Flow structures in large-angle conical diffusers measured by PIV

Flow in two different conical diffusers with large opening angles (30° and 18°) have been measured with stereoscopic Particle Image Velocimetry (PIV). The measurements were done in a cross section just after the exit of the diffuser. The Reynolds number was 100000 based on upstream diameter and mean velocity. The inlet condition was a straight pipe with a fully developed velocity profile. The diffuser with opening angle of 18° was also investigated with an inlet pipe with a 30° pipe bend shortly upstream of the diffuser. In general the flows show very high turbulence intensities, of the order of 100%. The cases with straight pipe inlet seem to have a high velocity core that moves to different positions in the cross section of the diffuser. Other parts of the cross section have flow separation. The time scale of these motions is an order of magnitude larger than the largest turbulent time scale found in fully developed flow in the downstream pipe, suggesting precession of the high speed core. For the inlet with a bent pipe, the high velocity regions and region with flow separation are found at more fixed positions and the time scale is similar to time scales in fully developed flows.