The Effect of the Evaporation Coefficient and Temperature Jumps on the Thermophoresis of a Particle in a Binary Gas Mixture in View of Thermodiffusion and Stefan Effects

A theory of uniform thermophoretic motion of a solitary spherical volatile particle in an unbounded incompressible binary mixture of gases with a phase transition of one of the components on the surface of the condensed phase is constructed on the basis of hydrodynamic method under slip conditions. Analysis is performed of the direct relative effect of the evaporation coefficient and boundary temperature jumps on the distribution of the velocity, temperature, and concentration of the volatile component and on the rate of thermophoresis of a moderately large highly viscous sphere. The thermodiffusion terms, Stefan effects, and the heat flux associated with convective transfer of evaporating mass are taken into account. A generalization is performed by taking into account the internal circulation of the matter of a large liquid one-component droplet and the thermocapillary phenomena. The inferences made in this paper disagree with those of the traditional theories in the cases of thermophoresis of a solitary aerosol particle under conditions of weak and moderately strong diffusion evaporation. Analysis has revealed that, in the case of weak evaporation of a large particle, the difference between the results increases as the binary gas mixture is saturated with vapors of the volatile component. Apparently, the calculation formulas for the rate of thermophoretic transfer have a wider range of validity than those previously obtained, all other things being equal.     

Gas amplification of proportional counters with cathodes of rectangular cross section

We have measured gas amplification factors in counters with rectangular cross section for a 90% argon 10% methane mixture (0.1–1.6 MPa) and methane gas (0.1–0.6 MPa). In the theoretical calculation, an effective cathode radius was introduced from Tomitani's treatment that the potential distribution near the the anode calculated by means of a conformal transformation of the Jacobian elliptic function coincided with that of a cylindrical counter. The experimental results showed that the formulas of Zastawny and of Charles could give a good fit for given counter conditions; therefore, the effective cathode radius for a counter with rectangular cross section is available in the calculation of the gas amplification factor.     

The influence of metal vapors resulting from electrode evaporation in a thermal puffer-type circuit breaker

The present work is a computational investigation of thermal plasmas in SF₆ switchgear, which burns in a mixture of working gas and PTFE and metal vapors resulting from nozzle ablation and electrode erosion, respectively. In order to solve the concentration of PTFE and electrode vapors conservation equations for the PTFE vapor and the metal vapor are solved together with the governing equations for mass, momentum, and energy of the gas mixture under the assumptions of local thermodynamic equilibrium (LTE) and local chemical equilibrium (LCE). The influence of metal vapor due to electrode evaporation on the arc behavior in SF₆ switchgear can be explained with some important parameters indicating the characteristics of SF₆ arc plasma. First, temperature in arc column is considerably different from that of the case without contact evaporation. The contamination by metal vapors broadens the arc radius and lowers the core temperature when compared to the case without evaporation. Second, the concentration of PTFE vapor from nozzle surfaces can more easily move to the expansion volume during high current period due to the existence of metal vapors near electrodes. Finally, the electrical conductivity in the arcing zone can be higher and may negatively affect the interruption limit of switchgears due to the increasing rate of breakdown by the metal vapors between the electrodes.     

Theory of Thermophoresis of Moderately Large Drops of Concentrated Solutions in Binary Gaseous Mixtures

We have derived a formula for the rate of thermophoresis of a volatile drop of a concentrated solution in a binary gaseous mixture with allowance for known effects that are associated with all types of slipping of the gaseous mixture along the drop surface and jumps of temperature and concentrations. An analysis of this formula has been performed for a number of limiting cases.     

Influence of the Evaporation Coefficient and Temperature Jumps on the Thermophoresis of a Moderately Large Volatile High‐Viscosity Sphere in a Binary Gas Mixture with Allowance for Volume Thermodiffusion and Stefan Effects

The theory of a uniform thermophoretic motion of a volatile highviscosity sphere with the phase transition of one component of a moderately rarefied binary gas mixture on its surface has been constructed on the basis of the hydrodynamic method in the regime with slip. The relative influence of the evaporation coefficient and the boundary temperature jumps on the distributions of the velocities, temperatures, and concentrations of the volatile component and the thermophoresis rate has been analyzed. Allowance has been made for the thermodiffusion terms, Stefan effects, and the heat due to the convective transfer of the substance of the condensed phase. The formula obtained has wider limits of application than the existing results. The conclusions of the traditional theories are successfully generalized to the cases of weak and moderately strong processes of diffusion evaporation of a single highviscosity droplet that moves in a nonuniformly heated binary mixture of gases.     

