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work, by the same research group, aimed at establishing a general theory on cross-ow fan operation and at outlining the design

guidelines for this particular type of turbomachines.

empirical methods deriving from previous applications.

The complexity of the ow eld pattern (Fig. 1) is the

the uid-dynamic equilibrium between the throughput

ow and the recirculating ow. For a given geometrical

conguration and rotational speed, the ow eld pattern

also signicantly changes under dierent throttling

progressively reduced and moves towards the internal

periphery of the impeller and in the direction of impeller

mance and at obtaining experimental indications on fan

design. A systematic experimental test program was

carried out in [1] to understand the inuence of Rey-

nolds number and size on cross-ow fan operation. Five

machine congurations having a similar shape but dif-

ferent sizes were tested at dierent rotational speeds.

The results showed that similarity laws can be applied

id Sc*Corresponding author. Tel.: +39-049-827-6747; fax: +39-049-827-main obstacle that makes a general theory about the

operation more dicult to formulate than for other

categories of fans. In fact, the motion of the blades

forms an eccentric vortex within the impeller, the posi-

tion and strength of which considerably aect fan per-

formance and eciency. The characteristics of the

vortex are inuenced by the geometry of the impeller

and of the casing as well, since the latter mainly governs

rotation, allowing more throughput streamlines to reach

the discharge section. At fully open operating condition,

the center of the vortex is close to one of the two casing

walls, which is named vortex wall, the other being called

rear wall.

This paper is part of an extensive work made by the

same group of authors, aiming at verifying the inuence

of dierent design variables on cross-ow fan perfor- 2004 Elsevier Inc. All rights reserved.

Keywords: Cross-ow fan; Flow eld; Pressure and velocity measurements; Aerodynamic probes

1. Introduction

Cross-ow fans are a unique type of turbomachinery,

since both suction and discharge occur radially. The

research in this eld, which includes several theoretical

and experimental studies, has not led yet to an ultimate

set of criteria about the design, which is still based on

conditions. Fig. 2 shows three pictures of velocity vec-tors at low, intermediate and high ow rates, obtained

by a numerical simulation of a sample conguration (the

simulation was performed with the commercial CFD

code FLUENT, using the RNG ke turbulence modeland a grid with about 100,000 cells). As ow rate in-

creases, the recirculation area around the vortex core isAn experimental investigationthe impeller of

Andrea Toolo, Andrea Lazza

Department of Mechanical Engineering, Univers

Received 19 April 2003

Abstract

Cross-ow fan performance is strongly inuenced by the geo

strength of the eccentric vortex that characterizes the operatio

mental investigation of the ow eld within the impeller at di

casing. Both pressures and velocities are measured using a three

helps determine the relationship between the design parameters

Experimental Thermal and Flu6785.

E-mail address: andrea.lazzaretto@unipd.it (A. Lazzaretto).

0894-1777/$ - see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.expthermusci.2004.02.007the ow eld pattern withincross-ow fan

o *, Antonio Dario Martegani

Padova, Via Venezia, 1, I-35131 Padova, Italy

pted 13 February 2004

of the casing, as the latter, in turn, aects the position and the

this category of fans. The paper presents a systematic experi-

t throttling conditions and for dierent geometries of the fan

ensional ve-hole probe that is inserted in the ow. This study

e casing and the ow eld pattern, and it is part of an extensive

ience 29 (2004) 5364

www.elsevier.com/locate/etfsabove critical Reynolds numbers that depend on casing

Nomenclature

c blade chord [m]D diameter [m]h height [m]L axial length [m]p pressure [Pa]

g DpQ=Tx eciency [dimensionless]q density [kg/m3]l dynamic viscosity [kg/m s]U Q=LD2u2 ow coecient [dimensionless]W Dp=0:5qu22 pressure coecient [dimensionless]

d discharge

s static

54 A. Toolo et al. / Experimental Thermal and Fluid Science 29 (2004) 5364p0 inlet (ambient) pressure [Pa]Dp p p0 pressure dierence [Pa]Q volumetric ow rate [m3/s]R radial coordinate [m]Rec u2cq=l Reynolds number [dimensionless]ReD u2D2q=l Reynolds number [dimensionless]s thickness [m]T shaft mechanical torque [Nm]u xR peripheral speed [m/s]a log spiral angle [deg]b blade angle [deg]geometry, and no strong scale eect was observed. An

original eective criterion for the parameterization of

machine geometry was formalized in [2]. In particular, a

restricted number of independent design parameters was

used to describe the shape and position of the rear and

vortex walls. The fan congurations resulting from the

systematic variation of the selected parameters for both

the impeller and the casing were tested in [3]. The resultsof these tests led to the identication of the most

important design parameters that aect fan performance

and eciency.

