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Mech. & Aerospace Eng. J., Vol. 3, No. 4, Winter 2008, pp. 61-67 61

Effect of Wind Break Walls on Performance of

a Cooling Tower Model

ABSTRACTEnvironmental cross winds usually distord the uniform distributions of both air flow and air resistance at inlet to

cooling towers and subsequently cause hot areas due to insufficient local cooling of water by air. Curative

devices, such as windbreaks, are developed to enhance the performance of the power station cooling towers. At

present experimental investigation, the effects of curative devices on performance of a 1/1000 scaled isothermal

model of a 660MW wet cooling tower were quantified, using intake dynamic and total pressure losses.

Dimensional simulitudes were used in the isothermal modelling of the wet cooling tower. A dimensionless

pressure loss coefficient in the tower was defined, as the performance indicator, which is the ratio of total

pressure drop in the heat exchanger region (i.e. combined packing and rain zone) to the dynamic pressure. Over

twenty three curative devices were designed, manufactured, and their effects on the pressure loss coefficient

were quantified. Pressure drop coefficient with no curative device was selected as the base condition and was

compared with the presence of curative devices. Up to 33% improvement was noticed in the pressure loss

coefficient. The top three most efficient curative devices were selected and presented here.

Key Words: Curative Device, Natural Draft Wet Cooling Towers, Windbreak Wall, Pressure Drop Coefficient

)

(Vr)

1- Asistant Professor

2- Asistant Professor3- Associate Professor (Corresponding Auther): [email protected]

J. Madad-Nia and H. Koosha M. Mirzaei

Sydney Univ. of Tech., Aerospace Eng. Dep t.Australia K.N.Toosi Univ. of Tech.

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62 Mech. & Aerospace Eng. J., Vol. 3, No. 4, Winter 2008

Nomenclature

pt Dynamic Pressure Drop in the ModelTower (= Htower)

pw Dynamic Pressure Drop in the WindTunnel (= Hwind)

p0 Total Pressure Drop in the Tower(Measured at the Reference Positions

Between Tower Outside and Inside = H0)

D Base Diameter of the Model Tower

h Tower Intake Height

a Circumference Gap Between 2 BreakWalls

b Length of a BreakWall

c Radial Gap Between the Centre ofWalls

and the Outer Edge of the Model Tower

d the Angle Between a BreakWall and

Radial Direction

e the Angle Between the BreakWall and it s

Vertical Directionf Height of the BreakWall

Air Density

vtower Air Speed Inside the Tower

Uwind Air Speed in the Wind Tunnel at the

Reference Location

Cp the Dimensionless Tower Inlet Pressure

Drop Coefficient

Vr Velocity Ratio ( towe/windr vUV )DCp Percentage ofPerformance

Improvement ( 100Cp

)CpCp(DCp

0

0 )

Cp0 Pressure Loss Coefficient for the Case

Without Device

1. Introduction

Atmospheric boundary layers distort uniformity of

air flow at inlet to the cooling towers, leading to

lower performance and higher coolant wanter

temperature from the cooling towers. The

application of curative devices such as windbreak

walls could ameliorate the performance of the

cooling tower in decreasing the pressure drop

coefficient [1-14]. The wind disturbs the natural

induced flow and the use of the curative devices

changes the air flow patern at the intake of thecooling tower which is more uniform flow at the

intake.

This work is part of a larger undertaking which

is to be run as a joint venture between the

University of Technology, Sydney (UTS), the

University of New South Wales (UNSW), and

Delta-Electricity which operates the 660 MWe

Mount Piper thermal power station in New South

Wales, Australia. Delta-Electricity collaborates

with UTS and UNSW to design, develop and apply

flow-conditioning devices on the cooling towers at

the Mount-Piper power station. The research

project involves combined studies on scaled and

numerical models and full-scale prototype tower.

