Stereo PIV and LDA Measurements at the Axial Fan Outlet

Abstract

Turbulent swirl flow in the field of hydraulic machines and systems occupies attention of many researchers in laboratories and industry, as it has significant influence on energy consumption and fan design. Physical understanding of this phenomenon would enlighten many unsolved problems and help developing new turbulent numerical models for CFD analysis. Distribution and mutual influences of the time averaged and fluctuation velocity fields to each other have been investigated in this paper. Test rig is consisted of circular pipe sections with total length 27.74·D, where D=0.4m is pipe inner diameter. Specially designed axial fan, which generates Rankine’s type swirl, is positioned at the test rig inlet following profiled free-bell mouth inlet. Stereo Particle image velocimetry (SPIV) and one-component Laser Doppler Anemometry (LDA) have been conducted in the measuring section x/D=3.35 from the test rig inlet. In this paper is presented vortex core dynamics for five Reynolds numbers defined with five equally distributed fan rotation numbers in the interval n=500 till 2500rpm. Criterion of minimum total velocity has been employed. Percentage of the unique points remain almost constant for all five regimes. Distribution of the statistical moments of higher order, determined on the basis of LDA measurements, gives better understanding of the turbulent swirl velocity field. Visualization with Nd:Yag laser, max 30fps camera and paraffin oil has been performed in the cross section x/D=26.31 for better understanding of the vortex core dynamics.