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 . 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*
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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 . 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
´ J.M. Fernandez Oro et al. / Experimental Thermal and Fluid Science...