The 13th International Conference on Fluid Flow Technologies
Budapest, Hungary, September 6-9, 2006 /
Development of improved blade tip end-plate concepts for low-noise operation in industrial fans
Alessandro CORSINI1, FrancoRISPOLI2, A. Geoff SHEARD31 Corresponding Author. Dipartimento di Meccanica e Aeronautica, University of Rome “La Sapienza”. Via Eudossiana18, I00184
Rome, Italy. Tel.: +390644585231, Fax: +39064881759, E-mail:
2 Dipartimento di Meccanica e Aeronautica, University of Rome “La Sapienza”. E-mail:
3Fläkt Woods Ltd. E-mail:
Abstract
The use of improved blade tip geometries is addressed as an effective design concept for passive noise control in industrial fans. These concepts, based on geometrical modifications of datum blade by means of profiled end-plates at the tip, are shown to reduce fan noise in its tonal and broadband components by changing the tip leakage flow behaviour. The three dimensional structures of tip vortical flow fields are discussed for a family of axial fans in fully-ducted configuration, to investigate on the aerodynamics of the proposed blade tip concepts. The study has been carried-out using an accurate in-house developed parallel finite element RANS solver, with the adoption of non-isotropic two-equation turbulence closure. The nature of the flow mechanisms in the fan tip region is correlated to the specific blade design features that promote reduced aerodynamic noise. It was found that the tip geometrical modifications markedly affect the multiple vortex leakage flow behaviours, by altering the turbulence and velocity fluctuations in the near-wall region as well along the blade span. The tip end-plates were demonstrated to influence also the rotor loss behaviour, in the blade tip region. The improvement of rotor efficiency curves were assessed and correlated to the control of tip leakage flows exploited by the tip end-plates.
Keywords:industrialfans,end-plates, tip leakage flow, noise
Nomenclature
Latin letters
k[m2/s2]turbulent kinetic energy
l.e.leading edge
PSpressure side
P[Pa]static pressure
r[mm]radius
SSsuction side
t.e.trailing edge
Uc[m/s]casing relative peripheral velocity
v, w[m/s]absolute and relative velocities
x, y, zCartesian coordinates
Greek letters
[deg]stagger angle
[m2/s3]turbulent dissipation rate
[-]total loss coefficient,
[-]efficiency
[-]hub-to-casing diameter ratio
t[ms-2]turbulent viscosity
i[s-1]absolute vorticity vector
s[-]absolute streamwise vorticity,
[-]blade solidity
[-]global flow coefficient (annulus area-averaged axial velocity normalised by Uc)
[-]rotor tipclearance
[-]pressure rise coefficient (p/(0.5 ))
[rad/s]rotor angular velocity
Subscripts and superscripts
a, p, raxial, peripheral and radial
ccasing wall
hhub wall
iCartesian component index
ininlet section
sstreamwise component
pitch-averaged value
1. 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, e.g. cooling fans, or in some cases for emergency operation at up to 400°C for two hours to extract smoke in the event of a fire, e.g. ventilating fans. As well known, the tip clearance plays a detrimental role affecting the rotor aero-dynamics [1-3], and, as anumber of studies pointed out, significantly contributingtothe aero-acoustic signatureofimpeller in low speed ventilating equipments. In this pictures the tip clearance flow is recognized to inflence the rotor noise spectra by discrete frequency noise due toperiodic velocity fluctuation and a broadband or high-frequency noise due to velocity fluctuation in the blade passage[4-6]. To this end there is a strong motivation to look for deliberate aerodynamic design in order to minimize the negative effects of tip gap and to manage the fan or compressor tip clearance flow to minimize its impact on performance. Thus techniques and concepts that help to reduce tipclearance noise without sacrificing aerodynamic efficiency are highly desired and needed.
By surveying the techniques for noise control in fans and compressors, it was found that the solutions proposedcould be grouped into active 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.
In the ambit of active control techniques for fans and compressors, recently a number of experimental studies reconsider the tip clearance flow control by means of fluid injection on the casing wall in axial compressor [7], and low-speed axial flow fan [8].
