Claus Riehle

Reflections on similarity laws concerning particle transport in electrical precipitators

Abstract Measurements in three geometrically similar wire-plate precipitators show that in the cases of constant current density at the collecting plates or constant electrical field strength, larger geometries give better grade efficiencies. If a dimensionless voltage (found by dimensional analysis) is held constant, the grade efficiencies are no longer dependent on the absolute precipitator size. Introducing an ‘electrical drift parameter’, a generalized representation of measured and (according to Deutsch) calculated grade efficiencies can be found which turns out to be a scale-up invariant in the case of constant dimensionless voltage. Overall, the results show that with decreasing particle size the measured efficiencies become increasing better than the calculated ones. For particles smaller than 1 μm, there is little difference between the Deutschian and Laminar models. When common models are applied with space averaged values for the electrical field strength (with consideration of the ionic space charge) instead of the simple field strength value, then a qualitatively correct scale-up behaviour emerges with respect to the grade efficiency trends. Two parameters are decisive: the electrical drift parameter, which includes the operational state of the precipitator, and the relative length of the collecting zone.

Zur mehrphasigen Strömung in einem Zick-Zack-Sichter

ZusammenfassungIn einem Zick-Zack-Sichter können Schüttgüter durch eine Gasströmung bei großen Massendurchsätzen mit einer hohen Trennschärfe klassiert werden. Gegenstand der Untersuchung ist die Gas-Feststoffströmung in einem Zick-Zack-Sichter mit Hilfe experimenteller Methoden und der numerischen Strömungssimulation. Die Luftströmung im Zick-Zack-Sichter wurde durch Zugabe von Tracerpartikeln (Rauch) in den Apparat sichtbar gemacht, so daß lokale, charakteristische Bereiche der Strömung zu unterscheiden waren. Die Messung repräsentativer Strömungsprofile des Gases erfolgte mit Hilfe eines Laser-Doppler-Anemometers (LDA). Außerdem wurde die einphasige Gasströmung mit unterschiedlichen Turbulenzmodellen als zwei- und dreidimensionales Strömungsfeld simuliert. Das Sichtgut bestand aus einer Glaskugelfraktion mit Partikeldurchmessern 0,1<dp<1 mm. Die Bahnen einzelner Partikeln und von Partikel-Clustern sind visualisiert und ausgewertet worden. Die von Sichtluftstrom und Feststoffbeladung abhängige, unterschiedliche Klassierwirkung wurde durch die jeweiligen Trenngrenzen und Trennschärfen charakterisiert. Zusätzlich wurden Partikelbahnen nach der Lagrangeschen Vorgehensweise simuliert und daraus Trennkurven für die Einzelpartikelbewegung im Sichter bei niedriger Beladung bestimmt. Die Ergebnisse von Simulation und Messung werden verglichen und Unterschiede dargestellt. Auf dieser Grundlage werden die derzeitigen Möglichkeiten der Strömungssimulation disperser Gas-Feststoffströmungen bewertet und notwendige Verbesserungen aufgezeigt.

Electrically stimulated agglomeration at an earthed surface

With the aid of a microscope and a video camera, settling particles have been observed when passing a classical point-plate arrangement and, because of the deflection in the electrical field, impacting on the wall. As the cinematographic studies show, the particles arriving form agglomerate chains orientated along the electrical field lines and increasing to a critical length, dependent on the electrical operation conditions. In the video tapes, it is clearly seen that agglomerate chains of critical length are pulled off by the electrical forces. The observed agglomeration phenomenon is governed decisively by the particle material itself, in particular particle resistivity. Graphite and limestone particles, and glass spheres in the size range around 30 μm have been investigated. In order to illustrate this electrically stimulated agglomeration the whole process has been video taped. These mainly phenomenological investigations reveal the physical background of a successful electro-mechanical two-stage system for the separation of diesel soot particles.

Measuring and calculating solid carry over in a CFB cold flow model for different materials

The goal of the paper is to demonstrate the importance of particle characterization when the solid carry over of a circulating fluidized bed (CFB) is to be calculated quantitatively. This is of practical relevance when theoretical models are applied to real processes or in process design tools. Therefore, the solid carry over of three materials different in particle size, particle size distribution and in particle shape have been studied in a CFB cold flow model. Plotting the solid carry over as function of gas velocity shows that measured and calculated curves have the same shape. But, generally, both curves are shifted and the shift depends on the particle size taken for the calculations. For that reason, an equivalent particle diameter was calculated by fitting the calculations to the measurements. These CFB-equivalent particle sizes were compared with the particle sizes resulting from other measurement methods such as sieving analysis, laser diffraction and fluidization experiments.

Mass flux and effective migration velocity in electrostatic precipitators

Abstract Particle transport in electrical precipitators is usually described in terms of an ‘effective migration velocity’, but the meaning of this term is subject to debate. This work, based on mass balance, is able to demonstrate very simply that the particle transport towards the collecting plates can be described by the total coarse mass flux. The equation for this new quantity is as simple as the equation for the effective migration velocity, and therefore it should be appropriate for common applications. In addition, a relationship is given for transforming the total coarse mass flux into an effective migration velocity and vice versa. The concept of total coarse mass flux is based completely on physical terms, which might lead to more insight into the processes in electrical precipitation than effective migration velocity does, thus avoiding time-consuming discussions. For completeness it is briefly mentioned that this concept can be extended to the description of local coarse mass fluxes as well.