EXPERIMENTAL AND NUMERICAL INVESTIGATIONS ON PASSIVE DEVICES FOR TIP-CLEARANCE INDUCED NOISE REDUCTION IN AXIAL FLOW FANS
A. Corsini – B. Perugini – F. Rispoli
Department of Mechanics and Aeronautics, University of Rome “La Sapienza”, Rome, Italy
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A. G. Sheard – I. R. Kinghorn
Fläkt Woods Ltd, Colchester, Essex, UK
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1
Abstract
This paper presents experimental and computational investigations on a family of axial fans, in fully-ducted configuration, to investigate an improved blade tip concept designed to reduce the tip-clearance induced noise. The nature of the flow mechanisms in the fan tip region is correlated to the specific blade design features that promote a marked reduction of the fan aero-acoustic signature in both tonal and broad-band noise components. The tip vortical flow structures are characterised, and their role in creation of overall stage acoustic emissions clarified. It was found that the tip geometrical modification markedly affects the multiple vortex behaviour of leakage flow, by altering the near-wall fluid flow paths on both blade surfaces. The rotor loss behaviour, in the blade tip region, was also discussed in order to assess the effect of blade tip passive device onto the rotor efficiency.
NOMENCLATURE
Hn normalized helicity,
blade chord
l.e. leading edge
PS pressure side
p static pressure
r radius
SS suction side
t pitch
t.e. trailing edge
Uc casing relative peripheral velocity
v, w absolute and relative velocities
x, y, z Cartesian coordinates
Greek letters
bLV leakage flow skew angle with respect to blade tip chord
z total loss coefficient,
hdyn efficiency based on dynamic pressure rise
hstat efficiency based on static pressure rise
n hub-to-casing diameter ratio
xi absolute vorticity vector
xs absolute streamwise vorticity,
F global flow coefficient (annulus area-averaged axial velocity normalised by Uc)
c rotor tip clearance
Y pressure rise coefficient (Dp/(r 0.5 ))
w rotor angular velocity
Subscripts and superscripts
a, p, r axial, peripheral and radial
c casing wall
i Cartesian component index
in inlet section
s streamwise component
LV leakage flow
INTRODUCTION
Often in axial flow fans the design specifications demand large tip gap according to the requirements of operating with variable stagger or pitch angles, or in some cases for hot gas handling in emergency operation. As well known, the tip clearance plays a detrimental role affecting the rotor aero-dynamics (Fukano and Takamatsu, 1986) (Storer and Cumpsty, 1991) (Furukawa et al., 1999), significantly contributing to the aero-acoustic signature of impeller. To this end the tip clearance flow is recognized to influence the rotor noise spectra by discrete frequency noise, due to periodic velocity fluctuations, and a broadband or high-frequency noise, due to velocity fluctuations in the blade passage (Fukano and Jang, 2004) (Jang et al., 2003) (Quinlan and Bent, 1998). Thus there is a strong motivation to look for methodical aerodynamic design in order to reduce the negative effects of tip gap, manage the fan or compressor tip clearance flow and minimize its impact on performance.
By surveying the solutions for noise control in fans and compressors, it was found that they include active (Bae et al., 2005) (Roy et al., 2005) and passive noise control techniques, conceptually designed to accomplish this goal by reducing the leakage flow rate or by enhancing the primary-secondary flow momentum transfer.
As far as the passive control techniques are concerned, the literature review puts in evidence the role of three approaches respectively focused on three-dimensional blade design (Wadia et al., 1998) (Corsini and Rispoli, 2004), and on geometrical modifications of the equipments in the gap region, e.g on the casing wall (Takata and Tsukuda, 1977) (Smith and Cumpsty, 1984) (Thomson et al., 1998). During the last decade several contributions have appeared proposing passive noise control concepts based on the blade tip modifications by means of anti-vortex appendices. To mention but a few, the end-plates investigated by Quinlan and Bent (1998) or Corsini and co-workers (2006a), and the solutions recently proposed by industrial patents for ventilating fans (Jensen, 1986) (Longet, 2003) (Mimura, 2003) (Uselton et al., 2005).
In this ambit the present paper aims to investigate on the use of profiled end-plates at the blade tip (Corsini et al., 2006a) (Corsini et al, 2006b), and it reports on the experimental and numerical assessment of the pay-off derived from the adoption of improved blade tip concepts.