Fig. 2. Flow eld pattern in a cross-ow fan at (a)

Vortex Wall

Rear Wall

Eccentric Vortex

Impeller

Inflow arc

Outflow arcDischargesection

Fig. 1. A cross-ow fan conguration.The present work is devoted to an experimental

investigation of the ow eld pattern within the impeller

for the most signicant cross-ow fan casing congura-

tions according to the results obtained in [3]. The con-

sidered congurations (see Section 4) are selected to

cover the ranges of the design parameters in which high

performances are obtained in terms of pressure rise,

eciency and ow rate. These often conicting designobjectives are fullled by quite dierent choices of the

main design parameters of the casing, which correspond

to dierent features of the ow eld pattern within the

impeller. The aim of the paper is to highlight this close

link through the analysis of the results of an extensive

t total

V vortex wallv vortex coref angular coordinate [deg]x rotational speed [rad/s]

Subscripts

1 internal

2 externalprogram of local ow measurements performed by a

three-dimensional ve-hole probe, in which pressure and

velocity elds are reconstructed at dierent values ofthe ow coecient for each of the considered congu-

rations.

2. Theoretical and experimental studies of the ow eld

pattern in the literature

From Mortiers patent in 1891 to the years afterWorld War II, none of the researchers or inventors who

low, (b) intermediate and (c) high ow rate.

action among the considered variables can be observed.

Tuckey et al. [15] proposed to subdivide the suction,

The test facility (Fig. 4) is the same used in [3] to

determine fan performance, and was built following the

UNI 10531 standard [17] on industrial fans test methods

and acceptance conditions (equivalent to ISO 5801 [18]).The suction of the tested fan is free, whereas the delivery

is connected to a plenum chamber. At the outlet of this

chamber the air passes through a Venturi nozzle for ow

rate measurement. An auxiliary fan is placed at the end

of the airway, after a honeycomb straightener duct, to

overcome the pressure losses generated by the ow

passage. Pressure measurements in the plenum chamber

and in the Venturi nozzle are performed using watermicromanometers having a 1/100 mm accuracy. The

tested fan is driven by a direct current motor, which

includes a tachimetric dynamo for rotational speed

measurements. Fan total eciency is determined by

torque measurement, using a load cell connected to the

motor stator (range 0.5 kg, sensitivity 16 mV/V and

rmal and Fluid Science 29 (2004) 5364 55studied or patented cross-ow fan congurations real-

ized the actual structure of the ow eld. Eck [4] was the

rst to discover in the 50s the existence of the eccentric

vortex within the impeller by means of visualization

studies. He also understood that the vortex can act as anaerodynamic seal to prevent the recirculation of the ow

from discharge to suction, and obtained much more

ecient and much less noisy fan congurations by

increasing the radial clearance between the vortex wall

and the impeller. Ecks patented fan [5] features a small

radial width rear wall and a thick vortex wall with

decreasing values of the radial clearance in the direction

of rotation: at free blowing, the corresponding ow eldshows the vortex core near the inner periphery of the

blade row, while under throttling the vortex moves to-

ward the interior of the impeller, losing much of its

strength. In the following decades, Datwylers patent [6]

and then the study by Porter and Markland [7] dem-

onstrated that higher total pressure coecients, but

quite low eciencies, are achieved when a strong vortex

is free to move along the inner periphery of the impellerat all ow rates: this ow eld pattern is obtained when

the radial width of the rear wall is increased and a at

thin vortex wall is used.

In the same years, several analytical models were

proposed to describe mathematically pressure and

velocity elds within and around the impeller, and then

to predict fan performance. However, they were all

based on over-simplied hypotheses: a simple potentialow with a single vorticity source [4,8,9], a combined

solution consisting of a forced vortex ow inside the

core and a potential ow outside [10], the potential ow

resulting from a couple of equal vorticity sources, one

inside and the other outside the impeller [11], an actu-

ator disc model [12].

