Bubble formation from a flexible hole submerged in an inviscid liquid

In the waste water treatment industry, a novel gas sparger based on flexible membranes has been used for the last ten years. The objective of the present work is to study the bubble formation generated from a flexible orifice (membrane). Firstly, the membranes are characterised with regard to their properties: wetting critical surface tension, expanding hole diameter, orifice coefficients, flexibility, critical and elastic pressures. The bubble formation phenomenon in an inviscid liquid at rest is studied experimentally for different membranes and gas flow rates. The variation in the bubble diameter, the bubble centre of gravity and the bubble spread on the membrane are determined as a function of time. An analytic model is proposed to describe the bubble growth and its detachment at a flexible orifice. This theoretical approach, developed by Teresaka & Tsuge (1990) for rigid orifices, is adapted to take into account the membrane features (elastic behaviour and wettability). The predicted bubble diameters at detachment agree with the experimental measurements; however, the model underestimates slightly the bubble formation times. The calculation of the various forces acting on the bubble in the vertical direction indicates that the real forces governing the bubble growth are the buoyancy force, the surface tension force, and near detachment the inertial force

Influence of liquid surface tension (surfactants) on bubble formation at rigid and flexible orifices

The influence of liquid surface tension on the bubble formation from both rigid and flexible orifice has been investigated. The liquid phases under test are aqueous solutions with butanol or surfactants (cationic, non-ionic and anionic); static and dynamic measurements of liquid surface tension have been performed to characterise them. This study shows that the effect of surface tension on the bubbles generated cannot be analysed only in terms of the static surface tension, but also depends on whether the bubbles are generated from a rigid orifice or from a flexible orifice. The kinetics of adsorption and diffusion of the solute molecules towards the bubble interface have to be taken into account insofar as their time scales are comparable to those of the bubble formation phenomenon

Dynamics of bubble growth and detachment from rigid and flexible orifices

The objective of this paper is to understand how and why the orifice nature (rigid or flexible) governs the bubble generation. The differences in orifice nature and properties have strong consequences on the bubbles generated. Indeed, the dynamics of the formation and the nature of the detached bubbles are fundamentally different depending on whether the bubbles are generated from the rigid orifice or from the flexible orifice. Keywords. Gas-Liquid reactors, aeration, rigid and flexible orifices, bubble formation dynamics. Résumé. L’objectif de cette étude est de comprendre comment et pourquoi la nature de l’orifice (rigide ou flexible) contrôle la génération de bulles. Les différences de nature et de propriétés entre les deux orifices ont des conséquences notables sur les bulles générées. En effet, la dynamique de formation et la nature des bulles détachées sont fondamentalement différentes selon si elles sont générées par un orifice rigide ou par un orifice flexible

Effect of surfactants on liquid side mass transfer coefficients: a new insight

Specific experiments are proposed to investigate the effect of surfactants on liquid side mass transfer coefficients. They are based on the determination of the liquid side mass transfer coefficient kL at a free gas-liquid interface, under controlled temperature and hydrodynamic conditions. Firstly, the methodology is validated in water at various rotation speeds and temperatures. In a second time, it is applied in aqueous and pure solutions of anionic surfactants: a decrease of kL with an increase of surfactant concentrations is then observed until leveling off when the CMC is reached. Deduced from experimental results, the equivalent diffusion coefficients describe an identical behavior. These results demonstrate that the lowest kL are directly linked to the presence of surfactants at the gas-liquid interface which makes the diffusion coefficients of oxygen be reduced. At last, a comparison is performed with the data of [1-2] obtained from a chain of bubbles having diameters above to 3.5 mm. A quasi-linear relation between the kL issued from both hydrodynamic configurations is revealed in the whole range of surfactant concentrations. Such findings would prove that, in both cases, the impact of surfactants on liquid side mass transfer coefficient is correlated with the changes in the diffusion coefficients of oxyge

