LDA Experimental Research of Turbulent Swirling Flow Behind the Axial Fans in Pipe, Jet and Diffuser

Abstract

In this paper is analyzed turbulent swirling flow generated by axial fans. Three cases will be presented. The first one is the axial fan impeller in-built in the pipe 27.74D long, where D = 0.4 m is pipe inner diameter. The second case is study of the turbulent swirling flow in the free jet generated by the axial fan impeller with casing. The third case is conical diffuser with the inlet diameter D = 0.4 m and total divergence angle of 8.6°, studied in the test chamber followed by the flow meter. Axial fan, even in the case of the same impeller geometry, had various duty points in all these installations. All regimes were adjusted by the continuous fan rotation speed regulation, as well as with the impeller blade angles. Only in the case of the installation with diffuser, additional regulating valve and booster fan were sometimes applied. In all these cases was used laser Doppler anemometry (LDA), one- and three-component systems. All generated flows are extremely turbulent and in the case of the pipe and diffuser occur reverse flows in the central flow region. In the paper will be discussed calculated integral values, such as average circulation or swirl number. Time-averaged circumferential velocity profiles revealed Rankine vortex type in pipe, while in the diffuser is generated, with the different type of impeller, solid body profile. Turbulence levels and their distributions are analyzed. In addition, statistical moments of the higher order, like skewness and flatness factors are discussed.