Author(s)

CARSTEN MEHRING

Abstract

The performance of a gas–liquid cylindrical cyclone separator for the separation of air bubbles from hydraulic fluid has been analyzed numerically using the commercial computational fluid dynamics flow solver CFX. Two-phase flow behavior is modeled based on an Eulerian–Eulerian approach, representing both liquid and dispersed gas phase as interpenetrating media with interphase momentum transfer captured based on existing bubble drag models. Only a single bubble size is considered for the dispersed phase and bubble coalescence is ignored. At the tangential inlet, a homogeneous gas– liquid mixture is assumed with specified mass flow and air/liquid volume fractions. Pressure conditions were imposed at both the upper gas outlet and the lower liquid outlet boundaries. Effect of changes in turbulence model and bubble drag model on analysis predictions was analyzed for selected operating conditions, that is, bubble sizes, air/liquid volume fractions and flow rates. Separation efficiency was high only for larger bubble sizes (100 mm) and high flow rates (250 L/min). For bubble sizes below 35 mm, the cyclone was ineffective. At high flow rates, cyclone performance suffers due to liquid carry over in the form of a swirling liquid wall film carried with the gas (bubble) phase through the top outlet. The Explicit Algebraic Reynolds Stress Turbulence Model together with the Grace Drag Model was found to be effective in exploring changes to the cyclone geometry. Here, improvements in separation efficiency were only achieved with significantly increased tangential velocities as a consequences of a reduced inlet port cross section. Predicted bubble size separation limits as a function of Reynolds number and axial-to-tangential velocity ratios were found to compare favorably with existing literature.

Keywords

bubbly flow, hydrocyclone, modeling, phase separation