Motion of gas bubbles in a liquid under the influence of a temperature gradient

This study deals with the motion of a gas bubble developing under the influence of surface-tension forces in an imponderable viscous liquid with a temperature gradient. A theory of steady-state motion of a bubble in a field with constant temperature gradient is given for the case of small Reynolds numbers. Experimental results that show qualitative agreement with the theory are presented.The authors wish to thank M. A. Lavrent'ev for formulating the problem and giving constant attention to their work.     

Convection of a binary mixture under conditions of thermal diffusion and variable temperature gradient

Instability of a plane horizontal layer of an incompressible binary gas mixture stratified in the gravity field under the action of a transverse temperature gradient modulated in time is studied. The case of solid impermeable boundaries of the layer, where the flux of matter vanishes, is considered. The analysis is based on the Floquet method applied to linearized equations of convection in the Boussinesq approximation. It is shown that there are regions of parametric instability at finite frequencies. In addition to the synchronous or subharmonic response to an external action, the instability may be related to quasiperiodic disturbances. Depending on the amplitude and frequency, modulation can stabilize the unstable basic state and also destabilize the equilibrium of the fluid. The threshold values of convection for modulations of temperature and translational vertical vibrations are compared.     

Turbulent thermal diffusion in a multi-fan turbulence generator with imposed mean temperature gradient

We studied experimentally the effect of turbulent thermal diffusion in a multi-fan turbulence generator which produces a nearly homogeneous and isotropic flow with a small mean velocity. Using particle image velocimetry and image processing techniques, we showed that in a turbulent flow with an imposed mean vertical temperature gradient (stably stratified flow) particles accumulate in the regions with the mean temperature minimum. These experiments detected the effect of turbulent thermal diffusion in a multi-fan turbulence generator for relatively high Reynolds numbers. The experimental results are in compliance with the results of the previous experimental studies of turbulent thermal diffusion in oscillating grid turbulence (Buchholz et al. 2004; Eidelman et al. 2004). We demonstrated that the turbulent thermal diffusion is an universal phenomenon. It occurs independently of the method of turbulence generation, and the qualitative behavior of particle spatial distribution in these very different turbulent flows is similar. Competition between turbulent fluxes caused by turbulent thermal diffusion and turbulent diffusion determines the formation of particle inhomogeneities.     

Motion of a piston under the influence of gas pressure in the presence of an initial temperature gradient

A study is made of the motion of a piston without initial velocity under the influence of gas pressure. Under the assumption that the temperature gradient is constant and fairly small, expressions are obtained for the distributions of the gas-dynamic parameters in the disturbed region between the piston and the leading edge of the sound wave propagating through the gas at rest.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 177–180, March–April, 1984.     

Statistics of the surface temperature field of an air/water interface under air flow

Experimental results are presented that reveal the relationship between the root mean square of the surface temperature field of an air/water interface (σ) and the heat flux (q′′) emanating from that interface, over a range of wind speeds. Experiments were conducted for wind speeds ranging from 1.0 to 4.0 m/s to determine if and how the σ versus q′′ relationship was affected by wind speed. Consistent surfactant coverage conditions were maintained for wind speeds ranging from 1.0 to 2.6 m/s, and these are the focus of the results presented herein. For wind speeds above 2.6 m/s the surfactant was consistently pushed downstream, resulting in an inhomogeneous surface condition for the air/water interface. For wind speeds less than 2.6 m/s the relationship between σ and q′′ is approximately linear and is weakly dependent on wind speed. The surface temperature field was obtained by infrared (IR) imaging. Sample IR images are presented in addition to the σ versus q′′ data. IR images are presented for surfaces covered with insoluble surfactants (liquid phase and solid phase), a soluble surfactant, and a clean water surface.

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Exergy efficiency of a counterflow air/air heat exchanger with vapour condensation

The exergy efficiency of a counterflow hot moist air/ambient air heat exchanger with vapour condensation as a function of the temperature and humidity of the hot moist air, the ratio of ambient air flow rate to hot moist air flow rate and the minimum temperature differences between the flows is determined. The ranges 20 to 120°C and 0.01 to 1.0 kg·kg^–1 for the temperature and humidity ratio of hot moist air respectively are considered. Constant ambient air conditions of 20°C and 0.01 kg·kg^–1 were assumed.

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Exergie-Wirkungsgrad eines Luft/Luft-Gegenstrom-Wärmetauschers mit Dampfkondensation

Zusammenfassung Der Exergie-Wirkungsgrad eines Gegenstrom-Wärmetauschers wird bestimmt für die Kombination: heiße Feuchtluft zu Umgebungsluft mit Dampfkondensation, und zwar als Funktion von Temperatur und Feuchtegrad der heißen Feuchtluft, des Verhältnisses von Umgebungsluft zu heißen Feuchtluftstrom und der minimalen Temperaturdifferenz zwischen beiden Strömen. Die Temperatur des heißen Feuchtluftstroms variiert im Bereich von 20 bis 120°C, sein Feuchtegrad im Bereich 0,01 bis 1,0. Bezüglich der Umgebungsluft wurden konstante Bedingungen angenommen, und zwar 20°C und ein Feuchtegrad von 0,01.

