The Control Of Turbulent Jet Based On The Pulsed Mini-jets

Turbulent round jet is widely applied in industry,such as power plants,aviation and metallurgy.Therefore,understanding and controlling jet mixing are fundamental to noise reduction,high efficient combustion and heat transfer enhancement.This paper studied turbulent jet mixing characteristics under the control of pulsed mini-jets.The disturbance of the round turbulent jet was achieved by deploying the radial mini-jets around the jet nozzle.The effect of this disturbance on the jet flow characteristics was explored,along with the mechanism of the jet mixing.This work reported an experimental investigation on the active control of a turbulent round jet using one and two unsteady radial mini-jets.The mini-jets were located at 17 mm upstream from the exit.The Reynolds number were 8 000 and 9 333,respectively.Four excitation parameters were examined,i.e.,the mass flow ratio Cm of the mini-jets to that of the main jet(0 ~ 20%),the excitation frequency fe(0 ~ 200Hz),the duty cycle(5% ~ 100%),and the separation angle between two mini-jets(60°,120° and 180°).The measurements of the velocity were performed with a hot-wire anemometer,and detailed flow visualization measurements were conducted in the injection plane,the non-injection plane and a number of cross-sectional planes.To obtain the optimal excitation parameters of the jet mixing,a number of experiments were carried out by changing Cm,fe and the duty cycle.The results show that the jet decay rate K depends on both Cm and fe/f0.The variation in K with Cm exhibits a strong dependence on the duty cycle and is both qualitatively and quantitatively very different at small duty cycle(< 15%)from that at large Cm(> 30%).When the duty cycle exceeds 30%,the variation in K with Cm shows only one peak.The formation process of the coherent structures becomes turbulent.At the duty cycle of 15%,this variation exhibits two pronounced peaks(fe/f0 = 1.0),one at Cm = 0.19%,and the other at 2.31%.The formation process of the coherent structures,albeit greatly enhanced,remains to be laminar.The excitation results in the formation of asymmetric jet structure and even the flapping of the jet column.And the jet decay rate K depends on the angle between the two mini-jets.