Compensation magnétique de pesanteur dans des fluides : synthèse des performances et contraintes

La communication présente les principauxrésultats théoriques et expérimentaux obtenus depuis unequinzaine d'années dans le domaine de la compensationmagnétique de pesanteur. Cette technique, destinéeprincipalement à des études de comportement de fluides enenvironnement spatial, utilise des sources de champ magnétiquegénéralement élevé. Les avantages et les limites de la lévitationsont présentés ; son application à des problèmes decomportement de fluides sous forme diphasique est décrite.</p

Visualization in cryogenic environment: Application to two-phase studies

11 pagesInternational audienceThis paper reviews recent technical developments devoted to the study of cryogenic two-phase fluids. These techniques span from simple flow visualization to quantitative measurements of light scattering. It is shown that simple flow pattern configurations are obtained using classical optical tools (CCD cam- eras, endoscopes), even in most severe environments (high vacuum, high magnetic field). Quantitative measurements include laser velocimetry, particle sizing, and light scattering analysis. In the case of mag- netically compensated gravity boiling oxygen, optical access is used to control the poistioning of a bubble subject to buoyancy forces in an experimental cell. Flow visualization on a two-phase superfluid helium pipe-flow, performed as a support of LHC cooldown studies, leads to flow pattern characterization. Visu- alization includes stratified and atomized flows. Thanks to the low refractive index contrast between the liquid and its vapor, quantitative results on droplet densities can be obtained even in a multiple scatter- ing regime

Nucleation and growth of a bubble pattern under vibrations in weightlessness

Liquid-vapor phase transition of hydrogen was investigated during vibrations of "high" frequency = [10\hbox 25.4] s-1 and "low" amplitude [0.3\hbox0.47] mm in a weightless environment. Gravity effects were compensated in a strong magnetic-field gradient. The experiments were performed near the liquid-vapor critical point and at liquid, slightly off-critical density. Vapor bubbles nucleate and grow in the liquid phase. During the initial stage, the mean bubble diameter, D, is lower than the viscous boundary layer thickness, $\delta$, and bubble growth is unaffected by vibrations, i.e. D ~ (time)1/3. When D > $\delta$, the bubbles and the liquid phase have different velocities and the bubble pattern orders in rows perpendicular to vibration under the influence of forces of hydrodynamic origin. An analysis in terms of Levy flight and superdiffusion gives a bubble evolution of D ~ (time)1/2, a result that compares reasonably well with the experimental data

Phase transition under forced vibrations in critical CO₂

Phase separation is investigated in CO2 under linear harmonic vibrations. The study is performed under weightlessness in a sounding rocket. The fluid is at critical density near its critical point to get benefit from universal behavior. Without vibration, phase separation is characterized by an interconnected pattern of vapor and liquid domains and a near linear growth law. Under vibration, three time regions have been identified. i) When the liquid-vapor domains are smaller than a few viscous boundary layer thickness, growth is unaffected by vibration. ii) Then the Bernoulli pressure across the interfaces makes the domains grow exponentially perpendicularly to the vibration direction while growth parallel to the vibration direction is unaffected. iii) When the domains reach the sample size, the pattern looks as periodic stripes perpendicular to the vibration direction and keep on growing parallel to the vibration direction. A theoretical approach of these phenomena is proposed

Dynamics of phase transition in H2 under high frequency vibrations

Can vibrations act in space as an artificial gravity? We investigate here the role of high frequency vibrations to accelerate the dynamics of phase transition of gas and liquid in space. Hydrogen is studied near its critical point (Tc =33 K). Gravity effects are compensated in a high magnetic field gradient as provided by a 10 T superconducting coil. The experiments are performed in the temperature range [0.17 – 1.1] mK from Tc, at critical and off-critical densities. The pattern shows up as interconnected gas-liquid domains or bubbles. When the domain size becomes larger than the viscous boundary layer, growth is accelerated and the domains eventually elongate in the direction perpendicular to the vibration (interconnected pattern case) or align in periodic planes in the same direction perpendicular to vibration (bubble pattern case). We explain the experimental findings by the presence of inertial velocity gradients between the vapor and liquid domains, which favor coalescence and fast domain growth

Magnetic gravity compensation

International audienceMagnetic gravity compensation in fluids is increasingly popular as a means to achieve low-gravity forphysical and life sciences studies. We explain the basics of the magnetic gravity compensation and analyzeits advantages and drawbacks. The main drawback is the spatial heterogeneity of the residual gravity field.We discuss its causes. Some new results concerning the heterogeneity estimation and measurement arepresented. A review of the existing experimental installations and works involving the magnetic gravitycompensation is given for both physical and life sciences

High-frequency driven capillary flows speed up the gas-liquid phase transition in zero-gravity conditions

Under weightlessness conditions, the phase transition of fluids is driven only by slow capillary flows.We investigate the effect of high-frequency vibrations to reproduce some features of gravity effects andshow that such vibrations can greatly modify the phase transition kinetics. The investigation is performedinH2near its critical point (critical temperature 33 K) where critical slowing down enables the phasetransition process to be carefully studied. Gravity effects are compensated in a strong magnetic fieldgradient

Magnetic compensation of gravity in fluids: performance and constraints

International audienceThe paper deals with the main theoretical and experimental results gathered over the past 15 years in the domain of magnetic compensation of gravity. Such a technique for which very high fields are required has mainly been used to investigate the behaviour of fluids under space conditions of weightlessness. Advantage and drawback of magnetic levitation are presented; its application for analyzing two-phase fluid behaviour is described