Observations from LDA Research of the Turbulent Swirl Flow on the Axial Fan Pressure Side

Abstract

Some aspects of the laser Doppler anemometry (LDA) investigation of the turbulent swirl flow behind axial fans in pipes, diffusers and jets are presented in this paper. All three installations have profiled bell mouth inlet, while the fans are built-in in different ways. Namely, in the first case axial fan is built-in into the pipe approximately 27.74D long, where D is average inner diameter D = 0.4 m. In the second case axial fan is builtin into the straight conical diffuser with the inlet diameter 0.4 m and total divergence angle 8.6⁰, while in the third case turbulent swirl jet flow behind the axial was observed. In all cases fan impeller rotation number is varied. One-, as well as, two- and thee-component LDA systems were applied. Thermal fog seeding was applied in all cases. Time averaged velocity profiles are analysed, as well as the statistical moments of the second till the fourth order. In addition, distributions of the Reynolds stresses are also presented and discussed. Investigations reveal structures, as well as complexity of the three-dimensional, non-homogenous, anisotropic turbulent swirl flows. This stochastic character on the instantaneous level was shown for all three cases. Kolmogorov “-5/3 law” is shown in the case of the circumferential velocity distribution for the turbulent swirl flow in the pipe for various measuring points. Phenomenon of the non-local turbulent transfer and nongradient turbulent diffusion is studied. The influence of the axial fan duty point on the generated turbulence structure is studied.