towards prediction of artificial monolayer performance for water conservation pam pittaway &...
TRANSCRIPT
Towards Prediction of Artificial Monolayer Performance for Water
Conservation
Pam Pittaway & Nigel Hancock
National Centre for Engineering in Agriculture
University of Southern Queensland, Toowoomba.
ARTIFICIAL MONOLAYER TECHNOLOGY:
• Potential for cost-effective water saving; BUT
• Averaged daily data indicates highly variable performance.
THIS PRESENTATION:
• Understand the cause of highly variable performance to predict optimal conditions for cost-effective monolayer application.
ARTIFICIAL MONOLAYER TECHNOLOGY IN PRACTICE
VARIATION IN MONOLAYER FIELD TRIAL PERFORMANCE (Craig et al 2005)
Location Storage size(km2)
Trial & monitoring period
(days)
Evaporation Reduction (%)
University of Southern Queensland, Toowoomba, Qld
0.0001 1- 6 days2- 8 days3- 6 days4- 7 days5- 7 days
38%17%10%38%40%
Capella Qld 0.042 1- 9 days2- 8 days3- 7 days4- 8 days
0%0%0%0%
Cubby Station, Dirranbandi, Qld
1.2 1- 5 days2- 10 days3- 8 days
31%27%0%
ARTIFICIAL MONOLAYERS FOR EVAPORATION REDUCTION
• Monomolecular fatty alcohol films compressing at water surface to retard evaporative loss
• Long-chain, saturated fatty alcohols form continuous condensed film
• Condensed film retards molecular transfer across liquid thermal and gaseous boundary layers
• Wind speeds >6 m sec-1 disrupt films
1. METEOROLOGICAL DRIVERS OF EVAP LOSS AT MACRO-SCALE (Hancock et al. 2011)
2. PLUS DRIVERS AT MONO-MOLECULAR SCALE (Hancock et al. 2011)
• Damping capillary waves reduces wind shear RG reduced and eddies (Rayleigh-Benard convection) RL reduced.
3. IMPACT OF ARTIFICIAL MONOLAYER AT MONO-MOLECULAR SCALE
Liquid thermal boundary layer(LTBL)
(Figure 7.1 Davies and Rideal 1963)GAS PHASE
A condensed monolayer increases RG , RI & RL
• Cold surface film –thermally unstable, strong eddies reduce RL.
• Warm surface film –thermally stable, no eddies increase RL.
4. IMPACT OF MICROMETEOROLOGY ON RESISTANCE TO EVAPORATIVE LOSS
Liquid thermal boundary layer(LTBL)
(Figure 7.1 Davies and Rideal 1963)GAS PHASE
5. METEOROLOGICAL DRIVES AT THE MACRO SCALE
Q* radiation flux
QE turbulent
latent heat flux
QH sensible
heat flux
(Δ heat storage)
(Δ water current heat transfer)(Fig 3.14 Oke 1987)
6. METEOROLOGICAL DRIVERS AT THE MICRO SCALE
If θa – θw> 0 induces a cold surface film (θ0–θw<0),small RL
induces evap loss.
Increasing wind speed to 1.5 m sec-1
increases the cold surface film, reducing RG & RL, increasing evap loss.
1= reservoir2 = 0 ms-1 wind3 = 0.5 ms-1 wind4 = 1.5 ms-1 wind
surface – subsurface C
Air – subsurface C
Fig 2.5, Gladyshev (2002)
6. METEOROLOGICAL DRIVERS AT THE MICRO SCALE concluded:
• Air–subsurface water (θa – θw) is a surrogate of QH
• Surface–subsurface water (θ0 – θw) is a surrogate of Liquid Thermal Boundary Layer resistance
• (θ0 – θw) <0 = cold surface film (thin LTBL, < RL)
• (θ0 – θw) >0 = warm surface film (thick LTBL, > RL)
TRIALS: IMPACT OF PHYSICAL COVERS ON MICROMETEOROLOGY & RESISTANCE
TO EVAPORATIVE LOSS
Trial 1 black Atarsan cover on x2 tanks, monolayer on x1 tank
Trial 2 white shade cloth cover on x2 tanks, monolayer on x1 tank
1: Pyranometer; 2: Air temperature and relative humidity probe; 3: Net radiation sensor; 4: Infrared temperature sensor; 5: Floating thermocouple; 6-9: Fixed thermocouples.