Dynamics and fragmentation of thick-shelled microbubbles

Localized delivery could decrease the systemic side effects of toxic chemotherapy drugs. The unique delivery agents we examine consist of microbubbles with an outer lipid coating, an oil layer, and a perfluorobutane gas core. These structures are 0.5-12 /spl mu/m in radius at rest. Oil layers of these acoustically active lipospheres (AALs) range from 0.3-1.5 /spl mu/m in thickness and thus the agents can carry a large payload compared to nano-scale drug delivery systems. We show that triacetin-based drug-delivery vehicles can be fragmented using ultrasound. Compared with a lipid-shelled contrast agent, the expansion of the drug-delivery vehicle within the first cycle is similar, and a subharmonic component is demonstrated at an equivalent radius, frequency, and driving pressure. For the experimental conditions explored here, the pulse length required for destruction of the drug-delivery vehicle is significantly greater, with at least five cycles required, compared with one cycle for the contrast agent. For the drug-delivery vehicle, the observed destruction mechanism varies with the initial radius, with microbubbles smaller than resonance size undergoing a symmetric collapse and producing a set of small, equal-sized fragments. Between resonance size and twice resonance size, surface waves become visible, and the oscillations become asymmetrical. For agents larger than twice the resonance radius, the destruction mechanism changes to a pinch-off, with one fragment containing a large fraction of the original volume.     

Effect of fluorine doping on the optical loss in MCVD fibers based on heavily doped germanosilicate glass

We have studied the size distribution of silica glass particles in the fabrication of heavily doped germanosilicate glass fiber preforms by modified chemical vapor deposition (MCVD) at different Freon 113 concentrations in the gas mixture. The addition of Freon 113 to the gas mixture is shown to reduce the particle size in the deposited core glass layer and the optical loss in the fiber. A mechanism is proposed which accounts for the effect of the initial particle size in the core glass layer on the anomalous scattering and total optical loss in heavily doped step- and graded-index fibers.     

Bending of curved sandwich beams

Curved sandwich beams in bending are analyzed with analytical elasticity methods and compared to simple analytical formulae and FEM calculations. Solutions to Airy's stress function in polar co-ordinates are used to obtain the stress distributions in the radial and circumferential directions. Plane stress and plane strain solutions are given with isotropic and orthotropic material models. It is shown that the properties of the core can have a significant influence on the circumferential stresses of the faces, but that the radial stress of the core is nearly constant with varying core properties. The radius of curvature can have a large influence on the circumferential stresses of the faces when the radius of curvature to sandwich thickness ratio is small to moderate. It is shown that simple analytical methods can be utilised to calculate the radial stresses in the core and the circumferential stresses of the faces for beams with large to moderate radii of curvature and thin to moderately thick faces.     

Experimental Evidence of the Emptying Core Mechanism During Combustion Synthesis of TiC Performed Under Isostatic Gas Pressure

Titanium carbide was synthesized from self-propagating high-temperature synthesis performed under isostatic gas pressure. Carbon and titanium particles of mean diameters of 7 or 20 m and 17 m, respectively, were used as reactants. The final product was analyzed by X-ray diffraction and scanning electron microscopy. Porosity was carefully determined by different methods. For a reactant mixture with coarse C particles, the large value of the closed porosity (22%) is shown to be compatible with the emptying core model. The origin of such a predominant mechanism in pure C and Ti mixture is tentatively discussed.     

Realization of high pressures and temperatures in the gas phase of a bubble liquid flowing through a nozzle

A steady flow of a bubble gas-liquid mixture in a nozzle having a circular cross section has been investigated. The possibility of realization of superhigh temperatures and pressures in the gas phase of the mixture in the region near the smallest cross section of the nozzle has been analyzed. The influence of the initial radius of the flow and the volume content of bubbles, determining the volumetric rate of flow of the liquid supplied to the nozzle, on the pattern of the flow has been considered. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 6, pp. 134–137, November–December, 2007.     

Tracing Thermal Creep and Thermophoresis in Porous Structures at Low Ambient Pressure and Low Gravity

We carried out two types of drop tower experiments to quantify gas and particle motion induced by temperature gradients inside a porous structure in a low pressure environment. In one setup 400 μm sized particles were traced inside heated channels at pressures of a few Pascal. Their motion is consistent with pure thermophoresis. In the second setup tracer particles were used to track the thermal creep gas flow through a porous dust bed. Here, the flow was traced outside of the dust bed and without thermophoretic motion. The results are consistent with a simple capillary model of the dust bed.     

