Bubble removal from water with superhydrophobic capillary channels and Thermal boundary layer visualization experiments for engineering education
Beheshti Pour, Negar
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Gas-liquid phase separation under microgravity conditions or in small-scale fluidic systems represents a challenge for two-phase liquid-continuous systems. In this study, capillary channels formed by 3-mm diameter stretched stainless-steel springs coated with a commercial superhydrophobic coating are used to remove air bubbles from water. A single channel is capable of absorbing air bubbles of various sizes and at different flow rates as well as at different points of incidence from water at a rate of over 50 bubbles/s at a single point on the channel. A horizontal three-channel array has been shown capable of absorbing impinging bubbles from a sparger at superficial gas velocities of 0.03 m/s. The channel permeability was found to be three orders of magnitude higher than the permeability of porous hydrophobic membranes with micro-scale pores. However, the disadvantage of such high permeability is that liquid intrudes to the interior of the channel because of the jet formed during bubble collapse. To address this issue, we have shown that an eccentric annulus with non-wetting walls formed by inserting an off-center rod inside the cylindrical channel could prevent liquid from occluding the channel and may cause liquid to be transported out of the channel. The capillary channels studied in this work are envisioned to be useful for removing bubbles from critical large-aspect-ratio surfaces such as electrodes or boiler tubes, especially under conditions where buoyancy is weak relative to surface tension such as under microgravity or at microscale. Arrays of such channels may also have application to bulk phase separation under microgravity where they could effectively filter bubbles from a liquid flow. As the broader impact and educational outcome of this project, we used the same experimental equipment and imaging system to make high magnification videos to allow students to visualize the thermal boundary layer around a heated cylinder. In addition, numerical simulations of the experimental system, and an ultra-low-cost desktop learning module based on shadowgraphy were created. Statistical analysis of pre- and posttest results show significant improvements in student understanding and motivation in learning heat transfer.