| The bladeless fan introduces a new blast mechanism in the fan field------annular slot jet.Due to its property of ?air multiplying?(the discharge air volume greater than the inlet volume),the importance of turbulence study,the study lack of the free gas annular slot jet with a nozzle having no blunt body and its good application in the future,the study of the outlet annular slot jet of the bladeless fan has high academic and application value.In the study,the two kinds of experiments with the hot-wire anemometer and ventilation multi-parameters detecter were conducted,along with the numerical simulation by RNG 6)- model and the turbulence basic theoery.Under the five Reynolds numbers,the paper investigated the turbulence field of the outlet annular slot jet of the bladeless fan comparatively allsidely and systematically.Through the profiles of the time-averaged flow and turbulent flow(including the small-scale turbulence)in the cross-sections and along the axis,the paper obtained the the flow property,flow field structure,turbulence statues and complete jet development for this annular slot jet.On this basis,the paper studied deeply the essential mechanism of the air multiplying------the movement of the coherent structures and its evolvement,and investigated its specific roles on each developing stage of the annular slot jet as well as the turbulence intermittency property,which is the aim of the paper.In addition,the paper developed a new off-centering slot structure to passively control this annular jet,and analyzed the influence of Reynolds number on the flow field.In the study,the phenomena and conclusions have been found as following.1.The study of the flow property and movement of coherent structure.The annular slot jet through the Coanda surface with high Reynolds number of the bladeless fan,forms the free shear layer with high turbulence intensity.Due to the coherent movement,the annular slot jet entrains the air in two aspects: one is to spanwise entrain the air front of the air-out frame along the way,the other is to forwardly entrain the air back of the air-outframe,which together enable the bladeless fan to air multiply.In the simulation,the air discharge volume is 26.6 times of the inlet air volume at 3m ahead the air-out frame.The front entrained air is 2.41 times of the back entrained air.The back air entrained forward enters the interior of the annular jet in the near field,then the coherent movement in this area produces an important influence on the whole flow field.Along the flow direction,the spanwise horseshoe-shape vortex entrains the environmental air,whose scale and the lower break all become larger and larger,and the shape is axial asymmetric in the vertical direction and basically axial symmetric in the horizontal direction.Hence,the time-averaged velocity field,fluctuation velocity field and pressure field all presents axial symmetry in vertical directions and basic axial symmetry in horizontal directions in the outlet flow field of the bladeless fan.The entrainment on the environmental gas increases with Reynolds number increasing,while the entrainment within the interior of the annular jet in the near field decreases with Reynolds number increasing.2.The study of the flow field structure and the small-scale turbulence.Within the interior of the annular slot jet in the near field,the forward entrainment of the annular slot jet on the back air decay gradually along the flow direction.About at the 0.5d(d is the throat diameter of the Coanda surface and based on the front end face of the out-wind frame)position at the flow direction,the spanwise horseshoe-shape vortex starts to entrain the basiscopic fluid.Along the flow direction,this kind of upward entrainment becomes stronger and stronger.About to the 1.5d position in the flow direction,the acroscopic large-scale coherent structures enable the annular slot jet to be combined upwards,and the basiscopic fluid is drawn to the acroscopic flow field.Simultaneously,due to the jet combination,the coherent movement start to enable the shear layer to produce very strong instability along the flow direction,hence,the small-scale turbulence is excited most fully at the position of 2.2d.After 2.2d position,the turbulence flow field decays slowly due to the decay of the coherent structures.About the 3d position at the flow direction,the annular slot jet finishes the combined process.The time-averaged velocity in the whole flow field and the turbulence intensity around 0.7-2.2d all increase with Reynolds number increasing.The small-scale turbulence flow parameters such as the turbulent kinetic energy dissipation rate and Kolmogorov scale ? etc.enter the self-preservation state earlier than time-averaged flow parameter,which proves that the small-scale turbulence is more sensitive to the flow field variation.3.The study of the combined single jet.There exists an essential difference between the combined jet of the annular slot jet and the classic round jet,and the combined jet have no axial symmetry about the jet expansion.Reynolds number has a little effect on the verticle jet expansion,while having positive effect on the horizontal jet expansion.High Reynolds number can enhance the energy content of the large vortexes of the combined jet,promote the combined flow field to develop fully,and also speed up the decay along the flow field.4.The study of the passive control of the annular jet.At present,there still exist three issues of the improvement of non-equilibrium state in the perpendicular flow field,ventilation rate augmentation and noise reduction for the bladeless fan.The paper proposes three new off-centering slot structures of S0.9,S0.75 and S0.6 type,which can improve the perpendicular equilibrium better in the way of passive jet control,and augment the ventilation rate to a certain degree,which is proved by the simulation with RNG k-e model verified through experiment.The new structure of S0.6 type control the middle and far jet field most ideally.The essential reason lies in suppressing the acroscopic coherent movement and reinforcing the basiscopic coherent movement in the near field;the annular interior region in the near field is an important region for the annular slot jet control. |
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