Yamafuji performed in [13] a very interesting series of

experiments on the formation of an asymmetrical oweld in a geometrically symmetrical impeller. He showed

that the throughow and the eccentric vortex arise for

any shape of the impeller blade prole (circular, circular

arc and radial proles were tested) when the blade

Reynolds number is higher than 250. Among the other

experimental studies which have appeared in the litera-

ture, the one by Murata and Nishihara [14], aimed at

understanding the relation between the ow eld andthe shape of the characteristic curve (total pressure

coecient Wt vs. ow coecient U), is by far the mostcomprehensive and is conceptually similar to the anal-

ysis performed in the present work. Dierent casing

shapes, in which the vortex moved along the inner

periphery of the impeller at all throttling conditions,

were investigated to determine the inuence on the ow

eld of several geometric parameters. However, theresults of this analysis cannot be interpreted in a

straightforward manner, as the examined casing shapes

A. Toolo et al. / Experimental Thewere not obtained using a set of independent designinterior and discharge regions of the ow eld according

to the nature of the ow (Fig. 3), and described by

means of visualization studies how these zones changedat dierent ow rates. Finally, Tsurusaki et al. [16]

performed an experimental analysis of the ow within

the machine through optical techniques (particle-track-

ing velocimetry) using a water model of a single fan

conguration. Their work mainly deals with the nature

of the eccentric vortex and the mechanisms by which

vorticity is supplied to it from the blades and is then

diused from the recirculating region.

3. The experimental apparatusvariables, and therefore mixed eects due to the inter-

core

stalled

recirculation

to jet

recirculation

throughflow

inflow(to suction)

rear wall

vortex wall

dischargearc

suctionarc

separation

jet

throughflow

throughflow

Fig. 3. Regions of the ow eld in a cross-ow fan (adapted from [15]).accuracy 1%). All tests are performed at a rotational

speed of 1000 rpm and with impellers having an external

diameter of 152.4 mm and an axial length of 228.6 mm

(the resulting Reynolds number, referred to the external

with an external bearing system. A circular plate, which

can be rotated by hand around machine axis, was built

to seal the impeller (acting as the missing impeller disc)

Fig. 4. Schematic of the test rig used to determine fan performance and eciency.

56 A. Toolo et al. / Experimental Thermal and Fluid Science 29 (2004) 5364diameter and the peripheral speed, is ReD 80; 000).The estimated uncertainty for the total pressure coe-

cient is within 1.5%, whereas for total eciency is within3%, since more measured quantities are involved in its

calculation.

The measurements of ow eld quantities within the

impeller are performed using a three-dimensional aero-

dynamic probe by United Sensor. If information on the

ow eld are to be collected by means of both pressure

and velocity measurements, there is no alternative to this

methodology, because neither hot wire anemometersnor LDV techniques are able to measure pressures.

A ve-hole cobra probe is used (Fig. 5), having

the following characteristics:

axial length: 16 in. 406.4 mm; probe diameter: 1/8 in. 3.175 mm; probe maximum radial size: 5/8 in. 15.9 mm.

The test facility has been modied to get a measure-

ment window out of the side wall opposite to the driving

shaft. The corresponding lateral disc of the impeller was

substituted with a ring that was adequately supportedFig. 5. Sketch of the probe head and of the probeand the measurement window as well. Four rectangular

holes were made on the cover at dierent radii to allow

the insertion of the probe head in the middle cross-section of the impeller. This apparatus lets the probe

rotate around the impeller axis at a xed radius, to

analyze ow quantities in dierent measurement points

on a circumference, and around its own axis, to be

manually oriented according to the yaw angle of the

ow. A sketch of the arrangement of the probe in the

test facility is provided in Fig. 5.

The probe is connected to four dierential pressuretransducers (range 1000 Pa, sensitivity 8 mV/V and

accuracy 1%); the output signals of the transducers are

amplied to a range between )10 and +10 V and aresent to the data acquisition card of a computer. A

LabViewTM virtual instrument automatically calculates

the actual total, static and dynamic pressures, according

to the calibration curves provided by the manufacturer

of the probe, and writes them on a spreadsheet.The pressure signals at probe outlet are time-aver-

aged signals, since the time constant of probe capillary

tubes is too long to follow the high-frequency variation

of an unsteady ow eld, such as the one inside thearrangement in the experimental apparatus.

impeller of a cross-ow fan. If the ow can be assumed

quasi-periodic in a generic location, with high-frequency

harmonics mainly due to blade passage, the uncertainty

on the nal measurements are within 2% for pressure

(total and static), within 2.5% for velocity magnitudeand within 1 for the yaw angle. However, if owbehavior tends to be chaotic, with large time and length

scales (this happens in the stall zone, see Fig. 3 and

Section 5) the uncertainties may grow up to 8% for

pressure, 10% for velocity magnitude and 5 for theyaw angle.

