CFD Prediction of Cooling Tower

Drift in an Urban Environment

R.N. Meroney

Drift

• Water droplets entrained in the air stream as it passes through a cooling tower are referred to as DRIFT.

• Droplets vary from a few to several thousand microns in diameter.

• Drift contains the minerals of the makeup water and often contain water treatment chemicals…some of which may be toxic or corrosive.

• Drift eliminators strip most of the water from the discharge stream….but some escapes.

Concerns About Drift

• Salt deposition on agricultural areas

• Icing and fog

• Legionnaires disease

• Corrosion inhibitors can cause cancer

Natural Draft Cooling Towers

Mechanical Draft Cooling Tower

0 400 8 0 1200 1400 2000

Droplet Diameter (µm)

Liq

uid

Ma

ss

Em

iss

ion

(g

m/s

ec)

10

-41

0-3

10

-21

0-1

1

Drift

emission variation

MAX

AVG

MIN

Parameters which affect prediction of drift deposition

• Drift particle exhaust distribution (~300% at 50 ::::m up to 2400% at 1000 ::::m)

• Particle median diameter (microns)..(~25%)

• Drift emission rate (g/sec)………….(~42%)

• Exhaust velocity (m/s)……………...(~13%)

• Exhaust temperature (T oC) ………...(~4.5%)

• Wind velocity (m/s)…………………(~15%)

• Wind temperature (humidity) (T oC, %)

– Web, Wheeler and Moore (1978), “Variations in

the Chalk Point Cooling Tower Effluent

Parameters and Their Influence on Drift Transport Modeling Results,” Cooling Tower

Environment –1978, Proceedings of

Symposium , May 2-4, 1978, Univ. of Maryland,

pp. 42-53.

Percentage variation in deposition

rate due to parameter changes

• Exhaust velocity (3.9 to 5.1 m/s)… …...""""20%

• Wind velocity (3.4 to 4.6 m/s)…………...""""20%

• Exhaust air temp (35.8 to 39.2oC)…… ..""""20%

• Air temp (23 to 27 C)………… ………….""""20%

• Source droplet spectrum (min to max)– From 0 to 1 km …………………………. """"1500%

– From 1 to 10 km…………………………… """"200%

– Beyond 10 …………………………………""""50%

– Web, Wheeler and Moore (1978), “Variations in the Chalk Point Cooling Tower Effluent Parameters and Their Influence on Drift Transport Modeling Results,” Cooling Tower Environment –1978, Proceedings of Symposium , May 2-4, 1978, Univ. of Maryland, pp. 42-53.

0.1 1 10 100

Distance (km)

Dep

osit

ion

Flu

x (

kg

/km

2/m

o)

1

10

10

210

310

4

Effect of drift droplet size

Max

Avg

Min

Drift Diameter Distribution

• Data from Marley Cooling Tower Publication 2002

• Rosin-Rammler distribution

– Cum mass fraction = Yd = exp[ -( d/dmean)n ], where

– dmean = 0.0001 m,

– Shape parameter, n = 1.0

Drift Animation 2-d Stack

Particle Drift from 2d Cooling Tower

CFD Conditions• 2d domain x=400 ft, y= 100 ft

• Velocity inlet & Outflow conditions, Ceiling symmetry

• Cooling Tower located at x=100 ft

• Cooling Tower dimensions, w=30ft, h=25ft

• Fan with pressure drop to produce internal flow

• Cooling Tower interior includes two porous fill regions

Flow Field Conditions• Velocity inlet = 3 m/s

• Turbulence inlet = 10%, L=25 ft

• DP across fan = 200 pascals

• Droplet injections of water from fan exit– Droplet sizes ranged from 0.0001- 0.01ft (0.03-1.5 mm)

• Droplet trajectories with stochastic Lagrangian model

Stream Function (kg/sec)

Static Pressure (pascals)

Velocity Magnitude (mph)

Turbulent Intensity (%)

Particle Diameter = 0.03 mmSmall drift effect

Particle Diameter = 0.30 mmLarge Drift Effect

Particle Drift with Diam

D=1.52mm D=0.30mm

D=0.15mm D=0.03mm

Drift Validation Exercise

Chalk Point Cooling Tower Dye

Experiment July 1977

Chalk Point Power Plant

• Hyperbolic Cooling towers are 400 ft (124 m) tall by 374 ft (114 m) diameter base by 90 ft (27.4 m) diameter exit.

• Drift loss ~0.002%

• Plume temperature = Tvp = 315.3 oK

• Ambient temperature = Tve = 295.3 oK

• Exhaust velocity = Vs = 4.5 m/s

• Measurements at night during 93% humidity, so negligible droplet evaporation.

• Rhodamine WT (fluorescent dye) source strength = 1.86 g/sec.

