Experimental Measurement of the Vortex Development Downstream of a Lobed Forced Mixer

Abstract

An experimental study was conducted to investigate the mixing processes downstream of a forced mixer. A forced mixer generates large scale, axial (stirring) vorticity which causes the primary and secondary flow to mix rapidly with low loss. These devices have been successfully used in the past where enhanced mixing of two streams was a requirement. Unfortunately, details of the mixing process associated with these lobed forced mixers are not well understood. Performance sensitivity to design variables has not been documented. An experiment was set up to investigate the mixing processes downstream of a mixer. Air flow was independently supplied to each side of the forced mixer by separate centrifugal blowers. Pressures were measured at the entrance to the lobes with a pitot-static probe to document the characteristics of the approaching boundary layer. Interior mean and fluctuating velocities were nonintrusively measured using a two-component Laser Doppler Velocimetry (LDV) system for velocity ratios of 11 and 2:1. The wake structure is shown to display a three step process where initially secondary flow was generated by the mixer lobes, the secondary flow created counter-rotating vortices with a diameter on the order of the convolute width, and then the vortices broke down resulting in a significant increase in turbulent mixing. The results show that the mean secondary motion induced by the lobes effectively circulated the flow passing through the lobes. This motion, however, did not homogeneously mix the two streams. Turbulent mixing in the third step of the mixing process appears to be an important element in the enhanced mixing that has been observed with forced mixers. The length required for the flow to reach this third step is a function of the velocity ratio across the mixer. The results of this investigation indicate that both the mean secondary motion and the turbulent mixing occurring after vortex breakdown need to be considered for prediction of forced mixer performance.

Numerical research on the mixing mechanism of lobed mixer with inlet swirl in linear radial distribution

“…It indirectly indicated that the vortices breakdown induced numerous smallscale turbulent spots. 13 The vortices' interaction, especially their breakdown, was also one of the most important factors for accelerating the jet mixing. In addition, both the entrained ambient and separation bubble on center-body caused the high TKE fluid in the corresponding area.…”

“…With the aggravation of entrainment, the normal vortices broke down at the streamwise vortices cores, as indicated by ''BD'' at x ¼ 1.0D plane in Figure 5(b). The vortices breakdown produced numerous small-scale turbulent spots, 13 and thus played very important role in accelerating the jet mixing. Conversely, this process enhanced streamwise vortices dissipation as well, which was shown by the sharp decreasing streamwise vorticity at streamwise vortex cores in x ¼ 1.0-2.0D planes.…”

“…Paterson 11 first discovered the large-scale streamwise vortices in lobed jet by using LDV. Werle et al 12 and Eckerle et al 13 reported that the lobed mixer first induced radial secondary flows, which then grew up into streamwise vortices. Elliott et al 14 suggested that the streamwise vortices and the increased initial interfacial area, due to the convoluted geometry of the lobed mixers, strengthened jet mixing effectively, compared to circular nozzle.…”

Parametric Effects on Internal Aerodynamics of Lobed Mixer-Ejector With Curved Mixing Duct

“…The streamwise vortices were reported to interact within a small distance downstream of the mixer trailing edge, but remain the largest scale fiow structure for three to ten lobe wavelengths downstream. Further investigations on the fundamental mechanism of mixing enhancement were carried out either experimentally or numerically by a lot of authors (such as Eckerle et al [11], McCormick and Bennett [12], Tsui and Wu [13], Glauser et al [14], Yu and Yip [15], Hu et al [16], etc. downstream of a lobed mixer were explained by McCormick and Bennett [12].…”

Investigations of flows in a new infrared suppressor

“…Effective mixing is accomplished through the generation of large scale axial vorticity. This is achieved by merging the two streams after they have been given moderate absolute but strong relative secondary flow components as a result of their passage over opposite sides of a gently transitioned planar-to-threedimensional surface, as described by Werle et al [5,[10][11][12][13][14][15][16].…”