John Hegseth

Growth of a dry spot under a vapor bubble at high heat flux and high pressure

We report a 2D modeling of the thermal diffusion-controlled growth of a vapor bubble attached to a heating surface during saturated boiling. The heat conduction problem is solved in a liquid that surrounds a bubble with a free boundary and in a semi-infinite solid heater by the boundary element method. At high system pressure the bubble is assumed to grow slowly, its shape being defined by the surface tension and the vapor recoil force, a force coming from the liquid evaporating into the bubble. It is shown that at some typical time the dry spot under the bubble begins to grow rapidly under the action of the vapor recoil. Such a bubble can eventually spread into a vapor film that can separate the liquid from the heater thus triggering the boiling crisis (critical heat flux).

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.

Thermal Response of a Two-Phase Near-Critical Fluid in Low Gravity: Strong Gas Overheating as Due to a Particular Phase Distribution

An experimental study of the thermal response to a stepwise rise of the wall temperature of two-phase near-critical SF6 in low gravity for an initial temperature ranging from 0.1 to 10.1 K from the critical temperature is described. The change in the vapor temperature with time considerably exceeds the change in the wall temperature (overheating by up to 23% of the wall temperature rise). This strong vapor overheating phenomenon results from the inhomogeneous adiabatic heating process occurring in the two-phase near-critical fluid while the vapor bubble is thermally isolated from the thermostated walls by the liquid. One-dimensional numerical simulations of heat transfer in near-critical two-phase 3He confirm this explanation. The influence of heat and mass transfer between gas and liquid occurring at short time scales on the thermal behavior is analyzed. A model for adiabatic heat transfer, which neglects phase change but accounts for the difference between the thermophysical properties of the vapor and those of the liquid, is presented. A new characteristic time scale of adiabatic heat transfer is derived, which is found to be larger than that in a one-phase liquid and vapor.

Gas wets a solid wall in orbit

When coexisting gas and liquid phases of a pure fluid are heated through their critical point, large-scale density fluctuations make the fluid extremely compressible and expandable and slow the diffusive transport. These properties lead to perfect wetting by the liquid phase (zero contact angle) near the critical temperature Tc. However, when the systems temperature T is increased to Tc, so that it is slightly out of equilibrium, the apparent contact angle is very large (up to 110°), and the gas appears to “wet” the solid surface. These experiments were performed and repeated on several missions on the Mir space station using the Alice-II instrument, to suppress buoyancy-driven flows and gravitational constraints on the liquid–gas interface. These unexpected results are robust, i.e., they are observed under either continuous heating (ramping) or stepping by positive temperature quenches, for various morphologies of the gas bubble and in different fluids (SF6 and CO2). Possible causes of this phenomenon include both a surface-tension gradient, due to a temperature gradient along the interface, and the vapor recoil force, due to evaporation. It appears that the vapor recoil force has a more dominant divergence and explains qualitatively the large apparent contact angle far below Tc.

Direct imaging of long-range concentration fluctuations in a ternary mixture

We used a direct imaging technique to investigate concentration fluctuations enhanced by thermal fluctuations in a ternary mixture of methanol (Me), cyclohexane (C), and partially deuterated cyclohexane (C*) within 1mK above its consolute critical point. The experimental setup used a low-coherence white-light source and a red filter to visualize fluctuation images. The red-filtered images were analyzed off-line using a differential dynamic microscopy algorithm that allowed us to determine the correlation time, τ, of concentration fluctuations. From τ, we determined the mutual mass diffusion coefficient, D, very near and above the critical point of Me-CC* mixtures. We also numerically estimated both the background and critical contributions to D and compared the results against our experimental values determined from τ. We found that the experimental value of D is close to the prediction based on Stokes-Einstein diffusion law with Kawasaki’s correction.Graphical abstract

Dimple coalescence and liquid droplets distributions during phase separation in a pure fluid under microgravity

Phase separation has important implications for the mechanical, thermal, and electrical properties of materials. Weightless conditions prevent buoyancy and sedimentation from affecting the dynamics of phase separation and the morphology of the domains. In our experiments, sulfur hexafluoride (SF6) was initially heated about 1K above its critical temperature under microgravity conditions and then repeatedly quenched using temperature steps, the last one being of 3.6 mK, until it crossed its critical temperature and phase-separated into gas and liquid domains. Both full view (macroscopic) and microscopic view images of the sample cell unit were analyzed to determine the changes in the distribution of liquid droplet diameters during phase separation. Previously, dimple coalescences were only observed in density-matched binary liquid mixture near its critical point of miscibility. Here we present experimental evidences in support of dimple coalescence between phase-separated liquid droplets in pure, supercritical, fluids under microgravity conditions. Although both liquid mixtures and pure fluids belong to the same universality class, both the mass transport mechanisms and their thermophysical properties are significantly different. In supercritical pure fluids the transport of heat and mass are strongly coupled by the enthalpy of condensation, whereas in liquid mixtures mass transport processes are purely diffusive. The viscosity is also much smaller in pure fluids than in liquid mixtures. For these reasons, there are large differences in the fluctuation relaxation time and hydrodynamics flows that prompted this experimental investigation. We found that the number of droplets increases rapidly during the intermediate stage of phase separation. We also found that above a cutoff diameter of about 100 microns the size distribution of droplets follows a power law with an exponent close to −2 , as predicted from phenomenological considerations.Graphical abstract

