ИСТИНА

Experimental and numerical investigations of a flow in a unseparated conical diffuser with a small tilt angle of the generatrix are carried out. A steady turbulent air flow was delivered at the diffuser inlet through a tube with a diameter ~0.3 of the diffuser inlet diameter. The velocity and turbulent stresses were measured at the outlet of the diffuser, at 600 < Reout < 3000. A strong influence of the channel expansion on the flow pattern was discovered. In contrast to a tube, where for Re < 2000 only laminar flow is observed as steady, at the outlet of the diffuser a turbulent flow is formed for Reout > ~1000. The formation of a turbulent flow is confirmed by both the results of Reynolds stresses measurements and calculations. While decreasing Reout below ~1000, flow becomes laminar and turbulent perturbations disappear. For 600 < Reout < 3000 the velocity and the turbulent stresses measured in the diffuser outlet are well described by the three-parameter model of shear turbulence. The same calculations showed, that the turbulent flow can be considered steady for values Reout > ~1300, this means that its outlet parameters are almost independent of the conditions at the inlet of the diffuser, while these conditions are sufficient for the formation of a turbulent flow.