This paper deals only with the design and

testing of curative devices to determine their effects

on the performance of a 1/1000 scale model cooling

tower in a wind tunnel.The important factors in the model tests are

tower inlet pressure loss, air velocity in the tower

and wind velocity. Pressure tubes were used to

measure the pressure distributions in the wind

tunnel and inside the tower model. The pressure

loss coefficient as a performance indicator was

quantified and plotted against the velocity ratio.It is

anticipated that the tower performance in windy

conditions can be predicted from a plot of the

experimentally determined tower inlet pressure loss

coefficient. Curative devices were designed and

installed on the model and their performance

enhancement effects were quantified. The three

most effective curative devices were selected and

reported here.

Zhenguo and Jinling [1] have observed the

unfavorable effect of natural wind on dry cooling

towers at a power plant in Shanxi Province.

Measures to overcome such negative wind

influence have been studied through model test in

wind tunnel. Several arrangements have been

investigated to improve the inlet airflow velocity

distribution and thereby the tower cooling

efficiency under natural wind conditions. They

observed that the addition of 4 rectangular guidewalls around the periphery of the tower air inlet

could greatly improve the tower performance.

Bender, Bergstrom and Rezkallah [2] have

examined the use of wind walls placed upstream of

the cooling tower to control the flow rate entering

the intakes. A 1/25 scale model cooling tower was

tested in the simulated atmospheric boundary layer

of a wind tunnel to investigate the effect of

different wind wall configurations. The results

show that a simple wind wall placed upstream of

the windward intake can be used to balance the

flow rate into the intakes.

The experimental results focused on the relativemagnitude of the airflow rates. An optimal wall

configuration was found by varying three wall

parameters: wall placement, wall height and wall

porosity.The best wind wall configuration based on

the tests appears to be a 10% porous wall, with a

rectangular shape and placed in front of the

modeled cooling tower. The location of the wall

and its porosity had the dominant effect on the

cooling tower intake flow rate, whereas the wall

height was less important.

Du Preez [3] studied the effect of cross winds

on the performance of natural draught dry-cooling

towers by means of model tests, numerical

simulations, and prototype measurement. He

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Effect of Wind 63

developed a correlation for the tower pressure loss

coefficient as a function of velocity ratio, diameter

to intake height ratio, and pressure drop in the heat

exchanger region. He noticed an increase in the

tower pressure loss coefficient when the heat

exchanger resistance decreased and argued that wasdue to a tendency for tower velocity distribution to

become more non-uniform. He also highlighted that

the tower performance generally becomes less

sensitive to the wind for a reduction in the heat

exchanger's pressure drop. For large values of the

intake pressure, tower performances become

independent of the pressure loss in the heat

exchanger.Gandemer [4] examined a wide variety

of wind breaks, including various wall geometries,

double walls, walls with ramps and walls with

directing fins. Gandemer s analysis was useful for

determining the design of the walls adopted in the

present study due to the number of characteristics

taken into account.

In a full scale measurements on the wet type

natural draft cooling towers, Amur et el. [12],

reported that for wind speed between 0 4 m/s, the

tower approach temperature decreased to 3K when

the wind was blowing from plant building sides.

Contrary to that about 3K rise in the approach

temperature was observed when the wind was

blowing at similar speed from the second tower

side. Amur et al. [13] also conducted experiment in

a wind tunnel on a model tower. They reported

similar results as the inlet pressure coefficient (Cpi)in the model tower was higher when the wind was

blowing from the second tower side compared to

that blowing from the plant building sides. By

using a curative device made of several radial or

angular walls, the tower Cpi improvements of up to

10% and 30% were observed for plant buildings

and second tower sides respectively. Amur et al

[14] also numerically modelled the wind effects on

cooling towers and noticed that the tower

performance was reduced when the wind is

obstructed by another cooling tower.

2. Design of the Curative DevicesA wide variety of curative devices were designed

and tested in the wind tunnel. The three most

effective curative devices were selected. The main

selection criterion was improving the performance

of the cooling tower without adverse effects in no-

wind conditions.

Each device was positioned at the intake of the

tower outside the rain zone. The following

parameters as outlined in the Table 1 and Fig.1 are

considered in the performance analysis:

Geometry of the curative device,

dimensions such as a,b,c, d and e,

porosity: porosity percentage, holes shape and

holes slope, and

wall material.

