Vadim Nikolayev

Water recovery from dew

Abstract The recovery of clean water from dew has remained a longstanding challenge in many places all around the world. It is currently believed that the ancient Greeks succeeded in recovering atmospheric water vapour on a scale large enough to supply water to the city of Theodosia (presently Feodosia, Crimea, Ukraine). Several attempts were made in the early 20th century to build artificial dew-catching constructions which were subsequently abandoned because of their low yield. The idea of dew collection is revised in the fight of recent investigations of the basic physical phenomena involved in the formation of dew. A model for calculating condensation rates on real dew condensers is proposed. Some suggestions for the ‘ideal’ condenser are formulated.

Using radiative cooling to condense atmospheric vapor: a study to improve water yield

An inexpensive radiative condenser for collecting atmospheric vapor (dew) was tested in Grenoble (France). The surface temperature measurements are correlated with meteorological data (wind velocity, air temperature) and compared to the corresponding surface temperature of a horizontal Polymethylmethacrylate (Plexiglas) reference plate located nearby. The condenser surface is a rectangular foil (1 x 0.3 m 2) made of TiO2 and BaSO4 microspheres embedded in polyethylene. The foil has an angle with respect to horizontal. The backside of the device, thermally isolated, faces the direction of the dominant nocturnal wind. Both a 2-D numerical simulation of the air circulation around the foil and experimental measurements shows that the 30° angle is a good compromise between weak wind influence, large light-emission solid angle and easy drop collection. The study was conducted from November 25, 1999 to January 23, 2001. In comparison to the reference plate, it is found that water yield can be increased by up to 20 % and water collection greatly facilitated.

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.

New Hydrodynamic Mechanism for Drop Coarsening

We discuss a new mechanism of drop coarsening due to coalescence only, which describes the late stages of phase separation in fluids. Depending on the volume fraction of the minority phase, we identify two different regimes of growth, where the drops are interconnected and their characteristic size grows linearly with time, and where the spherical drops are disconnected and the growth follows (time) 1/3. The transition between the two regimes is sharp and occurs at a well defined volume fraction of order 30%.

Dynamics of the triple contact line on a nonisothermal heater at partial wetting

The dynamics of the triple gas-liquid-solid contact line is analyzed for the case where the gas is the saturated vapor corresponding to the liquid. For partial wetting conditions, a nonstationary contact line problem where the contact line motion is caused by evaporation or condensation is treated. It is shown that the Navier slip condition alone is not sufficient to relax the hydrodynamic contact line singularity: the Marangoni term is equally important when the heat transfer is involved. The transient heat conduction inside the heater is accounted for. A multiscale problem of drop evaporation with freely moving contact line is solved in the lubrication approximation as an illustration of the proposed approach.

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).

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.

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.

Development and test of a cryogenic pulsating heat pipe and a pre-cooling system

The needs of thermal links in cryogenic applications are increasing, especially because of the use of cryocoolers which offer a reduced size cold finger. The Pulsating Heat Pipe (PHP) is a passive two-phase high performance thermal link. Like the conventional heat pipe, it features a closed tube filled with a two-phase fluid able to transfer heat from its hot part (evaporator) to the cold part (condenser). A general problem for any two-phase cryogenic thermal link is the pre-cooling of the evaporator to ensure the presence of liquid inside the evaporator to start the flow motion. In conventional heat pipes, this problem is by passed by the wick but in the case of PHPs it has to be specially addressed. We have designed, manufactured and tested a helium PHP associated to a novel pre-cooling system. The cool down time of the PHP evaporator is reduced significantly. The maximum transferred power of the PHP is 145 mW with a cold source at 4.2 K.

Contact line singularity at partial wetting during evaporation driven by substrate heating

We present a theoretical investigation of the evaporation of a liquid on a solid substrate into the atmosphere of its pure vapor. The evaporation is provoked by the overheating of the substrate above the saturation temperature. At partial wetting, the liquid forms a wedge ending at the triple liquid-vapor-solid contact line (CL). The wedge region is extremely important in all evaporation geometries (bubble, drop, meniscus in a capillary) for two reasons. First, in this region a significant part of the evaporative heat flux is spent to compensate the latent heat. Second, a strong meniscus curvature that occurs in this region leads to the apparent contact angle larger than its actual microscopic value. We show that unlike the conventional diffusive evaporation models, the evaporation rate at the CL is bounded and is defined by the CL velocity. In particular the evaporation rate vanishes at the CL when it is immobile. This means that the slip length is not essential for the contact line singularity relaxation. The pressure boundary conditions at the CL are also derived. An analytic expression for the apparent contact angle (valid in the asymptotic limit of vanishing overheating) is derived. It is compared to numerical results.