Integración de Ecuaciones Diferenciales rígidas de valor de contorno en problemas de combustión con cinética de reacción detallada

Modelos detallados de la llama de difusión plana, laminar de jets opuestos requieren la solución de las ecuaciones de conservación de cantidad de movimiento, energía y especies químicas en conjunto con una cinética de reacción química stiff. El problema tiene soluciones autosimilares y se puede resolver mediante integración numérica de un conjunto de ecuaciones diferenciales ordinarias stiff de segundo orden, con condiciones de contorno, cada una de las cuales tiene un punto de torno de primer orden debido a la ecuación de conservación de cantidad de movimiento. El uso de discretización en diferencias finitas standard (en el dominio espacial) y la expansión en serie de Taylor alrededor de la iteración anterior (en el espacio temporal) del término cinético de las reacciones químicas lleva a una ecuación de matrices en la cual la matriz de coeficientes es del tipo tridiagonal en bloques y muy grande de tamaño. También representa una matriz de banda y se puede obtener una solución al problema mediante una descomposición LU. Inestabilidades numéricas debidas a la malla finita de espaciamiento irregular y a la difusión de errores numéricos hacia los bordes forzaron el uso de una malla regular con un gran número de puntos. Esto restringió el estudio a la solución de problemas con esquemas cinéticos relativamente simples, debido a la alta necesidad de memoria de computador. Este dilema fue resuelto por el desarrollo de un esquema de diferenciación modificado, que asume que la solución puntual del problema está dada por una suma de exponenciales. A Usando esta nueva técnica fue posible hallar la solución de una llama con 150 reacciones envolviendo las ecuaciones de ccnservación de 70 especies químicas. En este artículo formulamos el problema y describimos resultados de los varios esquemas numéricos mediante los que se intentó resolver el mismo. El foco principal, sin embargo, está u dirigido hacia la modificación del esquema de diferencias finitas, acoplado con la expansión en serie de Taylor de los términos de reacción química.Detailed models of the flat, laminar, opposed jet diffusion flame involve the solution of the momentu, energy and species conservation equations coupled with stiff chemical kinetics. The problem has self similar solutions and can be solved through numerical integration of a set of second order, stiff, boundary valued, ordinary differential equations, each with a regular first order turning point arising from convection. Use of standard finite difference discretization (in the spatial domain) and expansion of the reaction rate source terms in a Taylor series abaut the backward iteration (in the temporal domain), lead to a matrix equation in which the coefficient matrix is a very large block tridiagonal matrix. It is also a band matrix and solution is obtained through LU descomposition. Instabiiities originating from the unevenly spaced grid and from diffusion of numerical errors towards the boundaries forced the use of a large number of equally spaced grid points which contrained the program to solution of relatively small kinetic problems (due to core storage limitations). This dilemma was resolved by developing a modified central difference discretization which assumes that the solution at a mesh point is given by the sum of exponentials. Using the new technique it was possible to obtain the solution of the opposed jet problem with 150 reactions and 70 species on the available CYBER 175 computer.Peer Reviewe

Integración de Ecuaciones Diferenciales rígidas de valor de contorno en problemas de combustión con cinética de reacción detallada

Modelos detallados de la llama de difusión plana, laminar de jets opuestos requieren la solución de las ecuaciones de conservación de cantidad de movimiento, energía y especies químicas en conjunto con una cinética de reacción química stiff. El problema tiene soluciones autosimilares y se puede resolver mediante integración numérica de un conjunto de ecuaciones diferenciales ordinarias stiff de segundo orden, con condiciones de contorno, cada una de las cuales tiene un punto de torno de primer orden debido a la ecuación de conservación de cantidad de movimiento. El uso de discretización en diferencias finitas standard (en el dominio espacial) y la expansión en serie de Taylor alrededor de la iteración anterior (en el espacio temporal) del término cinético de las reacciones químicas lleva a una ecuación de matrices en la cual la matriz de coeficientes es del tipo tridiagonal en bloques y muy grande de tamaño. También representa una matriz de banda y se puede obtener una solución al problema mediante una descomposición LU. Inestabilidades numéricas debidas a la malla finita de espaciamiento irregular y a la difusión de errores numéricos hacia los bordes forzaron el uso de una malla regular con un gran número de puntos. Esto restringió el estudio a la solución de problemas con esquemas cinéticos relativamente simples, debido a la alta necesidad de memoria de computador. Este dilema fue resuelto por el desarrollo de un esquema de diferenciación modificado, que asume que la solución puntual del problema está dada por una suma de exponenciales. A Usando esta nueva técnica fue posible hallar la solución de una llama con 150 reacciones envolviendo las ecuaciones de ccnservación de 70 especies químicas. En este artículo formulamos el problema y describimos resultados de los varios esquemas numéricos mediante los que se intentó resolver el mismo. El foco principal, sin embargo, está u dirigido hacia la modificación del esquema de diferencias finitas, acoplado con la expansión en serie de Taylor de los términos de reacción química.Detailed models of the flat, laminar, opposed jet diffusion flame involve the solution of the momentu, energy and species conservation equations coupled with stiff chemical kinetics. The problem has self similar solutions and can be solved through numerical integration of a set of second order, stiff, boundary valued, ordinary differential equations, each with a regular first order turning point arising from convection. Use of standard finite difference discretization (in the spatial domain) and expansion of the reaction rate source terms in a Taylor series abaut the backward iteration (in the temporal domain), lead to a matrix equation in which the coefficient matrix is a very large block tridiagonal matrix. It is also a band matrix and solution is obtained through LU descomposition. Instabiiities originating from the unevenly spaced grid and from diffusion of numerical errors towards the boundaries forced the use of a large number of equally spaced grid points which contrained the program to solution of relatively small kinetic problems (due to core storage limitations). This dilemma was resolved by developing a modified central difference discretization which assumes that the solution at a mesh point is given by the sum of exponentials. Using the new technique it was possible to obtain the solution of the opposed jet problem with 150 reactions and 70 species on the available CYBER 175 computer.Peer Reviewe

