A joint experimental and theoretical project has been initiated at Virginia Tech to study the effects of dual-mode combustion at high pressures for a two-dimensional Wolfhard-Parker burner. This thesis is the first stage of the theoretical part of the project, and contains a numerical study of laminar coflow diffusion flames stabilized on a confined Wolfhard-Parker burner. A global finite difference method is used where the nonlinear equations written on a stream function-vorticity formulation are solved with a flame sheet approach. The pseudotransient, approximative factorization method is utilized to solve the coupled system of equations. Adaptive gridding, numerical evaluation of Jacobians and iterations within time step are implemented for computational efficiency. Numerical results have been obtained for different fuels under different conditions. Comparison with measured data by Smyth et al. (1985) for a buoyancy dominated methane-air flame is made. The location of the flame front is accurately predicted. The temperature is over predicted in the fuel rich zone since pyrolysis and radiation effects have not been accounted for in the numerical model. Good agreement is observed for major species and velocities. As expected, large velocity increase and horizontal inflow of nitrogen and combustion products associated with buoyancy occur in the lower region of the flame. / Master of Science

Gas turbine engines provide thrust for aircraft engines and supply shaft power for various applications. They consist of three main components. That is, a compressor followed by a combustion chamber (burner) and a turbine. Both turbine and compressor components are either axial or centrifugal (radial) in design. The combustion chamber is stationary on the engine casing. The type of engine that is of interest here is the gas turbine auxiliary power unit (APU). A typical APU has a centrifugal compressor, burner and an axial turbine. APUs generate mechanical shaft power to drive equipments such as small generators and hydraulic pumps. In airplanes, they provide cabin pressurization and ventilation. They can also supply electrical power to certain airplane systems such as navigation. In comparison to thrust engines, APUs are usually much smaller in design. The purpose of this research was to investigate the possibility of combining the three components of an APU into a single centrifugal rotor. To do this, a set of equations were chosen that would describe the new turbomachinery cycle. They either were provided or derived using quasi-one-dimensional compressible flow equations. A MathCAD program developed for the analysis obtained best design points for various cases with the help of an optimizer called Model Center. These results were then compared to current machine specifications (gas turbine engine, gasoline and diesel generators). The result of interest was maximum specific power takeoff. The results showed high specific powers in the event there was no restriction to the material and did not exhaust at atmospheric pressure. This caused the rotor to become very large and have a disk thickness that was unrealistic. With the restrictions fully in place, they severely limited the performance of the rotor. Sample rotor shapes showed all of them to have unusual designs. They had a combination of unreasonable blade height variations and very large disk thicknesses. Indications from this study showed that the single radial rotor turbomachinery design might not be a good idea. Recommendations for continuation of research include secondary flow consideration, blade height constraints and extending the flow geometry to include the axial direction. / Master of Science

Neste trabalho, avaliou-se a queima do sebo bovino por meio de uma fornalha calorimétrica flamotubular instrumentada que opera à pressão atmosférica. Como parte do trabalho experimental, foram analisadas as curvas de rendimento térmico da queima do combustível, sebo bovino, e verificou-se seu comportamento quanto à transferência de calor e emissão de gases. Experimentalmente, a fornalha calorimétrica foi remodelada e adaptada para operar com sebo bovino, incorporando-se a esta um elemento de aquecimento prévio, trata-se de um queimador desenvolvido no NETeF possuindo um sistema de alimentação com aquecimento para o combustível, já que o sebo se encontra no estado sólido a temperatura ambiente. O queimador utilizado no experimento foi do tipo misto. Os resultados das medições do poder calorífico do sebo bovino mostraram-se altos: 39.985,5 kJ/kg (dados obtidos no laboratório do NETeF), fazendo com que o sebo se torne uma fonte menos poluente. / In this study, we evaluated the burning of tallow by a calorimetric flamotubular instrumented furnace which operates at atmospheric pressure. As part of the experimental work, the curves of thermal efficiency of fuel combustion (beef tallow) were analyzed, and it was possible to set their composition toward the transfer of heat and gas emissions. Experimentally, the calorimetric furnace was remodeled and adapted to operate with beef tallow by incorporating an element of this preheating, it is a burner developed in NETeF and it has a system to supply heating for fuel, as the tallow is solid at room temperature. The burner used in the experiment was of mixed type. The results of measurements of the calorific value of beef tallow were shown to be high: 39985.5 kJ/kg (data obtained in the laboratory of NETeF), causing the fat to become a viable source.

