Exergy optimization in a steady moving bed heat exchanger

Proceedings of: Interdisciplinary Transport Phenomena V: Fluid, Thermal, Biological, Materials and Space Sciences (ITP 2007), 14-19 of October, 2007, Bansko, Bulgaria (Oral paper nº 70)This work provides an exergy analysis of a moving bed heat exchanger to obtain for a range of incoming fluid flow rates the operational optimum and the incidence on it of the relevant parameters such as the dimensions of the exchanger, the particle diameter and the flow rate of the fluid. The MBHE proposed can be analyzed as a cross flow heat exchanger where one of the phases is a moving granular medium. In the present work the exergy analysis of the MBHE is carried out over operation data of the exchanger obtained in two ways: a numerical simulation of the steady state problem and the analytical solution of the simplified (avoiding conduction terms) equations. The numerical simulation is carried over the two steady energy equations (fluid and solid), involving for the solid the convection heat transfer to the fluid and the diffusion term in both directions, and for the fluid only the convection heat transfer to the solid. The analytical solution is the wellknown solution of the simplified problem neglecting conduction effects.Publicad

Thermal analysis and optimization of a heat regenerator composed of two coupled moving bed heat exchangers

This work presents a study to optimize the performance of a heat regenerator composed by two coupled moving bed heat exchangers (MBHE). A MBHE is used to recover heat, from a hot gas stream, and the other one is used to preheat an air stream. A direct application might be a gasifier. The heat exchangers performance was studied in two cases, considering or not the conduction heat transfer in the solid phase. When the solid conduction is taken into account, a numerical solution is obtained, while an analytical solution is possible when the conduction terms are neglected. In both cases, the optimum values of bed length (in the air flow direction) and particle diameter were obtained from an exergy point of view. Finally, an energy optimization of the heat regenerator was carried out, obtaining the optimal heat regenerator dimensions as a function of gas velocity and gas flow rate.Publicad

Coherent structures and bubble-particle velocity in 2-D fluidized beds

This work presents an experimental study to characterize ascending bubbles and granular velocity in the dense phase of a 2-D fluidized bed. Three different non-intrusive techniques based on images obtained with a high speed camera are developed, and applied to the images. First the bubble paths are characterized with time-average concentration maps and the bubble velocities are obtained, using a tracking algorithm over the mass centers of the bubbles. Finally, a PIV (particle image velocimetry) method is used to characterize the particle velocity vectors. This procedure is repeated for different bed aspect ratios, and different superficial gas velocities. This study analyzes the superficial gas velocity influence on the bed behavior, and how the bubble path configuration depends on the bed aspect ratio. The PIV measurements give us information on the location of the recirculation regions and the influence of the superficial gas velocity.Publicad

Conflicto armado y militares en Colombia. Cultos, símbolos e imaginarios

ResumenEl tema de la violencia, el conflicto armado y la cuestión de los militares resultan temas no sólo interesantes sino polémicos y centrales en sociedades como la colombiana. En este sentido, recogemos algunas de las ideas del trabajo de Elsa Blair "Conflicto armado y militares en Colombia. Cultos, símbolos e imaginarios", publicado en 1999 en coedición entre la Universidad de Antioquia y el CINEP y que se presenta como una rigurosa y pertinente propuesta sociológica alrededor de dicho fenómeno

Desinstitucionalización de los partidos y militarización de la política en Venezuela

El presente ensayo aborda el devenir de los partidos y el sistema de partidos en Venezuela desde 1958 hasta nuestros días, analizando el proceso de consolidación, institucionalización y estabilidad, y fundamentalmente la etapa de inestabilidad y desinstitucionalización de los partidos políticos venezolanos registrada en la última década que incluye un proceso de militarización de la política o instauración a partir de 1998 de un régimen menos representativo, más participativo, mesiánico, militarista y personalista.Palabras clave: Partidos políticos, democracia, militarización, Venezuela, Chávez

