A. A. Chernov

Gas Hydrate Formation in a Gas-Liquid Mixture Behind a Shock Wave

621 In this work, a new shock-wave technique for intensifying the process of hydrate formation of gases in gas‐liquid mixtures is proposed. There exist different methods for increasing the rate of this process, namely, fine dispersion of a jet saturated by a gas in a gas atmosphere [1‐3], intense stirring of water saturated by a gas dissolved in it [2, 4], exposure of a gas-saturated liquid to vibration [5], and exposure of a medium to ultrasound [6]. The main disadvantage of these methods is the low rate of gas hydrate formation and, as a consequence, the low efficiency of setups based on these techniques. In the absence of pipelines, one of the promising methods for transportation of natural gas consists in converting it into the gas hydrate state and then transporting it in a solid state at atmospheric static pressure and a low temperature (‐10 ° C to ‐20 ° C). The estimates made by Japanese and Norwegian scientists show that the gas hydrate technique for transportation and storage of natural gas is most cost-effective for small gas fields and shelf natural gas fields. A possible way of using the method proposed is the crystal hydrate method for demineralization of mineralized water. Using freon hydrates for this purpose is simplest from the engineering standpoint and most cost-effective [7]. In [8‐10] and some other books, the properties of gas hydrates and the main conditions and features of their formation are described and the mechanisms of gas hydrate formation and the types of their crystallization are presented. Much attention is given to physical and chemical methods for studying both man-made and natural gas hydrates. In this study, we experimentally investigate the process of gas bubble fragmentation and dissolution with the formation of freon-12 hydrate behind a shock wave of a moderate amplitude in water with gas bubbles. We carry out a theoretical analysis of the process of hydrate formation behind a steplike shock front and compare the results to the experimental data. The experiments were conducted in a shock tube. The test section was a 1.5 m-long vertical thick-walled steel tube with an inner diameter of 53 mm, which was limited from below by a rigid wall. The test section was filled with water containing freon-12 bubbles and was thermostatically controlled. The bubble dimensions were determined by capturing them with a digital camera equipped with additional optics through optical windows made in the upper part of the test section with a required time lag behind the shock front. The steplike pressure waves were generated by the breakdown of a diaphragm separating a 2 m-long high-pressure chamber and the test section. The pressure wave profiles were recorded by pressure transducers located along the test section and mounted flush with its inner wall. The local profile of the variation of the gas content (by volume) behind the shock wave was measured by a conductivity transducer placed at the middle part of the test section. Signals from the transducers were applied to an analog-to-digital converter and were then com

A Model of Magma Solidification During Explosive Volcanic Eruptions

The paper considers the problem of magma solidification during an explosive volcanic eruption, which is characterized by release of a large amount of gases from the magma. This leads to considerable cooling and, hence, solidification of the magma. It is found that solidified magma has the structure of porous glass with crystalline inclusions.

Dissolution and hydrate-formation processes behind the shock wave in a gas-liquid mixture

