Browsing by Author "Martin, Christopher Reed"

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    Reduced-Order Models for the Prediction of Unsteady Heat Release in Acoustically Forced Combustion
    Martin, Christopher Reed (Virginia Tech, 2009-12-04)
    This work presents novel formulations for models describing acoustically forced combustion in three disjoint regimes; highly turbulent, laminar, and the moderately turbulent flamelet regime. Particular emphasis is placed on simplification of the models to facilitate analytical solutions while still reflecting real phenomenology. Each derivation is treated by beginning with general reacting flow equations, identifying a small subset of physics thought to be dominant in the corresponding regime, and making appropriate simplifications. Each model is non-dimensionalized and both naturally occurring and popular dimensionless parameters are investigated. The well-stirred reactor (WSR) is used to characterize the highly turbulent regime. It is confirmed that, consistent with the regime to which it is ascribed for static predictions, the WSR is most appropriate to predict the dynamics of chemical kinetics. Both convection time and chemical time dynamics are derived as explicit closed-form functions of dimensionless quantities such as the Damk\"ohler number and several newly defined parameters. The plug-flow reactor (PFR) is applied to a laminar, burner stabilized flame, using a number of established approaches, but with new attention to developing simple albeit accurate expressions governing the flame's frequency response. The system is studied experimentally using a ceramic honeycomb burner, combusting a methane-air mixture, numerically using a nonlinear FEA solver, and analytically by exact solution of the simplified governing equations. Accurately capturing non-unity Lewis-number effects are essential to capturing both the static and the dynamic response of the flame. It is shown that the flame dynamics can be expressed solely in terms of static quantities. Finally, a Reynolds-averaged flamelet model is applied to a hypothetical burner stabilized flame with homogeneous, isotropic turbulence. Exact solution with a simplified turbulent reaction model parallels that of the plug flow reactor closely, demonstrating a relation between static quantities and the flame frequency response. Comparison with published experiments using considerably more complex flame geometries yields unexpected similarities in frequency scale, and phase behavior. The observed differences are attributed to specific physical phenomena that were deliberately omitted to simplify the model's derivation.
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    Simulation of Electrical Characteristics in Oxyfuel Flame Subject to An Electric Field
    Xu, Kemu (Virginia Tech, 2021-06-11)
    The oxyfuel cutting method is still widely used nowadays, even though it is not a fully autonomous process. Thisthesis presents a computational model to study ion and electron transport and current-voltage characteristics inside a methane-oxygen flame. By finding the relationship between current-voltage characteristics and critical parameters,such as standoff, fuel oxygen ratio, and flow rate, a control algorithm could be implemented into the system and make it autonomous. Star CCM+ software is used to develop preheat phase computational models by splitting the simulations into the combustion and electrochemical transport parts. Both the laminar and turbulent flows are considered. Several laboratory experiments are used to compare test data with the numerical results generated using this model. The initial and boundary conditions used in the simulation were to the extent possible similar to the experimental conditions in the laboratory experiment. In the combustion part, the general GRI3.0 mechanism plus three additional ionization reactions are applied, and the combustion part results are then used as input into the electrochemical transport part. A particular inspection line inside the domain is created to analyze the results of the electrochemical transport part. Ions, electrons number density, and current density are studied in the interval from -40V to 40V electric potential. The ions are heavier and more challenging to move than electrons. The results show that at both the torch and work surfaces, charged sheaths are formed, which cause three different regions of current-voltage relations to form in a similar manner as observed in the tests.
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    Systematic Prediction and Parametric Characterization of Thermo-Acoustic Instabilities in Premixed Gas Turbine Combustors
    Martin, Christopher Reed (Virginia Tech, 2006-09-29)
    This thesis describes the coincident prediction and observation of thermo-acoustic instabilities in a turbulent, swirl-stabilized research combustor using a stability model constructed from validated reduced-order component models. The component models included the acoustic response to flame heat release rate at various locations in the combustor, the turbulent diffusion of uneven fuel-air mixing, and the flame's response to perturbations in both inlet velocity and equivalence ratio. These elements are closed in a system-level model to reflect their natural dynamic coupling and assessed with linear stability criteria. The results include the empirical validation of each of the component models and limited validation of the total closed-loop model with a lean premixed gaseous fuel combustor not dissimilar to an industrial burner. The degree of agreement between the predictions and the measurements encourages the conclusion that the reduced-order technique described herein not only includes the relevant physics, but has characterized them with sufficient acuracy to be the basis for design techniques for the passive avoidance of thermo-acoustic instabilities.