(227s) Interfacial Dynamics of Two-Phase Stratified Flow in Microchannels

Two-phase viscous stratified flows are encountered in several applications of microchannels, especially in (bio)chemical processing and separations. In some cases, stable stratified flow is desired, as in solvent extraction where the phases may be easily separated if stratified. On the other hand, destabilization of stratified flow is desirable in certain applications, for which the alternative flow regimes of droplet and slug flow are more advantageous. Therefore, it is important to understand interfacial instabilities in stratified microflows – their origin, propagation and associated flow regime transitions.

Stratified flow in microchannels is laminar due to low Reynolds numbers. Therefore, instability mechanisms are primarily associated with the dynamics of the inter-fluid interface. To analyse this system in detail, we construct a low dimensional model by applying center manifold reduction (CMR) to the Navier-Stokes equations. This order reduction is based on the difference between the transverse and longitudinal length scales. An important advantage of the CMR model over classic lubrication theory is that it allows us to systematically account for finite inertial effects. These may be significant because stratified flows are usually attained at relatively high flow rates of the fluids and the Reynolds numbers can attain values in excess of 50. This model allows us to study the evolution of perturbations to the interface and understand how instabilities may cause a transition from stratified to slug flow. We also investigate the possibility of inducing instabilities by modifying the shape of the walls of the channel.

Keywords: stratified flow, slug flow, instability, microchannels, center manifold reduction