TP04-04, New Fan Blade Tip Design

8/3/2019 TP04-04, New Fan Blade Tip Design

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PAPER NO: TP04-04CATEGORY: VIBRATION

COOLING TECHNOLOGY INSTITUTE

NEW FAN BLADE TIP DESIGNREDUCES STRUCTURAL VIBRATIONSVERIFICATION IN PRACTICE

SANDER C. VENEMAHOWDEN COOLING FANS

CHRIS B. LAZENBY, SOUTHERN COMPANY SERVICES

The studies and conclusions reported in this paper are the results of the authors own work. CTI has not investigated, and CTIexpressly disclaims any duty to investigate, any product, service process, procedure, design, or the l ike that may be describedherein. The appearance of any technical data, editorial material, or advertisement in this publication does not constituteendorsement, warranty, or guarantee by CTI of any product, service process, procedure, design, or the like. CTI does not warrantythat the information in this publication is free of errors, and CTI does not necessarily agree with any statement or opinion in thispubl ication. The user assumes the ent ire r isk of the use of any information in this publ icat ion. Copyright 2004. All rights reserved.This paper has been reviewed by members of the Cool ing Technology Inst itute and approved as a valuable contr ibution to cool ingtower literature and presented bythe author atthe Annual Conference of CTL

Presented at the 2004 Cooling Technology Inst ituteAnnual ConferenceHouston, Texas - February 8-11, 2004


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New Fan Blade Tip Design Reduces Structural Vibrations-Verification in PracticeSander C. VenemaProduct SpecialistHowden Cooling [email protected]

Chris B. LazenbySenior EngineerSouthern Company [email protected]

IntroductionThe main purpose of this study is to understand the origin of vibration of the fan stack andto find ways to keep these vibrations within acceptable limits. Assuming that the vibrationof the fan stack is not caused by resonance, then the pressure field around the fan bladetip is the cause of stack vibration. Taking a fixed position at the fan stack as a referencepoint, it is evident that the fan stack experiences a pulse force each time a fan bladepasses by. To avoid unacceptable vibration levels, the pulse force should be kept below acritical level. The easiest way to achieve this is to restrict the aerodynamic performanceper blade. In practice, this translates into a limitation of "horsepower per blade". If themaximum horsepower per blade is an aerodynamic design criterion, this will often lead tomore blades per fan than is necessary to achieve the required duty point. A morechallenging way to keep the pulse force below a critical level is to redesign the blade tip insuch a way that the pulse force is reduced without losing aerodynamic performance. Thismakes it possible to apply fans with fewer blades without the risk of introducing a vibrationproblem.At the 2003 CTI Annual Conference the development of a new blade tip design and itspotential benefits was presented [CTI TP03-05: Vibration Control: New Fan Blade TipReduces Pulsation, Henk van der Spek, Howden Cooling Fans]. This paper gives theresults of verification tests in a full-scale practical application.

Pressure field around the blade tipThe principle of the working of a fan blade is basically due to the difference in air velocitybetween either sides of the airfoil. The shape of the airfoil leads to an air velocity at theconvex side of the blade, which is locally higher than at the concave side of the blade. Thisresults in a pressure difference over the blade called lift. The lift is a useful factor becauseit forces the air to flow along the flow resistance of the installation. It cannot be changedwithout influencing the performance of the fan.At the blade tip, there is a second factor contributing to the pressure field. Because theblade tip moves at high speed along the fan stack, an under pressure is generated in thegap between the blade tip and the fan stack. This under pressure acts on the surface areaof the tip and on the part of the fan stack where the fan blade passes by at that specificmoment. We call this factor the "surface velocity pressure".Both the lift and the surface velocity pressure are illustrated in figure 1. Figure 1a showsthe air velocity at both sides of the airfoil as well as the pressure components lift andsurface velocity pressure. Figure 1b illustrates the resulting pressure distribution. This isthe variation of the pressure at a fixed position on the fan stack as a fan blade passes.

