A Modified Extremum Seeking Closed-loop System For Jet Mixing Enhancement

As one of typical shear flows,jet mixing is widely seen in engineering applications,e.g.in aero and automobile engines,combustion,heat transfer and chemical reactors.Jet mixing control has drawn significant attention in the past few decades from scientists and engineers.The jet mixing control can be passive and active.It would be unpractical for any engineering application to implement frequently noncircular nozzle geometry alteration due to cost and physical constraints.In the active control,the turbulent shear flow control can be significantly more effective if a closed-loop control is applied.A control must interact with turbulent fluctuations in a jet since the random aspects of the fluctuations reduce the effectiveness of an open-loop control.The closed-loop may treat effectively the random-phase problem in turbulence dynamics,and allow jet mixing to be flexibly controlled.A modified extremum seeking closed-loop control system is applied on the jet mixing enhancement.The system introduces a variable perturbation component that may act to adjust adaptively the searching range of the optimal control signal.The actuator is a single pulsed mini-jet injected radially into the main jet prior to its issue.The mini-jet or excitation frequency fe is electromagnetic-valve controlled.The open-loop control measurement indicates that,given the duty cycle and the mass flow rate ratio Cm of the mini-jet to the main jet,the decay rate K of jet centreline mean velocity,which provides a measure for jet mixing,displays a maximum at the optimum frequency ratio fe/f0 = 0.5,where f0 is the preferred-mode frequency of the main jet.Comparison is made between the modified and traditional control systems.While both may find automatically the optimal fe/f0 and hence a maximum K,the modified system achieves a contraction in the fluctuating fe,when the control target is reached,by 70%,and subsequently an increase in K by 12%.Facing the larger disturbance,the new system further exhibits a better robustness than the traditional one due to the new parameter r.