Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Performance enhancement of heat exchanger coolers with evaporative cooling(2014) Popli, Sahil; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Air or water cooled heat exchangers (HX) are typically utilized as condensers or coolers for air-conditioning, refrigeration or process cooling applications in both commercial and industrial sector. However, air cooled heat exchanger performance degrades considerably with rise in ambient air temperature and water cooled coolers require considerable pumping power, a cooling tower and may consume a significant amount of water which may come from fresh water sources. Evaporative cooling offers a unique solution to this problem, where a small amount of wetting water evaporates on HX surface to boost performance in high ambient air temperature conditions. In this study, several evaporative cooling technologies were applied to three wavy-fin HXs to quantify capacity enhancement ratio (CER) and air-side pressure drop penalty ratio (PRΔPa) compared to respective dry case baseline values. Effect of varying wetting water flow rate, air velocity, fin spacing, hydrophilic coatings, spray orientation and inlet air temperature and relative humidity was investigated on hybrid heat exchanger performance. Several new performance comparison parameters were defined to compare different evaporative cooling approaches. Deluge cooling achieved overall highest CER but at a PRΔPa that was similar in magnitude to the CER. This limitation was found to be inherent to the nature of wetting water distribution method itself. Although front spray cooling tests indicated PRΔPa~1, front spray evaporative cooling technology was found to have up to 23-75 % lower CER at 60-100% lower PRΔPa compared to deluge cooling. In order to understand the wetting behavior a novel visualization method was proposed and implemented, which consisted of borescope assisted flow mapping of water distribution within the HX core as a function of air velocities and wetting water flow rates. It was found that up to 85% of HX volume remained dry during front spray cooling which accounted for lower capacity enhancement and deluge cooling forms non-uniform and thick water film which causes bridging and increased PRΔPa, A larger component level testing with HX size similar to commercial units allowed to identify constraints of different evaporative cooling methods, which would not be possible if tests were performed at a smaller segment or fin level. A novel spray cooling technology utilizing internal jet spray cooling within HX volume was both proposed and implemented and a provisional patent # 61/782,825 was obtained. Compared to front spray cooling at a given spray rate, internal spray cooling could either achieve up to 35% higher HX cooler capacity, or obtain same HX cooler capacity at approximately three times lower air-side pressure drop. Alternatively, at same air-side pressure drop wetting water savings of up to 68-97% are achieved. Internal spraying combines advantages of conventional technologies and overcomes the drawbacks, by getting CER of approx. 3.8, without film carryover and at PRΔPa=1, while getting maximum wetting uniformity. Intermittent cooling combined with internal spraying could reduce water consumption as evaporative cooling sustains though the brief period when spray is turned off. Thus, potential for significant energy and water savings, targeted cooling, and retrofit design offers significant commercialization opportunity for future hybrid evaporative coolers. Discussions are underway for the inclusion of this technology into product line up of a leading HX manufacturing company.Item Experimental Study of Hybrid Cooled Heat Exchanger(2011) Tsao, Han-Chuan; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A test system for a hybrid cooled heat exchanger was designed, and the test facility was constructed based on ASHRAE Standard 41.2-1987. A conventional air-cooled tube-fin heat exchanger was tested with and without application of wetting water. The baseline tests were conducted to investigate the heat exchanger performance improvement by applying evaporative cooling technology. The heat exchanger capacity and air side pressure drop were measured while varying operating conditions and heat exchanger inclination angles. The results show the heat exchanger capacity increased by 170% with application of the hybrid cooling technology, but the air side pressure drop increased by 130%. Additional research investigating air fan power was also conducted, which increased 120% from the dry condition to the hybrid cooled condition. In summary, the potential for improving the heat exchanger performance by applying hybrid cooling is shown in this research.Item Direct Numerical Simulation of Non-Premixed Flame Extinction Phenomena(2010) Narayanan, Praveen; Trouve, Arnaud C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Non-premixed flame extinction phenomena are relevant in a variety of com- busting environments, including but hardly limited to diesel engines, pool fires, and fire suppression scenarios. These disparate phenomena are controlled by various parameters that contain information on flame stretch, heat losses, composition of the fuel and oxidizer supply streams, etc. Direct Numerical Simulation (DNS) is used in the present study to provide fundamental insight on diffusion flame extinction under non-adiabatic combustion conditions. The list of DNS configurations include: (C1) counterflow laminar flames with soot formation and thermal radiation transport; (C2) coflow turbulent flames with soot formation and thermal radiation transport; (C3) counterflow laminar and turbulent flames interacting with a mist-like water spray. Configurations C1 and C2 use single-step chemistry while configuration C3 uses detailed chemistry (all cases correspond to ethylene-air combustion). Configuration C1 is also treated using large Activation Energy Asymptotics (AEA). The AEA analysis is based on a classical formulation that is extended to include thermal radiation transport with both emission and absorption effects; the analysis also includes soot dynamics. The AEA analysis provides a flame extinction criterion in the form of a critical Damköhler number criterion. The DNS results are used to test the validity of this flame extinction criterion. In configuration C1, the flame extinction occurs as a result of flame stretch or radiative cooling; a variation of configuration C1 is considered in which the oxidizer stream contains a variable amount of soot mass. In configuration C1, flame weakening occurs as a result of radiative cooling; this effect is magnified by artificially increasing the mean Planck soot absorption coefficient. In configuration C3, flame extinction occurs as a result of flame stretch and evaporative cooling. In all studied cases, the critical Damkohler number criterion successfully predicts transition to extinction; this result supports the unifying concept of a flame Damköhler number Da and the idea that different extinction phenomena may be described by a single critical value of Da.