VELOCITY-MEASUREMENTS DOWNSTREAM OF A LOBED-FORCED MIXER WITH DIFFERENT TRAILING-EDGE CONFIGURATIONS

Velocity and turbulence characteristics have been measured downstream of a lobed-forced mixer with three different trailing-edge configurations, using a two-component laser-Doppler anemometer at a Reynolds number of 2.27 x 10(4) (based on the bulk mean velocity of the two streams at 10 m/s, and a nominal wavelength of the lobe at 33 mm), and with velocity ratios of 1:1, 1:2, and 1:3 across the lobe. The three trailing-edge configurations under investigation have the shape of a square wave, a semicircular wave, and a triangular wave. The results for the velocity ratio 1:1 indicate that the mixing is not only affected by the strength of the secondary now shed by a lobe, but also by the boundary-layer thickness grown along the sidewalls of the lobe penetration and the subsequent shedding of the boundary layer to the wake region. No region of high turbulence was found within six wavelengths downstream of the trailing edge. The results for the higher velocity ratios, 1:2 and 1:3, reveal a very different flow development from those of an equal velocity ratio, A high-turbulence region appeared at around two to three wavelengths downstream of the trailing edge, and was followed by a gradual decay in magnitude. Analyzing the production terms in the Reynolds stresses equations at the corresponding location suggested that positive production of turbulent kinetic energy existed and was a consequence of large mean axial velocity gradients that coincided with shear stresses of the opposite signs. Thus, the present investigations suggest that the high-turbulence region responsible for rapid mixing may not be due to vortex breakdown (at six wavelengths downstream), but may be due to the positive production of turbulent kinetic energy at a location (at about two to three wavelengths downstream) earlier than previously suggested.