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Enhanced Extended-Surfaces for Two-Phase Pump-Less Cooling Loops
AdvisorWirtz, Richard A
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Pool boiling experiments are conducted to investigate the heat transfer performance of screen laminate-enhanced extended surfaces in water at reduced pressures. A lamination of fine-filament, wire mesh is an effective surface enhancement for boiling since the surface can be configured to provide a very high density of potential bubble nucleation sites. A coating of copper fine wire screen is bonded to an extended surface (fin). The coating thickness ranged from 0.21mm to 1.3mm. The porosity of the coatings is held constant at 0.43 and the pore hydraulic diameter of the coating varies from 39μm to 172μm. Pool temperatures are held at the saturation temperature for water at pressures of 0.2, 0.3, 0.5 and 1.0atm. A reduction in pressure reduces the boiling performance for all surfaces.The work began by investigating 4-layer laminates on a fin that has a 1cm2 base cross section area, a 5deg taper with heights of 1cm and 2/3cm. Boiling performance of a vertical and horizontal fin orientation is investigated. The 4 layer screen laminate enhanced fin with a 10K superheat sustains base heat fluxes of up to 140 W/cm<super>2</super> at 1.0atm, and up to 132W/cm<super>2</super> at 0.2atm. These extended surfaces outperform screen laminates enhanced plane surface by up to 4 fold. Vertically oriented fins outperform horizontal fins.The second part of the work was to further investigate screen laminates on the trapezoidal fin. Laminations of 8-layers are found to improve boiling performance over the four layers. Experimental results show that the thin, fine coatings on the fin surfaces enhance the boiling performance. At 0.2atm base heat fluxes in excess of 193W/cm<super>2</super> are seen and with moderate superheats of around 10K. A semi-empirical model of the fin is developed. The model implements a boiling correlation from a plane enhanced surface. Assessments of fin aspect ratio and taper are conducted using the model. Larger aspect ratios improve fin performance. The effects of the taper are negligible.The last part of the work was to investigate the boiling performance from laminations on a wavy fin array. Two surfaces are considered: a 0.75mm thick 4-layer laminate with approximately 4000 pores per cm<super>2</super> and a 0.42mm thick 8-layer laminate with approximately 26,000 pores per cm<super>2</super>. The results show that the 8-layer laminate outperforms the 4-layer laminate. A semi-empirical boiling model is developed. The model predicts the boiling performance of our data within an error of 30%. The model shows that shorter fins improve boiling performance.