Günter Wozniak

The shape, stability and breakage of pendant liquid bridges

Pendant liquid bridges are defined as pendant drops supporting a solid axisymmetric endplate at their lower end. The stability and shape properties of such bridges are defined in terms of the capillary properties of the system and of the mass and radius of the lower free-floating endplate. The forces acting in the pendant liquid bridge are defined exactly and expressed in dimensionless form. Numerical analysis has been used to derive the properties of a given bridge and it is shown that as the bridge grows by adding more liquid to the system a maximum volume is reached. At this maximum volume, the pendant bridge becomes unstable with the length of the bridge increasing spontaneously and irreversibly at constant volume. Finally the bridge breaks with the formation of a satellite drop or an extended thread. The bifurcation and breakage processes have been recorded using a high-speed video camera with a digital recording rate of up to 6000 frames per second. The details of the shape of the bridge bifurcation and breakage for many pendant bridge systems have been recorded and it is shown that satellite drop formation after rupture is not always viscosity dependent. Bifurcation and breakage in simulated low gravity demonstrated that breakage was very nearly symmetrical about a plane through the middle of the pendant bridge.

Particle-image-velocimetry applied to thermocapillary convection

Thermocapillary convection is studied experimentally using particle-image-velocimetry for flow visualization and analysis. This method offers the advantage of measuring the entire flow field (velocity field, streamlines etc.) in a selected plane within the fluid at a given instant of time in contrast to point by point methods like laser-Doppler-velocimetry (LDV). The paper describes the method and presents quantitative results for different Marangoni numbers.

On the influence of buoyancy on the surface tension driven flow around a bubble on a heated wall

The surface tension driven flow in the liquid vicinity of gas bubbles on a heated solid wall has been investigated both, in a reduced gravity environment aboard a sounding rocket, and in an earth-bound experiment. Both experiments deal with temperature gradients within the liquid surrounding of a bubble which cause variations of the surface tension. These, in turn, lead to a liquid flow around the bubble periphery termed thermocapillary or thermal Marangoni-convection. On Earth, this phenomenon is widely masked by buoyancy. We therefore carried out an experiment under reduced gravitational acceleration. In order to simultaneously observe and record the flow field and the temperature field liquid crystal tracers have been applied. These particles offer the feature of selectively reflecting certain wavelengths of incident white light depending on the crystals temperature. Although the bubble injection system did not perform nominally during the flight experiment, some interesting flow characteristics could be observed. Comparison of results obtained in microgravity to data measured on Earth reveal that due to the interaction of thermocapillarity and buoyancy a very compact vortex flow results on ground, while in microgravity the influence on the surface tension driven flow penetrates much deeper into the bulk. This result is of special interest regarding the production of materials in space.

Numerical Calculation of Particle-Laden Cyclone Separator Flow Using Les

Abstract Numerical flow calculations were carried out at various axial positions of a gas cyclone separator for industrial applications. Due to the nature of cyclone flows, which exhibit highly curved streamlines and anisotropic turbulence, we used the advanced turbulence model of Large Eddy Simulation (LES). The application of LES reveals better agreement with the experimental data, however, it requires higher computer capacity and longer running times when compared to standard turbulence models. These calculations of the continuous phase flow were the basis for modeling the behavior of the solid particles in the cyclone. Particle trajectories, pressure drop and the cyclone separation efficiency have been studied in some details. The paper is organized into five sections. The first section consists of an introduction and a summary of previous work. Section 2 deals with the LES turbulence calculations of the continuous phase flow. The third section treats modeling of the dispersed phase behavior. In section 4, computational issues are presented and discussed as applied grids, boundary conditions and the solution algorithm. In section 5, prediction profiles of the gas flow at axial positions are presented and discussed in some details. Moreover, pressure drop, particle trajectories and cyclone efficiency are discussed. Section 6 summarizes and concludes the paper.

