Analysis of heat and mass transfer in membrane-based absorbers with new working fluid mixtures for absorption cooling systems
- Autores: Faisal Asfand
- Directores de la Tesis: Mahmoud Bourouis (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2016
- Idioma: español
- Tribunal Calificador de la Tesis: Ahmed Bellagi (presid.), Youssef Stiriba (secret.), Marco Medrano Martorell (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: hdl.handle.net
- Resumen
Absorption refrigeration technology, which has the ability to utilize heat directly for cooling purposes, has been one of the most widely used technologies for refrigeration and cooling applications since the early stages of refrigeration technology. Working fluid mixtures employed in the absorption cooling systems are environmental friendly and do not contribute in green house gas emission when compared to vapour compression systems which also use costly mechanical energy input. However, high initial costs and bigger size are some of the main obstacles that impede their wide use in small scale residential buildings and transport sector. In order to overcome these obstacles, design and configuration of the system and its components need to be reinvestigated in order to achieve compact components and reduce the size of the system. Absorber is an important component of the absorption refrigeration system and plays a critical role in the overall performance, size, and capital cost of the system. Both heat and mass transfer take place simultaneously in the absorber. The design and configuration of the absorber significantly influence its performance. Absorber of an absorption cooling systems employing water as a refrigerant works under vacuum conditions and therefore the size of the equipment is usually very big. It is the focus of this thesis to investigate advance absorbers with new working fluid mixture in order to enhance the heat and mass transfer mechanism and reduce the size of the equipment. Use of membrane contactors in the form of hollow fiber membrane module or plate-and-frame membrane module is one of the alternatives to achieve compact absorbers. Heat and mass transfer performance of the components is significantly enhanced due to the higher area to volume ratio available. In this study, plate-and-frame membrane-based absorber is selected to investigate in detail the heat and mass transfer mechanism and fluid dynamics behaviour employing working fluid mixtures that contain water as a refrigerant. This work will provide a sound foundation to better understand the absorption process in a membrane based absorber. The performance of membrane based absorber depends on many parameters such as the thermodynamic and transport properties of the working fluid mixture, the operating conditions, the design parameters (such as the channel thickness, membrane material characteristics) etc. Most of the research in this area is done using the conventional working fluid mixtures, water/LiBr and ammonia/water with water as a cooling medium. Moreover, there is a growing interest for air-cooled absorption chillers, for which water/(LiBr+LiI+LiNO3+LiCl) mixture is proposed as a working fluid due to its larger range of solubility. In addition, for high temperature heat sources, aqueous solution of alkitrate salts (LiNO3+KNO3+NaNO3) has been proposed as an attractive alternative to effectively utilize the high temperature in the third stage of a triple effect absorption refrigeration cycle. However, there is very scarce information in the literature about the absorption process with these non-conventional working fluid mixtures. In this study, numerical analyses are performed to evaluate the performance of a plate-and-frame membrane contactor based absorber employing water/(LiBr + LiI + LiNO3 + LiCl) and water/(LiNO3+KNO3+NaNO3) working fluid mixtures. CFD tool ANSYS/FLUENT 14.0 is used to perform the simulation and investigate in detail the heat and mass transfer mechanisms and fluid dynamics behaviour at local levels in the channels. The simulation tool is very useful to perform detail analyses and can play an important role to predict absorption rate, concentration and temperature profiles of the solution at local levels. Results show that absorption rate can be significantly enhanced if the solution is confined in a thinner channel with higher mass flow Reynolds number. Although the solution channel thickness and solution mass flow rate can be independently controlled in a plate-and-frame membrane-based absorber however it was observed that the pressure drop increases exponentially with a decrease in the solution channel thickness, while it increases linearly with an increase in the solution velocity. In this study, an optimum value of 0.5 mm for the solution film thickness and a solution velocity in the range 0.003 – 0.005 m/s is recommended to achieve higher absorption rate with minimum pressure drop. In addition, it was observed that the percent pressure drop in case of water/(LiNO3+KNO3+NaNO3) working fluid mixture is significantly lower when compared to the water/LiBr and water/(LiBr+LiI+LiNO3+LiCl) working fluid mixtures because of the higher operating pressure. Therefore, the use of water/(LiNO3+KNO3+NaNO3) working fluid mixture in a plate-and-frame membrane-based absorber which operates at higher pressures cannot only allows higher solution mass flow rate but can also allows the reduction of the solution channel thickness to achieve compact absorber and higher absorption rate. Moreover, MATLAB code is used to investigate the effect of membrane material characteristics and operating conditions on the absorption performance of the absorber. This study recommends optimum operating and design parameters to effectively utilize the membrane based absorber.