The length of the break walls, their angle relative to

the radial direction, and the gap between break

walls were included in the performance studies.Wall geometry, the height of the break walls, the

angle relative to the vertical direction, the gap

between the walls and the tower, the porosity and

the wall material were not varied. Intake cooling

tower height of h was taken as the reference height

with h = 8.5mm in 1/1000 scale model. The break

walls were assumed to be rectangular with zero

porosity.

Table (1): Tested design parameters.

Length of the

walls (b)

Angle relative to the

radial direction (d)

Gap between 2

walls (a)

h/2 0 degree h/2

h 30 degree h

1.5h 45 degree 2h

3. Wind Tunnel Facility and Apparatus

Experiments were conducted in the open wind

tunnel in the Aerodynamic laboratory of the Faculty

of Engineering at University of Technology,

Sydney (UTS).

Fig. (1): A Schematic diagram of curativedevices

positioned on a plastic ring shape support.A schematic diagram of the test rig is presented in

Fig. 2. The Open Wind Tunnel is approximately

10.5m overall length and has an octagonal test

section measuring 610mm 610mm approximately

2m long. The range of air velocity in the tunnel

ranges from approximately 0 to 60m/s. The induced

draft fan it is a centrifugal type, which allowssimulating the air exit of the tower in the real

walla

b

c

d

ef

Plastic

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Cooling Tower Model

Centrifugal InducedDraught Fan

Fan

Wind

Direction

conditions. U-tube alcohol manometers, Pitottubes,

and pressure transducers were used to measure the

required pressures and velocities in the tower and in

the wind tunnel. Pressure transducers are continent

devices for direct and automatic data acquisition by

a computer. The arrangement of these deviceshasbeen calibrated using NACA0012 airfoil test case

which its exact pressure distribution for low speed

flows is available. The results of this test case

showed that the error of this arrengemnt is less than

1.2%.

The transducer outputs a linear voltage

proportional to the applied pressure. A LabView

virtual instrument (VI) software was developed to

acquire data from the transducers.

Fig. (2): Open wind tunnel.

3. Experimental Procedure

The first step and the most delicate is to set the

curative device and to well arrange it. The

simulated stands for packing have to be at the level

to the packing mesh representing the packing in the

cooling tower. The Fig. 3 shows a positionedcurative device. The arrangement of the device is

shown clearly in Fig.1.

Once the device was installed, the data

acquisition could begin The tests involved running

the induced draft fan fixed at a frequency of30Hz

whilst varying the speed of the wind tunnel fan

over the range from zero to maximum (60m/s)

using the motor speed controllers.

The wind tunnel fan was varied from 0 to

maximum while recording measurements at regular

intervals with the manometers. For each

measurement, it is necessary to wait at least 1

minute to allow the stabilization of the alcohol inthe manometers.

Then this was repeated 3 times to evaluate the

repeatability of the results.

Fig. (3):Positioned curative device.

4. Analysis ofResultsOver twenty three curative devices were designed

and manufactured using various design parameters

shown in Fig.1. Their effects on the pressure loss

coefficient were quantified.

In Fig s. 4-a and 4-b the pressure loss of the

devices (Cp) is compared with the pressure loss of

no-device case (Cp0). Fig s. 5-a and 5-b show

variation of DCp versus Vr. Negative DCp means

negative performance and positive DCp means that

the pressure loss decreases as the curative device is

used. The trend of the curves in Fig s. 5-a and 5-b

are the same. The performance of the devices is

negative at low wind velocity. As the wind velocity

increases the presformance increases to positive

value, reaches to its maximum and then decreases

to zero or negative value. Consequently none of the

curative device can be effective at nearly zero wind

velocity and also at high wind velocity. The trend

of the variation of the performance with with wind

velocity is justifiable according to characteristics of

the flow around the tower: When the wind velocity

is nearly zero, the viscous effects are dominated,

and the presence of the devices leads to more

resistace (friction) force. Consequently the Cp will

be greater than Cp0. This can be seen in Fig s. 5-aand 5-b. Increasing the wind velocity causes non

uniform flow around the tower for no device case

as shown in Fig. 6-a . The pressure loss will

increases due to this non uniform flow. For such

conditions using the curative device changes the

velocity distribution and consequently the pressure

loss may be lesser than that of the no device case.

