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Title:Plasma-coupled microcombustion
Author(s):Mackay, Kyle K.
Director of Research:Johnson, Harley T.; Freund, Jonathan B.
Doctoral Committee Chair(s):Johnson, Harley T.; Freund, Jonathan B.
Doctoral Committee Member(s):Stephani, Kelly A.; Elliott, Gregory S.
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):microcombustion
plasma-coupled combustion
dielectric barrier discharge
microplasma
surface chemistry
flame quenching
Abstract:Microcombustion, releasing the chemical energy in a sub-millimeter scale flame, has the potential to provide a high energy density portable power source. Microburners have not realized this potential because of detrimental thermal and chemical surface effects. Investigation of macro-scale (>mm) flames has shown that plasma coupling can be an effective method to stabilize and enhance combustion. The applicability of plasma in the particular case of micro-scale flames has yet to be investigated. In this dissertation, a multi-scale approach is used to build models for surface mechanisms to better predict their effect on flame quenching. We simulate the effects of plasma coupling on flame stability at the microscale and evaluate the efficacy of two proposed plasma-coupled microburner devices. Recombination of radical species on surfaces is a chain-terminating step that hinders combustion. Most models for calculating reaction rates on surfaces are based on the simple Langmuir isotherm for reaction and diffusion on an atomically flat surface. The effects of atomic-scale surface defects can be taken into account with molecular dynamics simulations, but these methods are limited by short accessible time scales. Using Grand Canonical Monte Carlo and Monte Carlo Variational Transition State Theory, we overcome the time scale challenge of atomic-scale methods while maintaining a high level of detail for surface structure and reaction dynamics. The recombination rate of hydrogen on silica is calculated using an atomic-scale Monte Carlo approach. Multiple reaction pathways are taken into account and calculated reaction rates are in agreement with experiments and molecular dynamics simulations. Solid surfaces have a limited number of active sites, so adsorption in multicomponent systems can be a competitive process. Species that bind strongly to surfaces will occupy active sites and prevent other species from adsorbing and reacting. In experiments, variability in surface reaction rates has been attributed to this "poisoning" effect, but the lack of a model for this complex process makes it difficult to verify these claims. We build a two-layer Langmuir isotherm model for competitive adsorption of water and hydrogen on silica surfaces. Reactive hydrogen atoms are able to chemically bond with the silica, creating hydroxyl groups on the surface. Polar water molecules form a strong bond with the surface in a physisorption layer on top of these hydroxyl groups. Water molecules block incoming hydrogen atoms from reaching the surface and reacting with adsorbed hydrogen. Hydrogen recombination rate is reduced by 10-100 times in our two-layer model when the mole fraction of water vapor is as low as 5% in the gas phase, consistent with experimental observations. Experiments and simulations have shown that sub-millimeter scale plasma devices are capable of sustaining plasmas with the field emission of electrons. We model a radio-frequency field-emission dielectric barrier discharge (FE-DBD) actuator in conditions relevant to hydrogen combustion using particle-in-cell simulations. Quantities of interest such as momentum transfer, radical generation, and Joule heating are calculated and the effects of gas temperature and composition are investigated. Source terms from the particle-in-cell model are added to a continuum model of hydrogen combustion. With our physics based models, we anticipate that FE-DBDs can sustain combustion in a non-premixed microburner and reduce the autoignition delay time of a stoichiometric, 950 K H2/O2 mixture by a factor of two. We simulate H2/O2 and CH4/O2 combustion in an experimentally tested non-premixed microburner. Flame structure and low combustion completeness (<50%) in the microburner is consistent with experimental observations for these fuels and flow rates. Notably, the cell flame instability is simulated for the highest flow rate with methane, the first time this phenomenon has been captured in a 3D simulation. FE-DBD source terms are added to the hydrogen burner to simulate the effect of plasma actuators in three separate configurations. In each case, plasma coupling increases combustion completeness, with the best performance (72% completeness) in the design that forces mixing of fuel and oxygen. Flame temperature is reduced by plasma coupling, a result of flame stretching and mixing. Finally, design and fabrication of an FE-DBD actuator is discussed. The actuator array is tested with an applied voltage frequency of 60 kHz, where the device operates in the microdischarge regime. Observations from these experiments are summarized and resulting damage to the device is characterized.
Issue Date:2018-09-05
Type:Thesis
URI:http://hdl.handle.net/2142/102396
Rights Information:Copyright 2018 Kyle Mackay
Date Available in IDEALS:2019-02-06
Date Deposited:2018-12


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