Lobed Mixer Research Articles

Downstream evolution of proper orthogonal decomposition eigenfunctions in a lobed mixer

A two-dimensional (one space and time) scalar adaptation of the proper orthogonal decomposition was applied to streamwise velocity data obtained in a lobed mixer flowfield, using a rake of 15 single-component hot wires. Through the application of the proper orthogonal decomposition, the amount of streamwise turbulent kinetic energy contained in the various proper orthogonal modes was examined for two different downstream locations (z/h = 2.6 and 3.9). The large eddy or dominant mode was shown to have a measurable decrease in the relative streamwise component of the kinetic energy between these two downstream locations. This indicates that the large eddy, as defined by the proper orthogonal decomposition, breaks down, and the flow becomes more homogeneous. A pseudoflow visualization technique was then employed to help visualize this process.

Multifractal analysis of a lobed mixer flowfield utilizing the proper orthogonal decomposition

We use the theory of multifractal measures to throw a fresh perspective on the data analysis of velocity time traces in a lobed mixer flow. A lobed mixer is a device that enhances mixing through secondary flows and streamwise vorticity. A specially designed rake of hot wires with high spatial resolution was used to collect the streamwise velocity data. Through the use of the proper orthogonal decomposition, a basic knowledge of the coherent structures, on an energy weighted basis, was studied. The instantaneous velocity traces that are necessary for calculating the generalized fractal dimensions are reproduced from the ensemble average measurements taken from the proper orthogonal decomposition. We compute the generalized fractal dimensions Dq and the /(a) multifractal spectrum of several proper eigenmodes for data samples from the velocity time trace of several hot-wire probes at different energy levels. This is a marked departure from previous multifractal theory applications to self-similar cascades that mainly dealt with the dissipation fields of turbulent kinetic energy. In the course of this exposition it will be pointed out that in certain cases a single dimension D may suffice to capture the entire spectrum of scaling exponents for the velocity time trace. We verify that in such cases the location of the hot-wire probes imply a lack of intermittency .

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.

Numerical investigation of the flow within a turbofan lobed mixer

A computational procedure is described for the calculation of the flowfield within a multilobed turbofan mixer. The predictions have been obtained by using a finite-volume solution procedure for the three-dimensional elliptic equations of fluid flow extended to allow the modeling of flows within complex geometries. Turbulence is modeled using the two equation k-e eddy viscosity model. Previous experimental investigations have shown that large scale secondary flows at exit from the lobes play a dominant role in the mixing process downstream of the lobe exit plane. The present work demonstrates the capability of the current method to predict the flow characteristics within the lobes and hence the formation of these secondary velocities. Finally, parametric studies of the effect of changes in the fan-side geometry are reported to investigate how these influence the flow structure, in particular the generation and amplification of the secondary flow patterns. The calculations provide useful information on the nature of the secondary flows and demonstrate how the procedure may be used to examine the likelihood of flow separation and the influence of the thickening of upstream boundary layers.

Turbulent measurements in the lobe mixer of a turbofan engine.

Turbofan engines may use a device known as a lobe mixer in the exhaust duct, in which the core flow and the fan flow are rapidly mixed with strong secondary flows. This mixing process provides a means of achieving significant performance gains and reducing jet noise. Although this fact has been known for some time, the details of the mixing process within the mixing duct and exhaust nozzle have not been well understood. We measured the three-dimensional turbulent flow in a scale model of an exhaust duct with a lobe mixer by using a hot-wire anemometer, and clarified, experimentally, the basic characteristics of the mixing process.

Calculation of lobe mixer flow with Reynolds stress model.

Lobe mixer is a special nozzle equiped in a turbofan engine to increase the thrust efftciency and suppress the noise. At present the design depends on systematic experiment because the flow field is too complex to be represented with an eddy viscosity model. Although it is neccesary to use a Reynolds stress model(RSM), conventional ones have not taken account of 3 dimensional application. We extended Launder-Reece-Redi model and constructed a new low Reynolds number type RSM. To determine unknown constants and to confirm the ability 2 dimensional channel flow was calculated. The agreement between the calculation and the experiment is satisfactorily good. Then we applied it to the lobe mixer and showed that it can reproduce the flow qualitatively.

Acoustically treated mixer for a mixed flow gas turbine engine

In a mixed flow gas turbine engine of the type which includes an outer nacelle spaced apart from a core engine to define an annular bypass duct therebetween, there is provided a convoluted lobed mixer having a plurality of cold chutes in flow communication with the bypass duct and hot chutes in flow communication with the core engine. In order to suppress engine turbomachinery and exhaust noise, the mixer is alternatively provided with various combinations of acoustically treated lobes, spool pieces, radial splitters, and/or circumferential splitters. These acoustic treatments act in conjunction with an acoustically treated nozzle assembly and an acoustically treated plug assembly to provide an effective low aerodynamic pressure loss noise suppression system.

Numerical Modeling of Three-Dimensional Flows in Turbofan Engine Exhaust Nozzles

An experimental and numerical study was conducted to investigate the use of three-dimensi onal viscous analysis for the design of internal mixers for jet noise suppression. Three flows, a full-scale free mixer, a fullscale lobed mixer, and a model-scale lobed mixer, were selected for study. Details of the experiments, turbulence modeling, and the determination of initial flow properties from engine operating properties are described together with a discussion of the usefulness of the analysis for mixing design. Overall the results are encouraging and, at least for the flows considered here, numerical analysis appears to compare favorably with model-scale tests as a simulation of the full-scale flow.