On the stability of thermally radiative magnetofluiddynamic channel flow

Summary The stability of three dimensional infinitesimal disturbances is examined for the laminar flow of a thermally radiating, viscous, electrically and heat-conducting fluid between parallel walls with transverse magnetic field. In addition to the classical criteria for the non-radiative case a further system of homogeneous differential equations with mixed boundary conditions arises yielding possible further eigenvalues and requirements for stable flows. For the overall configuration the analogue of Squire's theorem is shown to fail. The eigenvalue problem is of unusual type with the eigenvalue parameter appearing non linearly in both the differential equation and boundary conditions. A study of two-dimensional disturbances is carried out in detail for non-thermally conducting fluids and for a particular wave number the further eigenvalues are shown by exact analysis to be always stable. In channels with black walls at the same temperature extensive numerical calculations for a range of wave-numbers and a variety of other parameters all yield the same result.     

On the space-charge longitudinal wake field in perfectly conducting pipe of planar geometry

Longitudinal space-charge wake field is considered for a beam of rectangular cross-section propagating between two perfectly conducting planes. It is shown that the forces driving negative mass instability are weaker compared to circular configuration. A similar effect takes place for a flattened (ribbon) beam.     

On stability of shock waves in a compressible viscous heat conducting gas

Summary Shock waves for the compressible Navier-Stokes equations with heat conduction are analyzed. The shock wave is viewed as a strong discontinuity on which surface modified Rankine-Hugoniot conditions hold. These jump conditions are derived by the usual way from the system of viscous conservation laws. The linear instability of planar shock waves is proved. This points, in particular, to the inadequate character of numerical computations for viscous flows near blunt bodies with a bow discontinuous shock wave.     

导电聚合物纳米线的制备及气敏性能研究

首次采用简单的浸润多孔氧化铝(AAO)模板法制备了导电聚合物聚-3,4-乙烯二氧噻吩(PE-DOT)纳米线.导电聚合物溶液浸润AAO模板后,PEDOT吸附于孔道壁并进一步聚合生成导电聚合物纳米线.紫外-可见光-近红外光谱(UV-vis-NIR spec-trum)分析表明生成的纳米线处于掺杂态.采用四探针仪分析了导电聚合物纳米线的导电性能,结果显示纳米线电导率相比普通PEDOT材料有数量级增加,且表现出良好的掺杂/脱掺杂能力.研究了导电聚合物纳米线的气体敏感性能,发现其对挥发性醇类,尤其对甲醇在较低浓度下表现出优异的敏感性,对5×10-6甲醇气体的响应时间约为10～20s,测试可重复性超过20次,达到饱和吸附时的气体浓度明显大于普通PEDOT材料.表明PEDOT纳米线不仅提供了较大表面积供气体分子吸附,而且纳米线中导电通道取向一致,从而体现出较好的气体敏感性能.     

Features of the refractometry of turbulent gas flows

The features of the interference refractometry of turbulent gas flows, which are a special case of a randomly inhomogeneous medium, are considered. Recommendations are made on the quasi-optimal choice of the parameters of the refractometer system. Translated from Izmeritel’naya Tekhnika, No. 8, pp. 31–35, August, 1996.     

An oscillatory flow phenomenon in microtube flows of thermally responsive fluids

A simple asymptotic model for the flow of a thermally responsive fluid in a microtube is derived. At low temperatures these fluids behave as a Newtonian fluid; however, above a critical temperature they (reversibly) form gel-like structures. Also, because of the small length scales involved in microfluidic flows, viscous heating can become significant. This can lead to gelation simply from the temperature change due to viscous heating. Our model takes into account viscous heating, as well as possible conduction through the channel walls. The rheology of the thermally responsive fluid is modelled using a bi-viscosity model, with the gel phase being represented by a constant large viscosity. The model is then used to show that, when the viscous heating exceeds a critical level, an oscillatory flow behaviour can occur. These oscillations eventually become damped out as the system reaches a steady state; however, the time it takes for this to occur can become excessively large. The physical mechanisms that cause the oscillatory behaviour are examined, and the criteria for the oscillatory flow to occur are determined. Some analysis of the oscillations and the timescales involved therein are also presented.     

The geometry of the cartan space associated with general compressible fluid flows

In this paper, an attempt is made to develop a general theory of the motion of a compressible fluid by means of the geometry of the general space propounded by Cartan based on the concept of area. Assuming that the flow is irrotational and isentropic, it is shown in Section I, that the equation of motion of an inviscid compressible fluid is regarded as that of hypersurfaces in the Cartan space. Various hydrodynamical features of the compressible fluid motion including the “shock” phenomena are clarified by the metric properties of the space. The fluid motion in the general case, in which the above assumptions are not valid, is treated in Section 2. The physical quantities such as vorticity, entropy and the Croccian vector are expressed by means of the geometrical terminology of the Cartan space along lines parallel to the Section 1.     

Calculation of the temperature field in a plane channel with nonuniform heating of thermally conducting walls

A determination is made of the steady temperature of the walls of a channel with Poiseuille gas flow and with heat sources in the thermally conducting walls. The source power density has a parabolic distribution law.     

On the electrical anisotropy of conducting polypyrrole

The electrical properties of polypyrrole films prepared by electrochemical polymerization have been studied. The films were electrochemically and optically characterized. A very high anisotropy of the resistivities and mobilities, both parallel and perpendicular to the substrate, has been found. The results obtained are consistent with a hopping model for the conduction mechanism, with the interchain hopping much lower than the intrachain hopping, if it is assumed that the chains are oriented preferentially parallel to the substrate.     

