Study of Condensation of Refrigerants in Micro-Channels for Development of Future Compact Micro-Channel Condensers
by Sourav Chowdhury
Mini- and micro- channel technology has gained considerable ground in the recent years in industry and is favored due to its several advantages stemming from its high surface to volume ratio and high values of proof pressure it can withstand. Micro-channel technology has paved the way to development of highly compact heat exchangers with low cost and mass penalties. In the present work, the issues related to the sizing of compact micro-channel condensers have been explored. The considered designs encompass both the conventional and MEMS fabrication techniques. In case of MEMS-fabricated micro-channel condenser, wet etching of the micro-channel structures, followed by bonding of two such wafers with silicon nitride layers at the interface was attempted. It was concluded that the silicon nitride bonding requires great care in terms of high degree of surface flatness and absence of roughness and also high degree of surface purity and thus cannot be recommended for mass fabrication. Following this investigation, a carefully prepared experimental setup and test micro-channel with hydraulic diameter 700 microns and aspect ratio 7:1 was fabricated and overall heat transfer and pressure drop aspects of two condensing refrigerants, R134a and R245fa were studied at a variety of test conditions. To the best of author's knowledge, so far no data has been reported in the literature on condensation in such high aspect ratio micro-channels. Most of the published experimental works on condensation of refrigerants are concerning conventional hydraulic diameter channels (> 3mm) and only recently some experimental data has been reported in the sub-millimeter scale channels for which the surface tension and viscosity effects play a dominant role and the effect of gravity is diminished. It is found that both experimental data and empirically-derived correlations tend to under-predict the present data by an average of 25%. The reason for this deviation could be because a high aspect ratio channel tends to collect the condensate in the corners of its cross-section leaving only a thin liquid film on the flat side surfaces for better heat transfer than in circular or low aspect ratio channels.
Doctoral Dissertation
http://hdl.handle.net/1903/9225
Performance and Oil Retention Characteristics of a CO2 Heat Pump Water Heater
by Nicholas Edward Peter Fernandez
A CO2 heat pump water heater (HPWH) was investigated experimentally and analytically. In the first stage of the study, a baseline performance was measured, investigating the effect of operating parameters on the system performance under typical tank heating scenarios. In the second, the CO2 HPWH was modeled to investigate the effect of optimizing key components. In the third, the oil retention mass, the increase in pressure drop, and the COP degradation were measured as a function of oil mass fraction. In the fourth, two alternative system configurations were investigated for potential performance enhancement; a two-stage compression cycle with internal heat exchanger and a system with a suction line heat exchanger. Overall, the CO2 cycle seems uniquely suited for water heating. CO2 HPWHs have enormous energy savings potential if the cooling from the evaporator can be harnessed during the summer months, and rejected to the environment during the colder months.
Master's Thesis
http://hdl.handle.net/1903/9008
Design and Characterization of an Electrohydrodynamic (EHD) Micropump for Cryogenic Spot Cooling Applications
by Parisa Foroughi
High-temperature superconducting (HTSC) components are being incorporated into communication and monitoring electronic devices to increase their signal-to-noise ratio or their channel capacity. Those devices must be maintained at cryogenic temperatures to prevent the loss of their superconducting properties and retain their performance superiority. They are conventionally cooled via direct heat conduction, which leads to undesirable temperature differences among the various components being cooled. Compact micropumps capable of pumping liquid nitrogen at 77 K into liquid-cooling circuits would enable a much more compact and lightweight method of maintaining a uniform temperature across the cooling circuit. These pumps can also address the demand for delivering small doses of LN2 to particular spots in bioengineering applications.
One of the main objectives of the present study was to develop an electrohydrodynamic (EHD) ion-drag micropump with LN2 as the working liquid. EHD ion-drag pumping phenomenon refers to liquid motion caused by an interaction between electric and hydrodynamic fields in a dielectric liquid.
To investigate the effect of each design parameter on the performance of the micropump, several prototypes with four distinct designs were fabricated and packaged. The designs included a variety of emitter shapes, inter-electrode spacings, electrode-pair spacings, and channel heights. The micropumps were tested at different DC voltages ranging from 0 to 2.5 kV. Two test rigs with novel measurement techniques were also designed, built, and calibrated to measure the generated static pressure head, electric current, and flow rate with an acceptable level of accuracy.
The relationships between pressure/current (P-I) and pressure/voltage (P-V) for various designs were investigated experimentally. The results showed good agreement with the general analytical trends reported for EHD pumping in the literature. The experimental results also demonstrated that electrode geometry and gaps are effective in determining the pressure onset voltage. The results also show that a maximum static pressure head of 160 Pa at 1400 V is achievable for a design with a combination of a 50-μm emitter-collector gap, a 200-μm electrode-pair gap, and a saw-tooth shaped emitter/flat collector.
