Decompression induced pool and flow liquid degassing

Liquid flow degassing

Contant Person: R. Oikonomidou [rania.oik@gmail.com]
 

The term liquid degassing implies the formation of gas phase in a liquid volume due to the supersaturation of the liquid with dissolved gas. The maximum concentration of gas that can be dissolved in a liquid volume is proportional to the partial gas pressure. Therefore, when a gas saturated liquid is forced through a nozzle due to pressure drop, the liquid becomes gas supersaturated. As a result, gas desorbs from the liquid phase in the form of bubbles. Liquid degassing is applied in industry either to take advantage of the generated bubbles, or to get rid of the gases dissolved in a liquid. The characteristic small size of degassing bubbles makes them ideal for the efficient removal of low-density solid particles suspended in wastewater with the process of Dissolved Air Flotation. Moreover they improve the sparkling taste of carbonated beverages and the atomization of sprays. On the other hand, degassing bubbles promote the oxidation of wines and the growth of bacteria in oil recovery. They also accelerate the aging of pumps and create gas cavities in plastic molds, coatings etc. Apart from terrestrial applications, liquid degassing is also concerning several processes in space (e.g. cooling systems, propellant combustion and liquid storage).

The size of degassing bubbles is crucial for the efficiency of the relevant processes. In order to control bubble size, researcher studied the effect of several experimental parameters (nozzle geometry, liquid physical properties, liquid flow rate, supersaturation etc.) on bubble growth. The present project aims to study the effect of saturation pressure (partial gas pressure prior to injection) on the final size of degassing bubbles. Another goal is to study the effect of gravity on the growth of degassing bubbles. As long as we are concerned, liquid degassing is performed for the first time in hypergravity using the Large Diameter Centrifuge facility of ESA/ESTEC. A high resolution still digital camera is used to measure the size, the velocity and the trajectory of bubbles. The composition of the two-phase flow medium is determined by measuring the local instantaneous desorbed gas fraction with IVED electrical impedance spectroscopy technique.

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Figure 1,a: Schematic description of the experimental process, b: Liquid degassing under 500kPa (Video)

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Figure 2: Miniature liquid degassing experimental setup in the Large Diameter Centrifure room of ESA/ESTEC (experiments in hypergravity)  

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Figure 3: Bubble size distributions obtained
by optical measurements.

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Figure 4: Local volumetric gas fraction (ε) measurements using IVED electrical impedance technique.

 

 

 

Liquid pool degassing

Contant Person: V. Papadopoulou [virginie.papadopoulou07@imperial.ac.uk]

 

Vascular gas bubbles are routinely observed after scuba dives using ultrasound imaging, however the precise formation mechanism and site of these bubbles are still debated and growth from decompression in vivo has not been extensively studied, due in part to imaging difficulties. An experimental set-up was developed for optical recording of bubble growth and density on tissue surface area during hyperbaric decompression. Muscle and fat tissues (rabbits, ex-vivo) were covered with nitrogen saturated distilled water and decompression experiments performed, from 3 to 0 bar, at a rate of 1bar/min. Pictures were automatically acquired every 5s from the start of the decompression for 1h with a resolution of 1.75 μm. A custom MatLab analysis code implementing a circular Hough transform was written and shown able to track bubble growth sequences including bubble center, radius, contact line and contact angles over time. Bubble density, nucleation threshold and detachment size, as well as coalescence behavior, were shown significantly different for muscle and fat tissues surfaces, whereas growth rates after a critical size were governed by diffusion as expected. Heterogeneous nucleation was observed from preferential sites on the tissue substrate, where the bubbles grow, detach and new bubbles form in turn. No new nucleation sites were observed after the first 10min post decompression start so bubble density did not vary after this point in the experiment. In addition, a competition for dissolved gas between adjacent multiple bubbles was demonstrated in increased delay times as well as slower growth rates for non-isolated bubbles.