„Fouling“-Vorgänge bei der Verdampfung von Argon im waagerecht durchströmten Rohr

Zusammenfassung Bei Messungen des Wärmeübergangs beim Strömungssieden von Argon wurde ein zeitlicher Anstieg der Wandtemperatur beobachtet, während die Einstellparameter Wärmestromdichte, Massenstromdichte, Dampfgehalt und Systemdruck konstant blieben. Dieser Temperaturanstieg endete nach mehreren Stunden bei einer beträchtlich höheren Wandtemperatur oder mit dem Eintreten der Siedekrise. Als Ursache für dieses Phänomen wurde die Ablagerung von im Argon gelöstem CO₂ an der Heizfläche ermittelt, wobei zu beachten ist, daß die Reinheit des verwendeten Argons größer als 99,996 vol-% war. In weiteren Versuchen wurde der Einfluß der CO₂-Konzentration auf diesen Fouling-Vorgang bestimmt und ein Verfahren entwickelt, um das CO₂ aus dem Meßkreislauf zu entfernen. Der Ablagerungsmechanimus und der Einfluß der Betriebsparameter auf die Ablagerungsgeschwindigkeit können mit Modellvorstellungen zur Blasenbildung erklärt werden.

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Fouling-phenomena during the evaporation of argon within a horizontal tube

During heat transfer measurements at flow boiling of argon a transient increase of the wall temperature has been observed, though the parameters such as heat flux, mass velocity, flow quality and system pressure were kept constant. This temperature drifting ended after several hours at a considerably higher wall temperature or with the boiling crisis. Although the purity of the test substance was higher than 99.996 vol-%, this phenomenon was caused by the solidification of CO₂ on the heating surface. Further investigations showed the influence of the CO₂-concentration on the drifting as well as a method to remove the CO₂ out of the test fluid. Both, fouling mechanism and influence of the parameters on the fouling, can be explained satisfactorily by means of bubble-growth mechanisms.

     

Bubble coalescence in pure liquids

Solutions of the flow and deformation during the approach of two bubbles along their centre line are presented for the low Weber number case. When viscosity is absent, Weber number and radius ratio disappear from the equations under suitable transformations of the variables and a universal solution is obtained. This solution indicates the formation of a dimple, after which the thinning rate of the film between the bubbles tends to a constant high value. When liquid viscosity is included the Reynolds number, Re, enters the equations. A retardation of the coalescence process is found for Re <100, while for Re 1 dimple formation is suppressed. The influence of gas properties is considered briefly. An extrapolation of the inviscid results to Weber numbers of order unity suggests that the bubbles will bounce apart before coalescence is achieved.     

Untersuchungen an Bogenkathoden

Zusammenfassung Der Kathodenfall in Hg- und Xe-Hochdruckentladungen zwischen Wo-Elektroden ist bei Stromstärken unterhalb 10 A für niedrige Drucke beträchtlich grösser als für höhere. Er nimmt mit zunehmender Stromstärke ab und erreicht bei etwa 20 A einen vom Druck unabhängigen Endwert (Xe: 6,5 V; Hg: 9,5 V). Der Anodenfall beträgt ziemlich unabhängig von Druck und Stromstärke 1 V. Die kathodischen Stromdichten sind bei höheren Belastungen nur wenig von der Stromstärke abhängig. Sie nehmen mit dem Druck zu. Der Strom im Kathodenfallgebiet, das eine Ausdehnung von einigen 10^–6 cm hat, wird vorwiegend durch Elektronen getragen. In einem anschliessenden Ionisationsgebiet mit einer Ausdehnung von rund 10^–2 cm erfolgt der übergang zum thermischen Gas des stark kontrahierten Säulenendes. Die hohen kathodischen Stromdichten werden durch Glühemission der Kathode, die wegen der hohen elektrischen Feldstärke um mehr als eine Zehnerpotenz gegenüber der feldfreien Thermoemission angehoben ist, erzielt.     

Time resolved measurements of the flow generated by suction feeding fish

The majority of aquatic vertebrates are suction feeders: by rapidly expanding the mouth cavity they generate a fluid flow outside of their head in order to draw prey into their mouth. In addition to the biological relevance, the generated flow field is interesting fluid mechanically as it incorporates high velocities, is localized in front of the mouth, and is unsteady, typically lasting between 10 and 50 ms. Using manometry and high-speed particle image velocimetry, this is the first study to quantify pressure within and outside the mouth of a feeding fish while simultaneously measuring the velocity field outside the mouth. Measurements with a high temporal (2 ms) and spatial (<1 mm) resolution were made for several feeding events of a single largemouth bass (Micropterus salmoides). General properties of the flow were evaluated, including the transient velocity field, its relationship to pressure within the mouth and pressure at the prey. We find that throughout the feeding event a relationship exists for the magnitude of fluid speed as a function of distance from the predator mouth that is based on scaling the velocity field according to the size of the mouth opening and the magnitude of fluid speed at the mouth. The velocity field is concentrated within an area extending approximately one mouth diameter from the fish and the generated pressure field is even more local to the mouth aperture. Although peak suction pressures measured inside the mouth were slightly larger than those that were predicted using the equations of motion, we find that these equations give a very accurate prediction of the timing of peak pressure, so long as the unsteady nature of the flow is included.     