Dependence of Gas Phase Abundances in the ISM on Column Density

Sightlines through high- and intermediate-velocity clouds allow measurements of ionic gas phase abundances, A, at very low values of HI column density, N(HI). Present observations cover over 4 orders of magnitude in N(HI). Remarkably, for several ions we find that the A vs N(HI) relation is the same at high and low column density and that the abundances have a relatively low dispersion (factors of 2-3) at any particular N(HI). Halo gas tends to have slightly higher values of A than disk gas at the same N(HI), suggesting that part of the dispersion may be attributed to the environment. We note that the dispersion is largest for NaI; using NaI as a predictor of N(HI) can lead to large errors. Important implications of the low dispersions regarding the physical nature of the ISM are: (a) because of clumping, over sufficiently long pathlengths N(HI) is a reasonable measure of the_local_ density of_most_ of the H atoms along the sight line; (b) the destruction of grains does not mainly take place in catastrophic events such as strong shocks, but is a continuous function of the mean density; (c) the cycling of the ions becoming attached to grains and being detached must be rapid, and the two rates must be roughly equal under a wide variety of conditions; (d) in gas that has a low average density the attachment should occur within denser concentrations

Bubble formation at a flexible orifice with liquid cross-flow

In waste water treatment, biological processes for denitrification and nitrification are performed using oxidation ditches. In these reactors, the mixing and the aeration functions are dissociated: a bubble cloud is generated from flexible membrane spargers and is subjected to a horizontal liquid flow. The objective of this paper is to study the effects of the liquid crossflow on the bubble formation at a single flexible orifice in water. The several forces acting on the forming bubble have been modelled in order to understand the dynamics of the bubble growth and detachment. The bubble detachment is controlled by the drag force due to the liquid motion and not by the buoyancy force. The experimental analysis of the bubble growth has shown that, under liquid cross-flow conditions, the bubbles move downstream and are flattened during their growth (position of the bubble centre of gravity, bubble inclination angle). The bubbles spread over the orifice surface, and the advancing and the receding bubble angles were measured. The detached bubbles have significantly smaller sizes and higher frequencies when compared to bubble formation under quiescent liquid conditions

Structural Health Monitoring from Sensing to Processing

Providing the best availability of aircrafts is a key driver in aeronautics industry. Monitoring system able to detect signs of failure before they happen, thanks to sensors and diagnosis/prognosis algorithms, is key for improving aircraft operability. Since a suspension system is connecting the engine to the aircraft, after hard landing, aircraft companies need to know if the suspension system is safe or could have been damaged. This chapter presents an autonomous wireless load sensing recorder development that will enable maintenance operators to make a relevant diagnosis of the suspension system by measuring the load level seen after a hard landing by connecting a portable device near the embedded sensor system. The sensor integrates energy harvesting and RFID communication modules that have been developed for this application. Data acquisition is performed by an embedded microcontroller connected to sensors. The paper is firstly dedicated to the different energy sources available in the project application (engine pods). The second part gives a presentation of the various devices developed for converting ambient energy into electric power and SHM system. The last part presents real measurement of ambient energy level from real tests in comparison to the energy needed to power the system

Effect of surfactants on liquid side mass transfer coefficients

In the present paper, the effect of liquid properties (surfactants) on bubble generation phenomenon, interfacial area and liquid side mass transfer coefficient was investigated. The measurements of surface tension (static and dynamic methods), of Critical Micelle Concentration (CMC) and of characteristic adsorption parameters such as the surface coverage ratio at equilibrium (se) were performed to understand the effects of surfactants on the mass transfer efficiency. Tap water and aqueous solutions with surfactants (cationic and anionic) were used as liquid phases and an elastic membrane with a single orifice as gas sparger. The bubbles were generated into a small-scale bubble column. The local liquid side mass transfer coefficient (kL) was obtained from the volumetric mass transfer coefficient (kLa) and the interfacial area (a) was deduced from the bubble diameter (DB), the bubble frequency (fB) and the terminal bubble rising velocity (UB). Only the dynamic bubble regime was considered in this work (ReOR = 150 - 1000 and We = 0.002 - 4). This study has clearly shown that the presence of surfactants affects the bubble generation phenomenon and thus the interfacial area (a) and the different mass transfer parameters, such as the volumetric mass transfer coefficient (kLa) and the liquid-side mass transfer coefficient (kL). Whatever the operating conditions, the new kLa determination method has provided good accuracy without assuming that the liquid phase is perfectly mixed as in the classical method. The surface coverage ratio (se) proves to be crucial for predicting the changes of kL in aqueous solutions with surfactants