Flujo ventilador axial

Experimental Thermal and Fluid Science 32 (2007) 316–331 www.elsevier.com/locate/etfs

On the structure of turbulence in a low-speed axial fan with inlet guide vanes
´ ´ ´ ´ Jesus Manuel Fernandez Oro *, Katia Marıa Arguelles Dıaz, Carlos Santolaria Morros, ¨ Eduardo Blanco Marigorta
´ ´ ´ Universidad de Oviedo, Area de Mecanica de Fluidos, Edificio Departamental Zona Este, Campus de Viesques,33271 Gijon (Asturias), Spain Received 10 November 2006; received in revised form 20 April 2007; accepted 20 April 2007

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 differentaxial 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 unsteadinessin 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 unsteadysegregation 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 autocorrelationanalysis 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. Ó 2007 Elsevier Inc. All rights reserved.
Keywords: Turbulence; Low-speed axial fan; Stator–rotor; Unsteadiness; Integral length scale; Hot-wire anemometry

1. Introduction Total unsteadiness in a multistageenvironment is a key parameter in the performance of any axial turbomachine. In case of axial compressors, much effort has been focused on understanding the boundary layer transition from laminar to turbulent on blade surfaces [1]. The vortical disturbances that are created by wakes convected from blade rows further upstream may lead to boundary layer transition. This periodic impinging of incoming wakesonto the blades is a well-known ‘‘wake-induced’’ transition. In addi*

Corresponding author. Tel.: +34 985 182102; fax: +34 985 182098. ´ E-mail address: jesusfo@uniovi.es (J.M. Fernandez Oro).

tion, a high level of turbulence, rather than those periodic disturbances on the blade surfaces, can also be responsible for the turbulent shear layer to be set on [2]. Therefore, in order to achieve agood description for both mechanisms, it is necessary to segregate its relative influence on the development of the unsteady boundary layers on the blades. The flow field inside a turbomachine is characterized by its complex unsteadiness. When this unsteadiness is considered as a whole unique fluctuation, this total variation leads to the establishment of the classical Reynolds stresses into the meanflow. However, the total unsteadiness can also be considered as the contribution of both periodic and random components. The periodic fluctuation, usually known as ‘‘unsteadiness’’, consists of all nonuniformities

0894-1777/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2007.04.008

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