Abnormal long wave dispersion phenomena in a slightly compressible elastic plate with non-classical boundary conditions

A two parameter asymptotic analysis is employed to investigate some unusual long wave dispersion phenomena in respect of symmetric motion in a nearly incompressible elastic plate. The plate is not subject to the usual classical traction free boundary conditions, but rather has its faces fixed, precluding any displacement on the boundary. The abnormal long wave behaviour results in the derivation of non-local approximations for symmetric motion, giving frequency as a function of wave number. Motivated by these approximations, the asymptotic forms of displacement components established and long wave asymptotic integration is carried out.     

Bistable Boundary Reactions in Two Dimensions

In a bounded domain \({\Omega \subset \mathbb R^2}\) with smooth boundary we consider the problem

$\Delta u = 0 \quad {\rm{in }}\, \Omega, \qquad \frac{\partial u}{\partial \nu} = \frac1\varepsilon f(u) \quad {\rm{on }}\,\partial\Omega,$

where ν is the unit normal exterior vector, ε > 0 is a small parameter and f is a bistable nonlinearity such as f(u) = sin(π u) or f(u) = (1 ? u ²)u. We construct solutions that develop multiple transitions from ?1 to 1 and vice-versa along a connected component of the boundary ?Ω. We also construct an explicit solution when Ω is a disk and f(u) = sin(π u).

     

Orientability and Energy Minimization in Liquid Crystal Models

Uniaxial nematic liquid crystals are modelled in the Oseen–Frank theory through a unit vector field n. This theory has the apparent drawback that it does not respect the head-to-tail symmetry in which n should be equivalent to ?n. This symmetry is preserved in the constrained Landau–de Gennes theory that works with the tensor \({Q=s \left(n\otimes n-\frac{1}{3} Id\right)}\). We study the differences and the overlaps between the two theories. These depend on the regularity class used as well as on the topology of the underlying domain. We show that for simply-connected domains and in the natural energy class W^1,2 the two theories coincide, but otherwise there can be differences between the two theories, which we identify. In the case of planar domains with holes and various boundary conditions, for the simplest form of the energy functional, we completely characterise the instances in which the predictions of the constrained Landau–de Gennes theory differ from those of the Oseen–Frank theory.     

Theory of Disappearance of the Electric Signal Induced by an Aircraft Jet Engine in the Afterburner Regime

Electrical aspects of aircraft jet engine operation are considered. A phenomenon previously observed experimentally, namely, the disappearance of the engine current and alternating electric signal induced by the engine jet in the afterburner regime, is explained. It is shown that this phenomenon results from the detachment of electrons from negative ions when the gas temperature in the afterburner increases. This leads to an increase in the effective conductivity of the gas. As a result, the engine current circuit is closed on the internal duct walls and engine charging becomes insignificant.A physico-mathematical model of the electrical processes in the afterburner is formulated and model problems are solved.     

A physically based correlation for the effects of power law rheology and inclination on slug bubble rise velocity

A study has been made of the motion of long bubbles in inclined pipes containing viscous Newtonian and non-Newtonian liquids. A semi-theoretical expression for the rise velocity of air bubbles in water is derived on the hypothesis that the dominant factor is the momentum exchange of the bubble underflow, i.e. the bubble nose shape. The correlation calls on empirical inputs from established literature on bubble rise speeds at high Reynolds number. The effects of increasing Newtonian viscosity are analysed with reference to the momentum exchange and it is shown how viscosity reduces the inclination dependence of the bubble Froude number. Results from an experimental survey in seven different non-Newtonian liquids in three different diameter pipes are presented. These data are correlated so as to decouple the effects of surface tension and viscosity. An empirical relation is proposed for the effective shear rate in the fluid travelling around the bubble nose. Our correlation is compared to literature data from a broad range of Reynolds numbers with excellent agreement except at shallow angles.     

Interaction of pressure waves with a wet foam

Brest. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, No. 5, pp. 78–83, September–October, 1994.