4. The test program

The program of the experimental investigations is

conceived to determine the ow eld pattern of a set of

the impeller for a given conguration and at a given ow

coecient consists in a series of local measures to

determine the total and static pressures and the velocity

vector on a predened grid of points which lie on cir-cumferences of dierent radii at half of the impeller axial

length L. This grid is shown in Fig. 9 (in which thereference angular coordinate f is dened as well), andconsists of:

6 points (at angular intervals of 60) on the circum-ference of radius R 10 mm;

12 points (at angular intervals of 30) on the circum-

A. Toolo et al. / Experimental Thermal and Fluid Science 29 (2004) 5364 57Fig. 6. The most signicant parameters aecting fan performance andrepresentative cross-ow fan congurations. These are

characterized by dierent values of the casing design

parameters that were identied in [3] as the most sig-

nicant, according to the inuence on performance andeciency. These parameters are the angle a of the log-arithmic spiral that controls the radial width of the rear

wall (i.e. the angle between the tangent to the rear wall

prole and the tangential direction), the height hd andthe thickness sV of the vortex wall (Fig. 6).

The choice of the congurations to be investigated is

driven by the indications obtained in [3]. All the three

classes of radial width are considered, since each of themresults in a dierent shape of the fan characteristic

curve: small, intermediate and large radial width corre-

spond to unstable, nearly at and stable WtU curves,respectively [3]. The three rear walls used in this work

are shown in Fig. 7 as:

RE (small radial width Ecks patented rear wall madeup of two circular arcs, one of which centered onimpeller axis),eciency [2,3]. R2r (intermediate radial width log spiral rear wall,a 17:2) and

R3r (large radial width log spiral rear wall,a 23:6).

All the three rear walls are combined with the two

lower positions of the at thin (sv=D2 0:13) vortexwall used in [3], for which high performance and e-

ciencies are obtained. The two positions are indicated as

H1 (hd=D2 0:185) and H2 (hd=D2 0:316) in Fig. 7.Moreover, the small radial width rear wall RE is alsomatched with two thick vortex walls, the combination of

design parameters leading to the highest eciency [3]: in

Fig. 7 the two thick vortex walls are shown as S3H1

(a modular vortex wall having sv=D2 0:39 andhd=D2 0:185, see [3]) and VE (Ecks patented vortexwall). Although the highest eciencies were obtained

with impellers having the external and internal blade

angles equal to 25 and 90 respectively [3], all theselected casing shapes are matched with the impeller for

which the validity of the similarity laws has been veried

in [1], having b2 38 and b1 70 respectively. Thecurves of the total pressure coecient and the total

eciency for the eight resulting fan congurations are

reported in Fig. 8.

For each of the considered congurations, the total

and static pressure elds and the velocity eld aremeasured at the ow rates corresponding to ow co-

ecients U equal to 0.2, 0.4, 0.6, 0.8 and 1.0, providedthat these values are lower than the ow coecient at

free blowing. The investigation of the ow eld within

Fig. 7. The casing shapes selected for ow eld investigation.ference of radius R 20 mm;

1.0

2.0

3.0

58 A. Toolo et al. / Experimental Thermal and Fluid Science 29 (2004) 5364RE- S3H1

20

40

60

t%

1.0

2.0

3.0

t 18 points (at angular intervals of 20) on the circum-ference of radius R 30 mm;

24 points (at angular intervals of 15) on the circum-ference of radius R 40 mm.

The radius of the largest circumference is limited to

40 mm because of the radial size of the probe head(about 16 mm), the internal diameter of the impeller

being equal to 124.2 mm.

RE- H1

R2r - H1

R3r - H1

0

20

40

60

t%

0

20

40

60

t%

0.0

1.0

2.0

3.0

0.0

1.0

2.0

3.0

0.0 0.5 1.0

0.0 0.5 1.0

0.0 0.5 1.0

0.0 0.5 1.0

0.0

1.0

2.0

3.0

0

20

40

60

t%

0

0.0

1.0

2.0

3.0

0.0

t

0.0

1.0

2.0

3.0

t

0.0

1.0

2.0

3.0

t

0.0

Fig. 8. Total pressure coecient (circles) and total eciency (triangleRE- VEt t%

20

40

605. Results and comments

The data collected in the experimental investigation

have been grouped in tables according to the congura-

tion and the ow coecient. The ow elds have been

then reconstructed by an interpolation of the data in the

plane perpendicular to the impeller axis. The results of thisanalysis are shown in Fig. 10, in which the elds of the

local total pressure coecient are shown for each of the

RE- H2

R2r - H2

t%R3r - H2

0.0 0.5 1.0

0.0 0.5 1.0

0.0 0.5 1.0

0.0 0.5 1.0

t

t

t

0

20

40

60

t%

0

20

40

60

t%

0

20

40

60

0

s) curves for the congurations considered in the test program.

considered congurations at the investigated ow coe-

cients. The measured velocity vectors are superimposed tothe corresponding elds, according to a common scale

that refers to the impeller peripheral speed u2, which isindicated on the outer circumference of the represented

impellers.

