Rotation effects on axisymmetric sudden-expansion flows

The dump combustor of a ramjet propulsion system was simulated by a Plexiglas tube of 63.5 mm I.D. and 76.2 cm long. A convergent nozzle with an exit diameter of 43.2 mm was fitted to the inlet of the tube. Therefore, this arrangement provided a sudden expansion and gave a step height of 10.15 mm. Tube rotation effects on mixing in the dump combustor were examined by rotating the Plexiglas tube about its own axis at a constant speed. The flow downstream of the sudden expansion was studied using a one-color, one-component, laser-Doppler anemometer. Two conditions were investigated in detail, one with the tube rotating at 840 rpm and another with the tube stationary. In addition, three other experiments with different tube rotational speeds were carried out to determine the dependence of reattachment length on rotation. The average inlet velocity was set at ~ 10.6 m/s for all experiments. The results show that the primary reattachment length decreased as rotation increased because of the destabilizing effect due to the large shear created by the rotating tube wall. A detailed comparison of the flow characteristics with and without rotation present is made, and the effects of rotation on the flow are analyzed.