Anna Lyhne Jensen

Characterisation of rag properties and their transport in wastewater pumps: Calibration of a discrete element rag model to simulate clogging effects

Accumulation of sanitary refuse, such as wet wipes, in wastewater pumps deteriorates the pump performance, ultimately leading to break-down with need of manual cleaning. To prevent this, the ability of wastewater pumps to handle wet wipes, rags and similar materials in the wastewater is a key feature which must be taken into account as the pumps are designed. Today, simulations are widely used in the design of pumps and other turbomachines. However, the complexity of simulating a flexible material such as a rag in a pump has so far prompted experimental investigation of resistance to clogging rather than simulations. Using the Discrete Element Method, a model of a rag formed by bonded particles is developed. The model is calibrated based on experimentally obtained quantitative parameters describing the elongation-, bendingand drape properties of the rag. By implementing separate parameters for the normal bond stiffness and out of plane bending stiffness of the bonds connecting the particles which form the rag, the model successfully reproduces elongation, bending and drape obtained from experiments. By coupling the discrete element rag model to Computational Fluid Dynamics using a standard drag model for fluid-particle interaction, a preliminary simulation of the transport of a rag through a wastewater pump with a single channel impeller is achieved. Furthermore, this study experimentally characterises motion of textile material immediately upstream of a wastewater pump and inside a pump using artificial wastewater containing rags. Clear differences in the tendency of the rags to get stuck on the tongue, stay in the volute flow or exit the pump depending on the operating point have been identified and the flow conditions inside the pump rather than at the inlet have been observed to lead to different types of clogging at different pump operating points. The result of the preliminary simulation of rag transport through a wastewater pump resembles the motion of the rags through a pump which was observed experimentally.

Characterisation of textile shape and position upstream of a wastewater pump under different part load conditions

Abstract Accumulations of sanitary textile materials often lead to clogging of pumps in the wastewater system. Simulation of clogging phenomena may help to identify means of reducing the risk of clogging. In order to provide realistic initial conditions for clogging simulations, this study characterises textiles in artificial wastewater in the suction pipe to a dry-installed pump at nine different operating points. The textiles are recorded approximately 3.5 pipe diameters from the pump inlet and approximately three pipe diameters from the suction pipe bend at pipe Reynolds numbers in the range 300,000–900,000. Parameters of position, orientation, elongation and projected two-dimensional (2D) area are extracted using image analysis. The resulting parameters from the different operating points are compared using analysis of variance. The results show that the position, orientation and elongation do not change significantly with the operating point, while the projected 2D area decreases with an increased flow.

Validation of guidelines for cfd modelling of a single tube row and in-line tube bundles

This paper questions and improves commonly used guidelines for modelling a tube bundle in cross-flow at ReD = 3.4 · 104 and ReD = 1.1 · 105. Especially, when the locations of flow separation are of high interest. A major conclusion of this paper is that near-wall modelling (y+ 5 should be avoided in relation to flow separation behind tubes in cross-flow. CFD modelling of a tube bundle may be simplified with the use of symmetric or periodic boundary conditions to account for the full geometry. The present work reveals periodicity in vorticity formation between a double cylinder row, though the wake region behind a single cylinder row is neither characterised as in-phase nor reversed phase. Likewise, periodic boundary conditions may result in a modelling with large wake deflections for a full tube bundle. Furthermore, since there is no unequivocal answer to which turbulence model to apply for tubes in cross-flow, the RNG k-e, Realizable k-e, SST k-ω, and RSM turbulence models are tested and compared.Copyright