The mixing characteristics of forced mixers with scalloped lobes

An experimental investigation had been undertaken to study the mixing characteristics of forced mixers with scalloped lobes using a two-component fibre-optic laser-Doppler anemometer at a Reynolds number of 2.27 x10⁴ and at a velocity ratio 1:2 across the lobes. The trailing edge shape of the mixers, without scalloping, had the shape of a square wave, a semi-circular wave and a triangular wave. Scalloping of the lobes was achieved by eliminating up to 70 % of the sidewall area at the penetration region of each lobe. Aspect ratio of each lobe (lobe height to wavelength ratio) was at unity and the half angles at the penetration region were 22 degrees and 35 degrees for the scalloped and the aggressively scalloped mixers respectively. The wavelength was 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. Strength of the streamwise circulation near the trailing edge for respective scalloped mixers was higher than the nonscalloping cases, due largely to the formation of two pairs of streamwise vortices at each lobe. The subsequent decay rate with downstream distance was also found to be more rapid indicating a faster mixing rate can be actually 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.