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mailto:[email protected]:[email protected]:[email protected]:[email protected]


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. - - ~- pressure

distributionair velocityliftsurface velocity pressure

figure 1a: air velocity and pressure componentsaround fan blade tip

figure 1b: pressure distribution around fan bladetip

Strategy for the reduction of pulsationThe lift and the surface velocity pressure both contribute to the pulse force on the fanstack. Because the lift cannot be reduced without reducing the fan performance, this studyconcentrated on reducing the surface velocity pressure.As mentioned before, the surface velocity pressure acts on the surface area of the bladetip. The thought, then, was that minimizing the surface area of the tip using a plate airfoilmight help reduce this velocity pressure. However, to be considered properly designed thisairfoil would have to maintain the same fan performance. A new design (patent pending),named 'Aerotip' (figure 2), was developed and tested in both scale model versions andultimately in a full size 32-foot diameter fan laboratory facility. The results, as follows, wereencouraging.

- For a fan whose blades were equipped with the 'Aerotips', the vibration level of thefan stack was 30% lower than a fan with standard blade tips (figure 3).

- The 'Aerotips' resulted in an increased maximum pressure of up to 10%, and anincreased fan efficiency of up to 5% (figure 4).These results indicate that it is possible to reduce the pulse forces on the fan stack withoutsuffering a reduction of fan performance. On the contrary, the 'Aerotip' seemed to increasethe fan performance slightly.Details of the tip design and test set-up are given in TP03-05.

f igure 2: 'Aerotip'

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25 'Aerotip'. . . . .. ! ! ?E. . 20'">"C 15)

Q)C.Inc0 10; : : ;~.c's 5

03 40 50 600 tipspeed [ m / s ]

figure 3: Vibration level on the fan stack as a function of the fan rotation speed

e 0.28: : : IIng j 0,26. . .c.In 0.24In~ 0.22o' i i i 0.20c:Q)E 0.18: c-; 0.16~{ S 0.14

0.120.10+-------~-------r------------------------~------~o

60 -: : < !~~'">.50 f.)c:Q)'(3! EQ)40

300.05 0.10 0,15 0.20 0.25 0.30

Cf (dimensionless flow)

figure 4: Comparative performance of fan pressure and fan efficiency versus airflow for conventional fansand fans equipped with the 'Aerotips'

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Field testingIn collaboration with Southern Company Services and Mississippi Power, the fiberglassconstruction cooling tower of Plant Daniel Unit 4 (Moss Point, Mississippi, USA) wasselected to test cooling fans both with and without 'Aerotips' to verify the benefits of the'Aerotip' in a practical application. The existing cooling tower has 33'-0" diameter fans.Aerodynamic and vibration measurements were taken for several test fan configurationswith and without the 'Aerotip'. To get a correct comparison between the two tip shapes, allfans were equipped with an 'Aerotip' and the standard tip was constructed by filling up the'Aerotip' with specially created foam pieces. This way the blade angle and tip clearancewere kept exactly the same.Tests were conducted using two different models of fan blades, the "E" and the "Z" type.The "E" type has a wide blade chord, the "Z" type has a standard blade chord.

Aerodynamic measurement:The cross section of the fan stack just in frontof the fan was virtually divided into ten equalarea bands. The air velocity and the totalpressure were measured at four positions ineach of the ten equal area bands (figure 5).This distribution of measurement pointsmakes it possible to calculate the total fanperformance by averaging the individualmeasurement results. This method ofmeasurement is in accordance with the CTI"Recommended Practice For Airflow Testingof Cooling Towers" (PFM-143).

figure 5: measurement points for aerodynamicmeasurements (10 points per quadrant)

Figures 6a, 6b, and 6c give the results of the aerodynamic measurements. Each figuregives the measured pressure against flow with and without the 'Aerotip' for one fanconfiguration. The system resistance curve is based on the average of all performedmeasurements. The fan curve is copied from the manufacturer's selection program usingthe measured fan configuration; the number of blades, blade angle, and the fan speed asinput. For the "E" type, the selection program is based on blades with a standard blade tip,whereas the "Z" selection program data is based on blades equipped with an 'Aerotip'.In figures 6a thru 6c, actual measurements confirm the fan selection data provided by themanufacturer's fan selection software. In all three cases, the fan curve crosses themeasurement uncertainty field.Although the differences in the airflow of fans with and without the 'Aerotips' are within themeasuring tolerance, there are clear indications that the 'Aerotips' increase the fanperformance. The air velocity patterns as plotted in figures 7a, 7b, and 7c confirm this. Theair velocity at the blade tip is significantly higher for blades with an 'Aerotip' than for bladeswith the standard tip shape.