5.1. Vortex position

At a given ow coecient, the position of the vortex

center is deeply inuenced by the shape of the casing. As

already observed by other authors in the literature, thevortex does not abandon the inner periphery of the

impeller provided that the radial width of the rear wall is

sucient (R2r and R3r rear walls). On the contrary, for

casing shapes featuring a small radial width rear wall

(RE) the vortex is forced to move also in the radial

direction towards the impeller axis, no matter the vortex

wall thickness. A thicker vortex wall causes a reduction of

vortex eccentricity, especially at low ow rates (U 0:2).The height of the vortex wall plays an important role on

the eccentricity of the vortex as well. It appears from

Fig. 10 that the vortex is more eccentric in the RE-H2

casing than in RE-H1, because of the larger space avail-

able to vortex expansion in the discharge region.

In Fig. 14 the angular coordinate fv of the vortexcenter, extrapolated from the measured data, is shown

as a function of the ow coecient. For all the consid-ered congurations, the vortex tends to depart from the

vortex wall under throttling with a progressive move-

ment that shows an almost constant trend. The results in

[14] showed that the vortex maintains its angular posi-

Fig. 9. The measurement grid inside the impeller at half of the

axial length L and denitions of the angular coordinate and yawangle.

A. Toolo et al. / Experimental Thermal and Fluid Science 29 (2004) 5364 59Other sample diagrams of the yaw angle (dened as in

Fig. 9) and of the local total and static pressure coe-

cients are presented for easy reference in Figs. 1113,

respectively.Fig. 10. The elds of the local total pressure coecient and the velocity vector

tip speed vector shown at the outer periphery of the impeller.tion approximately unchanged for U > 0:8, or evenmoves away from the vortex wall approaching the

maximum ow rates, but these were not observed in the

present experimental study. At a given ow coecient,

s measured experimentally. Velocity vectors are scaled according to the

R2r - H1

-120-90-60-30

0306090

120

-180 -120 -60 0 60 120 180

yaw [] = 0.2 = 0.4 = 0.66 = 0.8 = 1.0

Fig. 11. The yaw angle measured along the outer circumference

(R 40 mm) for a sample conguration at dierent ow coecients.

R2r - H1

-6

-4

-2

0

2

4

6

-180 -120 -60 0 60 120 180

t = 0.2

= 0.4 = 0.6 = 0.8 = 1.0

Fig. 12. The total pressure coecient measured along the outer cir-

cumference (R 40 mm) for a sample conguration at dierent owcoecients.

Fig. 10 (continued)

60 A. Toolo et al. / Experimental Thermal and Fluid Science 29 (2004) 5364

A. Toolo et al. / Experimental Thermal and Fluid Science 29 (2004) 5364 61R2r - H1

-16

-12

-8

-4

0

4

8

= 0.2 = 0.4 = 0.6 = 0.8 = 1.0

sthe design parameter that exerts the most signicantinuence on the angular position of the vortex is the rear

wall radial width. In fact, the vortex is closer to the rear

wall as the radial width increases. On the other hand, for

the same rear wall, the vortex moves towards the rear

wall as the height of vortex wall diminishes. These ten-

dencies are also observed in Fig. 15, where the angular

distance DfvV of the vortex center from the tangent tothe vortex wall edge is shown as a function of the owcoecient. In fact, DfvV decreases for rear walls havingsmaller radial widths and for vortex walls being placed

in lower positions. Therefore, it appears that the recir-

culating ow tends to occupy all the available portion of

the discharge arc according to the shape of the casing

-20-180 -120 -60 0 60 120 180

Fig. 13. The static pressure coecient measured along the outer cir-

cumference (R 40 mm) for a sample conguration at dierent owcoecients.

-120

-100

-80

-60

-40

-20

0

0 0.2 0.4 0.6 0.8 1 1.2

RE-S3H1RE-VERE-H1RE-H2R2r-H1R2r-H2R3r-H1R3r-H2

v

Fig. 14. The angular coordinate of the vortex center at dierent ow

coecients for all the considered congurations.and to the space that is required for the passage of the

throughput ow rate. It is also worth noting that the

change in the vortex wall height from H1 to H2 pro-

duces a larger change in fv as the rear wall radial widthincreases, and that the corresponding change produced

in DfvV is on the contrary smaller. For thick vortexwalls, the recirculating ow is partially guided by the

vortex wall itself, and therefore the angular position ofthe vortex is inuenced also by the shape of the clear-

vV

0

20

40

60

80

100

120

0 0.2 0.4 0.6 0.8 1 1.2

RE-S3H1RE-VERE-H1RE-H2R2r-H1R2r-H2R3r-H1R3r-H2

Fig. 15. The angular distance of the vortex center from the tangent to

the vortex wall edge at dierent ow coecients for all the considered

congurations.ance and the thickness of the vortex wall. In particular, a

thicker vortex wall (VE) keeps the vortex core farther

away (Fig. 15), since the recirculation zone is wider.

