Yves Garrabos

Rayleigh and Raman scattering near the critical point of carbon dioxide

An experimental study of the behavior of depolarized Rayleigh and Raman spectra of carbon dioxide when approaching the critical temperature along the critical isochore is presented. The study of the depolarized Raman scattering was carried out on the Fermi diad ν1, 2ν2 and was supplemented by a study of the polarized Raman scattering. Contributions to the different physical mechanisms which can give rise to the depolarized spectra are carefully analyzed. We have shown that the orientational dynamics of CO2 molecules are unchanged by critical slowing down of the diffuse hydrodynamical mode associated to fluctuations of the order parameter in the range 0.01 °C<T–Tc<10 °C. Moreover we found that the frequency variation of the polarized Fermi diad can be well described using only density terms. The variation of the spectral intensities of the different polarized and depolarized bands is well explained in terms of only turbidity and multiple scattering phenomena owing to the intensive quasielastic polarized Ra...

Experimental Evidence of the Vapor Recoil Mechanism in the Boiling Crisis

Boiling crisis experiments are carried out in the vicinity of the liquid-gas critical point of H2. A magnetic gravity compensation setup is used to enable nucleate boiling at near critical pressure. The measurements of the critical heat flux that defines the threshold for the boiling crisis are carried out as a function of the distance from the critical point. The obtained power law behavior and the boiling crisis dynamics agree with the predictions of the vapor recoil mechanism and disagree with the classical vapor column mechanism.

Heat transfers and related effects in supercritical fluids

This book investigates the unique hydrodynamics and heat transfer problems that are encountered in the vicinity of the critical point of fluids. Emphasis is given on weightlessness conditions, gravity effects and thermovibrational phenomena. Near their critical point, fluids indeed obey universal behavior and become very compressible and expandable. Their comportment, when gravity effects are suppressed, becomes quite unusual. The problems that are treated in this book are of interest to students and researchers interested in the original behavior of near-critical fluids as well as to engineers that have to manage supercritical fluids. A special chapter is dedicated to the present knowledge of critical point phenomena. Specific data for many fluids are provided, ranging from cryogenics (hydrogen) to high temperature (water). Basic information in statistical mechanics, mathematics and measurement techniques is also included. The basic concepts of fluid mechanics are given for the non-specialists to be able to read the parts he is interested in. Asymptotic theory of heat transfer by thermoacoustic processes is provided with enough details for PhD students or researchers and engineers to begin in the field. Key space are described in details, with many comparisons between theory and experiments to illustrate the topics.

Gas spreading on a heated wall wetted by liquid

This study deals with a simple pure fluid whose temperature is slightly below its critical temperature and whose density is nearly critical, so that the gas and liquid phases coexist. Under equilibrium conditions, such a liquid completely wets the container wall and the gas phase is always separated from the solid by a wetting film. We report a striking change in the shape of the gas-liquid interface influenced by heating under weightlessness where the gas phase spreads over a hot solid surface showing an apparent contact angle larger than 90 degrees. We show that the two-phase fluid is very sensitive to the differential vapor recoil force and give an explanation that uses this nonequilibrium effect. We also show how these experiments help to understand the boiling crisis, an important technological problem in high-power boiling heat exchange.