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~::J~ 150~o,, g 100e ntJc~ 50 +-----.-.----..

ro200e : . .~i j; 150(f)~; 100 - System resistance curve .-----.-r"-----. l . 2 'oq.) /~ --- Fan curve (sel. pr7-----~ 5 : ~ - - . - - - -

o

ro 2000. . .~~ 150(f)~o,'-' 100~tJcJ !! 50

~200

250 -r- .. -.--.----------- .....-.-8 bladed "E" type 1 3.9 deg.

Aerotip~, : - . . . . . . . --_-----_- --

. . . . . . . . . . . . . . . ,

'~"~~~-standard tip- System resistance curve---Fan curve (sel. prog.)

o 300 60000 200 400 500 700 800 900air flow (m 3/s )

figure 6a: 8-bladed "E" measured pressure against f low

25 0

---------

6 bladed "Z" type 110.0 deg.

Aerotip standard tip

- System resistance curveFan curve (sel. prog.)

a 100 200 300 400 500 600 700 800 900air flow (m 3/s )

figure 6b: 6-bladed "Z" measured pressure against f low

250 f - 6 bladed r z : type /7.9 ;;;9. Aerotip ~ ~ " " " = - _ - - ---"'.------=-~ standard tip _..-1'--+''"i--1~~,---~~~I

~ " I

100 200 300 400 500 600 700 800 900air flow (m 3/s )

figure 6c: 8-bladed "Z" measured pressure against f low

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1412

:0'10E: ; 8.;:;0 6jj>'-.( ii 4

205

8 bladed"E" type f 3.9 deg.----------------------- -a-. Aerotip

~ standard tip

4 3 2 ofan radius (m)

figure 7a: 8-bladed "E" measured air velocity pattern (average of 4 quadrants)

1412

:0'10E;8.;:;0 6jj>'-.( ii 4

20 tip5 4

6 bladed "Z" type f 10.0deg.+-__ ---o---------o----o-o--------------------- .....Aerotip........tandard tip

3 2 ofan radius (m)

figure 7b: 6-bladed "Z " measured air velocity pattern (average of 4 quadrants)

1412

:0'10.s: z : . 8. ; :;0 6jj>'-0( i i 4

20

5

8 bladed "Z" type f 7.9 deg.-,..-Aerotip......standard tip

fan center line ioo_ ."~._..~4 3 2 o

fan radius (m)f igure 7c: 8-bladed "Z " measured air velocity pattern (average of 4 quadrants)

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Vibration measurement:The vibration amplitudes were measured by reading overall root mean square speedvalues (Voverall in mm / s rms) at the outside of the fan stack. This overall value includesvibrations of all frequencies ranging from 1 Hz-10kHz. Probe 1 was placed at the bladepassing level; probe 2 was placed above the stiffening ring (figure 8).

figure 8: vibration measurement posit ions at fan stack

The results of the 6-bladed "Z" are not usable to qualify the influence of the 'Aerotip'. Itturned out that the natural frequency of the fan stack was close to the blade passingfrequency for a 6-bladed fan at this specific fan speed (rpm). This resulted in very highvibration levels due to resonance up to 100 mm / s rms. During normal operation,resonance vibrations always should be avoided because of the high and uncontrolledvibration levels. This study did not focus on resonance phenomena. Therefore, thevibration levels measured with the 6-bladed "Z" are left out of the data.Table 1 gives the results of the vibration measurements with the 8-bladed "E" and 8-bladed "Z".

8-bladed "E" 8-bladed "Z"Voverall (mm/s rms) v overall (mm/s rms)

standard tip 'Aerotip' standard tip 'Aerotip'I probe 1 32 18 14 13probe 2 14 14 17 21table 1: results of vibration measurements

