École Nationale Supérieure de
Techniques Avancées

Abstract

The lift force is an important phenomenon for estimating bubble distribution in two-phase flows in industrial devices such as the primary circuit of a nuclear power plant. This force has been studied almost exclusively in upward flows and its evolution in inclined flows is unknown. We use direct numerical simulations (DNS) with the TRUST/TrioCFD code,employing a mixed Front-Tracking/VoF method to model the two-phase interface. The calculations are performed for different inclination angles θg varying from 0◦ to 180◦, within the framework of the Lighthill mechanism (quasi-spherical bubbles, moderate Reynolds). Two approaches are used and compared to calculate the lift coefficients : the fundamental principle of dynamics using the usual lift expression and the integration of the stress tensor over the interface. The results were first validated on the vertical reference case, showing good agreement with existing correlations for drag and lift coefficients. In inclined channels, significant discrepancies appear between the two calculation methods, reaching up to 200%for θg =90◦.Flow analysis revealed an inclination and intensification of the bubble wake, leading to vorticity amplification (up to +450%) and a reduction in wake efficiency in generating lift. These mechanisms explain the non-monotonic evolution of the lift coefficient, which presents a maximum around θg =30◦ then decreases to reach a minimum for horizontal pipe flow. The concept of effective wake length is proposed to integrate the loss of wake efficiency into the usual lift force expression. A study of the lift coefficient during transients is performed in the end.