Near-critical fluid boiling: Overheating and wetting films

The heating of coexisting gas and liquid phases of pure fluid through its critical point makes the fluid extremely compressible, expandable, slows the diffusive transport, and decreases the contact angle to zero (perfect wetting by the liquid phase). We have performed experiments on near-critical fluids in a variable volume cell in the weightlessness of an orbiting space vehicle, to suppress buoyancy-driven flows and gravitational constraints on the liquid-gas interface. The high compressibility, high thermal expansion, and low thermal diffusivity lead to a pronounced adiabatic heating called the piston effect. We have directly visualized the near-critical fluid’s boundary layer response to a volume quench when the external temperature is held constant. We have found that when the system’s temperature T is increased at a constant rate past the critical temperature Tc, the interior of the fluid gains a higher temperature than the hot wall (overheating). This extends previous results in temperature quenching experiments in a similarly prepared system when the gas is clearly isolated from the wall. Large elliptical wetting film distortions are also seen during these ramps. By ray tracing through the elliptically shaped wetting film, we find very thick wetting film on the walls. This wetting film is at least one order of magnitude thicker than films that form in the Earth’s gravity. The thick wetting film isolates the gas bubble from the wall allowing gas overheating to occur due to the difference in the piston effect response between gas and liquid. Remarkably, this overheating continues and actually increases when the fluid is ramped into the single-phase supercritical phase.

Finding the temperature using image analysis techniques

Many experiments used light scattering to visualize the fluctuations of fluids density. Fluids near the critical point are affected by gravity because the compressibility of the fluid is very large near the critical point. Therefore, microgravity experiments allowed new phenomena to be discovered by reducing convection, sedimentation and buoyancy phenomena. In order to study, fluctuation and phase separation processes near the critical point of pure fluids without the influence of the Earths gravity, a number of experiments were performed in microgravity. Our results refer to a set of experiments that studied local density fluctuations by illuminating a cylindrical cell filled with sulfur hexafluoride, near its liquid-gas critical point. Using image analysis, we estimated the temperature of the fluid in microgravity from the recorded images showing fluctuations of the transmitted and scattered light. Our method has the advantage of avoiding any reference to the spatial correlation of the pixels in the recorded images. We assumed that the variation of the scattered light intensity is proportional to the average value of the gray levels. Furthermore, we also assumed that a small fluctuation of the fluid density induces a change in the scattered light intensity that can be measured from average gray scale intensity of the image. We found that the histogram of an image can be fitted to a Gaussian relationship and by determining its width we were able to estimate the position of the critical point.

Large-scale geophysical flows on a table top.

Abstract: Results from two similar experimental systems that attempt to create laboratory geophysical analogs in spherical geometry are presented. In the first system, real time holographic interferometry and shadowgraph visualization are used to study convection in the fluid between two concentric spheres when two distinct buoyancy forces are applied to the fluid. The heated inner sphere has a constant temperature that is greater than the outer spheres constant temperature by ΔT. In addition to the usual gravitational buoyancy from temperature induced density differences, another radial buoyancy is imposed by applying an ac voltage difference, ΔV, between the inner and outer spheres. The resulting electric field gradient in this spherical capacitor produces a central polarization force. The temperature dependence of the dielectric constant results in the second buoyancy force that is especially large near the inner sphere. The normal buoyancy is always present and, within the parameter range explored in our experiment, always results in a large‐scale cell that is axisymmetric about the vertical axis. We have found that this flow becomes unstable to toroidal or spiral rolls that form near the inner sphere and travel vertically upward when ΔT and ΔV are sufficiently large. These rolls start near the center spheres equator and travel upward toward its top. In the second experimental system, the central force is applied to a highly compressible near‐critical fluid in weightlessness (parabolic flight) and normal gravity. Although a geophysically similar density distribution could not be obtained in the limited time of a parabolic flight, clear influences of the central force on the fluid were observed in both weightlessness and terrestrial experiments.

Critical boiling phenomena observed in microgravity

Singular thermodynamic properties of pure fluids near their critical point (diverging specific heat and isothermal compressibility, vanishing thermal conductivity,…) lead to poor thermal diffusion, a large convective sensitivity, and a special heat transfer process through adiabatic compression, the so-called piston effect. The discovery and extensive study of the piston effect were performed in conditions of weightlessness to avoid convection. Although its mechanism in the supercritical range is now well understood, its coupling with an inhomogeneous density distribution and mass transport in the two-phase regime has been relatively less studied and remains puzzling. Recent experiments performed in the French Alice 2 facility built by CNES onboard the Mir station showed undoubtedly that when a liquid-vapor system of SF6 near its critical temperature is heated in microgravity, the apparent contact angle becomes very large (up to 110°). In this slightly out-of-equilibrium configuration the gas appears to “...