Conference on Modelling Fluid Flow (CMFF’06)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, Franco RISPOLI2, A. Geoff SHEARD3

1 Corresponding Author. Dipartimento di Meccanica e Aeronautica, University of Rome “La Sapienza”. Via Eudossiana 18, I00184 Rome, Italy. Tel.: +39 06 44585231, Fax: +39 06 4881759, E-mail: alessandro.corsini@uniroma1.it2 Dipartimento di Meccanica e Aeronautica, University of Rome “La Sapienza”. E-mail: franco.rispoli@uniroma1.it3 Fläkt Woods Ltd. E-mail: geoff.sheard@Fläktwoods.com

ABSTRACTThe 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: industrial fans, end-plates, tip leakage flow, noise

NOMENCLATURE Latin lettersk [m2/s2] turbulent kinetic energyl.e. leading edgePS pressure sideP [Pa] static pressurer [mm] radiusSS suction side

t.e. trailing edgeUc [m/s] casing relative peripheral velocity v, w [m/s] absolute and relative velocitiesx, y, z Cartesian coordinatesGreek letters [deg] stagger angle [m2/s3] turbulent dissipation rate [-] total loss coefficient, [-] efficiency [-] hub-to-casing diameter ratioi [s-1] absolute vorticity vectors [-] 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 velocitySubscripts and superscriptsa, p, r axial, peripheral and radialc casing wallh hub walli Cartesian component indexin inlet sections streamwise component

pitch-averaged value

1. INTRODUCTIONOften 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 (Fukano and Takamatsu, 1986), (Storer and Cumpsty, 1991), (Furukawa et al., 1999), and, as a number of studies pointed out, significantly contributing to the aero-acoustic signature of impeller 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 to periodic velocity fluctuation and a broadband or high-frequency noise due to velocity fluctuation in the blade passage (Fukano and Jang, 2004) (Jang et al., 2003) (Quinlan and Bent, 1998). 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 tip clearance noise without sacrificing aerodynamic efficiency are highly desired and needed.

By surveying the techniques to control the noise of fans and compressors, it was found that the proposed solutions could 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 (Bae et al., 2005), and low-speed axial flow fan (Roy, et al., 2005).

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 (Wadia et al., 1997), (Corsini and Rispoli, 2004), (Corsini et al., 2004). A second family of control technique based on gap geometrical manipulation, the use 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 (Takata and Tsukuda, 1977) (Smith and Cumpsty, 1984), or stepped tip gaps (Thompson et al., 1998). Furthermore, in the ambit of fan technologies recirculating vanes, and annular rings have been proposed as anti-stall devices (Jensen, 1986). As a final tip treatment solution, during the last decade, has been proposed the blade tip modifications by means of anti-vortex

appendices such as the end-plates investigated by Quinlan and Bent (1998), or the solutions recently proposed by industrial patents for small ventilating fans (pat. ???) (pat. ???) (pat. ???).

In this respect, the present paper aims to investigate on the use of profiled end-plates at the blade tip (Corsini et al., 2006). 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 (rep. FW) or variable thickness end-plate (Corsini, 2006)

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 and off-design conditions for two configurations of the six-blade axial flow fan under investigation, namely: the datum fan, code AC90/6; the fan modified with the tip feature, code AC90/6/TF. 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 tested employing 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 (Corsini et al, 2003), (Corsini and Rispoli, 2005). Despite the steady-state condition, the RANS is considered an effective investigation tool for vortical structure detection (Inoue and Furukawa, 2002). The authors adopt a parallel multi-grid (MG) scheme developed for the in-house finite element method (FEM) code (Borello et al., 2002). 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 fansThe 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.

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 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. Figure 1 compares, in a qualitative view not to scale, the thickness distributions of the developed improved tip concepts against the datum base-line.

Fig. 1 Test fans and rotor blades (not to scale)

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. 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 (Inoue et al., 1986) and fan (Corsini et al., 2004). A recent investigation, carried out by Corsini and co-workers (2006), 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 (Corsini, 2006).

2.2. Numerical procedure and axial fan modelingThe Reynolds-averaged Navier-Stokes equations are solved by an original parallel Multi-Grid Finite Element flow solver [13]. 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 [14], here used in its topology-free low-Reynolds variant. This

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

AC90/6/TFvte

datum

AC90/6/TF

turbulence closure has been successfully validated on transitional compressor cascade flows, as well as high-pressure industrial fan rotors [15 - 16].

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 [17]. 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 2 10-

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 conditionsStandard 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 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 (Corsini and Rispoli, 2004). 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 for the 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 EXPERIMENTSThe aerodynamic and noise performance tests

were carried out at Fläkt Woods Ltd laboratory in Colchester.

