Experimental Study On The Mixing Mechanism Of Lobed Mixer With Inlet Swirl In Model Turbofan Engines

The lobed mixer is entensive used in the low and intermediate bypass ratio turbofan engines, resulting in enhancement in jet mixing efficiency, increase in net thrust, a reduction in fuel consumption, decrease in jet noise and, in some cases,reduced infrared signature. Since 1960s, considerable researches have been carried out on the jet mixing mechanism of the lobed mixer and geometry optimization at the design point of the turbofan engine, which means the inlet condition is uniform flow with the axial direction. While, at the off-design points, i.e. take-off, landing or with high inlet swirling flow due to the usage of high loading turbine blade, it is rarely to find the relative paper which studies the effect of inlet swirl on the mixing and performance of the lobed mixer. Therefore, this paper works on the lobed mixer of a real turbofan engine on a co-annular, low speed, open circuit wind tunnel at the Gas Turbine Laboratory of the National Research Council of Canada. The seven-hole probe, hot-wire and surface oil-flow visualization was used to measure the flow field at inlet planes, downstream of the lobed mixer or lobe's surface, and the simulation was also carried out to offer the detailed information of the jet mixing flow as an assistant of the measurement.In this paper, the jet mixing mechanism of the loed mixer is studied with the uniform inlet flow without swirl, firstly. And then, the swirl vanes were used to generate the inlet swirling flow with different swirl angle to stuty the effect of inlet swirl on the formation, development, interaction and break-down of the vortical system downstream of the lobed mixer, the aerodynamic performance of the lobed jet flow is estimated under the inlet swirl condition. Further more, this paper also studied the mixing mechanism of the scalloped lobed mixer with/without inlet swirl, and the effect of swirl vanes'wakes and the length of the mixing tube on the performance of the scalloped lobed mixer is discussed in this paper too.Based on the analysis on the jet mixing mechanism of the lobed mixer, the special 3D contour of lobed mixer induces the large scale streamwise vortices, with the length sacle of height of the lobe. With the diffusion of the jet flow, the interaction between streamwise vortices and normal vortices and their break-down generates lots of turbulent eddies, which dominate the jet mixing of the lobed mixer. Most of the jet mixing happens in the range of two equivalent hydraulic diameter downstream of the lobed mixer. Under the highly inlet swirl conditions, the interaction between the streamwise vortices and normal vortices, which are enhanced by the inlet swirl, still dominates the lobed mixing flow. The penetration flow around the lobe's trough stretches and deforms the normal vortices, even generates counter-clockwise rotating additional streamwise vortices by interaction with the core flow, and improves the jet mixing of un-scalloped lobed mixer. The leakage swirling flow between the centre-body and lobe's trough entrains considerable core fluids with high momentum, and enhances the self-interaction of outer vortical system by extruding the outer fluid. All of thses phenomena enhance the lobed jet mixing, and the effect of the leakage flow becomes more important with the increasing inlet swirl. However, under highly inlet swirl conditions, the separation bubble presenting on the the suction surface of the lobe and the large-scale recirculation zone appearing downstream of the centre-body due to the high swirling leakage flow causes increasing total pressure loss and thrust loss.The streamwise vortices is introduced gradually from the tip point of the scalloping curve, and the induced flow mixing bates the strength of the normal vortices. The normal vortices are stretched and deformed by the scalloped notch due to their own different diffusion rate downstream of the trailing edge of scalloped lobed mixer, and induced the formation of the small-scale additional streamwise vortices. The interaction between the addition streamwise vortices and the large-scale streamwise vortices may not increase the streamwise vorticity, but it really enhance the jet mixing due to the generation of considerable turbulent eddies during their interaction. Under highly inlet swirl conditon, the additonal streamwise voritces near the lobe's trough induced by the interaciton between the penetration flow and the core flow due to their opposite radial velocity. The leakage flow around the centre-body is not sensitive to the scalloping notch of the scalloped lobed mixer. On ther other hand, the scalloping notch decrease the resistance of scalloped lobed mixer to the inlet swirl, resulting in the larger separation bubble on the suction surface of the lobe.Anyway, when the inlet swirl angle is less than 20°, the inlet swirl enhances the jet mixing of lobed mixer, and the penelty of total pressure loss or thrust loss is acceptable. For the low inlet swirl, the scalloping cut can increase the aerodynamic performance of the lobed mixer; for the high inlet swirl, the lobe with a certain degree of metal angle or scarfing the lobe's trough may increase the resistance of lobe mixer to the high inlet swirl.