The study focuses on a family of commercially available fans and compares the aerodynamic and aeroacoustic performance of the datum blade against two improved tip geometries, respectively with constant or variable thickness end-plates. The comparative aerodynamic performance and noise experiments have been carried out at Fläkt Woods Ltd axial fan test rig and anechoic chamber. The tip flow characteristics are analysed by using a three-dimensional (3D) Reynolds-Averaged Navier-Stokes (RANS) solver. The authors adopt a parallel multi-grid (MG) scheme developed for the in-house finite element method (FEM) code (Borello et al., 2003). The FEM formulation is based on a highly accurate stabilized Petrov-Galerkin (PG) scheme specially designed for turbomachinery CFD (Corsini et al., 2005). By means of such a numerical investigation, the tip leakage flow structures of the fans are analysed in terms of vortical structure detection, and rotor loss behaviour. The assessment of the benefits related to the improved tip geometries in terms of efficiency and operating margin gains is emphasized. The overall objective is to investigate, via steady computational simulations, the technical merits of a passive control strategy for controlling the leakage flow and reducing tip clearance vortex/stator interaction noise and rotor-tip self noise.
test axial flow fans
The present study was performed on a family of commercially available highly efficient cooling fans. The in-service experiences indicated that this family of fans gives good acoustic performance with respect to the state-of-the-art configurations. The investigated fans have six-blade unswept rotor, with blade profiles of modified ARA-D geometry type originally designed for propeller applications. The blade profiles geometry is given in Table 1, for the datum fan AC90/6 at the hub, and tip sections respectively.
Table 1 AC90/6 fan family specifications. Blade profile geometry and rotor specifications.
AC90/6 fansblade geometry / hub / tip
/ t / 1.32 / 0.31
pitch angle (deg) / 36 / 28
camber angle (deg) / 46 / 41
fan rotor
blade number / 6
blade tip pitch angle (deg) / 28
hub-to-casing diameter ratio n / 0.22
tip diameter (mm) / 900.0
rotor tip clearance t (% span) / 1.0
rated rotational frequency (rpm) / 900 – 935
The studied blade configurations, for datum and modified rotors, feature a high tip pitch angle, i.e. 28 degrees, measured, as is customary in industrial fan practice, from the peripheral direction. This rotor angular setting has been chosen in order to exploit operating points where the vortical flow near the rotor tip dramatically affects the aerodynamic performance and noise characteristics of the fans under investigation.
Test fan tip end-plates
The fan blades are drawn in Figure 1, together with a detailed view of blade tip for the datum rotor, and the improved rotors developed for low noise emission labeled: AC90/6/TF and AC90/6/TFvte. Fig. 1 compares, in a qualitative view not to scale, the thickness distributions of the developed improved tip concepts against the datum base-line.
The improved blade tip geometry, for AC90/6/TF fans, was originally inspired by the technique developed for tip vortex control and induced drag reduction in aircraft wings, also used as anti-vortex devices for catamaran hulls.
Fig. 1 Test fan rotor blades and tip end-plates (not to scale)
The tip blade section was modified by adding an end-plate along the blade pressure surface, with a square tail trailing edge. By means of the introduction of the end-plate, the blade section is locally thickened of a factor 3:1 with respect to the datum blade maximum thickness at the tip. According to the theory behind the end-plate design, this dimension was chosen as the reference radial dimension of leakage vortex to be controlled that could be estimated to be in the range of 0.2 ¸ 0.1 of the blade span, as shown by former studies on rotors of axial compressor (Inoue et al., 1986) and fan (Corsini and Rispoli, 2004). A recent investigation, carried out by Corsini and co-workers (2006a), assessed the aerodynamic and aeroacoustic gains of rotor AC90/6/TF with respect to the datum one. The numerical experiments gave the evidences of a tip leakage vortex breakdown that affect the rotor AC90/6/TF in design operation. In order to correct this negative feature, responsible for a loss in fan efficiency, Corsini (2006) recently proposed a blade tip geometry, here labeled AC90/6/TFvte, that exploits a variable thickness distribution of the end-plate according to safe rotation number chord-wise gradient concept.
investigation techniques
The single rotor investigations are carried out at design operating condition for three configurations of the studied six-blade axial flow fan, namely: the datum fan, coded AC90/6, and the two fans modified by the adoption of tip features, respectively coded AC90/6/TF and AC90/6/TFvte.
The experimental and numerical studies have been carried out in ducted configuration, adopting a high tip pitch angle configuration, i.e. 28 degrees, where the fan provides the higher static pressure and flow rate of its operational range. The investigations compares datum, AC90/6/TF and AC90/6/TFvte fan rotors operated in near-design condition (D) with volume flow rate 7 m3/s and global flow coefficient F = 0.278. The Reynolds number based on tip diameter and rotor tip speed is 8.3´ 105, for normal air condition.
