The Experts below are selected from a list of 63594 Experts worldwide ranked by ideXlab platform
Thomas Ziegenhein - One of the best experts on this subject based on the ideXlab platform.
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lift Force Coefficient of ellipsoidal single bubbles in water
International Journal of Multiphase Flow, 2021Co-Authors: H Hessenkemper, Thomas Ziegenhein, Roland Rzehak, Dirk Lucas, Akio TomiyamaAbstract:Abstract For the simulation of bubbly flows, knowledge of the lift Force as an interaction between gas bubbles and a surrounding shear field is of great importance. The sign of the lift Coefficient C L changes with increasing bubble size, i.e. with more pronounced bubble deformation. Beside this, impurities in terms of surface-active components are well-known to change the complete hydrodynamic behavior of a bubble even if the amount is very small. In the present work, the lift Coefficient of single ellipsoidal bubbles is determined with a recently developed method, which is suitable to overcome difficulties connected to low viscous systems. In order to investigate the influence of impurities on the lift Force, we conducted experiments with single bubbles of different sizes in purified, deionized and tap water. Overall, the determined lift Coefficients show no difference between deionized and tap water but reveal differences to results obtained with purified water. As no significant differences in shape and velocity are found between the different water qualities, it remains unclear how the impurities cause the observed differences. For the deionized and tap water results that are more relevant in practice, a new correlation is proposed to account for the observed differences in comparison to data from the literature. It can be used to calculate C L of ellipsoidal bubbles in the investigated size range.
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the critical bubble diameter of the lift Force in technical and environmental buoyancy driven bubbly flows
International Journal of Multiphase Flow, 2019Co-Authors: Thomas Ziegenhein, Dirk LucasAbstract:Abstract The lift Force acting on particles, drops, and bubbles in a shear field is a well know effect that was extensively investigated since the 1980s. Experiments with single bubbles in a linear shear field reveal that the lift Force Coefficient has at a specific bubble size a zero where the Coefficient switches its sign from positive to negative. Solving the lateral Force balance for a polydisperse bubbly flow with the liquid flow field usually found in a bubble column, the lift Force causes a spatial separation of small, which tend to the wall, and big bubbles, which tend to the center. Bubbles with a zero lift Force Coefficient on the other hand are equally distributed over the cross section. In order to investigate the influence of the flow field on the Force balance, simulations with the Euler-Euler two-fluid model were conducted. The simulations are in good agreement with experiments and showed that the liquid flow field found in the used bubble column has a minor influence on the steady state distribution of the bubbles. From evaluating six different bubble column experiments conducted in air/water, the critical diameter at which the lift Force Coefficient is zero is determined between 5.1 and 5.2 mm. This result is very close to the critical diameter that we were able to determine in previous, single-bubble experiments. Therefore, the conclusion is possible that the lift Force Coefficients that are determined with single bubble experiments are applicable to the complex flow field found in polydisperse bubbly flows at low void fractions and low turbulence levels. Moreover, a very simple experimental setup is described to determine the critical diameter in complex flow situations, which is the most important point when it comes to modelling the lift Force. Such a setup is therefore beneficial when complex substance mixtures used in a specific industrial or environmental application needs to be investigated since lift Force correlations are only available for clean, ideal substances.
Dirk Lucas - One of the best experts on this subject based on the ideXlab platform.
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lift Force Coefficient of ellipsoidal single bubbles in water
International Journal of Multiphase Flow, 2021Co-Authors: H Hessenkemper, Thomas Ziegenhein, Roland Rzehak, Dirk Lucas, Akio TomiyamaAbstract:Abstract For the simulation of bubbly flows, knowledge of the lift Force as an interaction between gas bubbles and a surrounding shear field is of great importance. The sign of the lift Coefficient C L changes with increasing bubble size, i.e. with more pronounced bubble deformation. Beside this, impurities in terms of surface-active components are well-known to change the complete hydrodynamic behavior of a bubble even if the amount is very small. In the present work, the lift Coefficient of single ellipsoidal bubbles is determined with a recently developed method, which is suitable to overcome difficulties connected to low viscous systems. In order to investigate the influence of impurities on the lift Force, we conducted experiments with single bubbles of different sizes in purified, deionized and tap water. Overall, the determined lift Coefficients show no difference between deionized and tap water but reveal differences to results obtained with purified water. As no significant differences in shape and velocity are found between the different water qualities, it remains unclear how the impurities cause the observed differences. For the deionized and tap water results that are more relevant in practice, a new correlation is proposed to account for the observed differences in comparison to data from the literature. It can be used to calculate C L of ellipsoidal bubbles in the investigated size range.
