Denis Chatain

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.

Using magnetic levitation to produce cryogenic targets for inertial fusion energy: experiment and theory

We present experimental and theoretical studies of magnetic levitation of hydrogen gas bubble surrounded by liquid hydrogen confined in a semi-transparent spherical shell of 3 mm internal diameter. Such shells are used as targets for the inertial confinement fusion (ICF), for which a homogeneous (within a few percent) layer of a hydrogen isotope should be deposited on the internal walls of the shells. The gravity does not allow the hydrogen layer thickness to be homogeneous. To compensate this gravity effect, we have used a non-homogeneous magnetic field created by a 10 T superconductive solenoid. Our experiments show that the magnetic levitation homogenizes the thickness of liquid hydrogen layer. However, the variation of the layer thickness is very difficult to measure experimentally. Our theoretical model allows the exact shape of the layer to be predicted. The model takes into account the surface tension, gravity, van der Waals, and magnetic forces. The numerical calculation shows that the homogeneity of the layer thickness is satisfactory for the ICF purposes.

Magnetic Gravity Compensation

Magnetic gravity compensation in fluids is increasingly popular as a means to achieve low-gravity for physical and life sciences studies. We explain the basics of the magnetic gravity compensation and analyze its 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 are presented. A review of the existing experimental installations and works involving the magnetic gravity compensation is given for both physical and life sciences.

Study of fluid behaviour under gravity compensated by a magnetic field

Fluids, and especially cryogenic fluids like hydrogen and oxygen, are widely used in space technology for propulsion and cooling. The knowledge of fluid behaviour during the acceleration variation and under reduced gravity is necessary for an efficient management of fluids in space. Such a management also rises fundamental questions about thermo-hydrodynamics and phase change once buoyancy forces are cancelled. For security reasons, it is nearly impossible to use the classical microgravity means to experiment with such cryofluids. However, it is possible to counterbalance gravity by using the paramagnetic (O2) or diamagnetic (H2) properties of fluids. By applying a magnetic field gradient on these materials, a volume force is created that is able to impose to the fluid a varying effective gravity, including microgravity. We have set up a magnetic levitation facility for H2 in which numerous experiments have been performed. A new facility for O2 is under construction. It will enable fast change in the effective gravity by quenching down the magnetic field. The facilities and some particularly representative experimental results are presented.

Bubble spreading during the boiling crisis: modelling and experimenting in microgravity

Boiling is a very efficient way to transfer heat from a heater to the liquid carrier. We discuss the boiling crisis, a transition between two regimes of boiling: nucleate and film boiling. The boiling crisis results in a sharp decrease in the heat transfer rate, which can cause a major accident in industrial heat exchangers. In this communication, we present a physical model of the boiling crisis based on the vapor recoil effect. Under the action of the vapor recoil the gas bubbles begin to spread over the heater thus forming a germ for the vapor film. The vapor recoil force not only causes its spreading, it also creates a strong adhesion to the heater that prevents the bubble departure, thus favoring the further spreading. Near the liquid-gas critical point, the bubble growth is very slow and allows the kinetics of the bubble spreading to be observed. Since the surface tension is very small in this regime, only microgravity conditions can preserve a convex bubble shape. In the experiments both in the Mir space station and in the magnetic levitation facility, we directly observed an increase of the apparent contact angle and spreading of the dry spot under the bubble. Numerical simulations of the thermally controlled bubble growth show this vapor recoil effect too thus confirming our model of the boiling crisis.

Two-phase visualization at cryogenic temperature

This paper presents two different applications for two-phase visualization at low temperature. In the first application, a CCD video camera located inside vacuum is directly supported by the Pyrex pipe containing a two-phase superfluid flow. In the case of slightly positive slopes in which the flow is co-current but ascending, two different flow patterns have been seen, stratified and intermittent, depending on the vapor mass flow. Experimental investigations from stratified to intermittent flow have been made visually and compared to a code derived from the Taitler/Dukler model. The second application concerns phase transition of hydrogen near critical point (33 K) in zero gravity. The experiments have been performed in a cryostat equipped with a 10 T superconducting coil allowing the gravity compensation for hydrogen. Images of the condensation cell are shifted to the top of the cryostat with a specific cryogenic endoscope because CCD cameras do not work in high magnetic fields. The sample was enlightened with diffuse or parallel (coherent) light using a second endoscope. Images obtained in this apparatus are similar with those obtained in space.

Boiling Crisis Dynamics: Low Gravity Experiments at High Pressure

To understand the boiling crisis mechanism, one can take advantage of the slowing down of boiling at high pressures, in the close vicinity of the liquid-vapor critical point of the given fluid. To preserve conventional bubble geometry, such experiments need to be carried out in low gravity. We report here two kinds of saturated boiling experiments. First we discuss the spatial experiments with SF 6 at 46 ∘C. Next we address two ground-based experiments under magnetic gravity compensation with H 2 at 33 K. We compare both kinds of experiments and show their complementarity. The dry spots under vapor bubbles are visualized by using transparent heaters made with metal oxide films. We evidence two regimes of the dry spots growth: the regime of circular dry spots and the regime of chain coalescence of dry spots that immediately precedes the heater dryout. A recent H 2 experiment is shown to bridge the gap between the near-critical and low pressure boiling experiments.

First demonstration of multi-MeV proton acceleration from a cryogenic hydrogen ribbon target

We show efficient laser driven proton acceleration up to 14 MeV from a 62 μm thick cryogenic hydrogen ribbon. Pulses of the short pulse laser ELFIE at LULI with a pulse length of ≈350 fs at an energy of 8 J per pulse are directed onto the target. The results are compared to proton spectra from metal and plastic foils with different thicknesses and show a similarly good performance both in maximum energy as well as in proton number. Thus, this target type is a promising candidate for experiments with high repetition rate laser systems.

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 [0.08 − 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 compensation of gravitational forces in (p‐) hydrogen near its critical point: Application to weightlessness conditions

We report a study of the variation and compensation of gravitational forces in diphasic (p‐) hydrogen. The sample is placed on the axis of a superconducting coil near one of the ends, in order to benefit from a nearly uniform magnetic field gradient. A variable reduced effective gravity g* can thus be applied to the fluid. It is shown that the exact compensation of gravitational forces (“weightlessness conditions”) is limited by the inhomogeneity of the magnetic field gradient, which consists of a radial force field centered on the point of compensation and varying proportionally with the distance from this point. Such inhomogeneities can be quantified by an associated capillary length (lC). The effect of this remaining field is negligible when lC is smaller than the cell dimension. Near the critical point, the length lC tends to zero and the gas-liquid interface is deformed in a paradoxical pattern, in which the liquid phase lies at the center of the container with the gas phase at the walls. We vary the...