Publication: Transferencia de masa y calor en absorbedores adiabáticos con aplicación de la disolución nitrato de litio-amoniaco
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2009-07
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2009-09-18
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Abstract
El continuo cambio climático, la incertidumbre de que los combustibles fósiles pueden agotarse y el aumento de las emisiones de dióxido de carbono, han motivado a varios países a poner especial interés en la investigación de nuevas fuentes de energía y de técnicas que puedan aprovechar mejor los recursos energéticos. Entre las técnicas que apoyan el mejor aprovechamiento de los recursos energéticos, se encuentran los Sistemas de Refrigeración por Absorción, SRA, para producción de frío solar, que están teniendo mayor interés últimamente. Actualmente estos equipos son muy voluminosos y costosos, por lo que disminuye su competividad respecto a otros sistemas de refrigeración. El mejoramiento de la técnica de absorción en estos sistemas puede ser una buena opción para reducir su volumen. El trabajo de la presente tesis se centra en evaluar el potencial del método de absorción en láminas y gotas dispersas de diferentes diámetros en absorbedores adiabáticos, así como la capacidad de generación de vapor de un intercambiador de placas, usados ambos en sistemas de refrigeración por absorción empleando la disolución amoniaco-nitrato de litio. Esto con el fin de reducir el tamaño de dichas máquinas y a su vez contribuir a hacerlas más económicas y prácticas. Del absorbedor, se ha evaluado la transferencia de masa y calor en configuración de absorbedores adiabáticos, utilizando para ello diferentes tipos de atomizado: lámina, cono hueco, niebla y cono sólido. Este último, por ser el más comercial, se ha analizado a tres diferentes alturas. La valoración del potencial de absorción se ha realizado mediante los parámetros relación de absorción, factor de aproximación al equilibrio, coeficiente de transferencia de masa y subenfriamiento final. Los resultados han mostrado que en absorbedores adiabáticos, empleando la disolución amoniaco-nitrato de litio, el atomizado tipo niebla es el que mejores resultados de absorción de masa ofrece. Por otra parte, el atomizado tipo lámina también ofrece buenos resultados, aunque ligeramente inferiores. La capacidad de absorción del atomizado tipo cono sólido, aunque a bajos números de Reynolds no es alta, con números de Reynolds mayores a 1.300 se acerca hasta un 85% de la capacidad del tipo niebla. Lo anterior supone que, con un absorbedor adiabático que emplea un atomizador tipo cono sólido, puede alcanzarse la meta de reducir el tamaño del absorbedor de una máquina de absorción. La evaluación del generador se ha llevado a cabo mediante un análisis de la transferencia de calor y la pérdida de carga en flujo bifásico, variando para ello el flujo de calor y el flujo másico. Los resultados obtenidos muestran que, con calidad del vapor a la salida del generador menor de 3%, pueden alcanzarse coeficientes de transferencia de calor en flujo bifásico para la disolución amoniaco-nitrato de litio 20% mayores que los obtenidos en flujo monofásico. Por otra parte, se ha comprobado el efecto reductor que la mezcla de dos componentes (amoniaco y nitrato de litio) tiene sobre el valor del coeficiente de transferencia de calor obtenido durante la ebullición del amoniaco puro. _______________________________________________
The continuous climate change, the uncertainty about running out of fossil fuels and the increase in carbon dioxide emissions, have prompted several countries to take special interest in researching new energy sources and techniques that can better exploit the energy resources. Among the techniques that support the efficient use of energy resources are the Absorption Refrigeration Systems, ARS, for the production of solar cooling, which are taking more interest lately. Currently these devices are bulky and expensive, thus diminishing its competitiveness compared to other refrigeration systems. Improvement of the absorption technique in these systems can be a good option to reduce its volume. The work of this thesis focuses on evaluating the potential of the absorption method using flat sheet and dispersed drops with different sizes in adiabatic absorbers, and the capacity for steam generation of a plate heat exchanger, both used in absorption refrigeration systems using ammonia-lithium nitrate solution. This is done in order to reduce the size of such machines and at the same time helping to make them more economic and practical. The absorber has been evaluated in terms of the mass and heat transfer using an adiabatic absorber configuration, employing different types of atomization: flat sheet, hollow cone, fog jet and full cone. The latter, being the most commercial, has been analyzed at three different heights. The potential for absorption was assessed using four parameters: absorption ratio, approach to equilibrium factor, mass transfer coefficient and final subcooling. The results have shown that for adiabatic absorbers using ammonia-lithium nitrate solution, the fog jet spray is the best one respect to mass absorption. In addition, the flat sheet spray pattern also gives good results, although slightly lower. The absorption capacity of the full cone type, although at low Reynolds numbers is not high, with Reynolds numbers greater than 1300 is about 85% of the capacity of the fog jet spray. This means that an adiabatic absorber that uses a full cone type nozzle can reach the goal of reducing the size of the absorber of an absorption machine. The evaluation of the generator has been carried out by means of the heat transfer and pressure drop analyses in two-phase flow, changing the heat and mass fluxes. The results show that, with vapor qualities at the exit of generator lower than 3%, two-phase heat transfer coefficients achieved can be 20% higher than those obtained in single-phase flow using the ammonia-lithium nitrate solution. Moreover, it has been verified the sinking effect that the twocomponent mixture (ammonia and lithium nitrate) has over the value of the heat transfer coefficient obtained during boiling of pure ammonia.
The continuous climate change, the uncertainty about running out of fossil fuels and the increase in carbon dioxide emissions, have prompted several countries to take special interest in researching new energy sources and techniques that can better exploit the energy resources. Among the techniques that support the efficient use of energy resources are the Absorption Refrigeration Systems, ARS, for the production of solar cooling, which are taking more interest lately. Currently these devices are bulky and expensive, thus diminishing its competitiveness compared to other refrigeration systems. Improvement of the absorption technique in these systems can be a good option to reduce its volume. The work of this thesis focuses on evaluating the potential of the absorption method using flat sheet and dispersed drops with different sizes in adiabatic absorbers, and the capacity for steam generation of a plate heat exchanger, both used in absorption refrigeration systems using ammonia-lithium nitrate solution. This is done in order to reduce the size of such machines and at the same time helping to make them more economic and practical. The absorber has been evaluated in terms of the mass and heat transfer using an adiabatic absorber configuration, employing different types of atomization: flat sheet, hollow cone, fog jet and full cone. The latter, being the most commercial, has been analyzed at three different heights. The potential for absorption was assessed using four parameters: absorption ratio, approach to equilibrium factor, mass transfer coefficient and final subcooling. The results have shown that for adiabatic absorbers using ammonia-lithium nitrate solution, the fog jet spray is the best one respect to mass absorption. In addition, the flat sheet spray pattern also gives good results, although slightly lower. The absorption capacity of the full cone type, although at low Reynolds numbers is not high, with Reynolds numbers greater than 1300 is about 85% of the capacity of the fog jet spray. This means that an adiabatic absorber that uses a full cone type nozzle can reach the goal of reducing the size of the absorber of an absorption machine. The evaluation of the generator has been carried out by means of the heat transfer and pressure drop analyses in two-phase flow, changing the heat and mass fluxes. The results show that, with vapor qualities at the exit of generator lower than 3%, two-phase heat transfer coefficients achieved can be 20% higher than those obtained in single-phase flow using the ammonia-lithium nitrate solution. Moreover, it has been verified the sinking effect that the twocomponent mixture (ammonia and lithium nitrate) has over the value of the heat transfer coefficient obtained during boiling of pure ammonia.
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Keywords
Transmisión de calor, Fuentes de energía, Refrigeración