To meet the climate goals and reduce the negative effects of anthropogenic industrial activity, the human civilization must move toward sustainable energy sources. However, in hard-to-abate applications and to compensate for the intermittency of renewable energy sources, combustion will continue to be a crucial energy conversion mechanism. This requires modern combustion devices to have highly reduced emissions not only to abide by stringent regulations but also as a collective social responsibility. Usually, a reduction of NOx requires a lowering of flame temperatures, which leads to an increase in CO. However, Flameless/MILD combustion is a technology that has the potential for low NOx while attaining complete combustion. This is because of its unique requirements of having very high reactant temperatures (typically above autoignition temperatures) and having low oxygen concentration. The current work investigates the phenomenology of the combustion process in a jet-stabilized combustor, where this regime can be produced with fuel and oxidizer injected in a premixed manner. The work has three main components; the first deals with the autoignition chemistry of methane-air mixtures under exhaust gas vitiation conditions, the second investigates the flow field and turbulent interface of a turbulent-jet-in-coflow which is a canonical version of the flow conditions in a jet stabilized combustor and finally, experiments were done on a jet-stabilized combustor with methane-hydrogen blends to classify the flame stabilization mechanism and quantify emissions.

Autoignition characteristics of vitiated methane-air mixtures were studied by simulations in 0D reactor setup. Unlike most studies in literature, vitiation, in the context of flameless combustion, was generated as the hot combustion product of fresh reactants that also contained radicals that existed in equilibrium. Particularly, the effect of varying levels of vitiation and heat loss was studied on properties such as ignition delay time, reaction time scale, and the NOx and CO emissions. This revealed the most suitable conditions to achieve low emissions and distributed reaction zones for premixed reactants that are vitiated by exhaust gases. Further, a regime of multi-ignition was discovered where prior to the main ignition event, there is a pre-ignition event attributed to the initial pool of radicals in a vitiated mixture. The conditions of occurrence were mapped out, as well as the mechanism behind it was explained.

The mixing at the interface of the jet and the recirculation zone in a jet-stabilized combustor has an important role in determining the composition of the hot-diluted mixtures. Thus, the fluid mechanics of the turbulent-turbulent interface were studied in a canonical configuration of a turbulent-jet-in-turbulent-coflow. This is also a common configuration used to produce Flameless/MILD combustion under laboratory conditions. Although there is vast research on free turbulent jets, combustors operating in the Flameless Combustion regime would reach flow conditions where the ratio of coflow to jet velocity would increase. This work elucidates the evolution of such a flow field through Particle Image Velocimetry (PIV) measurements done along the axis of the jet in the range 0<x/D<42. Further, the interface is detected using an algorithm developed based on other image processing algorithms using vorticity as a criterion. This enables the assembly of conditional statistics with respect to the interface. The results show that the cases with higher coflow have a lower jet centerline velocity decay rate and reduced jet spreading. The mean axial velocity shows a region of deficit compared to the free jet near the interface region. Further, the case with higher coflow shows higher turbulence intensity and Reynolds Shear Stress close to the interface. The detailed results are presented as both unconditional and conditional statistics and the mechanism behind this effect is deduced.

Experiments were done on a jet-stabilized combustor capable of producing the Flameless Combustion regime. It was operated using methane-hydrogen fuel admixtures at varying equivalence ratios. The combustor performance was analyzed based on the stabilization of the flame zone and the emissions. This work presents a unique, comprehensive measurement of temperature, gas composition, velocity field, and chemiluminescence signal in a jet-stabilized combustor. The recirculation regions are visualized through PIV measurements and the recirculation ratio is quantified. The instantaneous flame images are used to identify flame kernels and construct probability density functions of the aspect ratio, rotation angle, and location along the combustor axis. An increase in hydrogen content in the fuel mixture shifts the stabilization mechanism from autoignition to flame propagation. There is also an increase in NO emissions. A similar effect is seen with the increase in equivalence ratio from lean to stoichiometric condition. Distributed reaction regimes with ultra-low NO and moderate flame temperatures are achieved at very low equivalence ratios. Such mixtures are stabilized better with the addition of hydrogen to the fuel mixture.

