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- All Subjects: Instability
- Genre: Academic theses
- Creators: Holcomb, Don
- Creators: Miller, Clark
Description
Recent literature indicates potential benefits in microchannel cooling if an inlet orifice is used to suppress pressure oscillations that develop under two-phase conditions. This study investigates the costs and benefits of using an adjustable microchannel inlet orifice. The focus is on orifice effect during steady-state boiling and critical heat flux (CHF) in the channels using R134a in a pumped refrigerant loop (PRL). To change orifice size, a dam controlled with a micrometer was placed in front of 31 parallel microchannels. Each channel had a hydraulic diameter of 0.235 mm and a length of 1.33 cm. For steady state two-phase conditions, mass fluxes of 300 kg m-2 s-1 and 600 kg m-2 s-1were investigated. For orifice sizes with a hydraulic diameter to unrestricted hydraulic diameter (Dh:Dh,ur) ratio less than 35 percent, oscillations were reduced and wall temperatures fell up to 1.5 °C. Critical heat flux data were obtained for 7 orifice sizes with mass fluxes from 186 kg m-2 s-1 to 847 kg m-2 s-1. For all mass fluxes and inlet conditions tested, CHF values for a Dh:Dh,ur ratio of 1.8 percent became increasingly lower (up to 37 W cm-2 less) than those obtained with larger orifices. An optimum orifice size with Dh:Dh,ur of 35 percent emerged, offering up to 5 W cm-2 increase in CHF over unrestricted conditions at the highest mass flux tested, 847 kg m-2 s-1. These improvements in cooling ability with inlet orifices in place under both steady-state and impending CHF conditions are modest, leading to the conclusion that inlet orifices are only mildly effective at improving heat transfer coefficients. Stability of the PRL used for experimentation was also studied and improved. A vapor compression cycle's (VCC) proportional, integral, and derivative controller was found to adversely affect stability within the PRL and cause premature CHF. Replacing the VCC with an ice water heat sink maintained steady pumped loop system pressures and mass flow rates. The ice water heat sink was shown to have energy cost savings over the use of a directly coupled VCC for removing heat from the PRL.
ContributorsOdom, Brent A (Author) / Phelan, Patrick E (Thesis advisor) / Herrmann, Marcus (Committee member) / Trimble, Steve (Committee member) / Tasooji, Amaneh (Committee member) / Holcomb, Don (Committee member) / Arizona State University (Publisher)
Created2012
Description
National infrastructure form the bedrock for economic growth and social security, both of which lowers conflict risks. This encourages states and international organizations to invest heavily in post-conflict infrastructure reconstruction efforts, believing that infrastructure provision will reduce future political instability. This belief is based largely on the perceived successes of reconstruction efforts in prior eras, especially after World War II. Today, post-conflict reconstruction efforts are much less successful in this regard and, overall, are not reducing political instability---Iraq being the quintessential example of such policy failure. In the face of both ongoing conflict and persistent needs for infrastructure reconstruction after conflicts, therefore, there is a critical need to understand two questions: Why are current reconstruction efforts failing to reduce political instability or even, in some cases, increasing it? And, how can reconstruction efforts be organized to do better? To address these questions, this dissertation examines infrastructure reconstruction across a wide range of national contexts. In doing this, an updated viewpoint is provided on the role of infrastructure in conflict-prone areas to include a long-term perspective on infrastructure system's role in society, technological integration, and relationship between the state and conflicting groups. This dissertation finds that though provision of different types of infrastructure might increase conflict risks in the short term, such provision can reduce conflict in the long run depending on how and where infrastructure is provided vis-a-vis excluded populations. These results provide crucial input towards the redesign of reconstruction policies to limit future political instability risks through infrastructure.
ContributorsMolfino, Emily Suzanne (Author) / Miller, Clark (Thesis advisor) / Fisher, Erik (Committee member) / Wood, Reed (Committee member) / Arizona State University (Publisher)
Created2014