5.2. The velocity vectors

As ow rate increases, in all the considered congu-rations the ow eld pattern seems to rotate jointly

around impeller axis, in the direction of impeller rota-

tion (Fig. 10). This phenomenon can also be observed in

Fig. 11 where the yaw angle on the external circumfer-

ence (R 40 mm) is shown at dierent ow coecientsfor a sample conguration. The curves, in fact, tend to

simply shift towards higher yaw angles and angular

coordinates. For thin vortex walls, the yaw angles arehigher when higher positions of the vortex wall and

smaller radial widths of the rear wall are used. This

trend can be easily explained when the path that the

throughow has to follow around the recirculation zone

is considered. For thick vortex walls, on the other hand,

the yaw angles are much higher than those measured

with thin vortex walls. The mean values of the yaw angle

in the zone covered by the measurement grid areapproximately equal to 0, 15 and 40 when the rear

rmalwalls R3r, R2r and RE, respectively, are matched with

the thin vortex wall. The same value approaches 75when the rear wall RE is matched with the thick vortex

walls S3H1 and VE. Therefore, the overall curvature of

the streamlines in the ow eld within the fan is muchlower when thick vortex walls are used.

Most of the analytical studies in the literature pro-

posed ow eld models in which the streamlines are as-

sumed to be concentric to the vortex core. It appears

from Fig. 10 that this happens only when a special

equilibrium exists between the throughow and the re-

circulating ow, depending on the casing conguration

and, to some extent, on the ow coecient. In particular,the streamlines tend to be more concentric as larger rear

wall radial widths and higher positions of the thin vortex

walls are used, that is when a larger space is available to

the recirculating ow. On the other hand, the streamlines

of the ow eld obtained with the rear wall RE and the

two thick vortex walls seem to be almost concentric.

The length of the velocity vectors in Fig. 10 clearly

shows that the highest velocities for a generic congu-ration are obtained on the boundary of the vortex core.

The highest velocity values increase with the ow coef-

cient, attracting larger and larger portions of the

throughput ow in the proximity of the recirculation

zone. For instance, the maximum velocities measured on

the largest circumference using the R2r-H1 casing are

equal to 2, 2.2, 2.4, 2.7 and 3.2 times the peripheral

speed at U 0:2, 0.4, 0.6, 0.8 and 1.0, respectively. Thehighest velocity values, and the vortex strength (i.e.

circulation) accordingly, also increase noticeably for

larger radial widths of the rear wall. For example, at

U 0:4 the maximum velocities are equal to 1.9, 2.2and 2.6 times the peripheral speed using the casings

RE-H1, R2r-H1 and R3r-H1, respectively.

5.3. Total pressure

Fig. 10 shows that for all the considered congura-

tions the local total pressure coecient is greater than

zero only in the angular sector containing about the

whole geometrical inow arc and the half of the geo-

metrical outow arc that is close to the rear wall. Con-

versely, near the vortex core the local total pressure

coecient drops to negative values, and for a givenconguration diminishes as the ow rate increases. The

minimum local Wt in the vortex core diminishes signi-cantly as the rear wall radial width increases. The mini-

mum depression in the vortex core is registered when the

rear wall RE is matched with the two thick vortex walls.

Due to these eects, the ow eld induced by the

vortex cannot be described using a potential ow. It

cannot even be approximated by a combined Rankinevortex, since within the impeller the value of the local

total pressure coecient increases with the distance

62 A. Toolo et al. / Experimental Thefrom the vortex core according to annular strips, moreor less concentric to the vortex center, the extension of

which depends on both the conguration considered and

the ow coecient. At low ow rates, the strips asso-

ciated with the higher energy are located on the sector of

the inner periphery of the impeller opposite to theeccentric vortex. As the ow rate increases, these strips

tend to expand towards the vortex center, while in the

vortex and in the strips adjacent to it the local total

pressure coecient continues falling. At the highest ow

coecients (U equal to 0.8 or 1), the strip of maximumenergy further concentrates around the vortex, whereas,