Novel combustion technologies ensuring low emissions, high efficiency and fuel flexibility are essential to meet the future challenges associated to air pollution, climate change and energy source shortage, as well as to cope with the increasingly stricter environmental regulation. Among them, Moderate or Intense Low oxygen Dilution (MILD) combustion has recently drawn increasing attention. MILD combustion is achieved through the recirculation of flue gases within the reaction region, with the effect of diluting the reactant streams. As a result, the reactivity of the system is reduced, a more uniform reaction zone is obtained, thus leading to decreased NOx and soot emissions. As a consequence of the dilution and enhanced mixing, the ratio between the mixing and chemical time scale is strongly reduced in MILD combustion, indicating the existence of very strong interactions between chemistry and fluid dynamics. In such a context, the use of combustion models that can accurately account for turbulent mixing and detailed chemical kinetics becomes mandatory.Combustion models for conventional flames usually rely on the assumption of time-scale separation (i.e. flamelets and related models), which constrain the thermochemical space accessible in the numerical simulation. Whilst the use of transported PDF methods appears still computationally prohibitive, especially for practical combustion systems, there are a number of closures showing promise for the inclusion of detailed kinetic mechanisms with affordable computational cost. They include the Partially Stirred Reactor (PaSR) approach and the Eddy Dissipation Concept (EDC) model.In order to assess these models under non-conventional MILD combustion conditions, several prototype burners were selected. They include the Adelaide and Delft jet-in-hot coflow (JHC) burners, and the Cabra lifted flames in vitiated coflow. Both Reynolds Averaged Navier Stokes (RANS) and Large Eddy Simulations (LES) were carried out on these burners under various operating conditions and with different fuels. The results indicate the need to explicitly account for both the mixing and chemical time scales in the combustion model formulation. The generalised models developed currently show excellent predictive capabilities when compared with the available, high-fidelity experimental data, especially in their LES formulations. The advanced approaches for the evaluation of the mixing and chemical time scale were compared to several conventional estimation methods, showing their superior performances and wider range of applications. Moreover, the PaSR approach was compared with the steady Flamelet Progress Variable (FPV) model on predicting the lifted Cabra flame, proving that the unsteady behaviours associated to flame extinction and re-ignition should be appropriately considered for such kind of flame.Because of the distributed reaction area, the reacting structures in MILD combustion can be potentially resolved on a Large Eddy Simulation (LES) grid. To investigate that, a comparative study benchmarking the LES predictions for the JHC burner obtained with the PaSR closure and two implicit combustion models was carried out, with the implicit models having filtered source terms coming directly from the Arrhenius expression. Theresults showed that the implicit models are very similar with the conventional PaSR model on predicting the flame properties, for what concerns the mean and root-mean-square of the temperature and species mass fraction fields.To alleviate the cost associated to the use of large kinetic mechanisms, chemistry reduction and tabulation methods to dynamically reduce their size were tested and benchmarked, allowing to allocate the computational resources only where needed. Finally, advanced post-processing tools based on the theory of Computational Singular Perturbation (CSP) were employed to improve the current understanding of flame-turbulence interactions under MILD conditions, confirming the important role of both autoignition and self propagation in these flames. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

Reduction of the pollutant emissions is a challenge for the gas turbine industry. A solution to this problem is to employ the low swirl burner which can operate at lower equivalence ratios than a conventional swirl burner. However, flames in the lean regime of combustion are susceptible to flow perturbations and combustion instability. Combustion instability is the coupling between unsteady heat release and combustor acoustic modes where one amplifies the other in a feedback loop. The other method for significantly reducing NOx and CO2 is increasing fuel reactivity, typically done through the addition of hydrogen. This helps to improve the flammability limit and also reduces the pollutants in products by decreasing thermal NOx and reducing CO2 by displacing carbon. In this work, the flammability limits of a low swirl burner at various operating conditions, is studied and the effect of pressure, bulk velocity, burner shape and percent of hydrogen (added to the fuel) is investigated. Also, the flame structure for these test conditions is measured using OH planar laser induced fluorescence and assessed. Also, the OH PLIF data is used to calculate Rayleigh index maps and to construct averaged OH PLIF intensity fields at different acoustic excitation frequencies (45-155, and 195Hz). Based on the Rayleigh index maps, two different modes of coupling between the heat release and the pressure fluctuation were observed: the first mode, which occurs at 44Hz and 55Hz, shows coupling to the flame base (due to the bulk velocity) while the second mode shows coupling to the sides of the flame. In the first mode, the flame becomes wider and the flame base moves with the acoustic frequency. In the second mode, imposed pressure oscillations induce vortex shedding in the flame shear layer. These vortices distort the flame front and generate locally compact and sparse flame areas. The local flame structure resulting from these two distinct modes was markedly different.