Conflicto armado y militares en Colombia. Cultos, símbolos e imaginarios

Resumen El tema de la violencia, el conflicto armado y la cuestión de los militares resultan temas no sólo interesantes sino polémicos y centrales en sociedades como la colombiana. En este sentido, recogemos algunas de las ideas del trabajo de Elsa Blair "Conflicto armado y militares en Colombia. Cultos, símbolos e imaginarios", publicado en 1999 en coedición entre la Universidad de Antioquia y el CINEP y que se presenta como una rigurosa y pertinente propuesta sociológica alrededor de dicho fenómeno

Exergy Optimization of a Moving Bed Heat Exchanger

The MBHE proposed can be analyzed as a crossflow heat exchanger where one of the phases is a moving granular medium. In the present work the exergy analysis of the MBHE is carried out over operation data of the exchanger obtained in two ways: a numerical simulation of the stationary problem and a simplified analysis. The numerical simulation is carried over the two steady state energy equations (fluid and solid), involving (for the fluid) the convection heat transfer to the solid and the diffusion term in the flow direction, and (for the solid) only the convection heat transfer to the fluid. The simplified analysis followed the well-known e-NTU method, taking the equipment as a crossflow heat exchanger with both fluids unmixed.Publicad

Solid conduction effects and design criteria in moving bed heat exchangers

This work presents a theoretical study of the energetic performance of a moving bed heat exchanger (MBHE), which consists of a flow of solid particles moving down that recovers heat from a gas flow percolating the solids in cross-flow. In order to define the solid conduction effects, two solutions for the MBHE energy equations have been studied: an analytical solution considering only convection heat transfer (and neglecting solid conduction) and a numerical solution with the solid conductivity retained in the equations. In a second part, the power requirements of a MBHE (to pump the gas and to raise the down-flowing particles) are confronted with the heat transferred considering the variation of design parameters, such as gas and solids’ velocities, solids particle diameter or MBHE dimensions. The numerical results show that solid conductivity reduces the global efficiency of the heat exchanger. Therefore, a selection criterion for the solids can be established, in which their thermal conductivity should be minimized to avoid conduction through the solid phase, but to a limit in order to ensure that temperature differences inside an individual solid particle remain small. Regarding the other energy interactions involved in the system, these are at least one order of magnitude lower than the heat exchanged. Nevertheless, for a proper analysis of the system the efficiency of the devices used to pump the gas and to raise the particles and the relative costs of the different energy forms present in the system should be taken into account.Publicad

El neoinstitucionalismo y la revalorización de las instituciones

ResumenNos proponemos abordar la discusión que en estos últimos años se viene dando en la ciencia política alrededor del neoinstitucionalismo como una de las perspectivas que hoy en día, aparte de retomar y revalorizar el estudio y el papel de las instituciones, está produciendo importantes planteamientos en sus diversas aproximaciones (neoinstitucionalismo económico, sociológico y politológico) que vinculan actitudes, racionalidad, actores y motivaciones con estructuras sociales, políticas y económicas, partiendo de que las instituciones son una variable y, además, una forma de penetrar en la explicación política es a partir del análisis neoinstitucional como pueden incorporase dinámicamente otras variables con lo cual podamos explicar las dinámicas tanto de estabilidad como de cambio institucional.Palabras clave: Institucionalismo, neoinstitucionalismo, institución, estructuras, racionalidad, cambio institucional.&nbsp

El desconcierto de la política. La revalorización de la política democrática

La política atraviesa momentos inciertos que la afectan, y en muchos casos la transforman como actividad noble y engrandecedora que fue. De manera que, paralelo a la transformación de la política, la democracia asume en ciertas sociedades y contextos una serie de desafíos. Parte de los extravíos actuales estriban en que la democracia al igual que la política fueron sometidas a procesos diversos que la han socavado, quedando reducidas a una rutinas electorales cada vez más vacías y que se presentan como rituales y pragmatismos muy alejados del verdadero sentido de la política y la democracia