in the region of the impeller opposite to the vortex, a

decrease in total pressure occurs due to the unfavorableincidence on the portion of the suction arc that is far

from the vortex wall. This phenomenon is more evident

when low positions of the thin vortex wall are used,

since the geometric suction arc is larger and the inci-

dence conditions of a higher number of streamlines are

worse. The existence of this low energy zone due to stall

can be identied in Fig. 12, where the local total pres-

sure coecient on the outer measurement circumference(R 40 mm) is shown at dierent ow coecients forthe R2r-H1 conguration. When U is greater than 0.6the curves show a relative minimum between a pair of

maxima in the region opposite to the vortex, instead of

an absolute maximum.

In thin vortex wall congurations, a small decrease of

total pressure can be seen at low ow coecients (0.2

and 0.4) in an annular strip of limited thickness in cor-respondence to the wake of the vortex wall edge. This

phenomenon is probably due to the mixing between the

recirculation ow and the inow and also appears in

Fig. 12, where two pairs of relative maxima and minima

can be identied.

5.4. Static pressure

The local static pressure coecient within the impeller

is largely negative in all the considered congurations.

The typical shape of the curves representing the local

static pressure coecient on the external measurement

circumference are more regular than those of the local

total pressure coecient, as shown in Fig. 13 at dierent

ow coecients. These curves simply feature an absolute

minimum in the points that are closest to the vortex core.The minimum value follows the same trend of the min-

imum recorded for the total pressure coecient, that is it

diminishes as the ow coecient or the radial width of

the rear wall increase. On the other hand, the maximum

values of the localWs are obtained in a region that is nearthe edge of the rear wall. This maximum values become

higher as the radial width of the rear wall decreases, up

to small positive values in a narrow angular sector withthe R2r rear wall and up to large positive values in the

angular sector in which the radial clearance is constant

and Fluid Science 29 (2004) 5364with the RE rear wall. This static pressure recovery is

7. Concluding remarks

walls are matched with the intermediate (R2r) and

large (R3r) radial width rear walls (see [3]) because

mediate radial width of the same wall (R2r). A minor

decrease in the eciency and performance is due

The directions for the development of the present

rmal and Fluid Science 29 (2004) 5364 63a larger space is available for vortex expansion inthe discharge zone and a stronger vortex is formed.

When the vortex has the maximum eccentricity and isWhen the measured ow eld patterns (Fig. 10) are

compared to the performance of the corresponding fan

congurations (Fig. 8), a relationship can be outlined

as follows:

Flow eld patterns in which the eccentricity and thestrength of the vortex are limited are obtained atlow ow rates using small radial widths of the rear

wall (RE) and thick vortex walls. Under these condi-

tions the total pressure coecient is very low, but the

total eciency is higher if compared to thin vortex

wall congurations.

Maximum eciency is achieved when the vortex hasthe maximum eccentricity (i.e. it lies on inner periph-

ery of the impeller) but a moderate strength, that iswhen a thick vortex wall is used in combination with

a small radial width rear wall at medium ow rates

(around 0.6). The total pressure coecient is also

quite high (around 2) in this case. Conversely, high

eciency values are not obtained when thick vortexprobably due to the deviation imposed to the ow by the

rst segment of the rear wall, the eects of which are

propagated towards the interior of the impeller.

6. Practical usefulness/signicance

Cross-ow fans show substantial dierences in terms

of geometrical characteristics but similar performance

and eciency if compared to centrifugal fans. The pos-

sibility of increasing the mass ow rate by simply

extending the rotor length, without increasing the dia-

meter or the rotational speed, makes them suitable forapplications in which constraints exist on radial space or

noise (for instance, electric device cooling, air condition-

ing blowers, etc.). The high number of industrial appli-

cations is not supported by a deep theoretical knowledge

in the literature about the behavior of these machines.

The physical phenomena involved in fan operation due to

the non-axisymmetric ow eld are much more complex

than in traditional turbomachinery. The practical use-fulness of having trends clear in the link between char-

acteristics of the ow eld and geometrical parameters

allows the basics to be set for a design pursuing one or

more objectives (maximum eciency, maximum energy

transfer, maximum ow rate) at the same time.

A. Toolo et al. / Experimental Thevery strong, the eciency is penalized whereas thework are:

The reconstruction of the link between the ow eldpattern and the performance/eciency curves by

numerical simulations of the entire ow eld to be

validated using the experimental data collected on

fan performance and on the ow quantities within

the impeller;

The investigation on the mechanism by which energyis transferred to and wasted by the streamlines cross-

ing the impeller, to evaluate precisely the losses due

to volumetric and aerodynamic causes;

The prediction of the performance/eciency of a gen-eric conguration for a given set of the main design

parameters, to search for the optimized set of design

parameters and corresponding optimal performances

according to multi-objective optimization techniques.