Experimental procedure
The aerodynamic and noise performance tests were carried out at Fläkt Woods Ltd laboratory in Colchester.
The aerodynamic tests were conducted according to the ISO 5801:1997 (1997), set up for fully ducted configuration and installation type D. This installation features ducted inlet and outlet regions and the fan is supplied with a properly-shaped inlet bell mouth. The noise performance test have been carried out in the anechoic chamber according to the British Standard BS 848-2.6:2000 (2000), employing a type A testing configuration. In this method the fan is placed downstream of a plenum chamber with a free outlet, in an arrangement similar to that used for compact cooling fans.
Numerical procedure
The Reynolds-Averaged Navier-Stokes equations are solved by an original parallel Multi-Grid Finite Element flow solver (Borello et al., 2003). Despite the steady-state condition, the RANS is considered an effective investigation tool for vortical structure detection (Inoue and Furukawa, 2002). The physics involved in the fluid dynamics of incompressible 3D turbulent flows in rotating frame of reference, was modelled with a non-linear k-e model (Craft et al., 1996), here used in its topology-free low-Reynolds variant. This turbulence closure has been successfully validated on transitional compressor cascade flows, as well as high-pressure industrial fan rotors (Corsini and Rispoli, 2005). The numerical integration of PDEs is based on a consistent stabilised Petrov-Galerkin formulation developed and applied to control the instability origins that affect the advective-diffusive incompressible flow limits, and the reaction of momentum and turbulent scale determining equations. The latter ones, respectively, are related to the Coriolis acceleration or to the dissipation/destruction terms in the turbulent scale determining equations (Corsini et al., 2005). Equal-order linear interpolation spaces (Q1-Q1) are used for primary-turbulent and constrained variables. This finite element space pair prevents from the undesirable pressure-checkerboarding effects and provides a second-order accurate numerical scheme.
Axial fan modeling and boundary conditions
The mesh has been built according to a non-orthogonal body fitted coordinate system, by merging two structured H-type grid systems, respectively the mesh in the main flow region, surrounding the blade, and an embedded mesh in the tip gap region. The mesh has 154´68´58 nodes, respectively in the axial, pitch, and span wise directions. In the axial direction the node distribution consists of 20%, 50% and 30% of nodes respectively upstream of the leading edge, in the blade passage and downstream of it. Moreover, there are 14 grid nodes to model the tip-clearance along the span. The computational grid is illustrated, in Figure 2, providing a detailed view at the tip of the mesh in meridional and blade-to-blade surfaces.
The mesh has an adequate cluster toward solid boundaries, with the ratio of minimum grid spacing on solid walls to mid-span blade chord set as 2´10-3 on the blade tip, casing wall, and blade surfaces. The adopted grid refinement towards the solid surfaces controls the normalized wall distance d+ value about 1 on the first nodes row.
Fig. 2 Computational grid of fan rotor, mesh details in the tip gap region
A standard boundary condition set has been adopted, already used in recent numerical studies on high performance fans (Corsini and Rispoli, 2004) (Corsini et al., 2004). The Dirichlet conditions for the relative velocity components are imposed at the inflow section, half a mid-span chord far upstream of the leading edge, as obtained from a flow simulation in an annular passage of identical hub-to-casing diameter ratio that includes an upstream spinner cone. The inlet distribution of the turbulent quantities are obtained from inflow turbulence measurements in ducted industrial fans (Corsini et al., 2003). Flow periodicity, upstream and downstream of the blade row, and Neumann outflow conditions complete the set of boundary data.
PERFORMANCE EXPERIMENTS
Aerodynamic tests
The primary measured performance parameters were the fan static pressure and the efficiency. Measurements of static and dynamic pressure were carried out with four taps, equally spaced on the casing wall, and a standard Pitot-probe. The probe was mounted on traverse mechanism fixed to the outer wall of the test rig. Furness Control digital multi-channel micro-manometer (Model FC012, Furness Controls Ltd, UK) with 2 kPa range and a resolution of 1 Pa, was used to read pressure data. The pressure measurements accuracy is ± 0.5% of read data. The efficiency was calculated as the ratio between the air power, either computed on static or dynamic pressure rise, and the electric power. The absorbed electric power was measured with an AC Power analyser having an accuracy of 0.24% of read data.