Facilitation of Homogeneous Combustion with Oxygen Enrichment for High-Temperature Industrial Furnaces

A substantial portion of literature discussing highly efficient and low pollutant emission combustion systems comprises of Homogeneous Combustion (HC) or its variants (MILD, FLOX, CDC etc.). The underlying theory among the aforementioned methods is the reduction of the Damköhler number (Da) to the order of unity. To attain high temperatures, industrial heating is often accomplished by using “enriched” oxidizer streams i.e. XO2 > 21%. Extending the concept of HC to applications of industrial heating (e.g. glass melting furnaces) is a desirable but a challenging task. Higher reactant concentrations increase reaction rates. Fast reactions lead to an increase in temperature which in-turn accelerates the reaction rates and the heat release rate. This self-accelerating cycle causes a shift to the conventional mode of combustion (higher Da) with high NOx emissions. The broad research goal in this work is to keep Da ~ 1 to facilitate HC with enriched oxidizers. The first strategy employed towards this was to enhance the heat removal from the reaction zone by enabling the presence of soot in the reaction zone. The conjecture was that presence of soot will augment heat radiation, reduce temperatures, and reduce NOx emissions; similar to what has been reported for highly luminous flames. Since natural gas (methane) does not soot except under high pressure environments, fuel blends containing small amounts of lightly sooting fuels like ethylene were investigated. It was found that while the presence of soot definitely improves radiation heat transfer and reduces specific NOx emissions, there is an optimal blend level. A multi-variate regression model was used to demarcate the radiation emanating from the wall and from the gaseous zone. The second strategy employed was to engineer the flow in the furnace to enhance mixing and reduce Da. Experimentally studying the confined turbulent jet(s) flow in the furnace with limited optical access was infeasible and hence computational simulations were utilized. A number of steps to reduce computational expenses were taken. These included utilization of furnace symmetry and writing external code to describe furnace recuperator operation. The 3-D flow within the furnace was described/understood by breaking it into a set of canonical flows. The utilization of a detailed mechanism (GRI Mech-3.0) enabled accurate capture of the NOx field to within a few ppm. It was discovered that optimizing geometry and flow is important to achieve HC with enriched oxidizers. The third strategy focused on further enhancing jet dilution by modifying nozzle design. It was found that altering the nozzle shape caused essentially no reduction in NOx emissions from the furnace. It was also found that NOx emissions were independent of the inlet temperature of the reactants. Another strategy was to have a localized swirling injection for the oxidizer jet. While swirl did help in reducing NOx, there existed an optimal swirl number beyond which NOx emissions were aggravated to levels even higher than configurations with no swirl. Swirl, even though localized, was seen to affect the in-furnace flow even in the far-field (~75 diameters). A mutual competition was seen between swirl assisted and entrainment driven dilution; and at higher swirl intensities, the reduction in the latter overwhelmed the gains accrued by the former (in terms of NOx emissions).PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/155272/1/kumaraa_1.pd

Universal Statistics of Competition in Democratic Elections

Elections for public offices in democratic nations are large-scale examples of collective human behavior. As a statistical physics problem with complex interactions among agents, we can anticipate that universal macroscopic patterns can emerge independent of microscopic details. Despite the availability of empirical election data, such universality, valid at all scales, countries, and elections, has not yet been observed. In contrast to all previous attempts in this direction, in this work, it is shown that the distribution of vote margins is driven by that of voter turnout. We use empirical data from $34$ countries to demonstrate that a scaled measure depending on margin and turnout leads to robust universality. Further, a voting model is introduced, which reproduces all the observed universal features. The deviations from universality indicate possible electoral malpractices. We argue that the universality is a stylized fact indicating the competitive nature of electoral outcomes.Comment: 10 pages, 6 figure

Inference from gated first-passage times

First-passage times provide invaluable insight into fundamental properties of stochastic processes. Yet, various forms of gating mask first-passage times and differentiate them from actual detection times. For instance, imperfect conditions may intermittently gate our ability to observe a system of interest, such that exact first-passage instances might be missed. In other cases, e.g., certain chemical reactions, direct observation of the molecules involved is virtually impossible, but the reaction event itself can be detected. However, this instance need not coincide with the first collision time since some molecular encounters are infertile and hence gated. Motivated by the challenge posed by such real-life situations we develop a universal -- model-free -- framework for the inference of first-passage times from the detection times of gated first-passage processes. In addition, when the underlying laws of motions are known, our framework also provides a way to infer physically meaningful parameters, e.g. diffusion coefficients. Finally, we show how to infer the gating rates themselves via the hitherto overlooked short-time regime of the measured detection times. The robustness of our approach and its insensitivity to underlying details are illustrated in several settings of physical relevance

Reversible Target-Binding Kinetics of Multiple Impatient Particles

International audienceCertain biochemical reactions can only be triggered after binding of a sufficient number of particles to a specific target region such as an enzyme or a protein sensor. We investigate the distribution of the reaction time, i.e., the first instance when all independently diffusing particles are bound to the target. When each particle binds irreversibly, this is equivalent to the first-passage time of the slowest (last) particle. In turn, reversible binding to the target renders the problem much more challenging and drastically changes the distribution of the reaction time. We derive the exact solution of this problem and investigate the short-time and long-time asymptotic behaviors of the reaction time probability density. We also analyze how the mean reaction time depends on the unbinding rate and the number of particles. Our exact and asymptotic solutions are compared to Monte Carlo simulations

First-passage times of multiple diffusing particles with reversible target-binding kinetics

International audienceWe investigate a class of diffusion-controlled reactions that are initiated at the time instance when a prescribed number K among N particles independently diffusing in a solvent are simultaneously bound to a target region. In the irreversible target-binding setting, the particles that bind to the target stay there forever, and the reaction time is the K-th fastest first-passage time to the target, whose distribution is well-known. In turn, reversible binding, which is common for most applications, renders theoretical analysis much more challenging and drastically changes the distribution of reaction times. We develop a renewal-based approach to derive an approximate solution for the probability density of the reaction time. This approximation turns out to be remarkably accurate for a broad range of parameters. We also analyze the dependence of the mean reaction time or, equivalently, the inverse reaction rate, on the main parameters such as K, N , and binding/unbinding constants. Some biophysical applications and further perspectives are briefly discussed