One of the important factors of Earth’s climatechange is an increase in the carbon-dioxide concentra-tion in the Earth atmosphere. A promising way of gasrecycling is its conversion into a gas–hydrate state andstorage at the ocean bottom at a low temperature andhigh static pressure [1]. One of the main parametersproviding the economic feasibility of this way is theformation rate of carbonic-acid hydrate. There are var-ious methods of intensification of the hydratization pro-cess for gases: the fine dispersion of the gas-saturatedjet in gaseous atmosphere [2], the intense hashing ofwater saturated with dissolved gas [3], the vibrationaction on the liquid saturated with gas [4], the ultra-sonic action on the medium [5], etc. The basic disad-vantage of the proposed methods is their low rate ofgas-hydrate formation.The authors of [6] propose a new shock-wavemethod of intensification of gas-hydrate formation. Itwas shown that the basic mechanism providing theintensification of the hydrate-formation process isshock-wave gas-bubble fragmentation. In [7] we exper-imentally investigated the dissolution and hydrate-for-mation processes behind a shock wave of moderateamplitude in water with Freon-12 bubbles at staticatmosphere pressure. We proposed a kinetic model ofthe hydrate-formation process behind the step-profileshock wave in a gas–liquid medium, when the thermaleffects can be neglected. In [8] the experimental data onthe shock-wave propagation in water with carbonic-gasbubbles are presented taking into account the processesof gas-bubble fragmentation, dissolution, and hydrateformation.In [9, 10], the effect of various surface-active sub-stances (SASs) on the hydrate-formation rate in amotionless medium under the condition of intensehashing of the medium and atomization of liquid in thegas phase is experimentally investigated. It is shownthat the presence of SAS in water results in increasingthe rate of gas-hydrate formation.In this study, we experimentally investigated theprocesses of dissolution and hydrate formation behindthe shock wave in water with carbonic-acid bubbles atvarious initial static pressures. The SAS effect on theprocesses of dissolution and hydrate formation in themedium is investigated. A theoretical model of the pro-cesses of dissolution and hydrate formation behind theshock wave in the gas–liquid medium is proposed tak-ing into account the convective and molecular diffusionof gas in the liquid and the convective and conductiveheat exchange due to the heat release at the interface asa result of the processes of dissolution and hydrate for-mation. The experimental data are compared to themodel calculation.We consider the liquid (water saturated with gas toan equilibrium state at this temperature and pressure)with gas bubbles in which the one-dimensional step-profile shock wave is propagated. We consider that thebubbles are crushed in the wave front into small gasinclusions, which form gas–liquid clusters. Because theliquid behind the wave front proves to be in an incom-pletely saturated state, the process of gas dissolution inliquid begins. We consider the case when the mediumbehind the shock-wave front occurs in the phase-statearea, where hydrate formation is possible. It results inthe formation and growth of hydrate shells on the gas-bubble boundary. In [11] it is noted that the hydrateformed on the surface of a gas bubble moving in watergrows in the form of separate crystals. The opinion wasstated that the film of hydrate crystals presents no sig-nificant obstacle for the interaction between water andgas, and there is always a free surface of gas–liquidcontact. Hence, it is possible to assume that the jointprocess of dissolution and hydrate formation behind theshock-wave front depends mainly on the heat and mass

Nonequilibrium mechanism of spherical crystal growth in overcooled melts, regarding substance shrinkage

A model of single spherical crystal growth in overcooled melt with regard for substance shrinkage during hardening is represented. A quasistationary solution to the problem is found. It is shown that in the case of equal phase densities, the solutions obtained in this paper and those obtained earlier by other authors coincide.

A Theoretical Model of Dissolution and Hydrate Formation Processes in Shock Waves

A theoretical model for the processes of dissolution and hydrate formation behind a shock wave in a gas-liquid medium with allowance for convective and molecular gas diffusion in the liquid and convective and conductive heat transfer caused by heat release at the interphase boundary due to dissolution and hydrate formation is proposed. A comparison of the model calculations with experimental data is made.

Opening of a system of cracks—on the mechanism of the cyclic lateral eruption of the St. Helens volcano in 1980

The dynamic behavior of a magma melt filling a slot channel (crack) in a closed explosive hydrodynamic structure is considered. The explosive hydrodynamic structure includes the volcano focal point with a connected vertical channel (conduit) closed by a slug and a system of internal cracks (dikes) near the dome, as well as a crater open into the atmosphere. A two-dimensional model of a slot eruption is constructed with the use of the Iordanskii–Kogarko–van Wijngaarden mathematical model of two-phase media and the kinetics that describes the basic physical processes in a heavy magma saturated by the gas behind the decompression wave front. A numerical scheme is developed for analyzing the influence of the boundary conditions on the conduit walls and scale factors on the melt flow structure, the role of viscosity in static modes, and dynamic formulations with allowance for diffusion processes and increasing (by several orders of magnitude) viscosity. Results of the numerical analysis of the initial stage of cavitation process evolution are discussed.

Analytical Solution of the Problem of Displacement of a Gas Dissolved in a Melt by Plane and Spherical Crystallization Fronts

Self‐similar solutions of the problem of displacement of a gas dissolved in a melt by plane and spherical crystallization fronts are found for the case where the crystal growth rate is inversely related to the square root of time. A criterion of the absence of gas displacement due to segregation is found. The problem for a plane crystallization front moving with a constant velocity is analytically solved by means of the Laplace transform method.

Model of spontaneous crystallization of a thin melted layer brought into contact with a massive substrate

A model of spontaneous crystallization of a thin melted metal layer brought into contact with a massive substrate is proposed. With invoking the Kolmogorov composite crystallization theory, the model allows one to predict the size distribution of crystallites across the layer, which provides a possibility of controlling the microstructure of the solidifying layer through a proper choice of substrates.