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Wind Engineeringfor HVAC Engineers
John J CarterVice [email protected]
CPP Inc. CPP Pty Ltd2400 Midpoint Drive 500 Princes HighwayFort Collins, Colorado St. Peters, NSWUSA
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Who is CPP?
• Founded in 1981 in Fort Collins from Colorado State University College of Engineering
– research dating back to early 1950s
• CPP Pty Ltd formed in 2008
Graeme Wood, Ph.D Matt Glanville, Ph.D
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What is wind engineering?
…the study of the interactions of wind and structures
…the mitigation of any negative effects of those interactions
To Provide
• Better efficiency
• Better reliability
• Better comfort
• Better safety
• Better value
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What is wind engineering?
Sub-disciplinesMechanical Engineering
– Dispersion• Pollutant• Thermal
– Internal airflow– natural ventilation
Structural Engineering– Cladding loads– Structural loads
Architectural– Pedestrian wind environment
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What is wind engineering?Tools of the trade
Experience
Computational models (simple to complex)
Atmospheric boundary layer wind tunnels
2226 )(
2
1exp
)(
2
1exp
2
1exp
2
10
z
rs
z
rs
yzy
hhzhhzyy
UQ
C
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Dispersion for HVACThe Problem
Exhaust Fume Re-entry(research, healthcare, commercial, residential)
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Typical Air Quality Sources of Concern
Cooling Tower
Diesel Generator
Animal Room Odor
Kitchen Grease Hood Odor
Helicopter
Traffic
Idling Diesel Vehicle
Fume Cupboard
Radioisotope
Biosafety CabinetIsolation Room
Boiler
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The ProcessIdentify Exhaust Sources of Concern
Develop Design Criteria
Concentration Predictions(Numerical Modeling; Physical Modeling; Full Scale Testing)
Design
Acceptable?
Modify Design
or Criteria,
Hazard
Assessment
Yes
GOOD DESIGNNo
Include Wind Statistics
For Risk Assessment or Energy Optimization
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Criteria
no such thing as “zero” re-entrainment
Gather information to develop allowable re-entrainment criteria
Frame as normalized concentration (C/m)health/odor
Exposure Limit Chealth
Odor Threshold Codor
Emission rate m
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Concentration Predictions
• Mathematical
• Wind Tunnel
• ComputationalFluid DynamicsCFD
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CFD Modeling
• Expensive and time consuming if done properly
• Needs same domain as wind tunnel
• Multiple simulations– Wind Directions: minimum of 16
(ideally +/- 5 degrees around critical)
– Wind Speeds: 3 to 7 at critical wind direction
– Large Eddy Simulations (LES) to capture turbulence/separated flow around buildings
• No established consistently conservative method
• Useful in some circumstances
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Mathematical Modeling
• Simple Gaussian models for simple environments(ASHRAE/AERMOD/AUSPLUME)
– Isolated building or tallest building
– Homogeneous surroundings
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Plume Rise
x
y
z
σz
hs+hr
3/13
,
3
, brmrr hhh
mf
hj
mmr h
U
xFh ,
3/1
22, ,3
min
bf
h
b
br hxU
Fh ,
3/1
2
32, ,2
3min
s
aseb
T
TTdgVFBuoyancy
4
)(:
2
4:
22 d
VT
TFMomentum e
s
am
e
hj
V
U 2.14.0
6.0
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Gaussian Dispersion Equation
C = Pollutant concentration (ug/m3)
Q = Pollutant emission rate (g/s)
hr = plume rise (m)
y /z = horizontal/vertical dispersion
coefficients
U = wind speed (m/s)
z = receptor height (m)
2226 )(
2
1exp
)(
2
1exp
2
1exp
2
10
z
rs
z
rs
yzy
hhzhhzyy
UQ
C
x
y
z
σz
hs+hr
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Plume Rise Example
INPUTSHb = 40 mHs = 90 mVe = 5 m/sTs = 430KTa = 279Kd = 2.1 mNeutral stability
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Building Wake Cavities
Wind
Upwind Recirculation Cavity
Rooftop Recirculation Cavity
Downwind Recirculation Cavity (wake)
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Wind Tunnel Modeling
Conservative method established (EPA Guideline)
• Most Accurate (EPA “analog computer with infinite resolution”)
– includes complex surroundings
– includes rooftop/local massing and architectural features (e.g. screens)
– multiple wind directions and wind speeds
• Allows for probabilistic risk assessment by incorporating local wind data
• Allows for optimization of exhaust energy consumption
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Complex model for complex environment
Wind from right in both videos
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Thermal PlumeHospital Chiller Application
With Wall WithOUT Wall
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Other things we have learned
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Prevailing Wind?
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“Gang” StacksAll stacks must operate
Most effective at increasing plume rise of small stack near large stack
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Entrained Air Exhaust - Concept
Bldg Exhaust
Wind Band
Flow & Velocity
Entrained Flow
Bypass
Fan Flow
Nozzle Velocity
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Entrained Air Exhaust - Reality
0.00
10.00
20.00
30.00
40.00
50.00
60.00
0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 180.00 200.00
Downwind Distance (ft)
Plu
me
Cen
terl
ine
Hei
gh
t (f
t)
Induced Air Specs
34850 cfm Wind Band
3622 fpm
Fan Specs
20500 cfm Exit
5125 fpm
Typical Fan Specs
20500 cfm Exit
3000 fpm
Briggs Complete
Final Rise Equation
(9.7 m3/s)
(26 m/s) (18 m/s)(9.7 m3/s)(16 m3/s)
(15 m/s)
(9.7 m3/s)
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Entrained Air ExhaustExtra Dilution?
2
1
2
1exp
2 zzyse
h
U
V
C
CD
If the flow, V, increases by 2 the
Ce decreases by 2.
Hence, no change in C if plume
rise is constant.
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Architectural ScreensNo Screen
Screen Effect
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Questions?
CPP, Inc. CPP Pty Ltd
2400 Midpoint Drive, Suite 190 500 Princes Highway, Suite 2
Fort Collins, CO 80525 St. Peters, NSW
+1 970 221 3371 +61 (0)2 9551 2000
www.cppwind.com @CPPWindExperts