Investigation On Mixing Enhancement Mechanism Of Jet Flow With Tabbed Nozzle

As one of effective methods of passive control jet flow, tabbed nozzle has been widely used in mixing enhancement, turbine blade coolant film insulation and screech elimination at exit of jet flow. Research on the mechanism of tabs'effect on the jet flow field is highly important for the optimization of tab's configuration, size and number; thereby improve entrainment behavior of the jet fluid with ambient fluid.Flow visualization, two-component hot wire anemometry and numerical simulation are combined to investigate the influence of the size and number of tabs on jet flow field and vortex structure generation. Using all these research methods, more information of the flow field has been obtained and further investigations of mechanism of tab's influence on the jet flow have been carried out.Main research work in this paper includes the followings:1. The flow field quality of the wind tunnel has been improved by mounting more damped screens, honeycomb and flow deflectors to eliminate the influence of fan blade. Design and manufacture various nozzle mounted tabs with different size and number. It includes 2, 4, 6 and 8-big-size-tabbed nozzle. And also design 4, 8, 16 and 32-small-size-tabbed-nozzle in order to study the effect of the size and number of tabs on jet flow control.2. Flow visualizations at different cross sections with different tabbed nozzle are carried out using artificial seeding smoke combined with laser sheet illumination. Longitudinal vortex generated by the tabs with different size and different number has been observed carefully from the flow visualization pictures. No.①longitudinal vortex pair which has been found in previous work is reconfirmed here. Furthermore it has been found that a series of reversed longitudinal vortexes No.②is induced by No.①. And there exists a pair of longitudinal vortexes No.③at the apex of the tab. Vortex No.③gets stronger as tabs get more.3. Instantaneous two-component velocity signals at different cross sections with different tabbed jet nozzles are finely measured using hot-wire anemometer. Longitudinal and transverse velocity components at different locations of jet flow are obtained from the output signals of hot-wire anemometer. Mean flow field parameters of tabbed jet flow such as mean velocity, turbulence intensity, azimuthal vortex and flow flux are analyzed and the effects of tabs with different configurations, sizes and numbers have been compared with circular jet flow. 4. k-εturbulence model based on Renormalization Group theory is introduced in the numerical computation study on the mean flow fields of circular jet and tabbed jets. More abundant information of the flow field has been achieved to be compared with experimental results; numerical results reexhibit the tridimensional circular and tabbed jet flow fields. Numerical computational studies combined with flow visualizations and hot-wire measurements, give a quantitative insight of the effect of longitudinal vortexes in various tabbed jet flows. Together with hot wire measurement results, azimuthal vortex fields of various jet flows have been investigated. It has been found that there exists reversed azimuthal vortex at the upstream of the azimuthal vortex. And as more tabs mounted to the nozzle, there will be the phenomenon of"double-row azimuthal vortex". Investigations on mechanism of mixing enhancement of tab are carried out from the view of longitudinal vortex and azimuthal vortex generation.