Doctoral Dissertation
http://hdl.handle.net/1903/8100
Manifold Microchannel Cooling of Photovoltaic Cells for High Efficiency Solar Energy Conversion Systems
by Elnaz Kermani
Several works have been published on the concentration of solar radiation by mirrors and lenses onto smaller sized solar panels, which reduce cost and increase conversion efficiency at higher concentration ratio. One of the challenges in this technology is active and uniform cooling of high heat flux solar arrays, because conversion efficiency is dependent on device temperatures and drops with increase in temperature.
This research is targeted at cooling small, high concentrated solar cells. Benefits of manifold microchannel are attractive for cooling of electronic devices but have not been studied for cooling of high concentrated solar cells which is the target of this thesis, where the microchannel can be microfabricated and etched on the backside of the silicon solar cell to form a sealed heat sink with the manifold fabricated in the silicon substrate. This design minimizes the pressure drop, and also maximizes the heat transfer on the device.
Master's Thesis
http://hdl.handle.net/1903/8974
System-level Analysis and Comparison of Long-Haul Truck Idle-Reduction Technologies
by Ethan Lust
As of January 1, 2008 idling of the main vehicle engine for the purpose of powering sleeper cabin amenities by any truck over 10,000 lbs (4,500 kg) within the borders of the state of California is prohibited unless strict emissions standards are met. In anticipation of tighter idling legislation and rising fuel prices nation-wide, idle-reduction technologies are garnering an increasing market share. These include auxiliary battery-electric power systems, primary vehicle battery systems, truck-stop electrification, diesel-fueled auxiliary power systems, and fuel-fired heaters.
The purpose of this thesis is to provide a concise, detailed compilation of currently-marketed idle-reduction technologies, propose methodologies for evaluation and comparison, develop transient energy system simulations of the most prominent idling alternatives the most suitable commercially available software, create a simple, flexible cost-comparison program, propose future developments and applications, and conduct a critical assessment from the parameters considered which technology has the greatest relative advantage.
Master's Thesis
http://hdl.handle.net/1903/8357
Integration of a Thermoelectric Subcooler into a Carbon Dioxide Transcritical Vapor Compression Cycle Refrigeration System
by Jonathan Michael Schoenfeld
A thermoelectric (TE) subcooler was designed and fabricated to subcool CO2 exiting a gas cooler of a transcritical vapor compression cycle test system. The thermoelectric modules operated at efficiencies greater than the baseline system, increasing capacity and the overall coefficient of performance (COP) of the entire system. Subcooling of the CO2 before the expansion device led to a reduced optimum high side pressure, resulting in greater COP improvement that cannot be achieved in conventional refrigerant vapor compression systems utilizing a TE Subcooler. Improvements in COP of 10% were demonstrated with a corresponding capacity increase of 13%. A capacity increase of 24% was demonstrated at a comparable COP as the baseline system. Theoretical analysis of a combined Expander-TE Subcooler system, in which the electric power required by the TE Subcooler is provided by an expander-generator, was shown to provide a 30% increase in COP with a corresponding 24% increase in capacity.
Master's Thesis
http://hdl.handle.net/1903/8726
Performance Investigation of Two-Stage Heat Pump System With Vapor-Injected Scroll Compressor
by Xudong Wang
Heat pumps provide cooling in summers and heating in winters. It is inevitable that the capacity and COP of the heat pumps degrade significantly in the case of high ambient temperatures in summers and low ambient temperatures in winters, when the maximum capacity is desired. Refrigerant vapor-injection technique has been well justified to improve the performance of systems in refrigeration applications, however, it has not received much attention for air conditioning applications, particularly with air-conditioning for hot climates and heat pumps for cold climates. The performance degradation of conventional residential equipment at extreme weather conditions warrants further investigation of the vapor-injection technique.
This dissertation is focused on the experimental and theoretical investigations of a two-stage heat pump system with an innovative vapor-injected scroll compressor. Unlike other research, a heat pump system without a liquid receiver has been studied in this research. A 3-ton R410A heat pump equipped with a conventional scroll compressor has been built, and tested to serve as a baseline. The heat pump has been modified to be a two-stage system with the cycle options of flash tank and internal heat exchanger configurations, and been tested under the same ambient conditions to the baseline. Both compressors have the same displacement volume. The operating options of the two-stage system have been compared, and analyzed. The vapor-injection effects on the subcomponents of the system have been addressed.
The vapor-injected compressor has been modeled using compressor-mapping method. A simulation model of the two-stage system has been built using VapCyc and CoilDesigner software packages developed by CEEE, and been validated using the experimental data. The model is able to predict the system performance with ±5% of deviation to the experimental results for most performance variables.
The results show that the vapor-injection technique can effectively increase the system performance. A cooling capacity gain of around 14% with 4% COP improvement at ambient 46.1°C, about 30% heating capacity improvement with 20% COP gain at -17.8°C and about 7% HSPF improvement in U.S. Department of Energy's northern Region 4 climate have been found for the vapor-injected heat pump system as compared to the conventional system.
Doctoral Dissertation
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