Forced Mixer Research Articles

Three-dimensional measurements in non-axisymmetric flow experiment using a one-or two-component laser-Doppler anemometer

A method to determine the velocity components in the third direction for steady-non-axisymmetric nozzle flow experiments has been developed in addition to the two components that could be directly measured by a one-or two-component laser-Doppler anemometer. With the aid of this method, rotation for either the test rig or the anemometer become unnecessary. Results obtained by this approach were used to compare with those directly measured in the downstream wake of a co-axial nozzle with a central five-lobe forced mixer where the flow is non-axisymmetric. Good agreement could be obtained with maximum discrepancy at about 5%. The present approach can be readily applied to many other non-axisymmetric nozzle flow situations with only some minor modifications.

Investigation of the mixing flow structure in multilobe mixers

A computational procedure is described to study the mixing flow in a multilobe turbofan mixer. The predictions have been obtained using a finite volume method to solve the density-weighted time-averaged Navier-Stokes equations. Turbulence is characterized by the k-e eddy viscosity model. To fit the irregular boundaries of the flowfield, curvilinear nonorthogonal coordinates are employed. The robustness of the computational procedure is enhanced by making use of nonstaggered grids. In the calculation the computational domain contains not only the mixing duct but also the lobe itself. Three kinds of configuration are under consideration : a confluent mixer, a convoluted mixer, and a forced mixer. Results show that the streamwise vortex generated at the trailing edge is the most prominent flow structure for the forced mixer, responsible for the effective mixing. The strength of the streamwise vortex can be characterized by the circulation. A parameter termed mixedness is defined to describe the effects of the mixing process. In addition to the streamwise vortex, normal vortices are generated by the velocity difference in the shear layer between the core and fan streams.

Some flow characteristics of lobed forced mixers at higher velocity ratios

The flow characteristics of two types of lobed forced mixers, the unscalloped and the scalloped mixers, have been examined at velocity ratios higher than unity, in relation to the variation of mass flux uniformity, the decay of the streamwise vorticity, the variation of turbulent kinetic energy and the growth of the shear layer with distance from the trailing edge. Three trailing edge configurations have also been considered for each type of mixer, namely a square wave, a semi-circular wave and a triangular wave. The analysis showed that the strength of the streamwise vorticity shed at the trailing edge and the subsequent decaying rate with downstream distance are found to be very important in studying the mixing effectiveness of the lobed mixers.

Some aspects of the flows behind lobed forced mixers

Flow visualization studies have been conducted using the smoke-wire and laser-sheet techniques to examine the flows behind lobed forced mixers of different trailing edge configurations (a square wave, a semi-circular wave and a triangular wave) at a matched velocity ratio across the lobe. The size of the wake region and the strength of the secondary flow shed by a lobe were found to be larger with lobe geometry consisting of straight parallel sidewalls. Formation of streamwise vortices were observed within the first four wavelengths downstream of the trailing edge in the square and semi-circular lobed mixers and these vortices were largely responsible for intense mixing at the corresponding locations. Similar observation was not found in the case of triangular lobed mixer. More importantly, no indication of vortex breakdown was found within seven wavelengths downstream of the trailing edge in the three cases.

Some 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 × 10 4 (based on a reference velocity of 10m/s, and a nominal wavelength of the lobe of 33 mm). The three trailing edge configurations under investigation have the shape of a square wave, a semi-circular wave and a triangular wave. Measurements for a velocity ratio of 1:1 indicate that the strength of the secondary flow shed by a lobe is not the only parameter which determines the effectiveness of mixing, as previously suggested. The boundary layer thickness which grows along the side-walls of the lobe penetration and the subsequent shedding of the boundary layer to the wake region are of equal importance. High turbulence regions do not appear downstream of the mixers within six wavelengths downstream of the trailing edge. The normal and shear stresses in two cases (the semi-circular and triangular wave lobes) are shown to have a similar trend of decaying in magnitude.

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

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 1:1 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 counterrotating 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.