HIGH-PERFORMANCE TUBULAR EVAPORATOR UTILIZING HIGH ASPECT RATIO MANIFOLD MICROCHANNELS
Jha, Vibhash Chandra
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Heat recovery using absorption chillers has not been economical for small scale applications due to high capital requirements and heavy weight/volume as deterring factors for its expanded use in waste heat-to-cooling applications. Development of advanced, high performance heat and mass exchanger components can significantly improve the competitive edge of heat activated absorption cooling systems, particularly with respect to weight reduction and size/volume of these systems. The main contribution of this thesis is demonstration of a novel high performance micro-grooved evaporator, as well as a solution heat exchanger, for use in a small-scale ammonia-water absorption cooling system. A compact tubular evaporator was developed which uses an innovative manifold/fluid feed system combined with a micro-grooved evaporator to realize substantially higher (4 to 5 fold) overall heat transfer coefficient of the evaporator; while requiring much less refrigerant charge per ton of cooling, when compared to conventional state of the art systems. The experimentally measured heat transfer coefficients reported in this study are record high, while pressure drops for the given capacity are modest. Additional contributions of the study included a detailed numerical study of single- stage absorption cycle with multiple cycle design enhancements to identify the controlling system parameters. A single-phase numerical study for manifold microchannel design was carried out to understand the effect of important geometrical parameters in support of design and development of the evaporator. The tubular evaporator was successfully fabricated and tested to the system pressure of 500 psi on the refrigerant-side and was experimentally evaluated with several microgroove surface made of aluminum and nickel alloys, and also with different flow header enhancements using R134a/water pair. For the experiments conducted, the microchannel width was typically in the range of 30-100 µm with a maximum aspect ratio of 10. The refrigerant flow rate was varied within 5-30 g/s and water flow rate was varied within 150-600 ml/s obtaining wide range of cooling capacity between 1- 5 kW for 2-12 °C LMTDs. The overall heat transfer coefficients greater than 20,000 W/m2-K was obtained which is roughly 4-5 times higher than state of art for given application. A maximum pressure drop of 200 mbars on water-side and 100 mbars on the refrigerant-side was observed at maximum mass flow rates. An alternative method for the evaporator design was also explored in form of flat plate evaporators which can further provide improved overall heat transfer coefficients. Manifold microchannels were used on both sides of the plates, with the aim to achieve overall heat transfer coefficient greater than 50,000 W/m2-K. The new micro-grooved evaporator has the potential to introduce a game-changing evaporative surface, with precise flow delivery and high heat transfer coefficients, driven by a combination of thin film evaporation, as well as convective boiling on the heat transfer surface.