On the degradation of conducting Langmuir-Blodgett films

Direct current through conducting Langmuir-Blodgett films was observed as a function of the applied voltage and alternating current as a function of frequency. In some cases voltage-current characteristics appeared to be quasi-ohmic but in other cases they exhibited tunnelling-like behaviour. This phenomenon was interpreted as being due to the progressive detachment of the film from the micro-electrodes, as confirmed by measurements of the conductivity as a function of time. This interpretation was also confirmed by estimating the tunnelling barrier parameters from voltage-current characteristics, by conductivity measurements before and after thermal treatment of the samples and by conductivity measurements of samples stored in air and under vacuum.     

Mixed conducting ceramic membranes for gas separation and reaction

The development of dense ceramic membranes for separating and reacting various gaseous components has been under intense investigation during the past several years. Such applications include the separation of oxygen from air, the partial oxidation of methane and other hydrocarbons, and the separation of hydrogen from synthesis gas or other hydrogen-containing process streams. The US Department of Energy is currently sponsoring major efforts in the development of these membranes for advanced power and fuels production applications. This article provides an overview of the current programme. Particular emphasis is placed on the separation of oxygen from air.     

On the geometry of polar varieties

We have developed in the past several algorithms with intrinsic complexity bounds for the problem of point finding in real algebraic varieties. Our aim here is to give a comprehensive presentation of the geometrical tools which are necessary to prove the correctness and complexity estimates of these algorithms. Our results form also the geometrical main ingredients for the computational treatment of singular hypersurfaces. In particular, we show the non–emptiness of suitable generic dual polar varieties of (possibly singular) real varieties, show that generic polar varieties may become singular at smooth points of the original variety and exhibit a sufficient criterion when this is not the case. Further, we introduce the new concept of meagerly generic polar varieties and give a degree estimate for them in terms of the degrees of generic polar varieties. The statements are illustrated by examples and a computer experiment.     

On the geometry of entangled states

The basic question that is addressed in this paper is finding the closest separable state for a given entangled state, measured with the Hilbert-Schmidt distance. While this problem is in general very difficult, we show that the following strongly related problem can be solved: find the Hilbert-Schmidt distance of an entangled state to the set of all partially transposed states. We prove that this latter distance can be expressed as a function of the negative eigenvalues of the partial transpose of the entangled state, and show how it is related to the distance of a state to the set of positive, partially transposed (PPT) states. We illustrate this by calculating the closest biseparable state to the W state and give a simple and very general proof for the fact that the set of W-type states is not of measure zero. Next we show that all surfaces with states whose partial transposes have constant minimal negative eigenvalue are similar to the boundary of PPT states. We illustrate this with some examples on bipartite qubit states, where contours of constant negativity are plotted on two-dimensional intersections of the complete state space.     

Time behavior of heat fluxes in thermally coupled turbulent dispersed particle flows

The effect on heat transfer produced by injection of solid microparticles with high thermal capacity in turbulent channel flow is analyzed. Convection is forced by letting the fluid flow between a hot plate and a cold plate under zero-gravity conditions. An Eulerian?CLagrangian approach based on direct numerical simulation of turbulence (shear Reynolds number Re*?=?150 and molecular Prandtl number Pr?=?3) and on point-particle tracking is used. Full momentum and energy coupling between fluid and particles is considered. Different particle sizes and different particle concentrations are examined.     

On the cathodic protection of thermally insulated pipelines

Thermal insulation and corrosion protection of heated pipelines used in moving oil/gas and heated products are accomplished industry-wide by using protective and insulating coatings supplemented by cathodic protection to protect any defected coating areas. A series of tests are carried out to study how the resistivity of the applied insulating layer polyurethane (PUR) can be affected by its specific gravity (as function of its porosity), as well as, the salt (NaCl) concentration in the surrounding electrolyte (water). The current densities required for cathodic protection of insulated steel pipe at the worst condition (i.e., lower resistivity of the insulating material) at ambient and elevated temperatures were determined. The results have showed: that a lower PUR foam density has lower percentage closed cells which lead to a decrease in the PUR foam resistivity. An increase in the salt concentration up to about 3.5% NaCl leads to about 40% increase in the current intensity required for CP of the insulated steel sheets. In this investigation zinc wire of diameter 5 mm was used as a sacrificial anode in two different manners parallel and spiral. The sacrificial anode was fixed adjacent directly to the electrical insulation under the PUR foam shows a complete protection of the pipes, i.e., about – 920 mV vs. Cu/CuSO₄, in the salty water with about 3.5% NaCl; this method guarantees optimum protection. The experiments performed at different temperatures up to 60 °C showed that increasing the temperature has a slight positive effect.     

On the formation of thermally sprayed alumina coatings

A model for the fomation of thermally sprayed alumina coatings is proposed. The spreading and crystallization of liquid droplets on impact with the substrate are analysed and the thermal history of individual particles related to the kinetics of nucleation of -Al₂O₃ to other forms. The results suggest that under the usual spraying conditions undercooling of the liquid droplets is such that -Al₂O₃ nucleates in preference to -Al₂O₃ and the cooling rate after solidification is sufficiently rapid to prevent transformation to -Al₂O₃ or -Al₂O₃. Transformation of initially formed -Al₂O₃ to -Al₂O₃ appears to be possible only if the lamellae formed on impact are thicker than about 10 m if the substrate is heated to about 1000° C, or if the thickness is greater than about 20 m on an unheated substrate. The -Al₂O₃ generally observed in thermally sprayed coatings is the result of crystallization from pre-existing nuclei arising from incomplete melting of the feed material.