Passive Control Of The Shear Layer Of A Jet And The Related Premixed Flames

Premixed combustion is of high efficiency and it provides the convenience for controlling the maximum temperature of flames and then effectively suppressing NO_X. Thus it is widely used in advanced combustors. Most of the flames in practical combustors are established in flow fields of the structure of the shear layer of jets. The shear layer of jets always contains large-scale coherent structures, which induce low-frequency velocity fluctuation. On one hand;this can augment flame wrinkles therefore enhance combustion, while on the other hand, this induces low-frequency, large-amplitude vibration and strong stretch of flame fronts and may easily lead to unstable and incomplete combustion. Considering the mechanism of turbulent premixed combustion, to increase the components of high-frequency, small-scale fluctuation in the shear layer will benefit the enhancement and at the same stabilization of the flames. The passive control method, which influences the shear layer and improve combustion by the homogeneous turbulence generated by a grid upstream from the nozzle, is feasible in practice. New premixed combustion techniques may be derived from it. In the present study, the influence of grid turbulence on the shear layer of jets and the relative flame structure was investigated by both theoretical analysis and experiments. The purpose is to deepen the understanding on the behind mechanisms and support the improvement of combustor design by providing concepts and foundational data.First of all, control of the shear layer of jets by grid turbulence was studied for isothermal flows. The result shows that grid turbulence evidently reduced turbulence intensity and scales in the shear layer, suppressed low-frequency velocity fluctuation, and increased the distribution of turbulent kinetic energy on small-scale fluctuation hence made the turbulence tend to be more homogeneous. This means that grid turbulence can effectively suppress the large-scale vortex and coherent structure in the shear layer. In theexperiments, it was also found that the ring stabilizer, which was also located upstream from the nozzle, also took the effect of reducing turbulence intensity of the shear layer. From the angle of spectrum and instability analysis, the mechanism of grid turbulence and the ring stabilizer modifying the structure of shear layer was investigated. It was found that the ring stabilizer applied periodical disturbance to the shear layer via vortex shedding, and hence changed the frequency distribution of instability in the shear layer and made the flow field to be more stable. The high-frequency, small-scale disturbance of grid turbulence may overwhelm the instability of a wide range of scales in the initial period of the shear layer, and subsequently suppress vortex pairing and influence the downstream development of coherent structures. At the same time, vortex shedding from the ring stabilizer can also be inhibited by grid turbulence.Then, on the basis of understanding the isothermal flow field, a conical premixed flame of high Reynolds number, which was dominated by the shear layer, was established. The influences of grid turbulence and the induced modification of the shear layer on flame structure were investigated. The results of analysis and experiments indicate that the influence of grid turbulence on flame structure lies on two aspects: on one hand, grid turbulence suppresses the low-frequency movement of flame wrinkles via changing the local flow pattern of the shear layer of jets;on the other hand, grid turbulence generates flame wrinkles and hence enhances combustion via applying disturbance on flame roots. This means that grid turbulence can at the same time have the two effects of enhancing and stabilizing the premixed flames in the shear of jets.