Particle flow behavior in models of branching vessels. II. Effects of branching angle and diameter ratio on flow patterns.

To further elucidate the role of fluid mechanical factors in the localization of atherogenesis and thrombogenesis, we have studied the 3-dimensional flow patterns in square T-junctions with branching angles theta from 30 degrees to 150 degrees and diameter ratios d/D (side: main tube) from 1.05/3.0 to 1.0. Cine films of the motions of tracer microspheres in dilute suspensions were taken at inflow Reynolds numbers from 15 to 400 and flow ratios (main: side tube) from 0.1 to 4.0. Flow patterns with suspension entering through the main tube were similar to those previously described in uniform 3 mm diameter T-junctions: paired vortices (spiral secondary flows) symmetrical about the common median plane formed at the entrances of the main and side daughter tubes. Particles circulated through the main vortex, some crossing above and below the mainstream into and through the side vortex. At the geometrical flow ratio, the main vortex became smaller and smaller as the branching angle (theta less than 90 degrees) and diameter ratio decreased, and was confined to a thin side tube was a minimum. In obtuse angle T-junctions the stagnation point shifted from the flow divider into the side tube, enhancing the flow disturbance there. The velocity distributions in main and side tubes were skewed towards the inner walls close to the flow divider. When flow entered through the side tube, a pair of recirculation zones formed in the main tube at the inner wall of the bend with a sharper angle.