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Jason Foust
Stuart Coulson
May 19th, 2011
Using Dissolved Oxygen Prediction Methodologies
in the Selection of Turbine Aeration
Equipment
Turbine Aeration| May 19th, 2011 | 2
• Motivation
• Overview of Aeration Techniques
Bubble Distributions and Performance Impacts
• Dissolved Oxygen Prediction Methodology (DBM)
• Aeration Case Study
Central, Peripheral, Distributed for Francis Turbine Application
• Discussion
Turbine Aeration for Water Quality Enhancement
Turbine Aeration| May 19th, 2011 | 3
• Hydro power very reliable with equipment lasting several decades
• Turbine Rehabilitation:
Changes to design technology (increased power and efficiency)
Lifecycle extension
Hydrology changes
• Turbine Rehabilitation also provides excellent opportunity to improve
environmental compatibility:
Fish Passage
Water Quality
Low dissolved oxygen levels in turbine discharges can harm
fish and other aquatic life.
Turbine Aeration for Water Quality Enhancement:
Motivation
Turbine Aeration| May 19th, 2011 | 4
• Warmer temperatures can cause thermal stratification
• Reservoir separates into layers, preventing natural mixing
Turbine Aeration for Water Quality Enhancement:
Motivation (continued)
Turbine Aeration| May 19th, 2011 | 5
• Generally recommended to keep tailrace dissolved oxygen levels above
4 to 5 mg/l
• New regulations (federal and state) are requiring minimum tailrace DO levels
(i) instantaneous minimums
(ii) daily average
(ii) 30 day average
• In order to improve tailrace dissolved oxygen levels,
air or oxygen gas can be introduced into the water
passing through turbine.
Turbine Aeration for Water Quality Enhancement:
Motivation (continued)
Turbine Aeration| May 19th, 2011 | 6
• Motivation
• Overview of Aeration Techniques
Bubble Distributions and Performance impacts
• Dissolved Oxygen Prediction Methodology (DBM)
• Aeration Case Study
Central, Peripheral, Distributed for Francis Turbine Application
• Discussion
Turbine Aeration for Water Quality Enhancement
Turbine Aeration| May 19th, 2011 | 7
• Auto-Venting Turbines
3 primary AVT methods:
• Central aeration (blue) – air injected through deflector
• Peripheral (yellow) – air injected through continuous slot in discharge ring
• Distributed aeration (green) – air injected through hollow blade
Distributed Air
water water
Peripheral
Air air/water mixture
DR
DE DEF
Central Air
Vacuum Breaker
Central Air
Vacuum Breaker
Distributed Air
Peripher
al Air
Turbine Aeration for Water Quality Enhancement:
Overview of Aeration Techniques (AVT)
Turbine Aeration| May 19th, 2011 | 8
• Aeration technique has a large influence of mixing characteristics
Aeration technique influences bubble distribution (Q/Qopt = 1.4)
Peripheral DistributedCentral
Uniformly distributed bubble
cloud
Well disbursed bubble
distributionInefficient bubble distribution
Turbine Aeration for Water Quality Enhancement:
Overview of Aeration Techniques (Bubble Distribution)
Turbine Aeration| May 19th, 2011 | 9
• Distributed aeration produces a fine bubble cloud within draft tube cone
• Smaller bubbles lead to a more efficient oxygen transfer
Turbine Aeration for Water Quality Enhancement:
Overview of Aeration Techniques (Aerating Runner)
Turbine Aeration| May 19th, 2011 | 10
• Laboratory investigation of peripheral point source injection vs. continous slot
• Two separate draft tube cones manufactured
Pointwise
Continuous Slot
Turbine Aeration for Water Quality Enhancement:
Overview of Aeration Techniques (Peripheral Aeration)
Turbine Aeration| May 19th, 2011 | 11
Point Source Continuous Slot
• Void Faction = 1%waterair
air
Q
Turbine Aeration for Water Quality Enhancement:
Overview of Aeration Techniques (Peripheral Aeration)
Turbine Aeration| May 19th, 2011 | 12
• Motivation
• Overview of Aeration Techniques
Bubble Distributions and Performance impacts
• Dissolved Oxygen Prediction Methodology (DBM)
• Aeration Case Study
Central, Peripheral, Distributed for Francis Turbine Application
• Discussion
Turbine Aeration for Water Quality Enhancement
Turbine Aeration| May 19th, 2011 | 13
• Discrete Bubble Model (DBM)
- Solves equations of mass transfer between two phase mixtures
- McGinnis and Little successfully applied to airlift aerator, Speece cone, and
bubble diffusers
- Model accounts for individual site characteristics, turbine geometry and
operating condition
- Voith Hydro has implemented DMB to aeration field data, providing unique
set of correlations for central, peripheral and distributed.
Turbine Aeration for Water Quality Enhancement:
Aeration Predictions
Turbine Aeration| May 19th, 2011 | 14
Input: Air quantity, turbine discharge, incoming DO+ DN, initial bubble size,
temperature, pressure & draft tube geometry.