For higher wind velocities flow separation occurs

(Fig.6-b) that raises the pressure loss for no device

case. For such conditions using curative devises

intensifies recirculation flows and the pressure loss

gets higher. This can be seen in Fig.7.

Curative

device

Turntable

Cooling

tower

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Effect of Wind 65

0

2

4

6

8

10

12

14

16

0.16 0.33 0.52 0.76 0.99 1.26 1.75 2.25 3.74

Vr

C

p

No device

Device 32

Device 26

Device 27

(a)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.16 0.33 0.52 0.76 0.99 1.26 1.75 2.25 3.74

Vr

Cp

No device

Device 19

Device 17

Device 18

Device 29

Device 31

(b)

Fig. (4): Pressure loss at the intake of the tower

versus velocity ratio,

(a)-Cases with bad performance and

(b)-Cases with good performance.

-50

-40

-30

-20

-10

0

10

20

30

40

50

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

Vr

DCp

Device 32

Device 26

Device 27

(a)

-30.00

-20.00

-10.00

0.00

10.00

20.00

30.00

40.00

50.00

60.00

0.00 1.00 2.00 3.00 4.00

Vr

DCP

Device 19

Device 17

Device 18

Device 29

Device 31

(b)

Fig. (5): Percentage of performanceimprovement

fordevices,

(a)-Cases with bad performance and

(b)-Cases with good performance.

Fig. (6): Flow around the tower at low speed flows.

0

10

20

30

40

50

60

0.00 1.00 2.00 3.00 4.00 5.00 6.00

Vr

Cp

No device

Device 29

Device 17

Device 19

Device 22

Device 32

Device 26

Device 28

Device 31

Device 20

Device 27

Fig. (7): Pressure loss at the intake at high wind

velocities.

Among the tested devices, three of them (device 27,

19, 29) have better performance as shown in Fig s.

4 and 5. The specifications of these devices aregiven in in table 2.

The maximum of efficiency is around 54%that

can be attained using device 19 at Vr around 1. It is

noticed that the cooling efficiency is not on all the

range but this maximum is located around the value

of the velocity ratio obtained in the real situation

[7]. The common characteristic of these devices is

the axial inclination. Its value is 30 degree.

It should be noted that only the length of the

walls, the angle relative to the radial direction and

the gap between 2 walls was tested. It is deduced

that:

The ideal angle relative to the radial is 30

degree. For this angle, the cooling rate is the

best [7] consequently the pressure loss

experiments carried out at this angle,

It is difficult to deduce the optimal value of

the length of the walls because the best devices

of these comparisons have not the same length

of the walls,

It is difficult to deduce the optimal value of

gap between 2 walls because the best devices

of these comparisons have not the same gap

between 2 walls, and

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The concept of many walls around the tower is

the best one.

Table (2): The 3 best devices.

* These dimensions are schematized on Fig.1.

5. Conclusion

As it is shown, the maximum of efficiency is

around 54%. This value is obtained with a dry

cooling tower. At long term, this study applies to a

wet coolin tower. It is why the obtained cooling

effecincy can be less than the cooling efficiency

with a wet cooling tower.

Complementary tests are required to determine the

optimal value for each characteristic. At present,

only the optimal value for the radial angle is

known: 30 degree which is determined based on

maximum cooling rateb [7].

Other characteristics could be tested as thevertical angle, the gap between the wall and the

tower, the height of the walls, the porosity and the

type of material.

It is known that the wind also affect the thermal

characteristics of the natural draft cooling towers.

Arrangmenof experimental model to study the

effects of the towers is more complicated than the

presented model. The future step of the others is to

develop and experimental thermal model and

dealing optimum arrrangment of the curative

devices to compenents the reduction oe the towers

thermal performance.

Acknowledgments

This research was funded by an ARC-Linkage

grant supported jointly by Australian Research

Council, and Delta Electricity. All these tests were

peryomed in a wind tunnel on a scaled model.

