Performance of Two-Stage CO2 Refrigeration Cycles
by Aydin Celik
The performance of four CO2 cycle options was measured for three different evaporating temperatures, 7.2, -6.7, and -23.3°C under the ARI Standard 520 for condensing units. The four cycle options were the baseline cycle, the cycle with suction line heat exchanger, the cycle with intercooler, and two-stage split cycle. The cycle operation at the low evaporating temperature was limited by the high discharge temperature for most cycle options except the two-stage split cycle. The compressor used in the testing was a hermetic, rotary type two-stage compressor.
The effect of cycles and individual cycle components on system capacity and performance was investigated. Cycle optimization was conducted by using mass flow rate ratio, intermediate pressure coefficient and power ratio.
Modeling of these four cycles by EES showed similar results with the test data and provided information on sizing the system components for different system capacities for maximum performance.
Master's Thesis
http://hdl.handle.net/1903/1857
Experimental and Theoretical Investigation of Oil Retention in Vapor Compression Systems
by Lorenzo Cremaschi
The design of any system needs to consider a number of parameters according different needs. In heating, ventilation and air conditioning systems the overall efficiency, the reliability of the components, the cost and volume, and the refrigerant/oil charge are only some examples of variables that can be optimized. An important aspect is the selection of lubricants that provide the same or improved characteristics relative to traditional mineral oils. In HVAC systems, the oil exists only because the compressor requires it for lubrication and sealing. Proper oil management is necessary in order to improve the compressor reliability, increase the overall efficiency of the system, and minimize the system cost by avoiding redundancy.
Several literature sources focused on oil/refrigerant properties (Thome, 1995), oil return characteristics (Biancardi et al., 1996) and oil transport phenomena (Mehendale, 1998). An analytical and experimental study of the oil retention has been developed for automotive air conditioning systems using carbon dioxide (Jun-Pyo Lee, 2002). However, a general comprehensive model for oil retention and oil distribution in heat pump systems using other refrigerant/oil mixtures does not exist and is of importance to future design considerations.
The purpose of this thesis is to experimentally and theoretically investigate the physics of oil retention and oil transport in different components of the system. Condenser, evaporator, suction and liquid lines are studied using different pairs of refrigerant-oil mixtures. Oil retention is measured directly using an experimental apparatus, and oil film thickness is estimated. At oil mass fractions of 8 wt.%, the pressure drops increase up to 40% in the suction line, 20% in the evaporator and 30% in the condenser as compared to oil-free operating conditions. New pressure drop correlations need to include this penalty factor due to oil retention. An analytical model for vapor and two-phase refrigerant flows utilizing minimal empirical data is developed. The model is able to estimate the oil distribution in the entire system providing good design guidelines for the selection of the proper refrigerant/oil mixture, the optimization of the component geometries, and the management of the oil/refrigerant charge.
Doctoral Dissertation
http://hdl.handle.net/1903/1773
Performance Characterization of Pool Boiling on Innovative Foams and Micro Structured Surfaces - Application to Direct Immersion Cooling of Electronics
by Lewis Elliot Gershen
This thesis is an experimental investigation of the thermal performance during boiling on copper foam and copper-finned microstructures as a thermosyphon evaporator. Copper foam is an open-celled porous media with interconnected copper ligaments of thermal conductivity up to 1.51 W/(cm2⋅K). The high thermal conductivity of this type of copper foam allows heat to rapidly spread through the foam causing widespread boiling. The boiling allows large transfers of energy from the heater (the source) to the working fluid (sink) with a low temperature difference between the heated surface and the working fluid.
The thermal performance of the copper foams was investigated as a function of parametric values of foam height, pore density (pores per inch - PPI), and porosity. Data showed the pore density and porosity of the foam significantly affected heat transfer by changing the pore sizes and ligament sizes while thickness in the range of 1 to 5 mm had little effect on thermal performance. Surface tension is shown by dimensionless analysis to be the dominating force within the foam. The data also indicated the heat transfer coefficient for boiling HFE-7100 from copper foam ranged from 2,000 to 9,000 W/(m2⋅K). The copper foams provided significant heat transfer from the source. For example, copper foams provided heat loads up to 70 W/cm2 at 90 °C superheat.