10m
0.1m
0.3m
0.5m
0.6m
0.7m
1.2
m
0.8
m
1 2 3
4
5
7
6
8
9
INSTRUMENTATION ABOVE AND UNDER PHYSICAL COVERS
NOT TO SCALE
Atarsun cover
18/1/10 25/1/10 1/2/10 8/2/10 15/2/10 22/2/10 1/3/10
Ra
infa
ll (m
m)
0
20
40
60
80
100W
ate
r d
ep
th (
mm
)
400
500
600
700
800
900
1000
White Cover
22/2/10 1/3/10 8/3/10 15/3/10 22/3/10 29/3/10
Wa
ter
de
pth
(m
m)
400
500
600
700
800
900
1000
Ra
infa
ll (m
m)
0
20
40
60
80
100
Covered tank depth Uncovered tank depth rainfall
EFFECT OF PHYSICAL COVERS ON EVAPORATIVE LOSS:
Black cover >> effective in reducing evap loss but …….
ADDING C18OH MONOLAYER NO IMPACT
Black
DIURNAL ENERGY BALANCE FOR SHALLOW WATER (Fig 3.15 Oke 1987)
Shallow water Japan
(QG is soil heat flux)
Atarsun Cover
14/2/2010 12 15/2/2010 12 16/2/2010 12 17/2/2010 12
Tem
pe
ratu
re (0C
)
0
20
40
60
80
Re
lativ
e H
um
idity
(%
)
0
20
40
60
80
100
White Cover
7/3/2010 12 8/3/2010 12 9/3/2010 12 10/3/2010 12
Tem
pe
ratu
re (0C
)
10
20
30
40
50
60
70
80
Re
lativ
e H
um
idity
(%
)
40
60
80
100
Temperature Relative Humidity
IMPACT OF COVERS ON QH UNDER LOW WIND (<6m sec-1)
Black cover absorbs & re-radiates heat (>> QH?)
White cover reflects heat (< < QH?)
Black
IM PACT OF MONOLAYER ON QE\ &/OR QH
(hourly data, for 3 days wind <6 m s-1)
NO EVIDENCE OF
ADDITIONAL EVAPORATION
REDUCTION.
-10 0 10 20 30 40
Tw
f,c - T
w,0
.5,c (
C)
-4
-2
0
2
4
6
8Clean Tank 1 C18OH Tank 2
Ta,c - Tw,0.5,c (C)
-6 -4 -2 0 2 4 6 8 10 12
Tw
f,c -
Tw
,0.5
,c (
C)
-1.5
-1.0
-0.5
0.0
0.5
1.0
Black cover
White cover
(θa – θw)
(θ0 –
θw)
(θ0 –
θw)
IMPACT OF COVERS ON LIQUID THERMAL BOUNDARY LAYER AT THE MACRO SCALE
Atarsun cover
Wa
ter
tem
pe
ratu
re (0 C
)18
20
22
24
26
28
Wa
ter
tem
pe
ratu
re (0 C
)
18
20
22
24
26
28
Uncovered
18/1/10 25/1/10 1/2/10 8/2/10 15/2/10 22/2/10 1/3/10
Wa
ter
tem
pe
ratu
re (0 C
)
18
20
22
24
26
28
22/02/10 1/03/10 8/03/10 15/03/10 22/03/10 29/03/10
Wa
ter
tem
pe
ratu
re (0 C
)
18
20
22
24
26
28
0.1 m 0.3 m 0.5 m surface
Black cover water thermal gradient
White cover water iso-thermal
IMPACT OF COVERS ON LIQUID THERMAL BOUNDARY LAYER concluded
Date
22/3/10 29/3/10 5/4/10 12/4/10 19/4/10 26/4/10
Tw
f,c
- Tw
, 0.5
(0 C
)
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Atarsun cover White shade cloth no cover
Warm surface film (thick LTBL)
Cold surface film (thin LTBL)
black
CONCLUSIONS
1. Monomolecular films increase R in liquid thermal (RL) & gaseous (RG) boundary layers
2. Calm conditions with thermally stable LTBL (warm surface film),
RL > Rmonolayer (no effect)
3. Light wind, thermally unstable LTBL,
RL < Rmonolayer (water savings)
1. Hourly analysis is ESSENTIAL to interpret R and drivers of evaporation (QH, QE, Q*)
THANK YOU