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the critical bubble diameter of the lift Force in technical and environmental buoyancy driven bubbly flows
International Journal of Multiphase Flow, 2019Co-Authors: Thomas Ziegenhein, Dirk LucasAbstract:Abstract The lift Force acting on particles, drops, and bubbles in a shear field is a well know effect that was extensively investigated since the 1980s. Experiments with single bubbles in a linear shear field reveal that the lift Force Coefficient has at a specific bubble size a zero where the Coefficient switches its sign from positive to negative. Solving the lateral Force balance for a polydisperse bubbly flow with the liquid flow field usually found in a bubble column, the lift Force causes a spatial separation of small, which tend to the wall, and big bubbles, which tend to the center. Bubbles with a zero lift Force Coefficient on the other hand are equally distributed over the cross section. In order to investigate the influence of the flow field on the Force balance, simulations with the Euler-Euler two-fluid model were conducted. The simulations are in good agreement with experiments and showed that the liquid flow field found in the used bubble column has a minor influence on the steady state distribution of the bubbles. From evaluating six different bubble column experiments conducted in air/water, the critical diameter at which the lift Force Coefficient is zero is determined between 5.1 and 5.2 mm. This result is very close to the critical diameter that we were able to determine in previous, single-bubble experiments. Therefore, the conclusion is possible that the lift Force Coefficients that are determined with single bubble experiments are applicable to the complex flow field found in polydisperse bubbly flows at low void fractions and low turbulence levels. Moreover, a very simple experimental setup is described to determine the critical diameter in complex flow situations, which is the most important point when it comes to modelling the lift Force. Such a setup is therefore beneficial when complex substance mixtures used in a specific industrial or environmental application needs to be investigated since lift Force correlations are only available for clean, ideal substances.
Michael Hartnett - One of the best experts on this subject based on the ideXlab platform.
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incorporation of a non constant thrust Force Coefficient to assess tidal stream energy
Energies, 2019Co-Authors: Lilia Flores Mateos, Michael HartnettAbstract:A novel method for modelling tidal-stream energy capture at the regional scale is used to evaluate the performance of two marine turbine arrays configured as a fence and a partial fence. These configurations were used to study bounded and unbounded flow scenarios, respectively. The method implemented uses turbine operating conditions (TOC) and the parametrisation of changes produced by power extraction within the turbine near-field to compute a non-constant thrust Coefficient, and it is referred to as a momentum sink TOC. Additionally, the effects of using a shock-capture capability to evaluate the resource are studied by comparing the performance of a gradually varying flow (GVF) and a rapidly varying flow (RVF) solver. Tidal-stream energy assessment of bounded flow scenarios through a full fence configuration is better performed using a GVF solver, because the head drop is more accurately simulated; however, the solver underestimates velocity reductions due to power extraction. On the other hand, assessment of unbounded flow scenarios through a partial fence was better performed by the RVF solver. This scheme approximated the head drop and velocity reduction more accurately, thus suggesting that resource assessment with realistic turbine configurations requires the correct solution of the discontinuities produced in the tidal-stream by power extraction.
Akio Tomiyama - One of the best experts on this subject based on the ideXlab platform.
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lift Force Coefficient of ellipsoidal single bubbles in water
International Journal of Multiphase Flow, 2021Co-Authors: H Hessenkemper, Thomas Ziegenhein, Roland Rzehak, Dirk Lucas, Akio TomiyamaAbstract:Abstract For the simulation of bubbly flows, knowledge of the lift Force as an interaction between gas bubbles and a surrounding shear field is of great importance. The sign of the lift Coefficient C L changes with increasing bubble size, i.e. with more pronounced bubble deformation. Beside this, impurities in terms of surface-active components are well-known to change the complete hydrodynamic behavior of a bubble even if the amount is very small. In the present work, the lift Coefficient of single ellipsoidal bubbles is determined with a recently developed method, which is suitable to overcome difficulties connected to low viscous systems. In order to investigate the influence of impurities on the lift Force, we conducted experiments with single bubbles of different sizes in purified, deionized and tap water. Overall, the determined lift Coefficients show no difference between deionized and tap water but reveal differences to results obtained with purified water. As no significant differences in shape and velocity are found between the different water qualities, it remains unclear how the impurities cause the observed differences. For the deionized and tap water results that are more relevant in practice, a new correlation is proposed to account for the observed differences in comparison to data from the literature. It can be used to calculate C L of ellipsoidal bubbles in the investigated size range.
Hansjoachim Warnecke - One of the best experts on this subject based on the ideXlab platform.
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vof simulation of the lift Force for single bubbles in a simple shear flow
Chemical Engineering & Technology, 2006Co-Authors: Dieter Bothe, M Schmidtke, Hansjoachim WarneckeAbstract:Bubbles in shear flows experience a lift Force, causing them to migrate sideways while they are rising. This lateral migration is investigated in numerical simulations, which are carried out with an extended version of the highly parallelized code FS3D, employing an advanced Volume-of-Fluid method. The movement of single bubbles in linear shear flows is simulated to obtain the magnitude of the lift Force - expressed by the lift Force Coefficient C L - for various bubble diameters and material data. Simulation results are in good agreement with experiments for medium liquid phase viscosities. An investigation of the dynamic pressure on the bubble surface explains why large bubbles migrate in the opposite direction compared to small bubbles.