ENGINEERING AND CFD

Turbulent multiphase flows are ubiquitous in a wide range of natural processes and engineering applications. Surfactants and polymers additives are usually added to multiphase turbulent flows separately or together to manipulate the flow structure and dynamics for various purposes such as drag reduction and emulsion stability. The long chain molecules of surfactants make bulk fluid viscoelastic while drag-reducing polymer additives often act as a surfactant in multiphase flows.

Despite its fundamental importance from scientific and applications point of views, the combined effects of soluble surfactant and viscoelasticity on turbulent bubbly flows have not been studied experimentally or computationally. The motive of this study was to perform extensive large-scale direct numerical simulations (DNS) of turbulent bubbly channel flows to examine combined effects of surfactant and viscoelasticity by using a fully parallelized 3D finite-difference/fronttracking method. The flow equations are solved fully coupled with the governing equations of interfacial and bulk surfactant concentrations and the FENE-P viscoelastic model. The latter coupling is achieved through a non-linear equation of state which relates the surface tension to the surfactant concentration at the interface.

Interface-resolved direct numerical simulations are performed to examine the combined effects of soluble surfactant and viscoelasticity on the structure of a bubbly turbulent channel flow. Effects of different types of surfactant are examined using the physical sorption kinetics. For Newtonian turbulent bubbly flow, effects of Triton X-100 and 1-Pentanol are examined (Fig. 1).

It is observed that the sorption kinetics highly affect the dynamics of the bubbly flow. A minute amount of Triton X-100 is found to be sufficient to prevent the formation of bubble clusters. On the other hand, 100 times more of 1-Pentanol surfactant is not sufficient to prevent the formation of the layers. For viscoelastic turbulent flow, it is found that the polymer drag reduction is completely lost for the surfactant-free case and the addition of small amount of surfactant (Triton X-100) restores the polymer drag reduction for the viscoelastic turbulent bubbly flows.

Then the polymer drag reduction of turbulent poly-dispersed bubbly flow is examined in the presence of soluble surfactant (Fig. 2).