Approximate analytical solutions for self-similar flows of a dusty gas with variable energy

Approximate analytical solutions are obtained for the unsteady one dimensional self-similar flow behind a strong shock wave propagating in a mixture of a gas and small solid particles at rest. It is assumed that variable energy is deposited at the shock and the total energy of the flow behind the shock is assumed to be varying with shock radius obeying a power law. The initial density of the medium is taken to be a constant. A simple integral method is employed to study the adiabatic and isothermal flows. The effect of the parameter characterizing the variation of total energy of the flow, on solutions of the flow field is investigated in detail. It as also shown that the similarity solutions of perfect gas with variable energy can be obtained as a particular case of this problem by taking the initial volume fraction of solid particles in the mixture as zero. Approximate solutions are compared with the numerical solutions and found a good agreement between them.     

Motion of a gas in a cyclone heat exchanger

A semi-empirical theory of turbulence is used to obtain relations for calculation of the field of gas velocity in the flow core and boundary layer in a cyclone chamber. To close the system of Navier-Stokes equations, the apparent shear stress is represented in the form of the gradient dependence in circulation. The relations for calculating the tangential component of velocity were derived using experimental data on the qualitative character of the distribution of apparent shear stress over the radius of the cyclone.Translated from Inzhenerno-fizicheskii Zhurnal, Vol. 60, No. 2, pp. 277–284, February, 1991.     

An approach for evaluating aerosol particle deposition from a natural convection flow onto a vertical flat plate.

An approach through numerical integration for evaluating aerosol particle deposition onto a vertical flat plate is proposed. The airflow was based on the assumption of a two-dimensional, incompressible and steady state laminar flow driven by a buoyancy force. The mechanisms of particle deposition were coupled from natural convection, Brownian diffusion, thermophoresis and electrophoresis due to constant electric strength. This approach demonstrated an easier method of prediction and produced a very good agreement with the thermophoresis exact solution. Results described the role of thermophoretic and electrophoretic forces on particle deposition. The thermophoresis effect was predicted to be particularly important for particles of d(p)>/=0.1 microm moving toward a cold surface or away from a hot surface at a given temperature gradient. The electrophoresis effect dominates the deposition of submicron particles.     

Helical Shear-Flow Instability in Phase-Separated Two-Component Bose-Einstein Condensates

We theoretically study the helical shear-flow instability in phase-separated two-component Bose-Einstein condensates by considering the case where one component forms a quantized vortex and the other component is localized in the vortex core with an uniform velocity. The analysis for a flat interface is applicable for a sufficiently large radius of the circular interface. On the other hand, when the interface radius is small, the analysis for flat interface is inapplicable and the instability is much affected by the outer vortex. The helical shear-flow instability is investigated with the interface wave model and the Bogoliubov-de Gennes model.     

Plasma-enhanced chemical vapor deposition of molybdenum

The plasma-enhanced chemical vapor deposition of molybdenum films from Ar-Mo(CO)₆ gas mixtures and H₂-Mo(CO)₆ gas mixtures was studied. Films deposited from the former mixture contain large quantities of carbon and oxygen before and after annealing, while their sheet resistance remained beyond the range of the four-point probe used here. As-deposited films from the latter mixture contain carbon, but oxygen is present apparently in the form of a hydroxide. On annealing, the resistivity reaches a minimum of 7.5 μΩ cm and oxygen is no longer present in the film.     

Analytical solution of the problem on thermal slip of second order for molecular gases

Results obtained using exact analytical methods in the problem on thermal slip of second order for molecular gases with allowance for the rotational degrees of freedom of molecules have been presented. Numerical calculations of the thermal-slip coefficient for a number of molecular gases have been carried out. The dependence of the velocity of thermal slip of second order of a molecular gas on the Prandtl number has been shown. The found value of the coefficient of thermal slip of second order theoretically confirms the existence of negative (in the direction of the temperature gradient) thermophoresis for molecular gases. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 3, pp. 190–194, May–June, 2006.     

Helium-propane as drift chamber gas

A light gas mixture, consisting of helium and propane (0.938:0.062) at atmospheric pressure has been tested in a large single-volume drift chamber. Contrary to the general belief that helium cannot be used as a drift gas due to its high ionisation potential, the above mixture was found to have stable operation with a spatial resolution of (260±40) μm.