Pulverized Coal Combustion: The Influence of Flame Temperature

A laboratory combustor was used to investigate the factors that influence the conversion of fuel nitrogen in coal during coal combustion. Fuel NO was isolated by experimentation utilizing Argon/Oxygen/Carbon Dioxide mixtures as the oxidant, and care was taken to compare cases with air at matched conditions. For both well mixed and slowly mixed flame types, fuel NO contributed over 75% of the total NO emissions for all conditions examined. Fuel NO was insensitive to temperature changes except when the adiabatic flame temperatures were above 2480 0 K (4000°F). At the highest adiabatic flame temperature, 2580 0 K (4200°F), a 10% increase in fuel NO was observed. Four different coals and one coal char were investigated. Fuel NO could not be correlated with fuel nitrogen content alone, even though aerodynamic conditions were kept constant. Fuel nitrogen conversion to NO during pulverized char combustion was 12-16% at a stoichiometric ratio of 1.15 compared to 28% for a pulverized coal of the same nitrogen content. Furthermore, in contrast to the coal results, NO emissions from char combustion were not greatly influenced by changes in injector design. The implication is that although conversion of fuel nitrogen to NO may be relatively low during the char burnout regime of coal combustion, the residual "char NO" may be especially resistant to abatement by modifications of the burner aerodynamics

The future of sovereignty in multilevel governance Europe: a constructivist reading

Multilevel governance presents a depiction of contemporary structures in EU Europe as consisting of overlapping authorities and competing competencies. By focusing on emerging non-anarchical structures in the international system, hence moving beyond the conventional hierarchy/anarchy dichotomy to distinguish domestic and international arenas, this seems a radical transformation of the familiar Westphalian system and to undermine state sovereignty. Paradoxically, however, the principle of sovereignty proves to be resilient despite its alleged empirical decline. This article argues that social constructivism can explain the paradox, by considering sovereign statehood as a process-dependent institutional fact, and by showing that multilevel governance can feed into this process

Mechanism and Kinetics of Lead Capture by Kaolinite in a Downflow Combustor

An 18 kW, 6 m long, gas-fired downflow combustor was used to examine the postflame reaction between lead vapor and kaolinite. Since the focus of this work was to evaluate the reaction with lead vapor, samples were extracted at a temperature above the metal dew point. The sample was rapidly diluted with nitrogen such that lead vapor homogeneously nucleated to form small particles in the presence of a pre-existing particle population. These small lead particles were easily distinguished from lead reacted to large sorbent particles; hence, multicomponent particle size distributions were used to determine the extent of lead capture. A parametric study was conducted in which sorbent/metal molar ratio and sorbent injection temperature were controlled. Temperatures and residence times were varied by changing the sorbent injection point in the combustor, which had the approximate time and temperature profile of practicalscale units. The effect of chlorine was evaluated by doping chlorine gas into the flame. Results show that lead capture by kaolinite was reduced at higher temperatures and in the presence of chlorine. A two-reaction mechanism is proposed to model the apparent temperature inhibition. In the primary capture reaction, lead oxide reacts with activated kaolinite and forms a lead aluminosilicate product. Subsequently, a reaction between this product and activated kaolinite acts to inhibit further lead capture. First-order rate expressions are proposed for each reaction, and kinetic parameters are estimated from experimental results. The primary capture reaction appears to have an activation energy that is approximately zero. The inhibition reaction has an activation energy of ϳ10 2 kJ mol ‫1מ‬ . To model the effect of chlorine, the reaction scheme is modified to account for the partitioning of lead between lead oxide and lead chloride. Based on experimental results, the concentration of lead chloride vapor in the system is significantly higher than predicted by equilibrium calculations. Introduction The emission of toxic elements from stationary combustion sources, such as incinerators and coalfired boilers, is a major concern. Of particular concern are semivolatile toxic elements, for example, lead and cadmium, which vaporize and condense within the combustion system. At high temperatures, lead vaporizes and is liberated from ash particles. As the temperature of the combustor decreases, the vapor condenses to form, in part, submicron particles. These submicron particles can penetrate conventional air pollution control systems, like baghouses and electrostatic precipitators, and be emitted to the environment One potential method to control toxic metal emissions is to inject a high-temperature sorbent into the postflame region. The sorbent powder, which is easily collected, reacts with metal vapors and prevents the subsequent vapor-to-particle transformation processes that form submicron particles. Previous researchers have examined the reaction of lead and a clay-based sorbent, kaolinite. In a series of benchtop experiments where large kaolinite flakes were exposed to lead chloride vapor, Uberoi and co-author