References

1. Zhenguo, Z. and Jinling, S., "The UnfavorableEffect of Natural Wind on Natural Draft Dry

Cooling Towers and its Improvement

Measures", China Institute of Water Resources

and Hydropower Research, IWHR, 1998.

2. Bender, T.J., Bergstrom D.J., and Rezkallah, K.S., "A Study on the Effects of Wind on the Air

Intake Flow Rate of a Cooling Tower, Part 2.,

Wind Wall Study", J. Wind Eng. and Industrial

Aerodynamics, Vol.64, No. 1, pp. 61-72, 1996.

3. Du-Preez, A.F., "The Influence ofCross-windson the Performance of Natural Draft Dry-

cooling Towers", Mech. Eng. Dep t., Univ. of

Stellenbosch, Stellenbosch, p. 7600, RSA, 1992.

4. Gandemer,J.,"The Aerodynamic Characteristicsof Windbreaks, Resulting in Empirical Design

Rules", J. Wind Eng. and Industrial

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and Hill, W., Wind Effects on the Operation of

a Natural Draft Wet Cooling Tower, a)

Performance Analysis" , The 12th IAHR Symp.

in Cooling Tower and Heat Exchangers, UTS,

Sydney, Australia, 2001.

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Natural Drought Wet Cooling Tower, b)

Influence of the Packing Design (Model Tests)",

12th IAHR Symp. in Cooling Tower and Heat

Exchangers, UTS, Sydney, Australia, 2001.

7. Madadnia, J., Bojnordi, M., and Koosha, H.,"Wind Effects on the Operation of a Natural

Drought Wet Cooling Tower, c) Effectiveness

Analysis of Performance Improving Devices

(Scaled Model)", The 12th IAHR Symp. inCooling Tower and Heat Exchangers, UTS,

Sydney, Australia, 2001.

8. Koosha, H., Madadnia, J., and Simoff, S.,"Development of a Remote Access Research

Site in Power Station Cooling Towers", The

12th IAHR Symp. in Cooling Tower and Heat

Exchangers", UTS, Sydney, Australia, 2001.

9. Samali, B. and Madadnia, J., Wind Simulationin an Environmental Wind Tunnel for Both

Structure and Performance Studies , The 12th

IAHR Symp. in Cooling Tower and Heat

Exchangers, UTS, Sydney, Australia, 2001.

10. Madadnia, J., Koosha, H., Bojnordi, M.,and Al-Wakeed, R., An Overview of Experimental and

Numerical Studies In Power Station Cooling

Towers , The 12th IAHR Symp. in Cooling

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Natural Draught Cooling Towers under

Crosswind , The12th IAHR Symp. in Cooling

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Australia, 2001.

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Effects on the Natural Draft Cooling Towers of

Device number

CriteriaDevice 29 Device 19 Device 17

Geometry type Rectangular Rectangular Rectangular

Number of walls 43 21 21

Height

(f)*h h h

Dimensions

Length

(b)*1.5h 1.5h h

Radial angle (d)* 30 30 30

Gap between 2

walls (a)*h 2h 2h

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Effect of Wind 67

Mount Piper Power Station in NSW Australia

Full scale Measurements , The Int. Conf. on

Electric Supply Industry in Transition, Issues

and Prospects for Asia, Bangkok Thailand,

2004.

13. Amur, G.Q., Madadnia J., and Milton, B,Study of Wind Effects on the Natural Draft

Cooling Towers of Mount Piper Power Station

in NSW Australia, Wind Tunnel Study , The

Int. Conf. on Electric Supply Industry in

Transition, Issues and Prospects for Asia,

Bangkok Thailand, 2004.

14. Amur, G.Q., Madadnia, J., Milton, B., Reizes,J., and Koosha, H., Role of Plant Buildings in

a Power Station Acting as a Barrier to the Wind

Affecting the Natural Draft Cooling Tower

Performance The 15th Australasian Fluid

Mechanics Conf., Univ. of Sydney, Sydney,

Australia, 2004.

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