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Heat transfer and fluid flow characteristics in various micro devices for the development of micro absorption heat pump systems

Authors Hu, Jinshan
Issue Date 2007
Summary This thesis presents a series of studies on heat transfer and fluid flow characteristics in various micro devices for the development of micro absorption heat pump systems. The study covered a feasibility analysis of an absorption micro heat pump system, convective condensation in micro condensers, electroosmotic (EO) micro pump, flow friction in microchannels and the performance of a laboratory scale micro absorption heat pump system. Mass, energy density, efficiency and the orientation issues of a micro absorption heat pump system were analyzed. It showed that the micro absorption heat pump system can have a quite high energy density and its efficiency can be comparable with that of macro absorption heat pump systems. Micro absorption heat pump systems are worth developing for many potential applications. Five condensation flow regimes were identified in micro silicon condensers, i.e., pure droplet flow, droplet/liquid slug flow, stratified flow, injection flow and slug-bubbly flow. A correlation was obtained to predict the transition of the flow pattern from slug-bubbly flow to the mixed flow patterns. Heat transfer property of the five flow regimes was analyzed. Slug-bubbly flow, pure droplet flow and droplet/liquid slug flow were identified to be the dominant flows. Heat transfer coefficients and pressure drops of slug-bubbly flow were studied with the effect of channel dimension, mass flux and heat flux. Heat transfer property of pure droplet flow and droplet/liquid slug flow was also studied via experiments and theoretical analysis. Size of droplets with the effects of steam mass flux, steam-surface temperature difference and channel dimension was investigated. Classical dropwise condensation theory was modified for the prediction of the dropwise condensation heat flux in microchannels. Condensation heat flux in microchannels was studied experimentally at various steam mass fluxes, steam-surface temperature differences and channel dimensions. The predicted heat fluxes and those from the experiments agreed with each other well. In the study of the EO micro pump, issues including fabrication, theoretical model and performance of the EO micro pump were investigated. A revised mathematical model was established in which Boltzmann equation, Debye-Huckle approximation and symmetric condition were avoided. A fabrication process was designed and EO micro pumps were fabricated successfully. Distribution of the potential and the ions in EO micro pumps were investigated via numerical simulation. It showed that when the EO pump was very thin, the error of traditionally used Poisson Boltzmann equation could be quite large while the error of the developed model remained to be acceptable. Performance of EO micro pumps with the effects of pump depth, temperature, electric field strength and bulk ionic concentration was investigated via experiments and numerical simulations. Experiments were also carried out to study the flow friction characteristic in microchannels for the development of expansion devices. The results showed that pressure drops in microchannels exhibit a linear behavior with the Reynolds number, which was in agreement with that in macro tubes. Friction factor was found to be proportional to Re-1 in microchannels as was observed in macro tubes. Friction factors in microchannels were found to be lower than those predicted by the classical laminar flow theory. The deviation of the friction factor from the theoretical value was dependant on the channel dimension. Based on the experimental data, a correlation was developed to predict the friction factors in smooth microchannels with hydraulic diameters from about 118 to 175 μm. We established a laboratory scale micro absorption heat pump system. Cooling was successfully generated by this micro absorption heat pump system. Performance of the micro absorption heat pump system was studied via experiments. It showed that the COP of the system was comparable to some macro absorption heat pump systems. Key Words: Micro Absorption Heat Pump System, Convective Condensation in Microchannels, Slug-bubbly Flow, Pure Droplet Flow, Droplet/ Liquid Slug Flow, Electroosmotic Micro Pump, Pumping Performance, Flow Friction in Microchannels
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007
Language English
Format Thesis
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