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The 8-bladed "E" equipped with the 'Aerotips' performed in accordance with theexpectations. The vibration speed at the blade passing level (probe 1) was about 44%lower than when applying a standard tip. At the upper level of the fan stack (probe 2),there was no change in vibration level; however, the values were already quite low.For the 8-bladed "Z' there was no significant difference in vibration level between a fanwith the 'Aerotips' and a fan with standard tips. At the upper level of the fan stack (probe 2)the 'Aerotip' even showed slightly higher vibration levels.To help explain why the 'Aerotip' seemed to have a very positive effect on the "E" bladewhile having a less positive, or even slightly detrimental effect, on the "Z" blade, it isnecessary to consider two major factors: the geometry of the airfoil and the aerodynamicperformance.The main difference between the "E" and the "Z" blades is the chord, and thus the surfacearea of the blade tip. The "E" blade tip has a surface area that is about 85% larger thanthat of the "Z" blade. Therefore, the 'Aerotip' effectively reduces the surface area of the "E"blade much more than it does for the "Z".With regards to aerodynamic performance, the 'Aerotip' causes a higher airflow at theblade tip for both the "E" and the "Z" designs.The effect of these two factors on the pressure distribution around the blade tip isillustrated in figure 9. The solid line is the pressure distribution around the standard tip.The dotted line indicates the pressure distribution around the 'Aerotip'. The difference inmaximum and minimum pressure is the pressure pulse (Ap). The 'Aerotip' decreases thenegative surface velocity pressure and increases the positive lift. The concept is that thechange in surface velocity pressure is dominant to the increase in lift for the "E", resultingin a decrease of the pressure pulse. For the "Z", the change in surface velocity pressureand the change in lift compensate each other. Although the pressure pulse effectively doesnot change, the fan performance increases. Laboratory tests confirm this difference in thepressure distribution around "E" and "Z" blades .

-- standard tip...................crotip

ilpstanuard

./'-'-'-'-'-'-'-'-/.'\\ilpAerotip

-- standard tip...................Acrotip

"E " " Z "figure 9: Pressure distribution around the blade tip for "E" blade and "Z" blade

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Additional testingAs mentioned in the Introduction, there are two ways to keep the pulse force on the fanstack within acceptable limits by restricting the aerodynamic performance per blade,(translated in a limitation in "horsepower per blade"), or by redesigning the fan blade tipshape. To verify if higher levels of power per blade are allowed for fans with 'Aerotips',additional laboratory tests were done. For two different fans, both with and without'Aerotips', the vibration displacement of the fan stack at blade passing level was measuredat several fan speeds. At each fan speed, the power consumption was measured. Fordetails about the test set-up, see CTI technical paper TP03-05.The results are given in figure 10. This figure shows that when an acceptable vibrationdisplacement of the fan stack is defined, more horsepower per blade can be allowed forfans with the 'Aerotips' than for other fans. The acceptable vibration displacement dependson the mechanical stiffness of the fan stack and on the vibration frequency.Depending on the configuration, an increase in acceptable horsepower per blade of 20%to 30% is possible.If for instance, the maximum acceptable vibration displacement for the tested fan stack isdefined to be 0.5 mm, than, the maximum allowed horsepower per blade can be increasedfrom 13 kW to 17 kW for the 3-bladed "E" type fan and from 15 kW to 20 kW for the 8-bladed "Z" type fan. This results in an increase of about 30% power for both fan types.

3-bladed "E" 8-bladed "Z"1.0E 'Aerotip' 0.9o S standard tip

. . l < : - 0.8III


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Conclusions: The actual fan performance was in line with the selection data provided by the

manufacturer's fan selection software. The 'Aerotip' increases the air velocity in the area of the blade tip. This improvesthe overall performance of the fan. For blades with a wider blade chord, like the "E" type, the 'Aerotip' directly results ina reduction of the vibration level of the fan stack, as well as an improved fan

performance. For blades with a smaller blade chord, like the "Z" type, the 'Aerotip' directly resultsin a fan performance improvement, while the vibration is maintained withinacceptable levels. If the increased fan performance is not required, a lower fan

speed or a reduced blade angle can be selected. This results directly into lowervibration levels of the fan stack. The 'Aerotip' generates lower vibration levels at equal power per blade. Therefore,

the 'Aerotip' allows fewer blades per fan without increasing the risk of vibrationproblems. Depending on the configuration, an increase in acceptable horsepowerper blade of 20% to 30% is possible.

As always, careful attention should be paid during the tower design phase toensure that there is no overlap between the natural frequency of the fan stack andthe blade pass frequency of the fan...

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TP04-04, New Fan Blade Tip Design