This thesis provides fundamental information on the chemistry and flow physics of the phenomenon in a jet-stabilized combustor followed by measurements from the operation of one. The data and conclusions are a suitable reference for future engineers designing jet-stabilized combustors for low NOx emissions and high combustion efficiency.@en

The jet-in-hot-coflow is a canonical combustion setup, which has been used in several studies to study Flameless/MILD combustion and auto-ignition of fuels. However, the NO_x and CO emission measurements from these combustion setups were not possible due to the entrainment of laboratory air and a lack of a well-defined physical system limit. These limitations have been overcome by a new enclosed jet-in-hot-coflow setup. The combustor was operated by injecting a mixture of CH₄-Air in the central jet, and the coflow comprised of hot products from CH₄-Air combustion in burners upstream. The coflow composition was further controlled by adding diluents such as N₂ and CO₂. Measurements were done using stereoscopic particle image velocimetry, suction probe gas analysis, thermocouples, and chemiluminescence imaging. Increasing central jet velocity and equivalence ratio led to lower NO_x and a reaction zone that enlarged and shifted downstream. The reduction in NO_x emission was attributed to the returning mechanism. Adding CO₂ and N₂ as diluents in the coflow resulted in a longer combustion zone and reduced temperatures in the combustion chamber, leading to decreased NO_x production and increased reburning. These experiments provide relevant flowfield and emissions data for modelers and help characterize combustion regimes such as Flameless/MILD.

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Flameless Combustion is an interesting low NO_x combustion technology for gas turbine engines. In order to design systems for stringent performance standards, it is important to understand emission formation in this regime. To this end, the characteristics of a combustor capable of operating in the Flameless regime are studied. Particle Image Velocimetry and thermocouple measurements were performed to obtain the velocity field and gas temperatures respectively, in the combustor under reacting conditions. Results from experiments were used to generate an "informed" chemical reactor network (CRN) model from which, temperature and species distributions were obtained. As such, this paper presents measured data and a methodology to combine it with CRN modelling to obtain gas composition and temperature. The temperature, NO_x, CO and O₂ mole fractions obtained at three different operating conditions shall be validated with gas composition measurements in the future.

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The Flameless Combustion (FC) regime has been pointed out as a promising combustion technique to lower the emissions of nitrogen oxides (NOx) while maintaining low CO and soot emissions, as well as high efficiencies. However, its accurate modeling remains a challenge. The prediction of pollutant species, especially NOx, is affected by the usually low total values that require higher precision from computational tools, as well as the incorporation of relevant formation pathways within the overall reaction mechanism that are usually neglected. The present work explores a multiple step modeling approach to tackle these issues. Initially, a CFD solution with simplified chemistry is generated [both the Eddy Dissipation Model (EDM) as well as the Flamelet Generated Manifolds (FGM) approach are employed]. Subsequently, its computational cells are clustered to form ideal reactors by user-defined criteria, and the resulting Chemical Reactor Network (CRN) is subsequently solved with a detailed chemical reaction mechanism. The capabilities of the clustering and CRN solving computational tool (AGNES—Automatic Generation of Networks for Emission Simulation) are explored with a test case related to FC. The test case is non-premixed burner based on jet mixing and fueled with CH4 tested for various equivalence ratios. Results show that the prediction of CO emissions was improved significantly with respect to the CFD solution and are in good agreement with the experimental data. As for the NOx emissions, the CRN results were capable of reproducing the non-monotonic behavior with equivalence ratio, which the CFD simulations could not capture. However, the agreement between experimental values and those predicted by CRN for NOx is not fully satisfactory. The clustering criteria employed to generate the CRNs from the CFD solutions were shown to affect the results to a great extent, pointing to future opportunities in improving the multi-step procedure and its application.@en