Chalk Point Fluent Results

Velocity Magnitude

Contours

Log K concentration Contours

Velocity Magnitude

Profiles

Turbulence Intensity Profiles

Accretion Results

Accretion Nodal Contours

Accretion Face

Contours

1000 m

500 m

Particle Tracks: RR

Time (seconds)

Accretion Comparison

CONCLUSIONS

• CFD successfully predicted cooling tower plume rise above the Chalk Point cooling tower.

• CFD predicted similar center-line cooling tower plume dilutions to the ISC plume algorithms.

• CFD predicted similar cooling tower plume ground level concentrations to those calculated from the ISC plume algorithms.

• The Discrete Particle Method with a Lagrangian stochastic option appears to be a satisfactory calculation technique for drift estimation.

Drift in an Urban Setting

Mechanical Draft Cooling Tower

Cooling Towers in an Urban Setting

Cooling Tower Vapor Clouds

Wind Tunnel Model (1:240)

Visualization

• U = 7.5 m/s

• α = 160o, 180o, 210o, & 240 o

• U = 5.0 m/s

• α = 130o

Isolated Tower CFD Conditions

• 3d domain: 2000 ft x 1000 ft x 500 ft

• Wind directions from 160o, 180o, 230o, & 240o from North

• Approach wind speed = 3 m/sec @ 10 m

• Approach turbulence = 10% , Hydraulic diameter = 25 m

• Cooling Tower Exhaust Velocity = 8.5 m/sec

• Cooling Tower H20 concentration = 0.01

Islolated BHHS Results

Static Pressure (pascals) Velocity (m/s)

Velocity Vectors (m/s) Turbulence Intensity (%)

Isolated BHHS Results

Log Kconc factorParticle tracers colored by Surface ID

Particle tracers colored by Surface ID Particle tracers colored by Surface ID

Isolated BHHS CT Results

Particle tracers colored by track time (sec) Contours of DPM deposition (kg/m2-sec)

Contours of DPM deposition (kg/m2-sec)Contours of DPM deposition (kg/m2-sec)

Drift in an Urban Setting

Mechanical Draft Cooling Tower

Wind Directions 160-240 degrees

Urban CFD Conditions• 3d domain: 2000 ft x 1000 ft x 750 ft

• Wind directions from 160o, 180o, 230o, & 240o from North

• Approach wind speed = 3 m/sec @ 10 m

• Approach turbulence = 10% , Hydraulic diameter = 25 m

• Cooling Tower Exhaust Velocity = 8.5 m/sec

• Cooling Tower H20 concentration = 0.01

BHHS 160 deg

Pressure Coefficients, Cp Velocity Magnitude (m/s)

Velocity Vectors Magnitude (m/s) Turbulence Intensity (%)

BHHS 160 deg

Log Kconc factor Log Kconc factor

Concentration Surface colored by Velocity Magnitude (m/s)

Concentration Surface colored by Velocity Magnitude (m/s)

BHHS 160 deg dpm

d = 0.0002 m

160o

BHHS 220 deg dpm

BHHS 220 deg dpm

230o

BHHS 240 deg dpm

240o

Zonal Deposition Rates

• Divide downwind region

into 250 ft wide

deposition zones.

• Accumulate accretion

rates (kg/sec) in each

zone for cooling tower

with/without surrounding

buildings.

• Calculate multiplying

(or amplification) factors

by taking ratio of

accretion rates with and

without buildings

present.

Multiplying Factors

Zonal Deposition: 160 degrees

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

0-250 250-500 500-750 750-1000 1000-1250

Distance Zones (feet)

Mu

ltip

lic

ati

on

Fa

cto

r

U= 2.5 mps

U = 5.0 mps

U = 7.5 mps

Zonal deposition Multiplication Factors (MF) for a 160o wind orientation

in downwind zones for approach wind velocities at 52 m of U = 2.5, 5.0and 7.5 mps.

Zonal Deposition: 240 degrees

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0-250 250-500 500-750 750-1000 1000-1250

Distance Zones (feet)

Mu

ltip

lic

ati

on

Fa

cto

r

U = 2.5 mps

U = 5.0 mps

U = 7.5 mps

Zonal deposition Multiplication Factors (MF) for a 240o wind

orientation in downwind zones for approach wind velocities at 52m of U = 2.5, 5.0 and 7.5 mps.

Zonal deposition Multiplication Factors (MF) in downwind zones for wind

orientations of 160o, 180o, 220o, and 240o.

Average zonal deposition in terms of Average Multiplication Factors (AMF) for wind orientations of 160o, 180o, 220o, and 240o

in downwind zones for approach wind velocities at 52 m of

U = 2.5, 5.0 and 7.5 mps.

CONCLUSIONS

• Buildings significantly deflect water vapor and water droplet distributions.

• Building turbulence draws particles to ground at a more rapid rate, but in some cases spreads them over a greater lateral distance.

• Particle sizes which intersect buildings and grounds downwind are primarily in diameter range from 0.001 to 0.0001 m.

• Multiplying factors ranged from 0.3 to 9.0 depending on wind direction and downwind distance.

Thank you for your attention!