     

Development of lattice turbulence in a stream with a constant velocity gradient

On the basis of the equations for the Reynolds stresses and the equation for the scale of the turbulence, an analysis is made of the development of lattice turbulence in a stream with a constant velocity gradient. The constants in the equations are determined under the assumption that, far from the lattice and with large Reynolds numbers, the structure of the turbulence tends toward a limiting state with constant values of the correlation coefficient, the degree of anisotropy, and the dimensionless velocity gradient. The constants in terms containing the viscosity are determined from a consideration of the flow beyond the lattice without a velocity gradient in the final stage of decay of the turbulence. The equations obtained were solved in an electronic computer. The calculation is in satisfactory agreement with the existing experimental data. For calculating flows with a variable velocity gradient, instead of the equation of the scale, it is proposed to use an equation for the frequency of the turbulent pulsations obtained in the present work. The computer calculations were made by S. I. Bekritskaya.     

Unsteady state diffusion of vapour in a closed tube

Results of one-dimensional calculations for unsteady state diffusion of a vapour in a closed tube are compared to calculations (a) for unsteady state diffusion of a vapour in an open tube, and (b) for heat conduction in a finite slab and an infinite slab. In the cases of (b) the effect of the convective term in the transport equation is absent.The total pressure is calculated and differences in the two cases are explained.An experiment is described in which the pressure increase in a closed tube due to the diffusion of water vapour in dry air is measured as a function of time. The experimental results agree quite well with the theoretical predictions. The results of this investigation may be used for the experimental determination of the diffusion coefficient of a vapour in a gas and in pressure measurements in systems with an evaporating liquid.Nomenclature a thermal diffusivity - D diffusion coefficient - w mean particle velocity - g particle flux in the laboratory system - g* particle flux in a system moving with the mean particle velocity w - L length of the tube - n number density of molecules - n _vs saturation number density of vapour molecules - p pressure - p _vs saturation pressure of the vapour - t time - z distance from the liquid surface - N total quantity of evaporated liquid     

Convective instability of Marangoni-Poiseuille flow under a longitudinal temperature gradient

An exact solution is obtained for the problem of steady flow in a system of two horizontal layers of immiscible fluids with a common interface. The stability of the flow is studied by a linearization method. It is shown that the occurrence of instabilities is due to the different governing parameters of the fluids (thickness, heating conditions, viscous and thermal conductivity of the fluids). It is found that under constant gravity conditions, the perturbations are monotonic, and in zero gravity, oscillatory thermocapillary instability occurs.     

The measurement of water vapour transfer rate through clothing system with air gap between layers

The experiments described in this paper are designed to test the water vapour transfer rates through outdoor clothing system with air gap between layers under conditions more closely actual wear. It was adopted distance of 5 mm to ensure no disturbance of the air gap thickness between layers throughout the measurement period with all fabrics. The results have indicated that the water vapour transfer rates of clothing system decrease very slightly with time, it is shown that they approached nearly equilibrium state throughout the experiment. It is revealed that the water vapour transfer rates of the clothing system were ordered into groups determined by the type of waterproof breathable fabric as a shell layer being ordered.     

具有恒定冲击载荷的梯度泡沫金属材料设计

多胞材料可通过大变形大量地吸收冲击能量,引入密度梯度可进一步提高其耐撞性。梯度多胞材料的宏观力学响应对材料密度分布极为敏感,不同类型的细观构型的影响也极为不同。已有的研究工作主要局限在对给定的密度梯度分析其动态响应,较少对耐撞性设计方法进行研究。本文针对梯度闭孔泡沫金属材料,基于非线性塑性冲击波模型发展了耐撞性反向设计方法,以维持冲击物受载恒定为目标,运用级数法获得了简化模型和渐近解。利用变胞元尺寸法构建了连续梯度变化的三维Voronoi细观有限元模型,并利用ABAQUS/Explicit有限元软件对理论设计进行数值验证。结果表明,反向设计理论简化模型的渐近解对于梯度闭孔泡沫金属材料的耐撞性设计是有效的,所提出的耐撞性设计方法在控制冲击吸能过程和冲击物受载方面具有指导意义。     

Steady-state heat transfer through a slab-on-ground floor over a constant temperature water table

Explicit analytical solutions are obtained for two-dimensional steady-state heat transfer rate from a slab-on-ground floor over a constant temperature water table. The floor heat transfer rate results calculated using the derived expression are in close agreement with a series expansion solution available in the literature. The current work offers accurate temperature field and floor steady state heat loss calculations for long-narrow slab-on-ground buildings over a water table at finite depth.     

