Graduate Thesis Or Dissertation
 

The experimental characterization of membrane behavior in two-phase gas extraction

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/v979v562s

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  • Understanding the behavior of hydrophobic membranes is important for applications where separating a gas from a liquid-gas mixture is beneficial. For example, in-situ vapor extraction can be used in microscale heat sinks to improve heat transfer and flow stability. In this study, gas is experimentally evaluated flowing through flexible polytetrafluoroethylene nanofiber membranes. The gas is extracted from liquid-gas combinations of liquid water, water vapor and air. For single-phase flow such as superheated vapor and air, an applied pressure difference across the membrane results in structural changes affecting properties including the membrane thickness, which restricts the flow rate. Two existing models predicting fluid transport in porous media are found to be insufficient to predict gas extraction flow rates from two-phase mixtures, even when accounting for membrane compaction and a proposed estimated reduced extraction area. The proposed estimated extraction area insufficiently accounts for bubble kinetic energy, bubble dynamics and the existence of thin liquid films. The thin liquid film must be ruptured to open membrane pores to extract the gas in the bubbles. At greater applied pressure differences, film rupture and three phase contact are accelerated and bubbles are extracted more quickly resulting in a reduced gas-membrane contact area. If the bubbles have too much energy associated with them, it may take multiple collisions with the membrane for the bubbles to establish stable three phase contact. To account for the hydrodynamics near the membrane surface, empirical parameters are developed as a function of thermophysical properties of the mixture near the membrane and the applied pressure differences across the membrane. In the case of liquid-air mixtures with high void fractions, bubbles are not completely extracted due to hydrodynamics near the membrane surface. For saturated liquid-vapor studies, full vapor extraction is achieved even at high void fractions due to different bubble energy and behavior than the liquid-air studies. One implication with liquid-vapor mixtures is that changes in the membrane flow resistance can be induced if the extraction vacuum pressure is cycled.
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