The phase transition of gases and liquids

The behavior of gases and liquids during a phase transition is severely affected by gravity effects which unavoidably drive the denser phase downwards and the lighter phase upwards. When such gravity effects are suppressed a number of new and surprising behavior are observed. The studies are performed mostly in the vicinity of a critical point where the behavior can be expressed in scaled units of a natural length scale (the correlation length of the order parameter fluctuations) and a natural timescale (the characteristic relaxation time of the order parameter fluctuations). We will review the main results obtained with binary liquids and simple fluids. Firstly, a phase change is initiated by quenching down the fluid from above the coexistence curve, where the fluid is homogeneous, to inside the coexistence curve, where its phase separates into two phases. The main result is the recognition that the hydrodynamics of coalescence eventually induces the pattern morphology and the phase transition kinetics. The volume fraction, is the key parameter which decides whether drops fuse because of random, Brownian collisions [pattern of drops, growth law in (time)1/3], or coalesce in a continuous process where the flow due to a coalescence event induces another coalescence (growth proportional to time, interconnected drop pattern). The presence of a wall modifies by its geometry and wetting properties the phase development. Coalescence is constrained and leads to new growth laws in the immediate vicinity of the wall. We emphasize that these phase ordering processes are quite general; they can be applied with success to quite different ordering situations, such as the sorting of embryonic tissues, an important process in morphogenesis. Secondly, when a gas–liquid, two-phase fluid, is heated from below the critical point to above, the process appears to be driven by the wetting and thermal properties of the boundary layer near the heating wall. Under terrestrial gravity, the dynamics is driven by a Rayleigh–Taylor instability. Under weightlessness, the liquid/vapor contact angle on the wall is modified in such a way that the gas seemingly “wets” the wall. We propose the vapor recoil force to be at the origin of this non-equilibrium “wetting” transition.

Depolarized light scattered near the gas–liquid critical point of Xe, SF6, CO2, C2H4, and C2H6

We report an experimental study of depolarized Rayleigh light scattered at 90° by two isotropic fluids (xenon and sulfur hexafluoride) and three anistropic fluids (carbon dioxide, ethane, and ethylene) near their respective gas–liquid critical points. All measurements were performed along the critical isochore, in the single phase region, that is to say for T≳Tc. The depolarization ratios, as shown by theory, are separable into two contributions. Far from the critical temperature single scattering of light is predominant but near the critical point double and multiple scatterings become the main phenomena. From measurements of polarized intensities in the temperature range corresponding to double scattering, the cross sections or the compressibility factors were calculated when a choice of a theoretical expression for the Rayleigh ratio was made.

Measurements of the Thermal Conductivity of HFC-125 in the Temperature Range from 300 to 515 K at Pressures up to 53 Mpa

Measurements of the thermal conductivity of HFC-134a made in a coaxial cylinder cell operating in steady state are reported. The measurements of the thermal conductivity of HFC-134a were performed along several quasi-isotherms between 300 and 530 K in the gas phase and the liquid phase. The pressure ranged from 0.1 to 50 MPa. Based on the experimental data, a background equation is provided to calculate the thermal conductivity outside the critical region as a function of temperature and pressure. A careful analysis of the various sources of errors leads to an estimated uncertainty of ±1.5%.

Master crossover functions for one-component fluids

By introducing three well-defined dimensionless numbers, we establish the link between the scale dilatation method able to estimate master (i.e., unique) singular behaviors of the one-component fluid subclass and the universal crossover functions recently estimated [Garrabos and Bervillier, Phys. Rev. E 74, 021113 (2006)] from the bounded results of the massive renormalization scheme applied to the Phi(d)(4)(n) model of scalar order parameter (n=1) and three dimensions (d=3), representative of the Ising-like universality class. The master (i.e., rescaled) crossover functions are then able to fit the singular behaviors of any one-component fluid without adjustable parameter, using only one critical energy scale factor, one critical length scale factor, and two dimensionless asymptotic scale factors, which characterize the fluid critical interaction cell at its liquid-gas critical point. An additional adjustable parameter accounts for quantum effects in light fluids at the critical temperature. The effective extension of the thermal field range along the critical isochore where the master crossover functions seems to be valid corresponds to a correlation length greater than three times the effective range of the microscopic short-range molecular interaction.

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.

Measurements of the Thermal Conductivity of HFC-32 (Difluoromethane) in the Temperature Range from 300 to 465 K at Pressures up to 50 MPa

New measurements of the thermal conductivity of HFC-32, made in a coaxial cylinder cell operating in steady state, are reported. The measurements were performed along several quasi-isotherms between 300 and 465 K in both the liquid and the vapor phases. The pressure ranged from 0.1 to 50 MPa. Based on the experimental data, a background equation is provided to calculate the thermal conductivity outside the critical region as a function of temperature and density. A careful analysis of the various sources of experimental errors leads to an estimated uncertainty of ±1.5%. Comparisons between calculated and experimental values from the literature are presented.