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Towards Prediction of Artificial Monolayer Performance for Water Conservation Pam Pittaway & Nigel Hancock National Centre for Engineering in Agriculture.

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Presentation on theme: "Towards Prediction of Artificial Monolayer Performance for Water Conservation Pam Pittaway & Nigel Hancock National Centre for Engineering in Agriculture."— Presentation transcript:

1 Towards Prediction of Artificial Monolayer Performance for Water Conservation Pam Pittaway & Nigel Hancock National Centre for Engineering in Agriculture University of Southern Queensland, Toowoomba.

2 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.

3 ARTIFICIAL MONOLAYER TECHNOLOGY IN PRACTICE

4 VARIATION IN MONOLAYER FIELD TRIAL PERFORMANCE (Craig et al 2005) LocationStorage size (km 2 ) Trial & monitoring period (days) Evaporation Reduction (%) University of Southern Queensland, