Confined jets in co-flow: effect of the flow rate ratio and lateral position of a return cannula on the flow dynamics

Co-axial tubes have been used to produce a co-flowing confined jet similar to that found in an Extracorporeal Membrane Oxygenation return cannula flow configuration. Particle Image Velocimetry was used to investigate the flow rate ratio between jet and co-flow as well as changes in flow characteristics due to cannula position. The flow was found to be dominated by three main structures: lateral flow entrainment, shear layer induced vortices and backflow along the wall. An increase in cannula flow rate amplified entrainment and recirculation, resulting in a decrease in length required to reach a fully developed flow. Changing cannula position relative the outer cylinder induced a significant reduction in recirculation zone as well as vortex formation on the side to which the cannula was tilted towards, whereas on the other side, the recirculating flow region was enhanced. Proper Orthogonal Decomposition demonstrated that the dominating structure found in the flow is the backflow, composing of several structures having different oscillation frequencies. The significance of the observed and measured flow structures is in enhancing mixing.