The second set of experiments in the present study involved testing of micro-finned structures. For such surfaces the important parameters we paid attention to were fin density (fins per inch) and aspect ratio (ratio of channel height to channel width) of the copper-finned microstructures. The data showed that aspect ratio and fin density substantially affect heat transfer performance through different channel cross-section sizes. The heat transfer provided by the micro-finned structures was substantially enhanced. For example, copper-finned microstructures provided heat loads up to 43 W/cm2 at 20 ºC superheat.
In this thesis, many chapters will discuss the reasoning behind this study and the results of it. The first chapter will cover the motivation and background information for this work. The next chapter discusses the experimental setup and how the results were obtained. The third chapter will review the results of the copper foams and discuss the cause of the results. The fourth chapter discusses the thermal performance of micro-finned structures. Finally, the conclusions of this study and suggested future work will be presented in the last chapter.
Master's Thesis
http://hdl.handle.net/1903/159
The Development of an Air-Cooled Absorption Chiller Concept and Its Integration in CHP Systems
by Xiaohong Liao
This dissertation focuses on the feasibility, crystallization issues, and the integration of LiBr-H2O air-cooled absorption chillers into Cooling, Heating and Power (CHP) systems. The concept of an air-cooled system is attractive because the cooling tower and the associated installation and maintenance issues can be avoided. However, crystallization of the LiBr-H2O solution then becomes the main issue in the operation of the unit, since the air-cooled absorber tends to operate hotter than the water-cooled absorber due to the relative heat transfer characteristics of the coolant leading to crystallization of the working fluid. Differently from the conventional approaches to air-cooled absorption chillers, novel temperature control strategies in conjunction with a specialized application is proposed. This prevents crystallization but presents unique system integration challenges and opportunities. A model to accurately reflect the thermodynamic characteristics of air-cooled absorption chillers and to facilitate control is developed as part of this research, and field experiments that simulate air-cooled conditions with a water-cooled absorption chiller, which was driven by the waste heat of a microturbine, were conducted to validate the feasibility of the air-cooled concept and the accuracy of computer model.
While CHP provides a good opportunity for the application of air-cooled absorption chillers, system integration issues need to be investigated. The capital cost of CHP equipment and the load fluctuation of a commercial building restrict the advantage of designing a unit sized for peak load. Therefore, the conventional Heating Ventilation and Air Conditioning (HVAC) system is needed to pick up the residual loads. Thus, the result of an extensive system integration analysis is that CHP should be arranged in series with the HVAC system to ensure obtaining more operating hours at its full capacity, so that the cost savings achieved through the recovery of waste heat are fully realized to repay its higher initial capital cost. The primary energy savings are presented for all potential configurations.
As a part of this research a fully integrated CHP system has been installed and instrumented at the Chesapeake Building. It is a commercial office building on the University of Maryland campus. The experimental setup, data processing, and experience gained are detailed here. Based on the computer simulation, extensive experiments, first hand installation, operation and maintenance experience, valuable guidelines on the integration of an air-cooled absorption chiller in CHP are developed. All the guidelines are also applicable to water-cooled absorption chillers.
Doctoral Dissertation
http://hdl.handle.net/1903/2176
Reliability Assessment of Rolling Piston Rotary Type Compressors
by Layla Monajemi
The performance of a compressor changes during its lifetime, depending on various parameters such as operating conditions, type of the compressor, working fluid and type of the oil being used. Most performance degradation of the compressor is due to wear on the moving mechanical parts inside the compression vessel. Wear develops on the contact point(s) or area(s) between the moving and fixed mechanical parts inside the compressor when there is a lack of lubrication. In a rolling piston rotary compressor, the most vulnerable regions are the vane and roller contact area, and the shaft and journal bearing area. As the amount of wear increases, leakage through the clearances inside the compression compartment increases as well. An increase in the amount of leakage degrades the volumetric efficiency and the overall performance of the compressor.
The objectives of this study are to predict the life span of rolling piston rotary-type compressors through the measurement of wear on the roller under accelerated test conditions, and predict the effect of wear on the degradation of compressor performance. In order to be able to estimate the amount of wear under different operating pressures, a correlation between operating pressures, amount of wear, and degradation of performance was developed. Then the change in compressor performance was estimated using a computer simulation program which takes into account the effect of leakage through the clearances inside the compression vessel based on the operating conditions provided.
