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- Creators: Barrett, The Honors College
- Creators: Holcomb, Don
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
The following study evaluated the effectiveness of the COR Bench, to determine if its unstable surface allows for both greater range of motion and greater tendency towards co-contraction, in comparison to a traditional flat bench. An initial session assessed the 19 participants’ (age, 22.364 ± 2.480 years; mass, 77.850 ± 18.328 kg; height, 1.787 ± 0.0923 m) current one- repetition maximum strength levels for the unilateral chest press. Four different conditions were used to test the kinematic variables such that each participant completed 5 repetitions of unilateral chest presses at 70% of their max with one and two dumbbells on both the COR Bench and traditional flat bench. Differences in ROM were evaluated by measuring an angular representation of the shoulder complex. Levels of co-contraction were determined by analyzing the middle trapezius EMG. The results indicate a significant difference in ROM between the conditions (p = .00); specifically, a greater ROM during the one dumbbell condition versus the two dumbbell condition on the COR and the flat benches (p = .002, 0.001 respectively); greater ROM with two dumbbells on the COR bench compared to the flat bench (p = .022). The middle trapezius EMG data showed a significant difference in co-contraction levels between conditions (p = .00); specifically, a greater co-contraction during the one dumbbell condition versus the two dumbbell condition on the COR and the flat benches (p = .001, 0.008 respectively); greater co- contraction with one dumbbell on the COR bench compared to the flat bench (p = .010). This lends support for the future use of the COR bench as a means for more efficient exercise through a full range of motion; however, it also suggests that more practice is necessary to master the movement on the bench to decrease co-contraction activity.
ContributorsOppenheimer, Nicole (Author) / Harper, Erin (Thesis director) / Broman, Tannah (Committee member) / Hoffner, Kristin (Committee member) / Barrett, The Honors College (Contributor) / School of Nutrition and Health Promotion (Contributor)
Created2013-12