Nitrogen oxide abatement by distributed fuel addition

The research reported here is concerned with the application of secondary fuel addition, otherwise known as reburning, as a means of NO{sub x} destruction downstream of the primary flame zone in boilers. This paper consists of two parts: First, results from a statistically correct design of parametric experiments on a laboratory coal combustor are presented. These allow the effects of the most important variables to be isolated and identified. Second, mechanisms governing the inter-conversion and destruction of nitrogenous species in the fuel rich reburning zone of a laboratory coal combustor were explored, using fundamental kinetic arguments. The objective here was to extract models, which can be used to estimate reburning effectiveness in other, more practical combustion configurations. Emphasis is on the use of natural gas as the reburning fuel for a pulverized coal primary flame. Then, reburning mechanisms occur in two regimes; one in which fast reactions between NO and hydrocarbons are usually limited by mixing; the other in which reactions have slowed and in which known gas phase chemistry controls. For the latter regime, a simplified model based on detailed gas phase chemical kinetic mechanisms and known rate coefficients was able to predict temporal profiles of NO, NH{sub 3} and HCN. Reactions with hydrocarbons played important roles in both regimes and the Fenimore N{sub 2} fixation reactions limited reburning effectiveness at low primary NO values

Nitrogen oxide abatement by distributed fuel addition

A combustor has been designed in order to retard the formation of nitrogen oxides by injection of reburning fuel. The design and the rebuilding of the new combustor was completed. Several new features were incorporated in the new design so that it would last longer. The design and construction of the furnace are discussed in this report. (VC

Pattern Analysis of Microtubule-Polymerizing and -Depolymerizing Agent Combinations as Cancer Chemotherapies

Subcellular distribution of mass can be analyzed by a technique that involves culturing cells on interferometers and digitizing their interference contours. Contour sampling resulted in 102 variables per cell, which were predictors of oncogenic transformation. Cell phenotypes can be deconstructed by use of latent factors, which represent the covariance of the real variables. The reversal of the cancertype phenotype by a combination of microtubule- stabilizing and -depolymerizing agents was described previously. The implications of these results have been explored by clinicians who treated patients with the combination of docetaxel and vinorelbine (Navelbine®). The current study was performed to determine the effects of different combinations on phenotype and in phases of the cell cycle other than mitosis. Combinations of paclitaxel with either colchicine, podophyllotoxin, nocodazole, or vinblastine caused phenotype reversal. Paclitaxel analogue, 7-deoxytaxol, by itself caused reversal. Factors #4, (filopodia), #5 (displacement and/or deep invaginations in the periphery), #8, and #12 took on values typical of normal cells, whereas the values of #7 (p21-activated kinase), and #13 (rounding up) shifted toward the cancer-type. All combinations altered microtubule arrangement at the cell edge. Delivery schedules and drug ratios used in clinical studies were subjected to analysis. Clinical response rates were better when the combination was not interspersed with a single agent (P=0.004). The results support the idea that efficacy depends upon simultaneous exposure to both agents, and suggest a novel mechanism for combination therapies. These therapies appear to restore in transformed cells some of the features of a contact-inhibited cell, and to impede progress through the cell cycle even when provided at nanomolar concentrations

Desempenho e manejo de capivaras em sistema semi-intensivo na Região costeira sul do Rio Grande do Sul.

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Marine protected areas, marine heatwaves, and the resilience of nearshore fish communities

Anthropogenic stressors from climate change can affect individual species, community structure, and ecosystem function. Marine heatwaves (MHWs) are intense thermal anomalies where water temperature is significantly elevated for five or more days. Climate projections suggest an increase in the frequency and severity of MHWs in the coming decades. While there is evidence that marine protected areas (MPAs) may be able to buffer individual species from climate impacts, there is not sufficient evidence to support the idea that MPAs can mitigate large-scale changes in marine communities in response to MHWs. California experienced an intense MHW and subsequent El Niño Southern Oscillation event from 2014 to 2016. We sought to examine changes in rocky reef fish communities at four MPAs and associated reference sites in relation to the MHW. We observed a decline in taxonomic diversity and a profound shift in trophic diversity inside and outside MPAs following the MHW. However, MPAs seemed to dampen the loss of trophic diversity and in the four years following the MHW, taxonomic diversity recovered 75% faster in the MPAs compared to reference sites. Our results suggest that MPAs may contribute to long-term resilience of nearshore fish communities through both resistance to change and recovery from warming events