Buoyancy and thermocapillary flow analysis by the combined use of liquid crystals and PIV

Thermocapillary and buoyancy convection is studied experimentally using particle-image-velocimetry with liquid crystal tracers for flow visualization and analysis. This method offers the advantage of measuring the entire flow field (velocity field, temperature distribution etc.) in a selected plane within the fluid simultaneously at a given instant of time in contrast to point by point methods like laser-Doppler-velocimetry (LDV). The paper describes the method and presents quantitative results of both, a thermocapillary and a buoyancy flow experiment. Data of the latter are compared with LDV-results and theoretical predictions, respectively.

Non-isothermal flow diagnostics using microencapsulated cholesteric particles

The temperature and the flow field of thermo-convective liquid flows are visualized using cholesteric liquid crystal material as tracer particles. This type of tracers offers the scientifically valuable feature of measuring the flow and the temperature field simultaneously. Three thermoconvective flow configurations have been investigated successfully using liquid crystals. The results are discussed in some detail. It turns out that the liquid crystal technique is a valuable tool for thermo-convective liquid flow analysis.

Image processing for laser speckle velocimetry using the 2-D fast Fourier transform

An algorithm is described for the fringe analysis in laser speckle velocimetry. Based on the 2-D fast Fourier transform, the method relies on inherent features in the fringe pattern to remove efficiently the influence of the diffraction halo. A windowing operation is performed to enhance the reliability and reduce the influence of various noise contributions.

Optical whole-field methods for thermo-convective flow analysis in microgravity

In this paper optical whole-field methods for the analysis of thermo-convective flow phenomena, with special emphasis on space experiments in microgravity, are described in some detail. These flows are usually investigated regarding their temperature and velocity field. Measurement methods and their potential for these field properties are discussed in view of microgravity applications; representative sample results of relevant experiments are presented.

Über die Anwendung von Flüssigkristallen zur thermischen Analyse von strömenden Fluiden und Festkörperoberflächen

ZusammenfassungDie Anwendung von Flüssigkristallen als Anzeigemedium und Temperaturfeldindikator von Oberflächen sowie Flüssigkeitsströmungen mit Temperaturverteilung hat in den letzten Jahren zunehmend an Bedeutung gewonnen. Insbesondere beim Einsatz zur Temperaturfeldanalyse haben sich Flüssigkristalle zu einem eleganten Meßverfahren entwickelt, welches es erlaubt, ausgedehnte Temperaturfelder sichtbar zu machen und zu analysieren. Hierzu macht man sich die temperaturabhängige Farbreflexion von speziellen Flüssigkristallmaterialien zunutze. Diese in weiten Bereichen der Forschung, Entwicklung und Prozesskontrolle noch wenig bekannte Anwendung ist Gegenstand des vorliegenden Aufsatzes.AbstractIn recent years the application of liquid crystals to the analysis of liquid flows and solid surfaces has evolved to an elegant measuring technique allowing to visualize and analyze extended temperature fields. For this one uses the temperature dependent colour reflection of special liquid crystal materials. The described work deals with this technique which is relatively unknown in research & development and in process control yet.

Application of a novel colour imaging technique to thermal convection under reduced gravity

The quantitative measurement performance and the robustness of a novel high-speed imaging system using a liquid crystal tunable filter have been verified by a fluid dynamic experiment in a reduced gravity environment. This new type of diagnostic tool is a combination of a monochrome high-speed CCD camera with fast ferroelectric liquid crystal control. The filter can be tuned to red, green and blue colour planes (RGB filter), which provides real colour images without loss of resolution. The scientific application was the investigation of the influence of buoyancy on the surface tension-driven flow around a bubble on heated wall. The flow velocity and temperature patterns were observed in gravity and microgravity environments. The measuring technique is based on particle image velocimetry and thermometry (PIV/T). The principle of this optical full-field technique relies on seeded thermochromic liquid crystals (TLCs) as signal particles, which change colour depending on their temperature. The experimental results of the flow investigations under 1-g and μ-g conditions are discussed and compared with one another.