Investigation of the flow in free and impinging air micro- and macrojets

Abstract

The aim of this work is an experimental study of the flow in subsonic free and impinging macro- and microjets of air. Complex measurements have been performed using the PIV method (Particle Image Velocimetry), hot-wire anemometry and flow visualization. In the experiments, we used the axisymmetric and flat micronozzles with the characteristic size of 250−600 μm. The measurement performed for the jets of the larger size of 1−8 mm are presented for comparison and generalization of experimental data. Air, whose thermodynamic parameters are close to the atmospheric conditions, was used as the working gas. Flow visualization allowed us to construct the spatial pattern of formation and decay of the laminar part of the jet and transition to turbulence. It was shown that the range of a free jet can equals 100−200 nozzle sizes. The Reynolds number of transition to turbulence in mini- and microjets takes high values (1000−2600), and this is two-three tens times higher than the Reynolds number of stability loss. In the impinging jet, an obstacle can change the coordinate of transition to turbulence, for instance, due to the feedback effect.