Large Eddy Simulation Of Jet Mixing With Pulsed Microjet Controlling

Jets have widely used in different industrial fields,for example,gas fuel mixing processes,micro-jet active cooling control,and aircraft noise control.Understanding jet mixing mechanism and controlling the mixing process is still worthy of further study.In this dissertation,the modification of jet flow structure under the control of different excitation parameters and the spatial development of the corresponding jet coherent structure are studied by simulating the process of pulsed micro-jet flow injecting into the main jet flow.Based on the CFD software Ansys Fluent,the numerical model of the jet with the control of pulsed micro-jet is established.The influence of the inlet condition on the accuracy of the numerical simulation is considered.Comparing with the natural jet's experimental data,the reliability of the numerical model is confirmed.The control parameters of micro-jet,such as mass flow rate,duty cycle ratio.and excitation frequency ratio,are examined.The numerical simulation shows that at the mass flow rate are 0.10%and 0.19%under the condition that the duty cycle ratio of 15%and excitation frequency ratio of 1,the effect on jet mixing is the best.At this excitation condition,changing the jet excitation frequency,the effect on jet mixing is decreased.At the mass flow rate is 0.1%,and changing the duty cycle of the jet,it is found that the best control parameter of mixing effect is duty cycle ratio of 15%,followed by 30%.Through the Q criterion to identify the coherent structure of the flow field,periodic micro-jet flow advances vortex rolling,and influences the production of the streamwise vortex structure on azimuthal location of vortex ring in jet flow near field,the jet flow spreads at the azimuthal location.It is found that there are several different mixing mechanisms under different excitation parameters,and depends on the interaction between the micro-jet produced vortex pairs and the vortex ring in the near field of the jet flow.When the injecting velocity of micro-jet is small,the micro-jet moves above the jet center line,and advancing the jet vortex rings pairing,when the injecting velocity of micro-jet is large,the micro-jet moves cross the jet center line,interacting with the vortex ring on the opposite side of injection.The adjacent vortex rings are partial paired and intertwined with each other,so that the upstream vortex structure continuously expands outwardly,resulting in a rapid spread of the jet and an increase in the centerline velocity decay rate.