No description available.

Biomass fast pyrolysis liquid (bio-oil) is a cellulose based alternative fuel with the potential to displace fossil fuels in stationary heat and power applications. To better understand the combustion behavior and emissions of bio-oil, a 10 kW spray burner was designed and constructed. The effect of swirl, atomization quality, ignition source (pilot) energy, air/fuel preheat and equivalence ratio on the stability and emissions of bio-oil spray flames was investigated. A blend of 80% pyrolysis liquid and 20% ethanol by volume was used during the tests and the results were compared to burner operation with diesel. It is important to have good atomization, thorough mixing and high swirl in order to stabilize ignition, promote the burnout of bio-oil and decrease CO, hydrocarbon and particulate matter emissions. The total amount of primary air and atomizing air that can be used to improve turbulence, mixing, droplet burnout and overall combustion quality is limited by the distillable fraction and narrow lean blow-out limit associated with pyrolysis liquid. Air and fuel preheat are important for reducing hydrocarbon and CO emissions, although subsequent fuel boiling should be avoided in order to maintain flame stability. The NOx produced in bio-oil flames is dominated by the conversion of fuel bound nitrogen. The particulate matter collected during bio-oil combustion is composed of both carbonaceous cenosphere residues and ash. Under good burning conditions, the majority consists of ash. Pilot flame energy and air/fuel preheat have a weak effect on the total particulate matter in the exhaust. Generally, these results suggest that available burner parameters can be adjusted in order to achieve low hydrocarbon, CO and carbonaceous particulate matter emissions when using pyrolysis liquid. Total particulates can be further mitigated by reducing the inherent ash content in bio-oil. Comparative burner tests with diesel reveal much lower emissions for this fuel at most of the operating points considered. This is due to the fully distillable nature, better atomization and improved spray ignition characteristics associated with diesel. Because of its superior volatility, diesel can also operate over a much wider range of primary air and atomizing air flow rates compared to bio-oil.

Biomass fast pyrolysis liquid (bio-oil) is a cellulose based alternative fuel with the potential to displace fossil fuels in stationary heat and power applications. To better understand the combustion behavior and emissions of bio-oil, a 10 kW spray burner was designed and constructed. The effect of swirl, atomization quality, ignition source (pilot) energy, air/fuel preheat and equivalence ratio on the stability and emissions of bio-oil spray flames was investigated. A blend of 80% pyrolysis liquid and 20% ethanol by volume was used during the tests and the results were compared to burner operation with diesel. It is important to have good atomization, thorough mixing and high swirl in order to stabilize ignition, promote the burnout of bio-oil and decrease CO, hydrocarbon and particulate matter emissions. The total amount of primary air and atomizing air that can be used to improve turbulence, mixing, droplet burnout and overall combustion quality is limited by the distillable fraction and narrow lean blow-out limit associated with pyrolysis liquid. Air and fuel preheat are important for reducing hydrocarbon and CO emissions, although subsequent fuel boiling should be avoided in order to maintain flame stability. The NOx produced in bio-oil flames is dominated by the conversion of fuel bound nitrogen. The particulate matter collected during bio-oil combustion is composed of both carbonaceous cenosphere residues and ash. Under good burning conditions, the majority consists of ash. Pilot flame energy and air/fuel preheat have a weak effect on the total particulate matter in the exhaust. Generally, these results suggest that available burner parameters can be adjusted in order to achieve low hydrocarbon, CO and carbonaceous particulate matter emissions when using pyrolysis liquid. Total particulates can be further mitigated by reducing the inherent ash content in bio-oil. Comparative burner tests with diesel reveal much lower emissions for this fuel at most of the operating points considered. This is due to the fully distillable nature, better atomization and improved spray ignition characteristics associated with diesel. Because of its superior volatility, diesel can also operate over a much wider range of primary air and atomizing air flow rates compared to bio-oil.

