Interactions between flow fields induced by surface dielectric barrier discharge arrays

This study investigates the flow field induced by a surface dielectric barrier discharge (SDBD) system, known for its efficient pollution remediation of volatile organic compounds (VOCs). We aim to understand the flow dynamics that contribute to the high conversion observed in similar systems. Experimental techniques, including schlieren imaging and particle image velocimetry (PIV), applied with high temporal resolution, were used to analyse the flow field. Complementary, fluid simulations are employed to investigate the coupling between streamer and gas dynamics. Results show distinct fluid field behaviours for different electrode configurations, which differ in geometric complexity. The fluid field analysis of the most basic electrode design revealed behaviours commonly observed in actuator studies. The simulation results indicate the local information about the electron density as well as different temporal phases of the fluid flow. The electrode design with mostly parallel grid line structures exhibits confined vortices near the surface. In contrast, an electrode design also used in previous studies, is shown to promote strong gas transport through extended vortex structures, enhancing gas mixing and potentially explaining the high conversion observed.Comment: submitted to Plasma Chemistry and Plasma Processing by Springer Natur

State of the Art on Two-Phase Non-Miscible Liquid/Gas Flow Transport Analysis in Radial Centrifugal Pumps Part C: CFD Approaches with Emphasis on Improved Models

Predicting pump performance and ensuring operational reliability under two-phase conditions is a major goal of three-dimensional (3D) computational fluid dynamics (CFD) analysis of liquid/gas radial centrifugal pump flows. Hence, 3D CFD methods are increasingly applied to such flows in academia and industry. The CFD analysis of liquid/gas pump flows demands careful selection of sub-models from several fields in CFD, such as two-phase and turbulence modeling, as well as high-quality meshing of complex geometries. This paper presents an overview of current CFD simulation strategies, and recent progress in two-phase modeling is outlined. Particular focus is given to different approaches for dispersed bubbly flow and coherent gas accumulations. For dispersed bubbly flow regions, Euler–Euler Two-Fluid models are discussed, including population balance and bubble interaction models. For coherent gas pocket flow, essentially interface-capturing Volume-of-Fluid methods are applied. A hybrid model is suggested, i.e., a combination of an Euler–Euler Two-Fluid model with interface-capturing properties, predicting bubbly flow regimes as well as regimes with coherent gas pockets. The importance of considering scale-resolving turbulence models for highly-unsteady two-phase flow regions is emphasized

A method for the coupling of compressible 3d flow simulations with a cavitation erosion model for ductile materials and assessment of the incubation time

A compressible 3D in-house flow solver with temporal nanosecond resolution is coupled to a simple material erosion model for ductile materials. Due to limited spatial resolution, not all details of collapsing wall adjacent single bubbles can be resolved, and thus a collapse detection algorithm based on the mass flux divergence is applied. Load collectives are statistically evaluated by the multitude of detected collapses and serve as input for the material model. Grid dependence is carefully assessed. Since the physical simulation time is much shorter than the realistic exposure time, a method for the time extrapolation of the wall load to capture realistic time scales together with a step-by-step implementation is presented. The simulation method is applied on an impinging water jet test case as well as on an ultrasound cavitation case. A validation on temporally highly-resolved pressure measurement data is performed. Limitations of the particular material model are pointed out. The coupled CFD – material model comprises one model parameter, in terms of the cell size of a reference computational grid to handle grid dependence, that needs to be case-dependently fixed e.g. on measurement data. We conclude, that for a more predictive method, the detailed spatial resolution of single bubble collapses seems indispensable

Numerical and Experimental Investigations on the Acoustic Characteristics of a Single-Stage Centrifugal Pump

