Search

Previous microgravity experiments have shown unique flame spread characteristics and spread mode transition phenomenon for ultra-thin samples (< 1 mm). Theory also predicts a thin-thickness limit for flame spread. Computational Fluid Dynamics (CFD) is used to explore these observations. Samples of fuel/inert composite with varying composition are simulated in a tunnel similar to that used in previous microgravity experiments. The sample is ignited at the leading edge and is allowed to burn. The thickness is parameterized by the area density of the fuel, ranging from 4 to 18.2 /2. Flames with lower area density cases quenched before steady spread. Flow speeds of 5, 15, and 30 cm/s are used to investigate the flow effects. The char yield of the solid samples is another parameter in the study. Flame spread rates and pyrolysis lengths are calculated and then compared. Solid and gas profiles are examined to understand the effects of the parameters.

The rate of spread of fire (RoS) depends on various topographical, weather and fuel parameters. The wind speed and slope of the terrain are among the two major weather and topographical factors that determine the rate of fire spread. These two parameters are investigated in this study to obtain an insight into grassfire behaviour in respect to wind and slope variables, which may then be used to improve operational and empirical models, improve prediction of real wildfires, and subsequently alleviate the risks of wildfire impact. A fully physics-based modelling study of grassfire behaviour over flat and sloped terrain has been conducted at field-scale. The purpose was to investigate the combined effect of slope and wind on grassfire behaviour and attempt to capture the physical processes governing fire propagation. The simulations are performed using physics-based model, Wildland-urban Interface Fire Dynamics Simulator (WFDS version svn 9977), at driving wind velocities of 12.5, 6, 3, 1 and 0.1 m.s–1 and slope angles varying from –30° to +30°. This research primarily analyses the RoS, alongside other quantities such as fire isochrones, fire intensity, flame dynamics, mode of fire propagation and heat transfer. Although the focus of this study is to compare quasi-steady fire behaviour with empirical models, an attempt is also made to analyse the dynamic beahviour of fire, as the fire intensity and the expansion of the isochrones even though RoS may remain roughly constant. Slope correction for grassfire propagation in empirical models are presented in two ways: multiplicative and additive. For the former, RoS is calculated for no-slope (under the influnce of wind) and then a multiplied by a slope correction factor developed from studies conducted with no-wind conditions (i.e. doubling RoS for every 10° slope as given by Noble et al. (1980)). For the latter, RoS under the influnce of no-wind is determined to which the wind and slope factors are added (Rothermel 1972 model). Within the simulations conducted in this research, the RoS and fire intensity are found to have a positive correlation with respect to the slope angles, besides with the wind velocity. RoS comparisons have been made with common empirical models: Australian models with ‘slope correction’ multiplication (CSIRO model and McArthur models MKIII and MKV), and Rothermel models (‘Original’ and ‘Modified’ models). At different slope angles and driving wind velocities, different empirical quasi-steady RoS broadly matches with particular dynamic maximum, minimum (the upper and lower bounds of instantaneous RoS are provided as whiskers to present uncertainty to the averaged RoS values) and averaged RoS values from this study. The dynamic nature of grassfire propagation and challenges related to capturing this dynamism in experimental studies are likely reasons for any observed discrepancies. Within the slope angles and driving wind velocities examined in this study, for higher wind velocities, a second-order polynomial relationship exists between the quasi-steady RoS and slope angle, which is also the case for Rothermel models. However, for lower wind velocities, the RoSslope angle relationship is closer to an exponential fit, consistent with the exponential relationship reported with most of the experimental studies that are conducted at no or very low wind speeds. The slope correction for RoS in Australian empirical correlations is also exponential, which are likely to be derived from very low wind laboratory-scale slope studies. The relative RoS (RoS on any slope divided by RoS at a reference slope) results shows that, at higher wind velocities, the relative RoS from WFDS are closer to Rothermel models than the Australian correlations. The Rothermel model shows very little slope-effect under the influence of strong wind, whereas, as the wind-velocity reduces, a greater effect can be observed. The opposite is noted for Australian slope function models. It is due to multiplicative nature of Australian correlations compared to Rothermel models’ additive nature. Under lower or near zero wind velocities, the Australian model’s slope function is closer to the WFDS quasi-steady RoS