Calculation starts at bubble inlet location,
continues through the draft tube,
and finishes when the bubble rises
to the tailwater elevation
Accounts for changes in pressure, bubble radius, buoyancy and
dissolved gas along path
Output: Outgoing dissolved oxygen & total dissolved gas levels.
Turbine Aeration for Water Quality Enhancement:
Aeration Predictions (DBM)
Turbine Aeration| May 19th, 2011 | 15
• Motivation
• Overview of Aeration Techniques
Bubble Distributions and Performance impacts
• Dissolved Oxygen Prediction Methodology (DBM)
• Aeration Case Study
Central, Peripheral, Distributed for Francis Turbine Application
• Discussion
Turbine Aeration for Water Quality Enhancement
Turbine Aeration| May 19th, 2011 | 16
• Dissolved oxygen at many sites are well below 1 mg/l during low
dissolved oxygen seasons
• During investigation, aeration performance (air flows, tailrace dissolved oxygen
levels, efficiency and power impacts) evaluated for three methods of auto-venting
turbine aeration.
• Unit centerline set 5.0 ft above TWE
Turbine Aeration for Water Quality Enhancement:
Case Study
H/Hopt
[-]
Q/Qopt
[-]
1.0 0.8
1.0 1.0
1.0 1.2
Table 1: Flow range corresponding to typical operation of a
Francis turbine.
Turbine Aeration| May 19th, 2011 | 17
• Air flow predictions
Static pressures predicted within
the water passage
Turbine Aeration for Water Quality Enhancement:
Case Study
H/Hopt
[-]
Q/Qopt
[-]
Central
Φ
[%]
Peripheral
Φ
[%]
Distributed
Φ
[%]
1.0 0.8 2.4 3.5 7.5
1.0 1.0 1.6 2.6 6.5
1.0 1.2 1.6 2.8 6.4
Table 2: Air flow prediction summary for natural aspiration.
Turbine Aeration| May 19th, 2011 | 18
• Tailrace dissolved oxygen levels
• Distributed aeration only method that can provide desired tailrace dissolved
oxygen levels across the operating range.
Turbine Aeration for Water Quality Enhancement:
Case Study
H/Hopt
[-]
Q/Qopt
[-]
Central
Tailrace Dissolved
Oxygen
[mg/l]
Peripheral
Tailrace Dissolved
Oxygen
[mg/l]
Distributed
Tailrace Dissolved
Oxygen
[mg/l] 1.0 0.8 3.0 3.5 6.3
1.0 1.0 2.5 3.4 6.1
1.0 1.2 2.5 3.8 6.1
Table 3: Predicted tailrace dissolved oxygen levels.
Turbine Aeration| May 19th, 2011 | 19
• Impact of aeration on turbine efficiency
NOTE: Compressor is required for central and peripheral aeration methods.
Turbine Aeration for Water Quality Enhancement:
Case Study
Efficiency Impacts to Reach 6.0 mg/l tailrace DO*
*Air flows requirements vary for each method
Turbine Aeration| May 19th, 2011 | 20
• Motivation
• Overview of Aeration Techniques
Bubble Distributions and Performance impacts
• Dissolved Oxygen Prediction Methodology (DBM)
• Aeration Cases Study
Central, Peripheral, Distributed for Francis Turbine Application
• Discussion
Turbine Aeration for Water Quality Enhancement
Turbine Aeration| May 19th, 2011 | 21
• Distributed aeration is most effective for 0.8 ≤ Q/Qopt ≤ 1.2
largest potential for air flow and DO uptake
lowest impact on turbine efficiency
• If distributed not available, peripheral next alternative
for Q/Qopt ≥ 0.8, air flows, DO uptakes andefficiency impacts more favorable than central
• Central aeration third choice
effective below Q/Qopt = 0.8
• AVT combinations can be used to maximize uptake while minimizing performance impacts
Turbine Aeration for Water Quality Enhancement:
Conclusions
Turbine Aeration| May 19th, 2011 | 22
• New direction:
Recently Voith designed and installed a peripheral aeration system on
a Kaplan turbine
No water quality
requirements, but discharge ring
being replaced
Air flow predictions and
DBM modeling performed to
evaluate effectiveness
Turbine Aeration for Water Quality Enhancement:
Conclusions
DT air box
Silencer
Air inlet
Turbine Aeration| May 19th, 2011 | 23
Turbine Aeration| May 19th, 2011 | 24
Aeration Influence on Turbine Performance (Q/Q opt)
70
75
80
85
90
95
0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
Q/Qopt
Ov
era
ll E
ffic
ien
cy
- %
Central Aeration
Distributed Aeration
No Aeration
Aeration Influence on Turbine Performance (Generator Output)
70
75
80
85
90
95
10 15 20 25 30 35
Generator Output - MW
Ov
era
ll E
ffic
ien
cy
[%
]
Central Aeration
Distributed Aeration
No Aeration
Aeration Performance
Turbine Aeration| May 19th, 2011 | 25
Turbine Aeration for Water Quality Enhancement:
Overview of Aeration Techniques (Influence of Aeration on Turbine
Performance)
Turbine Aeration| May 19th, 2011 | 26
• Cost of aeration
• Important to optimize dissolved oxygen needs with efficiency impacts.
