Visualization of entry flow separation for oscillating flow in tubes

Results of visualization experiments are presented for the entry flow to circular tubes under oscillatory flow conditions. Geometries and conditions have been chosen to simulate the flow in a Stirling engine with straight heat exchanger tubes. Since oscillating flow in Stirling engines is unavoidably strongly influenced by the entry conditions, such documentation is useful when engine designs are being considered and is needed when test results are being interpreted. Two entry geometries are explored, one with unrestricted entry to a squared-edged tube and another with entry from one side. The visualization technique is by illumination of neutrally-buoyant, helium-filled soap bubbles with laser light, capturing with still photography. Each picture is an ensemble of exposures from 150 cycles. Each entry to the ensemble is taken at the same range in crank position, typically five degrees. Thus, one picture may visualize the flow from 75 to 80 degrees of crank rotation, for instance. Such documentation is done for representative points throughout the cycle. The figures show that, upon entry, there is separation of streamlines from the wall with a separation bubble length of few diameters. During the acceleration phase of the cycle, this separation zone becomes larger with increasing velocity and Reynolds number. During the deceleration phase of the cycle, the scale of the separation region is roughly constant. It does not grow as might be expected. Circuitous bubble trajectories in photos taken during later portions of the deceleration phase hint that eddy circulation is intense. Such circulation may be curtailing the separation bubble growth. When the entry flow is restricted to one side, the recirculation zone on the side from which the flow enters becomes considerably larger than when the flow entry is unrestricted. In this case, the separation bubble does grow somewhat during the period of strong decelaration.