transit. The goal of this thesis is to understand the development of a spark kernel issued into a non-premixed flow and the sensitivities of the ignition process. To this effect, a stratified flow facility for ignition experiments has been fabricated utilizing a high speed schlieren and emission imaging system for visualizing the kernel motion and ignition success. Additionally, OH chemiluminescence and CH PLIF were used to track chemical species during the ignition process. This facility is also used to control the important variables regarding the flow and spark kernel interaction to quantify the influence on ignition probability. A reduced order model employing a perfectly stirred reactor (PSR) has also been developed based on experimental observations of the entrainment of fluid into the evolving kernel. The simulations provide additional insight to the chemical development in the kernel under different input conditions. This model was enhanced by introducing random perturbations to the input variables, mimicking a practical situation. A computationally efficient support vector machine was trained to replicate the numerical model outputs and predict ignition probabilities for nominal input conditions, providing comparison to experimental results. Experimental and numerical results show that initial mixing with non-flammable fluid quickly reduces the ability for the kernel to ignite the flammable flow, resulting in a strong influence of the inlet temperature and the kernel transit time on the probability of ignition. Once the kernel reaches the flammable mixture, entrainment of this flow occurs, which requires on the order of a vortex turn-over time before chemistry can begin. Initial chemical reactions include endothermic fuel decomposition, further reducing the kernel temperature prior to heat release, creating a competition between the cooling effect of additional mass entrainment and the delayed heat release reactions. CH PLIF results show that flame chemistry is initially confined to a thin region that corresponds to the interface layer where the flammable gases mix with the hot kernel fluid from the vortex entrainment of ambient gas. The dependence of the ignition probability to variations in flow conditions is captured reasonably well by the reduced order model, validating the PSR approach and the probability prediction tool. The development of this reduced order model is a major contribution of this work with the ability to predict the effects of the important physical ignition processes, which can be used when considering an ignition system's feasibility. This work will provide knowledge to guide the use and design practices in industry, as well as a simple model to test ignition feasibility based on mixing, entrainment, and chemical reactions. Furthermore, the flow facility is well characterized, and a database has been developed that can provide validation points for future computational simulations. Future modeling will be important to further understand fluid dynamic effects that are difficult to measure experimentally, and study a broader range of conditions.

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The effect of combustion chamber design on combustion rate has been investigated experimentally and theoretically. The experimental work concentrated on the measurement of cylinder pressure and flame speed using a piezo-electric pressure transducer and multiple ionisation probes together with a data acquisition/processing system. A total of twenty one chamber designs of varying shape, compression ratio and spark plug arrangement were tested over a range of operating conditions on a single cylinder S.I. engine. The pressure data were analysed to obtain values of pressure rise rate, cyclic dispersion and combustion (mass burn) rate whilst the ionisation data were processed to yield flame travel angles and flame dispersion. The results obtained show that for a given compression ratio, the flame speed is not significantly affected by chamber design. In contrast, the combustion rate and pressure parameters are highly dependent on the chamber design; more compact arrangements giving higher combustion rates and reduced cyclic dispersion. A computer simulation model of the compression, combustion and expansion phases of the engine cycle was developed to predict the effects of the combustion chamber design parameters. Based on the experimental results, the model assumes that the ratio of laminar to turbulent burning velocity is independent of chamber design. The influence of chamber shape on the burnt volume, flame front surface area and heat transfer surface areas is modelled using a simple but effective geometric integration technique. This technique allows an infinite variation of the design parameters to be specified for a large range of chamber shapes with a minimum of input data being required. The model predicts that chamber design does have a major effect on combustion rate and cylinder pressure but shows that the influence of individual design is highly dependent on the setting of all other parameters. The effect of squish area is shown to be due to it changing the compactness of the chamber, optimum squish area being about 50% for conventional engines with higher areas being suited to higher compression ratio designs. Spark plug arrangement is predicted to be the most effective way of controlling the combustion rate with a single centrally located spark plug or alternatively, dual spark plugs, giving large increases in combustion rate. Computer model predictions have been compared directly with experimental results obtained in this study and with experimental results reported by two other independent workers. Good agreement was obtained thereby giving support to the assumption of the flame speed being unaffected 'by chamber design. The model was also used to predict squish velocities in fired engines. The results show that the velocities and, in particular the reverse squish, can be significantly modified by the combustion process with a strong dependence on ignition timing being evident. The predictive model has been modified to yield a heat release program capable of analysing experimental pressure time data to predict combustion rate, flame speed, turbulent burning velocity and many other variables. The predicted flame speeds were in good agreement with corresponding experimental values obtained from ionisation probes. In conclusion, the study has confirmed the importance of combustion chamber design as a means of improving the combustion rate but has shown that the flame speed is not affected by chamber shape (i.e. squish). The semi-empirical simulation model has been shown to predict the effects of the chamber design parameters to an acceptable degree of accuracy.

NOx produced during combustion can persist in the exhaust gases of a gas turbine engine in quantities significant to induce regulatory concerns. There has been much research which has led to important insights into NOx chemistry. One method of NOx reduction is exhaust gas recirculation. In exhaust gas recirculation, a portion of the exhaust gases that exit are redirected to the inlet air stream that enters the combustion chamber, along with fuel. Due to the presence of NOx in the exhaust gases which are subsequently introduced into the burner, knowledge of the effects of NOx on combustion is advantageous. Contrary to general NOx research, little has been conducted to investigate the sensitizing effects of NO2 and N2O addition to methane/oxygen combustion. Experiments were made with dilute and real fuel air mixtures of CH4/O2/Ar with the addition of NO2 and N2O. The real fuel air concentrations were made with the addition of NO2 only. The equivalence ratios of mixtures made were 0.5, 1 and 2. The experimental pressure range was 1 - 44 atm and the temperature range tested was 1177 – 2095 K. The additives NO2 and N2O were added in concentrations from 831 ppm to 3539 ppm. The results of the mixtures with NO2 have a reduction in ignition delay time across the pressure ranges tested, and the mixtures with N2O show a similar trend. At 1.3 atm, the NO2 831 ppm mixture shows a 65% reduction and shows a 75% reduction at 30 atm. The NO2 mixtures showed a higher decrease in ignition time than the N2O mixtures. The real fuel air mixture also showed a reduction. Sensitivity Analyses were performed. The two most dominant reactions in the NO2 mixtures are the reaction O+H2 = O+OH and the reaction CH3+NO2 = CH3O+NO. The presence of this second reaction is the means by which NO2 decreases ignition delay time, which is indicated in the experimental results. The reaction produces CH3O which is reactive and can participate in chain propagating reactions, speeding up ignition. The two dominant reactions for the N2O mixture are the reaction O+H2 = O+OH and, interestingly, the other dominant reaction is the reverse of the initiation reaction in the N2O-mechanism: O+N2+M = N2O+M. The reverse of this reaction is the direct oxidation of nitrous oxide. The O produced in this reaction can then speed up ignition by partaking in propagation reactions, which was experimentally observed.

Thesis (M.S.)--University of Missouri--Rolla, 2007. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed November 29, 2007) Includes bibliographical references (p. 153-155).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1959. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (M.S.)--University of Wisconsin--Madison, 1969. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (M.S.)--West Virginia University, 2009. / Title from document title page. Document formatted into pages; contains v, 37, [20] p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 35-37).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1969. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.