Mixing characteristics of forced mixers with scalloped lobes

An experimental investigation has been undertaken to study the mixing characteristics of forced mixers with scalloped lobes using a two-component fiber-optic laser Doppler anemometer at a Reynolds number of 2.27 × 104 and at a velocity ratio 1:2 across the lobes. The trailing edge of the mixers, without scalloping, had the shape of a square wave, a semicircular wave, and a triangular wave. Scalloping was achieved by eliminating up to 70% of the sidewall area at the penetration region of each lobe. The aspect ratio of each lobe (lobe height-to-wavelength ratio) was at unity and the half-angles at the penetration region were 22 and 35 deg for the scalloped and the aggressively scalloped mixers, respectively. The wavelength and lobe height were the same for both types of mixer. The results showed that lobe configurations appeared to be more important than the penetration angles for the benefit of scalloping to occur. Strengths of the streamwise circulation near the trailing edge for respective scalloped mixers were higher than the nonscalloping cases, largely because of the formation of two pairs of streamwise vortices at each lobe. The subsequent decay rates with downstream distance were also found to be more rapid, indicating that a faster mixing rate can actually be achieved by the scalloped mixers. Additional production of turbulent kinetic energy appeared at about two to three wavelengths downstream of the trailing edge and then asymptote to lower level in the far-field region. Spatial uniformity of the mass flux at the wake region could be achieved at two wavelengths further downstream of the high turbulent kinetic energy region.