Characterization of heat/mass transfer in a single microchannel absorber

  • Yunshan Liu

Student thesis: Master's Thesis

Abstract

Study on microscale-based absorption refrigeration system has sprung up motivated by the need of efficient energy utilization. Heat-driven absorption systems offer a possibility of generating both power and cooling with environment friendly refrigerants, such as ammonia/water and LiBr/water. However, these systems are often large in size and low in COP especially in single stage absorption systems. These characteristics of absorptions systems make them unattractive in most cases. This work introduces the utilization of micro-channel enhanced surfaces as heat exchangers to enhance the component and system performance, to reduce the size and cost of the system as well. In this work, a new concept of enhancing heat and mass transfer processes is applied in the absorber part of the absorption cycle by using a single micro-channel. Due to its merit of high area to volume ratio, microchannel technology has been well theoretically validated to be a very effective and potential choice for enhancing heat transfer performance. But there is a lack of research work on the mass transfer performance in micro-channels. This work investigated simultaneous mass and heat transfer characteristics of a novel microchannel absorber that uses LiBr/water as the working fluid through both numerical and experimental methodologies. A microchannel with hydraulic diameter of 0.7 mm is employed in this characterization study. Velocity distribution, pressure drop, concentration and temperature profile inside the microchannel as well as effects of the inlet absorbent concentration, flow rate and temperature together with the refrigerant flow rate on the heat/mass transfer are predicted from the simulation. Feasibility of this novel absorber design was proved via this study as the mass transfer taking place inside the mixing channel was observed to achieve the identical performance but with a size reduction by 1/27 compared to a conventional falling film absorber. Enhancement of 7 times in the heat transfer coefficient was also achieved with the comparison of a macro-scale based absorber configuration. This study also reveals the significance of the inlet solution sub-cooling as the overall heat transfer coefficient descends when the inlet solution temperature increases. Key words: microchannel, absorber, heat transfer, mass transfer, LiBr/water.
Date of Award2012
Original languageAmerican English
SupervisorEbrahim Al Hajri (Supervisor)

Keywords

  • Applied sciences
  • Heat transfer.
  • Heat transmission
  • Mass transfer
  • Mechanical engineering
  • 0548:Mechanical engineering

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