Acoustically Enhanced Condensation in Horizontal Tubes
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Hughes, Matthew Thomas
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Abstract
Condensation is a ubiquitous heat transfer process in heating, ventilation, air-conditioning, and refrigeration (HVAC&R) and chemical processing. However, while condensation is an effective heat transfer mechanism, the heat transfer coefficient decreases as the vapor quality decreases; therefore, several methods to enhance condensation heat transfer have been explored over the past several decades. This study focuses on active techniques to achieve enhancement in condensation in regions where the heat transfer coefficient is typically lower. A review of the pertinent literature on active enhancement of condensation is conducted, from which one promising and relatively unexplored method, enhancement by actuation of the flow with acoustics, is selected for detailed investigation. Acoustic actuation can enhance the heat and mass transfer during condensation by agitating the phases and providing an additional mixing mechanism for the two phases, which in turn reduces the thermal resistance in the condensate. In this study, a tube-in-tube condenser test section is coupled to an acoustic actuator at the inlet and a Helmholtz volume at the outlet. A flow visualization study is performed to characterize the flow regimes, oscillations in the fluid motion that modify the regimes, and void fraction. A phase-change heat transfer test facility is constructed to measure the heat transfer coefficient and frictional pressure gradient of the actuated flow compared with the baseline flow. The effects of mass flux, actuation frequency, actuation amplitude, and degree of subcooling are studied to understand the relevant heat transfer enhancement mechanisms. Heat transfer enhancement of up to 32% is observed, allowing for size reduction in common liquid-coupled condensers by up to 10%. A pressure drop penalty of ~6.5 is observed, but only in the low-quality region of the condenser where local pressure drops are smallest. Additionally, length reduction due to heat transfer reduction will help decrease the overall condenser pressure drop, making the overall change in pressure drop nearly negligible as heat transfer at low qualities is enhanced. Based on these experimental results, a model is developed to predict the heat transfer and pressure drop to aid in condenser design. Insights from these experiments and analyses are extended to provide a guide to the future development of more compact condensers and the potential use of acoustics to enhance heat flux.
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Date
2023-06-30
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Dissertation