Experimental Study On Aerodynamic Characteristics Of Tornado-like Vortex

Tornado-like vortex, similar to tornado, is a specific strongswirling flow, which has high swirling intensity, internal vacuum degree, radial energy separation, and velocity gradient on the boundary between swirling flow and environment. In order to understand aerodynamic characteristics of tornado-like vortex and explore generation conditions deeply, the flow visualization and parameters measurement were presented. The cold flow field characteristic and sensitive analysis were studied, which provided important basis for researching the interaction of combustion and tornado-like vortex.Based on Ranque-Hilsch principle, a series of vortex generators were designed, which have different swirling chambers, nozzles and shapes of tube. Laser sheet combined with tracer particle is adopted to realize flow visualization. Longitudinal and transverse section images of flow field demonstrate axial flow evolution and radial structure. Pressure and temperature were measured by pressure probe and thermocouple, and 3-D velocity was obtained by using a 3-D phase Doppler anemometer(PDA). The results are as followings:(1) According to the measurement results of swirling, chambers’ vacuum degree(namely, primary vacuum degree, PVD), tangential inlet and Archimedean spiral nozzles are beneficial to enhance PVD, which results inproducestronger swirl.(2) Analysis of velocity field and flow visualization showthat the swirl flow field can be divided into three areas: initial-field, strong-swirl-field, transition-field and far-field. There is high vacuum degree and temperature difference in the center of near-field and main-field, which means that Ranque-Hilsch effect has happened. The central vaccum is positive feedback from strong swirl. Two inverse effects—vacuum abstraction and centrifugal expansion—both derive from intensive swirling, leading to minor jet angle and superior aerodynamic leakproofness than normal swirling-jet. The vacuum and axial velocity distributions are both “M” type traverse distribution. It was studied that the centrifugal force caused by rotation and high axial velocity both lead to high vacuum in the center.There is a great velocity gradient in the outer space of the axial velocity and the tangential velocity profiles. The streamline of atmosphere in the visualization image turn around suddenly in the boundary of swirl field. In addition, the small scale entrainment eddies and clear boundaries of swirl also show that interaction between swirling flow and environment is very small. However, the difference between near-field and main-field is also significant. There is a bullet shaped center recirculation zone(CRZ) in the near-field, thus velocity varies dramatically in the radial direction. The velocity profiles are irregular in near-field, while axial velocity in main-field has self-similarity. Furthermore, the swirl divergence angle in near-field is obviously large than ones in main-field.(3) Flow visualization images show that the large scale eddies is formed and Shedding in the boundary of far-field. Other phenomena, such as vacuum degree, temperature drop, and tangential velocity tending to zero, flat distribution of axial velocity and non-expansion in the boundary, suggest that the swirling flow gradually dissipated by the resistance of the environmental impact. Turbulent fluctuation near peak velocity in near-field and main-field is intense, while turbulence in far-field can be considered to be isotropic.(4) The research results indicate that high inlet pressure leads to a greater rotation intensity and vacuum degree, better energy separation and flow rigidity and weaker interaction between swirl and environment. Although research has showed tube length have much influence in energy separation, the optimum value hadn’t be found in this work, which will be studied in the future.