Study Of Active Jet Control Using Unsteady Minijets

With the rapid development of China‘s economy,energy demand and environmental pollution become more and more serious.The efficient and clean use of energy has become an inevitable choice.The rapid mixing of fuel and air directly affects the combustion efficiency and pollutants emissions.On the other hand,the development of China‘s civil aviation engine lags behind,which seriously restricts the development of civil aviation industry.Breaking through the key technical bottleneck of aeroengine and breaking the international monopoly will help to improve the scientific and technological innovation ability and the comprehensive national strength.The engine noise reduction technology is one of the key technologies of advanced aeroengine,and the rapid mixing of engine wake and air can effectively reduce the noise of engine wake.Therefore,the jet mixing is one of the basic scientific problem relates to efficient and clean combustion,and engine noise reduction.Study on the jet mixing and its control is of greatly importance to the key technology breakthrough of strategic industries,e.g.energy saving and environmental protection,and high-tech equipment.Unsteady minijets have many advantages in jet control,including small lift loss,a wide range of excitation frequencies and magnitudes.In spite of previous investigations,a systematic study of the unsteady minijets has yet to be performed,especially the experimental investigations is not enough.The interaction between the minijets and main jet and the control mechanism are not well unde rstood.Thus,this work experimentally investigated active manipulation of a round jet using unsteady radial minijets.Six unsteady minijets are azimuthally separated along jet perimeter,prior to the issue of the main jet.The Reynolds number based on jet exit diameter is of 8000.The excitation parameters include the mass flow rate ratio of the minijets to the main jet Cm,the ratio of the minijet excitation frequency to the preferred mode frequency of the main jet fe/f0,the minijet number N and configurations.Flow structures with and without excitation are measured using hot-wire,flow visualization and particle image velocimetry(PIV)techniques in order to gain a thorough understanding of the flow structure evolution and the control mechanism behind the effective jet manipulation.The unsteady minijets excitation could greatly increase the jet centerline decay rate K,which exhibits a strong dependence on fe/f0,Cm,N and minijet configurations.Firstly,the maximum of K occurs,regardless of N or configurations,at fe/f0 = 1,i.e.,the synchronization of the excitation frequency with the preferred mode frequency of the main jet,which enhances the formation and pairing of the large-scale coherent structures.As a result,the jet entrainment is enhanced and thus a rapid jet decay rate is achieved.Secondly,as Cm increases,K varies greatly,and is more sensitive in the lower range.At small Cm,the jet decay rate is associated with the minijet penetration depth.The maximum K is achieved at the deepest penetration.The unsteady minijets act to excite the jet instabilities,enhancing the entrainment by large-scale coherent structures and thus increasing the jet decay rate.At large Cm,the minijet impinges on the opposite wall of the nozzle for N = 1 or the minijets collide with each other for other cases.The generated turbulence is enhanced with increased Cm,resulting in a monotonic rise in K.Finally,the minijet number or configurations have a great effect on the jet decay rate K.The excitation of two minijets separated by 60° and fe/f0 = 1,Cm=2.0% produces the largest maximum K,about 11 times that in the unexcited jet,suggesting the most effective manipulation.Three control mechanisms are identified in this work.Firstly,under the excitation of single minijet or two asymmetrically arranged minijets(separated angle ? = 60° or 120°),the excited jet exhibits inclined ring vortices and stagger-arranged vortex pairing.As a result,the jet flapping motion occurs.The excitation of ? = 60° produces a completed flapping jet,that is,two streamwise fluctuating velocity signals on the opposite sides of the jet exhibit complete ly anti-phased.The jet flapping motion produced by asymmetrical perturbation increases dramatically the spread along the flapping direction,thus resulting in the increased jet decay rate.Secondly,under the excitation of two symmetrically arranged minijets(? = 180°)or N = 3-5,the flow structure of the excited jet is highly three-dimensional,consisting of contorted but enhanced ring vortices,azimuthally fixed counter-rotating steamwise vortex pairs and sequentially ?tossed-out‘ mushroom-like structures.The vigorous interactions among the three types of vortical structures are responsible for the rapid jet decay rate.Finally,under the excitation of six minijets,the shear layer rolls up along the entire jet perimeter,forming ?unsmoothed‘ ring-like structures,much nearer to the nozzle exit than other cases.The rapid growth and three-dimenstional development of ring-like structures accelerates the entrainment and spread along the entire perimeter,leading to the increased jet decay rate.The flow visualization results reveal that jet spread exhibits a significant change,depending on N,showing distinct ?branches‘.Jet spread always occurs along the injection plane for N =1 but along the non-injection half-plane for N = 2~5.The number of ?branches‘ varies with N,i.e.,two branches for N = 1 and 2,three for N = 3,two for N = 4,one for N = 5.Finally,the branch is absent at N = 6.The ?branching‘ phenomenon is related to the number of azimuthally fixed counter-rotating streamwise vortex pairs with ?out-flow‘ sense of rotation.Each pair acts to induce jet fluid between two vortices away from the jet center,producing a large spread and thus a ?branch‘.In summary,this work conducted an experimental investigation on active control of a round jet using unsteady minijets.The effect of fe/f0,Cm,N and minijet configurations on the main jet flow has been measured using hot-wire,flow visualization and PIV techniques.Based on the experimental data,the effect of minijet parameters on the flow structure is examined,three control mechanisms are identified,and the conceptual models of the flow structure under excitations are proposed.As such,some fundamental issues are addressed to improve the unsteady minijets technique,which is benefit for the key technology breakthrough of revelant industries.