results, whereas there are significant discrepancies in the results for higher wind velocities. Hence, the multiplicative nature of the Australian model’s slope function (especially for upslopes at higher wind velocities) may need strong scrutiny. Generally, the RoS, headfire width, fire intensity and flame length are all strongly correlated to wind velocity and slope angle. This study also analyses the RoS-fire intensity and fire intensityflame length relationships. In most cases, the RoS-fire intensity relationship is found to be linear (as expected from Byram’s correlation for no-slope scenarios). However, the relationship deviates from linearity for higher upslopes. Generally, Byram intensity (Q = fuel load x heat of combustion x RoS) was satisfied with approximately 42% fuel load consumed instead of 100% fuel load. A power-law correlation is found between the simulated flame lengths and fire intensities as observed by many previous researchers. However, various researchers proposed different exponents. The flame length results from this study match closest to the empirical model proposed by Anderson et al. (1966). This study also analyses the grass fire propagation with varied ignition fire line widths, at different upslope angles, at 1 m.s–1 wind speed. Comparing the pyrolysis region contours for varied ignition fire lines, for higher upslopes, the contour pattern with wider ignition lines appears to give sharper convex curvature. The simulation results, in general, show similarity with the experimentally observed results of Dupuy et al. (2011). The narrower the ignition fire line is, the slower is fire propagation and a convergence is obtained at about 30 m ignition line in terms of RoS, fire intensity and flame length. The interaction of wind and fire on a sloped terrain is always complex due to mechanisms of heat transfer and flame dynamics. Heating of unburned vegetation by attached flames may increase the rate of spread. The relative intensities of convective and radiative heat fluxes may change the fire behaviour significantly. This study presents a detailed analysis of flame dynamics, mode of fire propagation and surface radiative and convective heat fluxes on sloped terrain, at various wind speeds. With the increase of slope angles and wind velocity, the plume inclines more towards the ground and becomes elongated in upslope cases, whereas in downslopes, the plume rises from the ground at a shorter distance from the fire line. The flame dynamics results at higher wind velocities indicate that eruptive growth of fires occur in higher upslopes. For higher wind velocities, the flame and near-surface flame dynamics appear to be up-rising, even though the plume is attached. The flame contour results for higher wind velocities indicate that the near-surface flame dynamics are difficult to characterise using the Byram number analysis. Finally, the heat transfer analyses demonstrated that the convective heat fluxes are more relevant at wind-driven fire propagation and at higher upslopes, whereas, both fluxes are equally significant at lower driving wind velocities compared with higher wind velocities. This tendency agrees with that observed in the literature. Overall, the finding from this research provide insight into physical and parametric observations with fire isochrone progression, RoS, the flame dynamics, mode of fire propagation and heat flux parameters. This can lead to further extensive studies to improve the approach of combined wind and slope effects within operational models.

Methane as a fuel is becoming more and more interesting for engine applications due to ever stricter pollutant and greenhouse gas emission regulations and market requirements in both mobile and stationary applications. The use of methane enables the reduction of nitrogen oxides, soot and CO2 emissions from combustion compared to Diesel and gasoline and has the potential to increase efficiency through increased knock resistance. Biogenic sources and the conversion of excess electricity from renewable sources into methane in power-to-gas plants can further improve the CO2 balance and give methane a long-term perspective. An additional increase in efficiency and a further reduction in the knock tendency as well as a minimization of nitrogen oxide emissions by lowering the peak temperature can be achieved through lean operation and/or exhaust gas recirculation. The increased inert gas content reduces the reactivity of the mixture, together with the high molecular stability of methane and the higher pressures at the spark timing, this results in greatly increased demands on the ignition systems for gas engines. This can lead to unwanted mis res and high cycle-to-cycle fluctuations. Hydrogen, on the other hand, has a very high reactivity, a wider in flammability range and a much higher laminar flame speed. Mixtures of methane and hydrogen have the potential to extend operating limits to enable even more efficient and lower CO2 emission operating concepts. Up to now, investigations on the e effects of admixture of hydrogen to methane on combustion were focused mainly on engine investigations of