One unit of rolling piston rotary type compressor was tested under accelerated operating conditions. An initial increase in the capacity and volumetric efficiency of the compressor was achieved due to aging affects. Then the performance of the compressor started to degrade due to increase in the amount of refrigerant leakage from suction pocket to discharge pocket of the compression compartment."
Master's Thesis
http://hdl.handle.net/1903/1696
Electrohydrodynamics(EHD) Pumping of Liquid Nitrogen & Application to Spot Cryogenic Cooling of Sensors and Detectors
by Mihai Catalin Rada
Superconducting electronics require cryogenic conditions as well as certain conventional electronic applications exhibit better performance at low temperatures. A cryogenic operating environment ensures increased operating speeds and improves the signal-to-noise ratio and the bandwidth of analog devices and sensors, while also ensuring reduced aging effect. Most of these applications require modest power dissipation capabilities while having stricter requirements on the spatial and temporal temperature variations. Controlled surface cooling techniques ensure more stable and uniform temporal and spatial temperature distributions that allow better signal-to-noise ratios for sensors and elimination of hot spots for processors. EHD pumping is a promising technique that could provide pumping and mass flow rate control along the cooled surface. In this work, an ion-drag EHD pump is used to provide the pumping power necessary to ensure the cooling requirements.
The present study contributes to two major areas which could provide significant improvement in electronics cooling applications. One direction concerns development of an EHD micropump, which could provide the pumping power for micro-cooling systems capable of providing more efficient and localized cooling. Using a 3M fluid, a micropump with a 50 mm gap between the emitter and collector and a saw-tooth emitter configuration at an applied voltage of about 250 V provided a pumping head of 650 Pa. For a more optimized design a combination of saw tooth emitters and a 3-D solder-bump structure should be used.
The other major contribution involves the application, for the first time, of the EHD pumping technique to cryogenic liquids. Successful implementation of these cooling techniques could provide on-demand and on-location pumping power which would allow tight cryogenic temperature control on the cooling surface of sensors, detectors and other cold electronics. Pumping heads of up to 1 Pa with mass flow rates of 0.8 g/s were achieved using liquid nitrogen. Although the pressure head results seem relatively small, the corresponding liquid nitrogen mass flow rate meets the targeted heat removal requirements specific to superconducting sensors and detectors.
Doctoral Dissertation
http://hdl.handle.net/1903/212
Experimental and Computational Analysis of an Electrohydrodynamic Mesopump for Spot Cooling Applications
by Amir H. Shoushtari
As electronic products become faster, more compact, and incorporate greater functionality, their thermal management becomes increasingly more challenging as well. In fact, shrinking system sizes, along with increasing circuit density, are resulting in rapid growth of volumetric heat generation rate and reduction in surface area for adequate heat dissipation. Moreover, system miniaturization by employing microfabrication technology has had a great influence on thermal and fluid research Smaller systems have many attractive characteristics and can be more conveniently fabricated using batch production technologies.
One of the fields showing promising potential in microsystems and electronics cooling is the use of the phenomenon of electrohydrodynamics or EHD defined as a direct interaction between the electric and hydrodynamic fields where the electric field introduces fluid motion.
The objectives of the present study were to identify the physics of these phenomena as related to the present study, to simulate it numerically, and to verify the modeling through experiments. More specifically, the goals were to develop a novel numerical methodology to simulate the highly complex interaction between fluid flow and electrical fields. Next, to verify the model a mesoscale ion-injection pump was designed and fabricated, followed by a set of experiments that characterized the pump's performance. The experiments will also demonstrate the application potential of the concept in electronics cooling and particularly for spot cooling applications.
Experimental tests were conducted on an EHD ion-injection mesopump to measure the flow rates and generated pressure heads with HFE -7100 as working liquid. It is shown that the results of two different flow rate measurement techniques that were employed, are in agreement. The experimental results also show that maximum flow rate of about 30 ml/min and pressure head of 270 Pa for the electrode gap of 250 µm and voltage of 1500 V are achievable. A novel numerical modeling method was developed that incorporates both the injection and dissociation of ions. This modeling method is used to simulate the EHD mesopump. The numerical results show a fairly good agreement with experimental data.
Doctoral Dissertation
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