3.1. Aerodynamic testsThe 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

rotor hub

l.e.

t.e.

bell mouth. The primary performance parameters measured were the fan static pressure and the efficiency. Fig. 3 compares the static pressure and efficiency characteristic curves for datum and AC90/6/TF rotors.

The analysis of static pressure curves of Figure 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 ADD interpretation of performance reduction TE06. On the other hand, rotor AC90/6/TF shows an efficiency improvement in the range of volume flow rate higher than the design one. In terms of peak value, the AC90/6/TF rotor features a 72.9% efficiency with a 1.25% of improvement. Moreover the efficiency curve comparison gives evidence that the adoption of the improved tip concept results in the appearance of an efficiency plateau that shift the peak volume flow rate towards the rotor stall margin.

Fig. 3 Static pressure and efficiency characteristic curves (dashed lines: datum fan; solid lines: AC90/6/TF fan; line-symbols: AC90/6/TFvte fan)

The rotor performance were assessed along the operating line. The predicted overall performance for 900 rpm rotational frequency are compared in Table 2 to the experimental data measured at Fläkt Woods Ltd measurements according to the fan performance test standards ISO5801:1997 for installations type D with inlet bell-mouth.

Efficiency is computed in terms of static pressure rise. The comparison confirms the validity of the predicted performance at the chosen setting angle, where the blades of this fan are the most loaded and more readily prone to flow separation.

Table 2 Predicted and measured fan overall

performanceMeasurements Predictions

pstat

(Pa) pstat

(Pa)

datum 134.8 0.490 133.3 0.510AC90/6/TF 126.2 0.510 126.1 0.504AC90/6/TFvte 133.3 0.486 126.1 0.504

It is also worth noting that the prediction of performance parameters have been referred to axial sections respectively located at the inlet of the domain, and 1.2 midspan chord downstream the blade trailing edge. The comparison confirms the validity of the predicted performance.

3.2. Noise testsThe noise performance test have been carried

out in accordance with the British Standard BS484, employing a type A testing configuration.

102 103 10450

55

60

65

70

75

80

85

90

95

100

datumAC90/6/TFAC/90/6/TFvte

102 103 10430

35

40

45

50

55

60

65

70

75

80

85

90

95

100

datumAC90/6/TFAC/90/6/TFvte

Fig. 3 Sound power level spectra in one-third-octave band. a) un-weighted spectra and b) A-weighted spectra(dashed lines: datum fan; solid lines: AC90/6/TF fan; line-symbols: AC90/6/TFvte fan)

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.

4 5 6 7 830

40

50

60

70

80

4 5 6 7 80

50

100

150

200

250

4 5 6 7 80

50

100

150

200

250

volume flow (m3/s)

D

frequency (Hz)

dB

dB(A)

b)

a) frequency (Hz)

datumAC90/6/TFAC90/6/TFvte

datumAC90/6/TFAC90/6/TFvte

p(Pa)

Figure 4 compares the measured power spectra in one-third-octave band. Fig. 4.a and Fig. 4.b respectively show the measured sound power level and the A-weighted sound power level spectra for the frequency-dependent human audition. The noise tests have been done in order to compare the rotors aeroacoustic signature for identical static pressure rise, e.g. 190 Pa close to the peak pressure operation. As shown in Fig. 4, the effectiveness of the improved tip concept is demonstrated by the reduction of the rotor aeroacoustic signature both in terms of tonal noise and broad-band noise. These noise components are related to the main recognized tip noise generation mechanisms in axial decelerating turbomachinery. The convection of the primary tip vortex and its interaction with the statoric structures produces mainly tonal noise, while the oscillating tip vortex behaviour could be linked to the production of broadband self-generated noise (Khourrami and Choudari, 2001).

ADD coments on the compaative performance

Finally, Table 3 compares the overall soud power levels for the family of fans under investigation, for

Table 3 Overall sound power levels @ 190 Paun-weighted SWLdB

A-weighted SWLdB(A)

datum 94.6 88.7AC90/6/TF 93.0 86.3AC90/6/TFvte 91.5 88.1

ADD coments on the compaative performance

4. INNER WORKINGS OF END-PLATESIn axial decelerating turbomachines, the

distinctive feature of the tip clearance flow structure is classically the occurrence of a roll-up into a tip leakage vortex (Inoue and Furukawa, 2002). This prompted a comparative investigation onto the presence and structure of the tip leakage vortex and other systems of secondary vorticities for the datum fan and the fan with blade tip feature. The effectiveness of the passive tip device is analysed by comparing the static pressure fields within the gap, the normalized stream wise vorticity map chordwise evolution, and the tip vortex core paths. These analyses are complemented with the evalutaion of tip leakage flow energy contents, affecting the rotor aeroacoustic signatures, and with the presentation of loss coefficient map evolution within the blade passage to assess the influence of the geometrical modification at the blade tip and the rotor aerodynamic efficiency along in operating range of the fans. ADD