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 and on geometrical modifications of the equipments in the gap region. The first concept makes use of sweep technique in blade design, recognized as a remedial strategy to improve the aerodynamic limits in compressor and low-speed axial fan rotors owing to the capability of affecting the rotor stall margin by reducing secondary flows effects and the flow leakage over the blade tip [9-11].
A second family of control technique based on gap geometrical manipulation, theuse of casing treatments in the shroud portion over the blade tip which is reported since the early 70s to improve the stable flow range by weakening the tip leakage vortex. Noticeable contributions deal with the use of grooves and slots [12-13], or stepped tip gaps [14]. Furthermore, in the ambit of fan technologies recirculating vanes, and annular rings have been proposed as anti-stall devices [15].
A final tip treatment solution, appeared during the last decade, proposed the blade tip modifications by means of anti-vortex appendices such as the end-plates investigated by Quinlan and Bent [6], or the solutions recently proposed by industrial patents for ventilating fans [16-19].
In this respect, the present paper aims to investigate on the use of profiled end-plates at the blade tip[20]. The study focuses ona family of commercially available fansand compares the aerodynamic and aeroacoustic performanceof the datum blade against two improved tip geometries, respectively with constant or variable thickness end-plate [21].
The objective of the paper is to report on the experimental and numerical assessment of the pay-off derived from the blade tip concept developed at Fläkt Woods Ltd with respect to the aerodynamic performance of a class of low noise level industrial fans.
The single rotor investigations are carried out at design operating condition for three configurations of the six-blade axial flow fan under investigation, namely: the datum fan, coded AC90/6; two fans modified by the adoption of tip features, respectively coded AC90/6/TF and AC90/6/TFvte. The studies have been carried 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 comparative aerodynamic performance experiments have been carried out according to ISO 5801 for type D fully ducted configuration set-up. The noise performance test have been carried out in accordance with the British Standard BS484, Part 2 for outlet noise hemispeherical measurement. The fans have been testedemploying a type A configuration, in the Fläkt Woods Ltd anechoic chamber at the design operating conditions.
The tip flow characteristics are analysed by using a three-dimensional (3D) steady Reynolds-Averaged Navier-Stokes (RANS) formulation, with use of first order non-isotropic turbulence closure successfully validated for fan rotor flows [22-23]. Despite the steady-state condition, the RANS is considered an effective investigation tool for vortical structure detection [24]. The authors adopt a parallel multi-grid (MG) scheme developed for the in-house finite element method (FEM) code [25]. The FEM formulation is based on a highly accurate stabilized Petrov-Galerkin (PG) scheme, modified for application to 3D with equal-order spaces of approximation.
By means of such a numerical investigation, the tip leakage flow structures of the fans are analysed in terms of vortical structures detection, leakage flow energy and loss behaviours. Emphasis is laid on the assessment of the benefits related to the improved tip geometry in terms of efficiency and operating margin gains. 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.
2. Test apparatus and procedures
2.1. Test 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 / 0.22
tip diameter(mm) / 900.0
rotor tip clearance (% span) / 1.0
rated rotational frequency (rpm) / 900 – 935
The studied blade configurations, for datum and modified rotors, feature a high tip stagger 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 investigated fans.
The fanbladesaredrawn 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 by preventing 3D flows in aircraft wings, also used as anti-vortex devices for catamaran hulls.
Fig. 1 Test fans and rotor blades (not to scale)
The tip blade section was modified by adding an end-plate along the blade pressure surface that ends on the blade trailing edge with a square tail. By means of the introduction of the end-plate, the blade section is locally thickened of a factor 3:1 with respect to the maximum thickness at the tip of datum blade. 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 in the range 0.2 0.1 blade span, as shown by former studies on rotors of axial compressor [26] and fan [11].A recent investigation, carried out by Corsini and co-workers [20], assessed the aerodynamics and aeroacoustics gains of rotor AC90/6/TF with respect to the datum one. Nonetheless, the numerical simulation also founded the evidences of a tip leakage vortex breakdown affecting rotor AC90/6/TF at the design operating condition. To this end the AC90/6/TFvte blade tip geometry has been proposed that exploits a variable thickness distribution of the end-plate according to safe rotation number chord-wise gradient concept [21].
