Converged statistics for time-resolved measurements in low-speed axial fans using high-frequency response probes

Abstract

Fast-response probes in multistage turbomachinery are typically used to measure unsteady flows and turbulence in a number of traverse locations throughout the machine (rotor–stator inter-regions, inlet and outlet sections, tip clearance gaps…). When used intensively, they provide complete information of time-resolved flow structures, including wake patterns, wake mixing, wake–wake and rotor-wake interactions or turbulence transport in 2D planes and even 3D pictures if the raw signals are post-processed accurately.The segregation between deterministic, unsteady features and turbulent scales is essential to understand the unsteady mechanisms responsible for the energy transfer and requires an accurate selection of the sampling frequencies and the total length of the measured traces to assure a valid statistical reduction. Similar considerations must be made if they are filtered in a frequency basis (for example, filtering low-scale turbulence or extracting only BPF components), employing well-designed periodograms or power spectra with minimum scatter and large periods of time integration.This work presents the effect of number of periods (ensembles), resolution in which the averaged periods are reconstructed and turbulence intensity on the experimental accuracy of ensemble-averaged measurements in low-speed axial fans using fast-response probes. In particular, the statistical analysis is established in terms of convergence (residuals) between time-resolved traces retrieved using different sampling frequencies and number of total samples. The possible effects of three-dimensionality, the measured regions (hub, tip, midspan) or the sensibility to turbulence levels is also explored.A technique to quantify the convergence of the phase-locked averaging (PLA) processes is applied to a low-speed axial fan, with twin configurations of rotor–stator and stator–rotor arrangements. As a starting point, a concise survey of usual practices employed by other authors in the literature for axial fans and compressors is firstly reviewed, in order to identify fundamental parameters and values typically adopted to guarantee convergence. Finally, typical requirements are given as a function of the variable analyzed, the wake pattern to be described or the global disorder of the flow structures inside axial flow fans.