Mechanical Engineering
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Item ENABLING CO2 ISOTHERMAL COMPRESSION USING LIQUID PISTON AND INTEGRATED GAS COOLER(2022) Kim, Timothy; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)New avenues of decreasing environmental impacts and increasing the efficiency of HVAC systems are constantly being explored in the race to reduce carbon emissions and global warming. These new avenues have led to the exploration of the use of carbon dioxide as a refrigerant in refrigeration applications. Many researchers have also investigated ways to reduce the power consumption of compressors, which is typically the main source of power draw for HVAC systems. One theoretical process to achieve this is through isothermal compression. This thesis explores the idea of isothermally compressing CO2 by using a liquid piston and integrated gas cooler to achieve higher efficiencies with this transcritical cycle. A test facility was designed, sized, constructed, and calibrated to emulate the suction and discharge conditions of a typical CO2 system for air conditioning applications. A prototype of the liquid piston and integrated gas cooler chamber was designed and constructed as well. A simulation model was built in Engineering Equation Solver in order to properly design the gas cooler chamber. Other critical components have been carefully chosen to ensure smooth operation of the system. Results show isothermal efficiencies of up to 82.7% during steady-state operation and an isothermal efficiency of 91.2% during steady-state operation with the additional help of evaporative cooling. Comparing this to other conventional compressors give up to 34.2% absolute improvement in the isothermal compressor efficiency. These results show sufficient performance to warrant the design of a fully working prototype despite efficiency/capacity tradeoffs in the system. Challenges had been encountered such as the loss of refrigerant through the liquid piston, which will be accounted for in the next prototype. Discussion of the next prototype includes the use of a double-acting piston and a smaller tubed fractal heat exchanger design.Item VENTILATION IMPACT ON AIRBORNE TRANSMISSION OF RESPIRATORY ILLNESS IN STUDENT DORMITORIES(2018) Jenkins, Sara T; Srebric, Jelena; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work presents a study of the effect of ventilation rates on the bioaerosols that cause upper respiratory illness. A network of 147 sensors was placed in a pair of dormitories on a college campus to measure carbon dioxide concentrations over two semesters. The concentration results served as input into multi-zone ventilation models of the two buildings, which had different heating, ventilation, and air conditioning (HVAC) systems. The dormitory with a central mechanical ventilation system had, as expected, a higher turnover of fresh air compared to the other, which relied on exhaust fans and infiltration. This well-ventilated building also contained far fewer occupants with recorded upper respiratory illness incidence in comparison to the poorly ventilated building. The central ventilation system increased dorm room ventilation rates by 500%, while decreasing respiratory illness incidence by over 85%. Comparative studies have shown similar findings with increased ventilation reducing incidence of upper respiratory illness by an order of magnitude.Item Enhanced Gas-Liquid Absorption Utilizing Micro-Structured Surfaces and Fluid Delivery Systems(2014) Ganapathy, Harish; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Despite intensive research and development efforts in renewable energy in recent years, more than 80% of the energy supply in the year 2040 is expected to come from fossil fuel-based sources. Increasing anthropogenic greenhouse gas emissions led the United States to legislatively limit domestic CO2 emissions to between 1000-1100 lb/MWh for new fossil fuel-fired power plants, thus creating an urgent need for efficient gas separation (capture) processes. Meanwhile, the gradual replacement of coal with cleaner burning natural gas will introduce additional challenges of its own since nearly 40% of the world's gas reserves are sour due to high concentrations of corrosive and toxic H2S and CO2 gases, both of which are to be separated. Next-generation micro-structured reactors for industrial mass and heat transfer processes are a disruptive technology that could yield substantial process intensification, size reduction, increased process control and safety. This dissertation proposes a transformative gas separation solution utilizing advanced micro-structured surfaces and gas delivery manifolds that serves to enhance gas separation processes. Experimental and numerical approaches have been used to achieve aggressive enhancements for a solvent-based CO2 absorption process. A laboratory-scale microreactor was investigated to fundamentally understand the physics of multiphase fluid flow with chemical reactions at the length scales under consideration. Reactor design parameters that promote rapid gas separation were studied. Computational fluid dynamics was used to develop inexpensive stationary (fixed) interface models for incorporation with optimization engines, as well as high fidelity unsteady (deforming) interface models featuring universal flow regime predictive capabilities. Scalability was investigated by developing a multiport microreactor and a stacked multiport microreactor that represented one and two orders magnitude increase in throughput, respectively. The present reactors achieved mass transfer coefficients as high as 400 1/s, which is between 2-4 orders of magnitude higher than conventional gas separation technologies and can be attributed to the impressive interfacial contact areas as high as 15,000 m2/m3 realized in this study through innovative design of the system. The substantial enhancement in performance achieved is indicative of the high level of process intensification that can be attained using the proposed micro-structured reactors for gas separation processes for diverse energy engineering applications. This dissertation is the first comprehensive work on the application of micro-structured surfaces and fluid delivery systems for gas separation and gas sweetening applications. More than ten refereed technical publications have resulted from this work, part of which has already been widely received by the community.Item Performance and Oil Retention Characteristics of a CO2 Heat Pump Water Heater(2008) Fernandez, Nicholas Edward Peter; Radermacher, Reinhard K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.