performance, efficiency, combustion duration and pollutant emissions, considering various influences such as hydrogen content, fuel-air equivalence ratio, ignition timing, etc. Optical measurements are mainly limited to constant volume cells, where the determination of the laminar flame speed and the stability of the flame are in the spotlight. Optical investigations on the influence of hydrogen admixture on ignition and early flame formation were lacking until now. In the context of this work, an ignition cell is therefore set up to analyze gas mixtures under different boundary conditions by means of spark-induced breakdown spectroscopy. In addition, the influence of different hydrogen admixture rates on the early flame kernel and on the development of the flame front under engine-relevant conditions are investigated in an optically accessible rapid compression expansion machine. Spectroscopic investigations of the spark plasma have shown that the hydrogen admixture up to 50 vol% has no noticeable influence on the ignition voltage requirement. It could be shown that intensity ratios of spectral emission lines of Hα/C2 correlate with the hydrogen admixture rate in methane, but no clear tendencies could be determined for molecular emission spectra in reactive mixtures with air. Spatially and temporally resolved spark-induced breakdown spectroscopy has shown that ratios of atomic emission lines of Hα/O777 and Hα/N746 allow determining local fuel-air equivalence ratios in methane-air mixtures. Moreover, a slight dependence on pressure at spark timing was found under rich conditions. In addition, it could be shown that the broadening of the atomic emission lines is mainly pressure-dependent. Finally, the methodology of spark-induced breakdown spectroscopy was successfully applied to an optical spark plug, enabling the transfer of the methodology to a running engine. Experiments on the rapid compression expansion machine were carried out under quasi-laminar conditions, whereby the interaction of the plasma channel and the early flame kernel with the flow induced by the piston movement was analyzed. These measurements have shown that the hydrogen addition has a noticeable influence on the flame displacement speed, but also on the stretch rate and the curvature of the flame front. Hydrodynamic and thermodiffusive instabilities were monitored by probability density functions of flame curvature, stretch rate and displacement speed. Cellular instabilities started earlier with increasing hydrogen admixture rates and leaner mixtures. The increasing width of the probability density functions of curvature and stretch rate together with increasingly negative values was proposed as a suitable characteristic for the transition to a thermodiffusive unstable flame. An additional air injection at under-expanded conditions shortly before compression was used as turbulence generation mechanism. The resulting tumble flow was visualized by flow field measurements. It was shown that turbulent fluctuation speeds between 3.5 and 5.5 m/s can be achieved at ignition timing and during early flame generation by varying the pressure ratio across the injector.

Isolator/combustor interactions are measured in a direct-connect dual-mode ramjet-scramjet experiment. An operating point approach is used to create a mapping of the coupling effects between the isolator geometry, inlet flow conditions and fuel injector behavior. The resulting isolator/injector coupling map provides a description of the response of the isolator to particular injector performance, and the effective blockage it induces on the isolator flow. Existing models and correlations predicting the pressure rise across a pseudo-shock, and its resultant length, were evaluated through comparison with measurements made in a heated-flow isolator duct that is coupled to a hydrogen-air combustor. The observation of a normal-to-oblique shock-train transition mechanism has lead to the development of a revised shock-train operating regime description that takes into account the impact of Mach number and maximum pressure recovery on the shock configurations present in the isolator. The behavior of a ram-scram transition was examined along with pressure measurements and high-speed laser interferometry. The work quantifies the sudden change in the wall static pressure profile and flame position that occurs as the downstream boundary condition abruptly changes when the flow becomes unchoked. Transition was studied in three ways; as a quasi-steady phenomenon, or as caused by rapid variations in either fuel flow-rate or test-section wall temperature. A regime diagram was measured that plots the ram-scram transition boundary. Under certain conditions some periodic low-frequency oscillations of the flame position occur and they are shown to be correlated with oscillations of the upstream pre-combustion pseudo-shock. A self-sustaining shear-layer instability, associated with the flameholding cavity, is identified as the mechanism perpetuating this behavior. The relevant time scales associated with the ram-scram transition and the flameshock interactions are discussed.

