화학공학소재연구정보센터
International Journal of Multiphase Flow, Vol.116, 26-38, 2019
The critical bubble diameter of the lift force in technical and environmental, buoyancy-driven bubbly flows
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. (C) 2019 Elsevier Ltd. All rights reserved.