Investigation Into The Characteristics Of Flow Field Downstream Of The Lobed Mixer For Mixing Enhancement

Aiming at developing efficient mixing technology which could achieve rapid mixing in a short distance, a rectangular lobed mixer, which acts as a passive mixing enhancing device is investigated both numerically and experimentally in terms of the characteristics of mixing and flow field downstream of the lobed mixer by employing hybrid RANS/LES method, Nanoparticle-based Planar Laser Scattering(NPLS) method and Particle Image Velocimetry(PIV).Combining the former research on the lobed mixer with the requirements and limitations of present investigation, a rectangular lobed mixer is designed in the first place. The experiments are carried out in a supersonic suction type wind tunnel with calibrated convective Mach number of 0.22. The static pressure of the two streams at the outlet of the nozzle is matched in the experiments.Through the standard simulating case of three dimensional supersonic mixing layer, the hybrid RANS/LES method employed in present investigation is validated. Based on the simulating results, the streamwise vortices and K-H vortices are successfully identified by using the Q criterion of vortex identification, indicating that there are three large scale streamwise vortices downstream of the lobed trailing edge, which are generated in the near field of three parallel vertical walls of the lobed mixer. The streamwise vortices are the dominant flow structures, whose rotations have significant influence on the motion of K-H vortices while their own movements are almost kept unaffected. The interaction of streamwise and K-H vortices is different from that under incompressible conditions where the stream and K-H vortex tubes are distorted by each other. Of the three streamwise vortices, the middle one is the largest in scale, whose rotation can dramatically entrain the lower stream upward on one side of the vortex while entrain the upper stream downward on the other side. The rotation of the middle vortex also has slight affect on the movements of the other two streamwise vortices, with one moving upward while the other moving downward when convecting downstream.According to the distribution of static pressure and spanwise velocity, it is found that the generation of large scale streamwise vortices is driven by the spanwise and transverse pressure disparity, which is brought by the converse effect of the ramp of the lobe and able to amplify the initial instability.The instantaneous NPLS images shows that K-H vortices will roll up rapidly after a short distance of laminar region downstream of the trailing edge, which is very similar to that in supersonic mixing layer. Comparing with streamwise vortices, the K-H vortices are much smaller in scale and grow slower. The initial K-H instability has significant influence on the structure of the flow field in the form of orderly distributed vortex clusters which is composed of a large number of small scale vortices. The break-up of the streamwise vortices is tightly related to the secondary instability occurred in the rim of the vortices, which is induced by the transverse shear of two streams for the rotation of the streamwise vortices. The streamwise vortices break up from the rims of the vortices which is different from that in incompressible flow where the large scale streamwise vortices split into more smaller streamwise vortices directly.The scalar mixedness grows nearly linearly with the distance to the trailing edge, and the scalar mixedness can be improved significantly by increasing the lobe height, which is brought by the promotion of several mixing enhancement related factors, including streawise vortices, interface area and turbulent kinetic energy. The drawback is obvious as well, with total pressure loss been increased due to the shock wave.