2.2.Numerical procedure and axial fan modeling
The Reynolds-averaged Navier-Stokes equations are solved by an original parallel Multi-Grid Finite Element flow solver [25]. The physics involved in the fluid dynamics of incompressible 3D turbulent flows in rotating frame of reference, was modelled with a non-linear k- model [27], 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 [10, 23].
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, related to the Coriolis acceleration or to the dissipation/destruction terms in the turbulent scale determining equations [22]. Equal-order linear interpolation spaces (Q1-Q1) are used for primary-turbulent and constrained variables, implicitly eliminating the undesirable pressure-checkerboarding effects. Concerning the solution strategy, a hybrid full linear MG accelerator was built-in the in-house made overlapping parallel solver. The Krylov iterations in the smoothing/solving MG phases are parallelized using an original additive domain decomposition algorithm. The message passing operations were managed using the MPI libraries. By that way, the fully coupled solution of sub-domain problem involves an efficient non-conventional use of Krylov sub-space methods. The preconditioned GMRes(5) and GMRes(50) algorithms were respectively used as smoother and core solver.
Fig. 2 Computational grid of fan rotor, mesh details in the tip gap region
The mesh has been built according to a non-orthogonal body fitted coordinate system, by merging two structured H-type grid systems. The mesh in the main flow region, surrounding the blade, and an embedded mesh in the tip gap region. The mesh has 1546858 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 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 detailed view at the tip of the mesh in meridional and blade-to-blade surfaces.
The mesh has an adequate stretch toward solid boundaries, with the ratio of minimum grid spacing on solid walls to mid-span blade chord set as 210-3 on the blade tip, casing wall, and blade surfaces. The adopted grid refinement towards the solid surfaces controls the dimensionless distance + value about 1 on the first nodes row.
2.3.Boundary conditions and investigated flow conditions
Standard boundary condition set has been adopted, already used in recent numerical studies on high performance fans [10-11].
The Dirichlet conditions for the relative velocity components are imposed at the inflow section half a mid-span chord far upstream the leading edge. The velocity profile has been obtained from 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 kinetic energy k is obtained from axi-symmetric turbulence intensity (TI) profile derived on the basis of former studies on ducted industrial fans [10]. The TI profile features a nearly uniform value in the core region (about 6 percent) and it grows markedly approaching the endwalls (about 10 percent). The inlet profile of turbulence energy dissipation rate is basedon the characteristic length scale l set to 0.01 of rotor pitch at mid-span. Flow periodicity upstream and downstream the blading, and Neumann outflow conditions (homogeneous for k and and non-homogeneous forthe static pressure) complete the set of boundary data.
The numerical investigation compares fan leakage flow patterns for 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 = 0.278. The Reynolds number based on tip diameter and rotor tip speed is 8.3 105, for normal air condition.
3. Performance experiments
The aerodynamic and noise performance tests were carried out at Fläkt Woods Ltd laboratory in Colchester.
3.1.Aerodynamic tests
The aerodynamic tests were conducted according to ISO 5801 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 primary performance parameters measured were the fan static pressure and the efficiency. Figure3 compares the static pressure and efficiency characteristic curves for datum and AC90/6/TF rotors.The analysis of static pressure curves of Fig.3, gives the evidence of a small performance reduction in rotor AC90/6/TF with improved tip concept, e.g. about 2% at 6m3/s,according to the interaction between the tip clearance flow and the suction side near surface fluid [28]. On the other hand, rotor AC90/6/TFvte shows a performance recovery that increases remarkably by throttling the rotor toward the peak pressure.
Concerning the efficiency curves itis evident that both the modified rotors featurean efficiency improvement in the range of volume flow rate higher than the design one. Moreover the efficiency curve comparison gives evidence that the adoption of the improved tip concepts results in the appearance of a clearefficiency plateau that shift the peak volume flow rate towards the rotor stall margin.