Yasuhiko H. Mori

Behavior of clathrate hydrate formation at the boundary of liquid water and a fluorocarbon in liquid or vapor state

Observational studies have been carried out to reveal the mechanistic aspects of the formation of clathrate hydrate, crystalline solid, at the boundary of water in the liquid state and a fluorocarbon (HFC-134a; CF3CH2F) in the liquid or vapor state. The surface morphology of the hydrate layer formed at the interface depends strongly on the degree of saturation of the water phase with the fluorocarbon. Whether the fluorocarbon is in a liquid state or in a vapor state matters little to the mechanical structure of the hydrate layer. The structure seems to be affected more significantly by the hydrodynamic conditions near the interface.

Pool Boiling of n-Pentane, CFC-113, and Water Under Reduced Gravity: Parabolic Flight Experiments With a Transparent Heater

A series of pool boiling experiments have been conducted under reduced gravity condition (the order of 10 -2 times the terrestrial gravity) available in an aircraft taking parabolic flight. A transparent resistant heater, a transparent indium oxide film plated on a glass plate, was employed so that the vapor/liquid behavior interacting with the heater surface could be observed from the rear side of the heater simultaneously with the side view of vapor bubbles above the heater surface. The experiments were performed for three different fluids-n-pentane, CFC-113, and water-under subcooled conditions. The critical heat fluxes for both n-pentane and CFC-113 under the reduced gravity were lowered to about 40 percent of the corresponding terrestrial values. Although the heat transfer characteristics in a low heat flux nucleate boiling regime for both n-pentane and CFC-113 showed no more than a slight change with the reduction in gravity, a significant heat transfer deterioration was noted with water in the reduced gravity boiling. The observation from the rear side of the heater suggested that this particular difference in the gravity dependency of heat transfer was ascribed to a considerable difference, between the organic fluids and water, in the behavior of attachment to the heater surface of the bubbles grown up, while the behavior of attachment must depend on the surface tension of each fluid and the wettability of the heater surface with the fluid.

Clathrate hydrate formation at the interface between liquid CO2 and water phases—a review of rival models characterizing “hydrate films”

Abstract The clathrate hydrate formation at the interface between liquefied carbon dioxide (CO 2 ) and liquid water is one of the key processes in the course of direct CO 2 disposal into deep seas—an option to mitigate the emission of CO 2 into the atmosphere. Eight different models have been proposed so far on the formation and metabolic self-preservation of a hydrate film at the interface and also the mass transfer of CO 2 across the hydrate film. This paper reviews those rival models one by one and illustrates how they are discrepant. Each model is critically examined, and if any, its weakness in physical reality or mathematical formulation is pointed out. The state of the art of hydrate-film modeling thus revealed suggests the necessity of more careful consulting of pertinent experimental observations to establish our physical view about hydrate films, which should serve as the base of any further work on hydrate-film modeling.

Clathrate hydrate crystal growth in liquid water saturated with a hydrate-forming substance: Variations in crystal morphology

This paper reports on our interpretation of our visual observations of the variations in macroscopic morphology of hydrate crystals growing in liquid water saturated with a guest substance prior to the hydrate formation. The observations were made in a high-pressure cell charged with liquid water and gaseous CO2. They revealed distinct variations in the morphology of hydrate crystals depending on the system subcooling ΔT sub, the temperature deficiency inside the cell from the triple CO2–hydrate–water equilibrium temperature under a given pressure. When ΔT sub ≳ 3 K, a hydrate film first grew along the CO2–water interface; then hydrate crystals with dendritic morphology grew in large numbers into the liquid-water phase from that hydrate film. When ΔT sub ≲ 2 K, the dendritic crystals were replaced by skeletal or polyhedral crystals. We present a non-dimensional index for such variations in hydrate crystal morphology. This is based on the idea that this morphology depends on the growth rate of hydrate crystals, and their growth rate is controlled by the mass transfer of the hydrate–guest substance (CO2 in the present experiments), dissolved in the bulk of liquid water, to the hydrate crystal surfaces. The morphology variations observed in the present and previous studies are related to this index.

Pool boiling of a non-azeotropic binary mixture under microgravity

Abstract Pool boiling experiments with non-azeotropic water-ethanol mixtures were conducted under 10-second microgravity conditions available from a drop shaft facility with a 490 m free fall. Comparing with the terrestrial condition, the boiling heat transfer was enhanced under microgravity. The observation from the back side of a transparent glass plate heater suggests that the enhancement could be ascribed to the Marangoni flow induced by an ethanol concentration gradient due to preferential evaporation of ethanol along the surface of enlarged vapor bubbles detached from, but still kept close to, the heater surface under microgravity.

Configurations of gas-liquid two-phase bubbles in immiscible liquid media

Abstract The configuration of a “two-phase bubble” constituted of a gas phase and a liquid phase in an immiscible liquid medium is classified into three types: complete engulfing of a gas bubble inside a liquid shell, partial coalescence of a gas bubble and a liquid drop forming a three-phase contact line, and non-coalescence whereby a gas bubble and a liquid drop remain separated. Simple criteria have been presented by which the favorable type of configuration in a given system is predicted from the values of the spreading coefficients characterizing the system. Experiments using some combinations of liquids as well as air suggest the general validity of the criteria.

