Analysis of the vortex core and turbulence structure behind axial fans in a straight pipe using PIV, LDA and HWA methods

The dissertation investigates the structure of turbulent swirl flow generated by the axial fan impellers. The complexity of three-dimensional, non-homogeneous, anisotropic turbulent velocity fields required complex experimental and theoretical approach, associated with the complex numerical procedures. Mathematical interpretation of the structural analysis of turbulence is presented using the correlation-spectral theory of turbulence. This theoretical consideration provides adequate physical interpretation of complex interactions between the average and fluctuating velocity fields that characterize the processes of turbulent transfer. Analysis of the vortex core and the statistical characteristics of turbulent swirl flow in straight pipe behind axial fans is based on the latest experimental researches. In this sense, modern measurement systems that include classical probes, stereo particle image velocimetry (SPIV), high speed SPIV (TR PIV), laser Doppler anemometry (LDA) – one- and two-component and original hot-wire anemometers (HWA) were all applied. Measurements and measurement principles are discussed along with sophisticated numerical-measurement methods for data acquisition and statistical processing of measured data and together with calibration and error analysis and measurement uncertainty. Numerous experiments were carried out in the modified existing test rig and in the entire newly built experimental test rig. Based on the original measurement results, the PhD thesis examines in detail the influence of the type and operating regime of axial fan on turbulence structure and turbulent transfer mechanism. In particular, the phenomenon of the vortex core precession as well as the phenomenon of non-local turbulent transfer and non-gradient turbulent diffusion is studied. In addition, the effects of Reynolds and swirl number, and the effects of rotation speed and blade angle on these phenomena are investigated. The physical interpretation of experimental data indicates significant structural properties of a turbulent vortex core and a shear layer. Experimental and correlation analysis examines the evolution of statistical characteristics and correlation moments, which is the basis for drawing conclusions about the extreme turbulence non-homogeneity and anisotropy. The measured distributions of turbulent stresses enabled the formation of anisotropy invariant maps for various fan blade angles, so the important conclusions about the influence of fan duty