References

[1] L. Lazzarotto, A. Lazzaretto, A. Macor, A.D. Martegani, On

cross-ow fan similarity: eects of casing shape, J. Fluids Eng. 123to higher positions of the thin vortex wall, which

result in a stronger and more eccentric vortex.

The combination of the small radial width rear wallwith the lower positions of the thin vortex wall is par-

ticularly unfavorable, because the vortex is not eccen-

tric enough to produce high performance, but it is

strong enough to penalize the eciency.

The results of qualitative non-standard noise mea-surements performed in [3] showed that noise genera-

tion is lowered by increasing vortex wall thickness

and by reducing rear wall radial width. This trend

can be explained by the reduced interaction between

the vortex inside the impeller and the blade cascade,

due to the lower strength and eccentricity of the vor-

tex itself (Fig. 10), as already suggested by the few

authors who have reported experimental noise mea-surements in the literature [4,7].

8. Recommendations and future research needs

The nal objective that are still to be achieved in

cross-ow fan research are the formulation of a general

theory for fan operation and an ultimate set of design

criteria according to the dierent objectives that could

be considered (maximum eciency, maximum energy

transfer, maximum ow rate).total pressure coecient increases towards its highest

values. This can be noted with large radial width of

the rear wall (R3r) and, to a lesser extent, with inter-(3) (2001) 523531.

[2] A. Lazzaretto, A criterion to dene cross-ow fan design

parameters, J. Fluids Eng. 125 (4) (2003) 680683.

[3] A. Lazzaretto, A. Toolo, A.D. Martegani, A systematic exper-

imental approach to cross-ow fan design, J. Fluids Eng. 125 (4)

(2003) 684693.

[4] B. Eck, Fans, Pergamon Press, Oxford, 1973.

[5] B. Eck, UK Patent 757543, 1956.

[6] G. Datwyler, UK Patent 988712, 1965.

[7] A.M. Porter, E. Markland, A study of the cross ow fan, J. Mech.

Eng. Sci. 12 (6) (1970) 421431.

[8] R. Coester, Theoretische und experimentelle untersuchungen an

querstromgeblase, Mitteilungen aus dem Institut fur Aerodynamic

ETH 28, 1959.

[9] H. Tramposch, Cross-ow fan, ASME Paper No. 64-WA/FE-25,

1964.

[10] H. Ilberg, W.Z. Sadeh, Flow theory and performance of tangen-

tial fans, Proc. Inst. Mech. Eng. 180 (19) (1965) 481496.

[11] H. Ikegami, S.A. Murata, Study of cross ow fan. I. A theoretical

analysis, Technol. Rep. Osaka Univ. 16 (1966) 557578.

[12] K. Yamafuji, Studies on the ow of cross-ow impellers2nd

report, analytical study, Bull. JSME 18 (126) (1975) 14251431.

[13] K. Yamafuji, Studies on the ow of cross-ow impellers1st

report, experimental study, Bull. JSME 18 (123) (1975) 1018

1025.

[14] S. Murata, K. Nishihara, An experimental study of cross ow

2nd report, movements of eccentric vortex inside impeller, Bull.

JSME 19 (129) (1976) 322329.

[15] P.R. Tuckey, M.J. Holgate, B.R. Clayton, Performance and

aerodynamics of a cross ow fan, International Conference on

Fan Design and Applications, Paper J3, Guilford, England,

September 79, 1982.

[16] H. Tsurusaki, Y. Tsujimoto, Y. Yoshida, K. Kitagawa, Visual-

ization measurement and numerical analysis of internal ow in

cross-ow fan, J. Fluids Eng. 119 (5) (1997) 633638.

[17] UNI 10531, Ventilatori industrialimetodi di prova e condizioni

di accettazione (in Italian), Milan, 1995.

[18] ISO 5801, Industrial fansperformance testing using standard-

ized airways, 1993.

64 A. Toolo et al. / Experimental Thermal and Fluid Science 29 (2004) 5364

An experimental investigation of the flow field pattern within the impeller of a cross-flow fanIntroductionTheoretical and experimental studies of the flow field pattern in the literatureThe experimental apparatusThe test programResults and commentsVortex positionThe velocity vectorsTotal pressureStatic pressure

Practical usefulness/significanceConcluding remarksRecommendations and future research needsReferences