Costs of Providing Additional Dissolved Oxygen Uptake
0
5
10
15
20
0 1 2 3 4 5 6
Q/Qopt = 0.8
Q/Qopt = 1.0
Q/Qopt = 1.1
Δη
/mg
/l D
O U
pta
ke
DO Uptake [mg/l]
Turbine Aeration for Water Quality Enhancement:
Conclusions
Turbine Aeration| May 19th, 2011 | 27
2008 Aeration Field Data (Air Flow Concentration)
O sage Units 1 and 7 Average Aerating Runner Air Concentrations (2008)
2
3
4
5
6
7
8
3000 3500 4000 4500 5000
Unit D ischarge [cfs]
[%
]
TW E = 565 ft
TW E = 560 ft
TW E = 555 ft
waterair
air
Q
φ [
%]
Turbine Aeration| May 19th, 2011 | 28
2008 Aeration Field Data (Dissolved Oxygen Uptakes)
O sage M easured D issolved O xygen C oncentrations (2008)
1.0
2.0
3.0
4.0
5.0
6.0
7.0
3000 3500 4000 4500 5000
Unit D ischarge [cfs]
DO
Co
nc
en
tra
tio
n (
mg
/l)
TW E = 566 ft
TW E = 562 ft
TW E = 555 ft
Tailrace
Intake
Turbine Aeration| May 19th, 2011 | 29
2008 Aeration Field Data (Aeration Influence on Performance)
Aerating
Runner
Uniformly distributed bubble cloud
Δη
[%
]
φ [%]
Aeration Influence on Unit 1 and 7 Performance (Q/Qopt = 0.8)
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7
Predicted
TWE = 565 ft
TWE = 560 ft
TWE = 555 ft
Q/Qopt = 0.8
Aeration Influence on Unit 1 and 7 Performance (Q/Qopt = 1.0)
Δη
[%
]
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7
Predicted
TWE = 565 ft
TWE = 560 ft
TWE = 555 ft
Q/Qopt = 1.0
φ [%]
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7
Predicted
TWE = 565 ft
TWE = 560 ft
TWE = 555 ft
Δη
[%
]
φ [%]
Q/Qopt = 1.1
Aeration Influence on Unit 1 and 7 Performance (Q/Qopt = 1.1)
Turbine Aeration| May 19th, 2011 | 30
NrCPHKdt
dMiiiL
Gi 24)(
Recently, the Discrete Bubble Model was developed by several investigators to
predict mass transfer between different phases in an air/water mixture
McGinnis and Little successfully applied to airlift aerator, Speece cone, and
bubble diffusers
Governing equations:
(1)
Partial pressure for each gasLocal dissolved gas concentrationChange in moles of gas per time
subscript i indicates gas type (oxygen, nitrogen)Surface area for N bubbles per second (flux)Liquid mass transfer coefficientDriving gradient leading to mass transferHenry’s constant
Vb
HPi Ci
Oxygen
Csaturation
DO Uptake: Dissolved Oxygen Prediction Methodology
Turbine Aeration| May 19th, 2011 | 31
NrCPHKdt
dMiiiL
Di 24)(
jiiiL
j
Di
j
DiNrCPHKtMM
21
4
DO Uptake: Dissolved Oxygen Prediction Methodology
Expressions for KL and Hi
KL = 0.6r [m/s] r < 0.667 mm
KL = 0.0004 [m/s] r ≥ 0.667 mm (r = radius)
HO = 2.125 - 0.05021T + 0.000577T2 [mole/m3bar]
HN = 1.042 – 0.02450T + 0.0003171T2 [mole/m3bar] (T = temperature)
Equation (1) divided into j time steps (Δt) and solved using Euler’s method:
(3)
Turbine Aeration| May 19th, 2011 | 32
Turbine Aeration| May 19th, 2011 | 33
• Air flows into the turbine (continued)
- Static pressures adjusted for amount of air present
Aeration Performance
Air Influence on Local Drawing Pressure
0
1
2
3
4
5
6
7
8
0.7 0.8 0.9 1 1.1 1.2 1.3 1.4
Q/Qopt
p/
pre
f
Distributed Aeration (a)
Central Aeration (b)
Peripheral Aeration (c)
Q/Qopt
waterair
air
Q
Turbine Aeration| May 19th, 2011 | 34
• Air flows into the turbine (continued)
Aeration Performance
Drawing Pressure vs Q/Qopt
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3
Q/Qopt
Dra
win
g P
res
su
re a
t a
ir in
jec
tio
n lo
ca
tio
n [
psi]
Distributed Aeration (a)
Central Aeration (b)
Peripheral Aeration (c)
Solid lines: without aeration ( = 0)
Dashed lines: with aeration ( = 3)
Q/Qopt
Dashed lines: non-aerating (φ/φref = 0)
Solid lines: aerating (φ/φref = 1)
Drawing Pressures Under Aerating and Non – Aerating Conditions
Recommended