On the structure of turbulence in a low-speed axial fan with inlet guide vanes

Abstract

This paper analyzes the structure of turbulence in a single stage, low-speed axial fan with inlet guide vanes. Turbulence intensity values and integral length scales have been obtained using hot-wire anemometry for three different operating points and two different axial gaps between the stator and the rotor. These measurements were carried out in two transversal sectors, one between the rows and the other rotor downstream, covering the whole span of the stage for a complete stator pitch. Since total unsteadiness is composed of the contribution of both periodic and random unsteadiness, a processing data method was developed to filter deterministic unsteadiness in the raw velocity traces. Velocity signals were transformed into the frequency domain by removing all the contributions coming from the rotational frequency, the blade passing frequency and its harmonics. Consequently, coherent flow structures were decoupled and thus background levels of turbulence-RMS values of random fluctuations-were determined across the stage. Additionally, this unsteady segregation revealed further information about the transport of the turbulent structures in the unsteady, deterministic flow patterns. Therefore, anisotropic turbulence, generated at the shear layers of the wakes, could be identified as the major mechanism of turbulence generation, rather than free-stream, nearly isotropic turbulence of wake-unaffected regions. Finally, spectra and autocorrelation analysis of random fluctuations were also used to estimate integral length scales-larger eddy sizes-of turbulence, providing insight on the complete picture of the turbulent flow.

Non-deterministic kinetic energy within the rotor wakes and boundary layers of low-speed axial fans: frequency-based decomposition of unforced unsteadiness and turbulence

“…In these studies, complemented also with numerical simulations [33][34][35], a multi-point traversing system was employed to obtain detailed distributions of the velocity field in several windows covering the passage span for a complete stator pitch. The data set obtained from this facility has been used to investigate many different flow phenomena including wake dispersion and wake transport [35][36], mean flow, turbulent structures and integral length scales [36][37], deterministic stresses [38] and secondary flows and tip vortex in similar fans [39][40]. At present day, as an alternative to the costly experimental campaigns, computational modelling has become an extended, well-mature discipline, especially useful in the design process.…”

Experimental investigation on the effect of load distribution on the performances of a counter-rotating axial-flow fan

“…It would be very interesting to better characterize these interactions with phase-locked time-resolved measurements such as described in Refs. [12,13]. The effect of the number of blades in each rotor may also be addressed in forthcoming experiments, with JW1 rotors of same blade cascade parameters but different blade numbers, and focusing on acoustical behaviour of the counter-rotating stages.…”

“…Strong unsteady interactions between the rotors may lead to large acoustic noise emission [11] and may affect the performances of the stage [12,13]. When designing a counter-rotating stage, some parameters such as the distribution of the total work between the two rotors or the ratio of the rotation rates have to be chosen, this choice being most often somehow arbitrary.…”

A Novel Method for Developing Compressor’s Characteristic Curves Due to the Guide Vane Stagger Angle Variation

“…In [6] a Lagrangian method was used to find the flow characteristics of an airfoil. Many of today's compressors have guide vanes with variable stagger angles [7][8][9]. Variations in stagger angle of guide vane including geometrical variations of the stage would be resulted in variation of the stage's characteristic curves.…”