4.1. Helicity distributions and vortex cores The tip leakage vortical structures are now

investigated by using the normalized helicity Hn

(Furukawa et al. 1998) (Inoue and Furukawa, 2002) as the detection tool. Hn is defined and normalized as: with i = 1, 3, where i

and wi are the Cartesian components of the absolute vorticity and relative velocity vectors, and their norms. The absolute vorticity is used because the secondary flows in the rotor frame of reference are dominated by the advection of its components along the local relative flow directions. Figure 8 shows the normalized helicity distribution in the blade tip region by comparing the contours on cross flow planes in near-design operating condition. The probing planes are located, respectively, at 0.25, 0.43, 0.65, 0.89 and 1.2 blade chord from the tip section leading edge. The normalized helicity distribution is plotted with the vortex cores colored by the its local magnitude.

For both the investigated fan rotors, a clear vortex cores identification is only observed for the leakage flow structures emerging in the front portion of the tip blade sections. In the multiple vortex behaviour of datum fan rotor, Fig. 8.a, the helicity field shows that the main vortical structure TLV1 develops as a clock-wise vortex with a high skewing angle with respect to the blade surface.

Fig. 4 Normalized helicity Hn contours on cross sections and vortex cores at the tip, D operating point: a) datum rotor, b) AC90/6/TF rotor, and c) AC90/6/TFvte

Fig. 5 Comparison of tip vortex trajectories

4.2. Leakage flow energy

Fig. 6 Rotational kinetic energy isolines in the rotor tip gap @ R = 0.998, D operating point: a) datum rotor, b) AC90/6/TF rotor, and c) AC90/6/TFvte

Fig. 7 Turbulence intensity TI contours on cross sections and tip vortex streamlines, D operating point: a) datum rotor, b) AC90/6/TF rotor, and c) AC90/6/TFvte

4.3. Loss at the rotor tip

Fig. 8 Evolution of total pressure loss coefficient inside the blade passage: a) datum rotor, b) AC90/6/TF rotor, and c) AC90/6/TFvte

5. CONCLUSIONSIt is compulsory to summarise the results

discussed in the paper.

6. SUMMARYIt is compulsory to summarise the results

discussed in the paper.

ACKNOWLEDGEMENTSThe present research was done in the context of

the contract FW-DMA03, between Flakt Woods Ltd and Dipartimento di Meccanica e Aeronautica University of Rome “La Sapienza”. The authors gratefully acknowledge Mr. I. Kinghorn and Mr. B. Perugini for their contribution to the experiments.

REFERENCESNo numbering in the title (of Style “Heading

1”) is applied.References must be included at the end of the

paper, in the order to which they have been referred in the text. Examples for the format of references in the body of the paper: [1] or [1, 2] or [1-9]. (The latter is to be used when simultaneously more than two references are referred to.) Names of the authors of a reference can be emphasized in the body of the text if necessary. For example: “Recently, Corsini et al. [12]…”.

Examples for format of references in the reference list, using the basic format of the Style “References”, are as follows.

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[2] Yamaguchi, N., Tominaga, T., Hattori, S., and Mitsuhashi, T., 1991, “Secondary-Loss Reduction by Forward-Skewing of Axial Compressor Rotor Blading”, Proc. Yokohama International Gas Turbine Congress, Yokohama, Japan, pp. II.61 - II.68.

[3] Kuhn, K., 2000, “Experimentelle Untersuchung einer Axialpumpe und Rohrturbine mit gepfeilten Schaufeln”, Dissertation Technische Universität Graz, Institut für Hydraulische Strömungsmaschinen.

[4] Lakshminarayana, B., 1996, Fluid Dynamics and Heat Transfer of Turbomachinery, John Wiley & Sons, Inc.

[5] Spalart, P., and Allmaras, S., 1992, “A One-Equation Turbulence Model for Aerodynamic Flows”, Technical Report AIAA-92-0439.

[6] Vad, J., and Bencze, F., 1998, “Three-Dimensional Flow in Axial Flow Fans of Non-Free Vortex Design”, Int J Heat Fluid Flow, Vol. 19, pp. 601-607.

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