The acoustic properties of a single-stage centrifugal pump with low specific speed are investigated by means of compressible 3D CFD simulations (URANS) and experiments. In order to determine the pump’s acoustic transmission and excitation characteristics, a four-pole approach in the frequency domain is used. The transmission parameters determined by simulation are compared to experiments in water and air as functions of the Helmholtz number. The results indicate that the acoustic transmission characteristics within the experiments are significantly influenced by the structural compliance of the volute casing in terms of a fluid–structure interaction (FSI). A modelling approach for a one-dimensional representation of the centrifugal pump’s acoustic transmission characteristics in the time and frequency domains is applied to the current pump. As one model parameter, the effective speed of sound in the 1D model needs to be reduced to 607 ms−1 to account for the FSI. The agreement of the simulation results and the experiments underlines the above statement about the influence of the FSI. In a last step, the acoustic excitation parameter, depicted as monopole and dipole amplitudes, at two different blade-passing frequencies (fBP≈[111;169] Hz) are determined for several operating points. Especially for dipole amplitudes, a good agreement between experiments and simulations can be seen. The monopole amplitudes are also of similar orders of magnitude, but show stronger deviations. The cause of discrepancies between the 3D CFD simulations and experiments is believed to be the neglected influence of the FSI and surface roughness as well as the inaccurate reproduction of flow separation at the volute’s tongue due to the use of wall functions. A final important observation made during the numerical investigations is that the excitation mechanisms at the blade-passing frequency are probably independent of the piping system’s acoustic impedance

High-Performance Flow Simulation and Scale-Adaptive Turbulence Modelling of Centrifugal Pumps

While for the design point operation of centrifugal pumps, where an essentially steady flow field is present and statistical turbulence models yield an appropriate prediction of the characteristics, the flow field gets increasingly unsteady towards off-design operation. Special designs as e. g. sewage pumps are characterised by a single-blade impeller and show significantly unsteady characteristics even in the design point. For such highly-unsteady and turbulent flow fields, statistical models tend to fail. On the other hand, Large-Eddy Simulation models, where the large-vortex part of the turbulent spectrum is directly resolved, show a much better flow prediction. However, the spatial resolution and thus computational effort are too high for engineering real pump applications. Therefore, we provide an assessment of scale-adaptive turbulence simulation (SAS) models that recover a statistical flow solution in regions of low unsteadiness and – like Large-Eddy Simulation – resolve a part of the turbulent spectrum down to the available grid resolution for highly unsteady flow regions. After a thorough validation on standard turbulence test cases e. g. the periodic hill case, it is shown that with a moderately higher computational effort than statistical models, the SAS yields a considerable improvement of the prediction of the turbulence field in part load operation of a centrifugal pump while the mean flow field could be well predicted even with a well-established statistical model

Assessment of statistical eddy-viscosity turbulence models for unsteady flow at part and overload operation of centrifugal pumps

The accuracy of linear eddy-viscosity URANS turbulence modelling is assessed for off-design operation of a single stage radial centrifugal pump. The open source software OpenFOAM is utilized. Hot wire probes have been utilized for measurement of spatially and temporally resolved validation data at the impeller discharge of the air-operated pump. The ensemble-averaged mean flow angle is in qualitatively good agreement to data for the entire operation range, while minor deviations occur close to the volute tongue due to impeller-volute interaction. Turbulence statistics from the simulation are compared to ensemble-averaged velocity RMS from measurements. RMS distribution is also qualitatively well reproduced for near-design and overload operation. A more pronounced RMS mis-prediction occurs in part load in the region of high impeller-volute tongue interaction and reveals limitations of turbulence modelling in highly unsteady flow regions of centrifugal pumps

Three-Dimensional Flow Simulation by a Hybrid Two-Phase Solver for the Assessment of Liquid/Gas Transport in a Volute-Type Centrifugal Pump with Twisted Blades

A hybrid two-phase flow solver is proposed, based on an Euler–Euler two-fluid model with continuous blending of a Volume-of-Fluid method when phase interfaces of coherent gas pockets are to be resolved. In a preceding study on a two-dimensional bladed research pump with reduced rotational speed, the transition from bubbly flow to coherent steady gas pockets observed in optical experiments with liquid/gas flow could be well captured by the hybrid solver. In the present study, the experiments and solver validation are extended to an industrial-scale centrifugal pump with twisted three-dimensional blades and elevated design rotational speed. The solver is combined with a population balance model, and a scale-adaptive turbulence model is employed. Compared to the two-dimensional bladed pump, the transition from agglomerated bubbles flow to attached gas pockets is shifted to larger gas loading, which is well captured by the simulation. The pump head drop with increasing gas load is also reproduced, showing the hybrid solver’s validity for realistic pump operation conditions