Doctor of Philosophy (PhD) / Ultra-lean methane combustion in porous burners is investigated by means of a pilot-scale demonstration of the technology supported by a computational fluid dynamics (CFD) modelling study. The suitability of porous burners as a lean-burn technology for the mitigation of methane emissions is also evaluated. Methane constitutes 14.3% of total global anthropogenic greenhouse gas emissions. The mitigation of these emissions could have a significant near-term effect on slowing global warming, and recovering and burning the methane would allow a wasted energy resource to be exploited. The typically low and fluctuating energy content of the emission streams makes combustion difficult; however porous burners—an advanced combustion technology capable of burning low-calorific value fuels below the conventional flammability limit—are a possible mitigation solution. A pilot-scale porous burner is designed expressly for the purpose of ultra-lean methane combustion. The burner comprises a cylindrical combustion chamber filled with a porous bed of alumina saddles, combined with an arrangement of heat exchanger tubes for preheating the incoming methane/air mixture. A CFD model is developed to aid in the design process. Results illustrating the operating range and behaviour of the burner are presented. Running on natural gas, the stable lean flammability limit of the system is 2.3 vol%, a considerable extension of the conventional lean limit of 4.3 vol%; operating in the transient combustion regime allows the lean limit to be reduced further still, to 1.1 vol%. The heat exchanger arrangement is found to be effective; preheat temperatures of up to 800K are recorded. Emissions of carbon monoxide and unburned hydrocarbons are negligible. The process appears stable to fluctuations in fuel concentration and flow rate, typically taking several hours to react to any changes. A CFD model of the porous burner is developed based on the commercial CFD code ANSYS CFX 12.0. The burner is modelled as a single 1-dimensional porous domain. Pressure loss due to the presence of the porous solid is accounted for using an isotropic loss model. Separate energy equations for the gas and solid phases are applied. Models for conductive heat transfer within the solid phase, and for convective heat transport between the gas and solid phases, are added. Combustion is modelled using a finite rate chemistry model; a skeletal mechanism for ultra-lean methane combustion is developed and incorporated into the model to describe the combustion reaction. Results from the model are presented and validated against experimental data; the model correctly predicts the main features of burner behaviour. Porous burners are found to show potential as a methane mitigation technology.

Doctor of Philosophy (PhD) / Ultra-lean methane combustion in porous burners is investigated by means of a pilot-scale demonstration of the technology supported by a computational fluid dynamics (CFD) modelling study. The suitability of porous burners as a lean-burn technology for the mitigation of methane emissions is also evaluated. Methane constitutes 14.3% of total global anthropogenic greenhouse gas emissions. The mitigation of these emissions could have a significant near-term effect on slowing global warming, and recovering and burning the methane would allow a wasted energy resource to be exploited. The typically low and fluctuating energy content of the emission streams makes combustion difficult; however porous burners—an advanced combustion technology capable of burning low-calorific value fuels below the conventional flammability limit—are a possible mitigation solution. A pilot-scale porous burner is designed expressly for the purpose of ultra-lean methane combustion. The burner comprises a cylindrical combustion chamber filled with a porous bed of alumina saddles, combined with an arrangement of heat exchanger tubes for preheating the incoming methane/air mixture. A CFD model is developed to aid in the design process. Results illustrating the operating range and behaviour of the burner are presented. Running on natural gas, the stable lean flammability limit of the system is 2.3 vol%, a considerable extension of the conventional lean limit of 4.3 vol%; operating in the transient combustion regime allows the lean limit to be reduced further still, to 1.1 vol%. The heat exchanger arrangement is found to be effective; preheat temperatures of up to 800K are recorded. Emissions of carbon monoxide and unburned hydrocarbons are negligible. The process appears stable to fluctuations in fuel concentration and flow rate, typically taking several hours to react to any changes. A CFD model of the porous burner is developed based on the commercial CFD code ANSYS CFX 12.0. The burner is modelled as a single 1-dimensional porous domain. Pressure loss due to the presence of the porous solid is accounted for using an isotropic loss model. Separate energy equations for the gas and solid phases are applied. Models for conductive heat transfer within the solid phase, and for convective heat transport between the gas and solid phases, are added. Combustion is modelled using a finite rate chemistry model; a skeletal mechanism for ultra-lean methane combustion is developed and incorporated into the model to describe the combustion reaction. Results from the model are presented and validated against experimental data; the model correctly predicts the main features of burner behaviour. Porous burners are found to show potential as a methane mitigation technology.