Vaporization of single liquid drops in an immiscible liquid part II: Heat transfer characteristics

Heat transfer characteristics during the vaporization process of a pentane or furan drop in an aqueous glycerol of high viscosity has been studied. With the progress of vaporization, the overall heat transfer coefficient related to the liquid-liquid interfacial area of a two-phase bubble increases monotonically, and influences of initial drop diameter and temperature difference reduce. Some convection or circulation seems to occur in the unvaporized-liquid phase.ZusammenfassungIn dieser Arbeit wird der Wärmeübergang während der Verdampfung von Pentan- und Furan-Tropfen in einer wässerigen Glyzerinlösung hoher Viskosität untersucht. Mit fortschreitender Verdampfung steigt der Wärmeübergangskoeffizient, bezogen auf die Grenzfläche flüssig-flüssig der zweiphasigen Blase monoton an, wobei Einflüsse des anfänglichen Tropfendurchmessers und der Temperaturdifferenz abnehmen. In der nichtverdampften Flüssigkeitsphase scheint Konvektion oder Zirkulation aufzutreten.

Formation and dissociation of clathrate hydrate in stoichiometric tetrahydrofuran–water mixture subjected to one-dimensional cooling or heating

Abstract Formation and growth, or melting, of a polycrystalline layer of tetrahydrofuran (THF) hydrate from, or into, a liquid solution having the same composition as that of the hydrate have been observed in a macroscopically one-dimensional heat-transfer system under atmospheric pressure. Experiments were performed with either the liquid THF–water solution or the THF hydrate initially filling a 260 cm 3 cell which was sealed top and bottom by temperature-controlled copper plates and by glass plates on the side. In one group of experiments, the cell filled with the solution was initially adjusted at the hydrate–solution equilibrium temperature, T eq (4.4°C). The top copper plate was then cooled quasi-stepwise, while the temperature at the bottom copper plate was either unchanged or increased quasi-stepwise, resulting in the growth of a planar polycrystalline hydrate layer down from the surface of the top plate. In another group of experiments, the cell was initially filled with a polycrystalline THF-hydrate phase at a temperature slightly lower than T eq . Successively, the temperature at the bottom plate was increased quasi-stepwise to exceed T eq , resulting in the melting of the hydrate phase from the bottom. The behavior of such growth and melting of the hydrate layers observed in the experiments is in general agreement with that predicted by relevant theoretical/numerical analyses of transient conductive and/or free-convective heat transfer from/to the hydrate–solution interface, where the temperature is assumed to be fixed at T eq . Also described in this paper is an unexpected finding in a particular experimental condition—the formation of column-like hydrate crystals extending almost across the 20-mm spacing between the top and bottom plates, which precedes the growth of a planar polycrystalline layer.

Dissolution of liquid CO2 into water at high pressures: A search for the mechanism of dissolution being retarded through hydrate-film formation

Abstract The rate of dissolution of CO 2 from a liquid CO 2 phase to an adjoining liquid water phase is known to be retarded significantly by the formation of a thin clathrate hydrate film spread over the interphase boundary. Much account is currently made of this “barrier” effect in the investigation of ocean disposal, or sequestration, of CO 2 , an option to mitigate the emission of CO 2 into the atmosphere. However, the primary, dominant mechanism of the “barrier” effect provided by hydrate films is still unclear and sometimes delusively explained. This paper aims to discuss possible mechanisms of the effect and reasonably estimate the dominant one. The mechanism thus estimated is a reduction of the solubility of CO 2 in liquid water in thermodynamically stable hydrate/liquid-water coexisting conditions at a temperature below T h , the hydrate/liquid/liquid equilibrium temperature under a given pressure, from that in metastable ( T h ) or stable (⩾ T h ) hydrate-free conditions, which causes a smaller driving force for the diffusive, or convective, CO 2 transfer from hydrate-film surfaces to the adjoining liquid water phase compared to that for the transfer from hydrate-free CO 2 /water interfaces. It is also suggested that liquid CO 2 drops, freely buoying up in the sea, may also suffer an additional mechanism of retarding dissolution: a hydrate film covering each drop should suppress the tangential mobility of its surface, thereby causing a decrease in the surface-to-seawater mass transfer coefficient for CO 2 .

Microscopic observations of clathrate-hydrate films formed at liquid/liquid interfaces. I. Morphology of hydrate films

Abstract This study aims to obtain cross-sectional views of clathrate-hydrate films each formed at the interface between a liquid-water phase and a hydrophobic hydrate-former phase. For this purpose, an experimental scheme was devised which permitted us to observe, through a high-resolution microscope, cross sections of a ring-shaped hydrate film formed over the surface of each discoid drop of HCFC-141b (CH 3 CCl 2 F) held stationary in a narrow space between two transparent plates filled with flowing, or quiescent, water. We found that the hydrate films, once exposed to a shear flow of water and then held in a quiescent medium of water, continued to thicken at a much higher rate than the films which had been held exclusively in a quiescent water medium since their formation. The former films kept their surface/internal texture much coarser, in the course of their thickening, than the latter films which showed very fine texture throughout. When continuously exposed to a steady water flow, the hydrate films kept their thickness constant. The steady-state thickness thus recognized showed a negative dependency on the flow velocity. A temperature rise, slightly exceeding the thermodynamic stability limit for the hydrate, caused each hydrate film once grown at a lower temperature to undergo crystal dissociation simultaneously throughout its thickness, in which numerous HCFC-141b droplets, a few micrometers in diameter, were evolved and migrated into the adjacent water phase.