Vibrational convection in a plane layer with a longitudinal temperature gradient

An exact solution is obtained for the equations of vibrogravitational convection in an arbitrarily oriented plane fluid layer with a longitudinal component of the temperature gradient. It is shown that in the absence of a static field inclined vibration leads to the development of plane-parallel convective flow. On the other hand, gravitational plane-parallel convective flow can always be suppressed by an appropriate choice of the direction and amplitude of vibration.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 12–15, July–August, 1990.     

Analysis of piezoelectric semiconductor fibers under gradient temperature changes

Piezoelectric semiconductors(PSs) possess both semiconducting properties and piezoelectric coupling effects, making them optimal building blocks for semiconductor devices. PS fiber-like structures have wide applications in multi-functional semiconductor devices. In this paper, a one-dimensional(1D) theoretical model is established to describe the piezotronic responses of a PS fiber under gradient temperature changes. The theoretical model aims to explain the mechanism behind the resistance chang...     

On the motion of particles in a fluid under the influence of a large velocity gradient

The motion of seeding particles as used in laser Doppler anemometry is investigated in the presence of a large velocity gradient across aerodynamic shocks under different flow conditions. Experimentally obtained results are presented and compared with theoretical predictions based upon the size distribution of the seeding particles used. It is found that the agreement of experimental and theoretical results depends on the flow conditions as well as on the particle material.     

Chloride diffusion in porous concrete under conditions of variable temperature

A diffusion model is presented in this study for describing diffusion of chloride ion in porous concrete under nonisothermal conditions. The process is to simulate chloride diffusion in concrete exposed to daily cyclic temperature change. The effects of cyclic and constant temperatures on the chloride diffusion behavior in concretes are investigated. The time for chloride concentration to reach the corrosion threshold of the reinforcing steel is determined for a given water-to-cement ratio and thickness of concrete layer. Results indicate that the temperature and water-to-cement ratio play a vital role in governing the chloride diffusion process.

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Chlorid-Diffusion in porösem Beton unter dem Einfluß von Temperaturänderungen

Zusammenfassung Es wird ein Diffusionsmodell vorgestellt, das die Diffusion von Chloridionen in porösem Beton unter nichtisothermen Bedingungen beschreiben soll, wobei sowohl die täglichen zyklischen, wie auch sprunghaften Temperaturänderungen zugrunde gelegt werden. Berechnet wird die Zeitspanne bis zum Erreichen der Korrosionsschwelle an der Stahlarmierung, und zwar als Funktion des Wasser-Zement-Verhältnisses und der Schichtdicke des Betons. Die Ergebnisse zeigen, daß der Chlorid-Diffusionsprozeß hauptsächlich vom Temperatur- und Wasser-Zement-Verhältnis regiert wird.

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Nomenclature b amplitude parameter in the cyclic temperature function - C chloride concentration - Cp specific heat capacity - D effective chloride diffusion coefficient - D _o preexponential factor - E activation energy - k thermal conductivity of concrete - L thickness of concrete slab - R gas constant - t time - T temperature - T _o initial temperature - w water-to-cement ratio - x spatial coordinate - thermal diffusivity,k/( Cp) - density of concrete     

Effects of radial temperature gradient on the stability of flow in a narrow-gap annulus with constant heat flux at the inner rotating cylinder

The stability of Couette flow of a viscous, imcompressible fluid between two concentric rotating cylinders in the presence of a radial temperature gradient due to constant heat flux at the inner cylinder is studied. Assuming the flow that takes place in a narrow gap, the eigenvalue problem is solved numerically. The critical values ofa (the wave number) andTa (the Taylor number) are listed in a table and these are also shown graphically in the case of the rotation of the cylinders in clockwise and counter-clockwise direction. It is observed that greater amount of heat flux has a stabilising effect.

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Einflüsse des radialen Temperaturgradienten auf die Stabilität einer Strömung in einem engen Ringspalt mit konstantem Wärmestrom am inneren rotierenden Zylinder

Zusammenfassung Es wird die Stabilität der Couette Strömung eines viskosen inkompressiblen Fluids zwischen zwei konzentrischen rotierenden Zylindern in Anwesenheit eines durch Wärmeabgabe vom inneren Zylinder hervorgerufenen Temperaturgradienten studiert. Unter der Annahme, daß die Strömung in einem engen Spalt vor sich geht, wird das Eigenwertproblem gelöst. Es werden die kritischen Werte vona (Wellenzahl) und vonTa (Taylorzahl) in Tabellen aufgelistet und diese Ergebnisse werden auch graphisch dargestellt für den Fall, daß die Zylinder gleichsinning und gegensinnig rotieren. Es wurde beobachtet, daß zunehmender Wärmestrom einen stabilisierenden Effekt hat.