Lean premixed combustors have been designed to lower NOx and other pollutant levels in land based gas turbines. These combustors are often susceptible to thermo-acoustic instabilities, which manifest as pressure and heat release oscillations in the combustor. To be able to predict and control these instabilities, it is required that both the acoustics of the system, and a frequency-resolved response of the combustion process to incoming perturbations be understood. Currently, a system-level approach is being used widely to predict the thermoacoustic instabilities. This approach requires simple, yet accurate models which would describe the behavior of each dynamic block within the loop. The present study is directed toward using computational fluid dynamics (CFD) as a tool in developing reduced order models for the dynamics of laminar flat flames and swirl stabilized turbulent flames. A finite-volume based approach is being used to simulate reacting flows in both laminar and turbulent combustors. The study has been divided into three parts -- the first part involves the modeling of a self-excited combustor (the acoustics of the combustor are coupled with the unsteady heat release); the second part of the research aims to study the effect of velocity perturbations on the unsteady heat release rate from a burner stabilized laminar flat flame; the third and final part of work involves an extension of the laminar flat flame study to turbulent reacting flows in a swirl stabilized combustor, and study the effects on the turbulent heat release due to the velocity perturbations. A Rijke tube combustor was selected to study self-excited combustion phenomenon. A laminar premixed methane-air flat flame was stabilized on a honeycomb flame-stabilizer. The flame stabilizer was placed at the center of the 5 feet vertical tube. The position of the flame at the center of the tube leads to a thermoacoustic instability of the 2nd acoustic mode. The fundamental thermoacoustic frequency was predicted accurately by the CFD model and the amplitude was reasonably matched (for a flow rate of Q = 120 cc/s and equivalence ratio phi = 1.0). Other characteristics of the pressure power spectrum were captured to a good degree of accuracy. This included the amplitude modulation of the fundamental and the harmonics due to a subsonic pulsating instability. The flat flame study has been being conducted for Q = 200 cc/s and equivalence ratio phi = 0.75. The objective has been to obtain a frequency response function (FRF) of the unsteady heat release rate (output) due to incoming velocity perturbations (input). A range of frequencies (15 Hz - 500 Hz) have been selected for generating the FRF. The aim of this part of the study has been to validate the computational model against the experimental results and propose a physics based interpretation of the flame response. Detailed heat transfer modeling (including radiation heat transfer) and two-step chemistry models have been implemented in the model. The FRF generated has been able to reproduce the experimentally observed phenomena, like the low frequency pulsating instability occurring at 30 Hz. A heat transfer study has been conducted to explain the pulsating instability and a fuel variability study has been performed. Both the heat transfer study and the fuel variability study proved the role of heat transfer in creating the pulsating instability. The final part of the study involves simulation of reacting flow in a turbulent swirl stabilized combustor. The effect of velocity perturbations on the unsteady heat release has been studied by creating an FRF between the unsteady velocity and the unsteady heat release rate. A Large Eddy Simulation (LES) approach has been selected. A swirl number of S = 1.19 corresponding to a flow rate of Q = 20 SCFM with an equivalence ratio of phi = 0.75 have been implemented. Reduced reaction chemistry modeling, turbulence-chemistry interaction and heat transfer modeling have been incorporated in the model. The LES of reacting flow has shown vortex-flame interaction occurring inside the combustor. This interaction has been shown to occur at 255 Hz. The FRF obtained between unsteady velocity and unsteady heat release rate shows good comparison with the experimentally obtained FRF.

Lean, premixed combustion has enjoyed increased application due to the need to reduce pollutant emissions. Unfortunately, operating the flame at lean conditions increases susceptibility to thermoacoustic (TA) instability. Self-excited TA instabilities are a result of the coupling of the unsteady heat release rate of the flame with the acoustics of the combustion chamber. The result is large pressure oscillations that degrade performance and durability of combustion systems. Industry currently has no reliable tool to predict instabilities a priori. CFD simulations of full-scale, turbulent, reacting flows remain unrealizable. The work in this paper is part of a study that focuses on developing compact models of TA instabilities, i.e. acoustics and flame dynamics. Flame dynamics are defined as the response in heat release to acoustic perturbations. Models of flame dynamics can be coupled with models of combustor enclosure acoustics to predict TA instabilities. In addition, algorithms to actively control instabilities can be based on these compact models of flame dynamics and acoustics. The work outlined in this thesis aims at determining the flame dynamics model experimentally. Velocity perturbations are imparted on laminar and turbulent flames via a loudspeaker upstream of the flame. The response of the flame is observed through two measurements. Hydroxyl radical (OH*) chemiluminescence indicates the response in chemical reaction rate. Tunable Diode Laser Absorption Spectroscopy (TDLAS), centered over two water absorption features, allows a dynamic measurement of the product gas temperature. The response in product gas temperature directly relates to the enthalpy fluctuations that couple to the acoustics. Experimental frequency response functions of a laminar, flat-flame burner and a turbulent, swirl-stabilized combustor will be presented as well as empirical low-order models of flame dynamics.