Decompression induced pool and flow liquid degassing

Liquid flow degassing

Contant Person: R. Oikonomidou []

The term liquid degassing implies the formation of gas phase in a liquid volume as the result of liquid supersaturation with dissolved gas. The solubility of gases in liquids increases proportionally to the partial gas pressure. Therefore, when a gas saturated liquid passes through a nozzle, it decompresses and becomes supersaturated with gas. As a result, gas desorbs from the liquid phase in the form of bubbles. The technique of liquid degassing is applied in several industrial processes either to take advantage of the generated bubbles, or to remove dissolved gases from liquids. The utilization of degassing bubbles for the efficient removal of suspended solid particles from wastewater is the working principle of Dissolved Air Flotation technique. The sparkling taste of carbonated beverages and the efficient spray atomization is attributed to liquid degassing. On the other hand, degassing bubbles promote wines oxidation and bacteria growth in crude oil. They also accelerate pumps aging and cause imperfections in plastic molds, coatings etc. Apart from terrestrial applications, liquid degassing participates in several applications of space missions (e.g. liquid propellants storage and combustion, cooling of microelectronics, lubrication systems).

The size of degassing bubbles is often crucial for the efficiency of applications. In order to control bubble size, researchers studied the effect of several functional parameters (nozzle geometry, liquid physical properties, liquid flow rate, supersaturation etc.) on bubble growth. In terms of the present research project (that is funded by the European Space Agency, NPI 4000108790/13/NL/PA), the dynamics of degassing bubbles are examined under the effect of different dissolution pressures (partial gas pressure prior to injection), gravitational accelerations and liquid flow rates. For this study, a combination of optical and electrical diagnostics are applied. A high resolution still digital camera is used to investigate the time evolution of degassing bubbles size, population and velocity along the height of a degassing column. A unique patented electrical impedance spectroscopy technique (IVED) equipped with a multiplexer, is used to examine the volumetric gas phase distribution along the column height and thus characterize the resulting two-phase flow.

Video1: Degassing of a flowing liquid jet under 300kPa pressure drop

Figure 1: a) Degassing column equipped with optical and electrical diagnostics, for two phase flow characterization at 4 different measuring channels, b) The resulting volumetric gas fraction (ε) time evolution along the column height, under 300kPa dissolution pressure and 0.21 L/min liquid flow rate.

Video2: Degassing bubbles ascension in a 4g acceleration environment

Figure 2: a) Large Diameter Centrifuge facility of ESA/ESTEC for the imposition of artificial hypergravity, b) Small-scaled degassing experimental set up for tests in hypergravity, c) Size distribution of degassing bubbles under a wide range of gravitational accelerations, at 300kPa dissolution pressure.


Liquid pool degassing

Contant Person: V. Papadopoulou []


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.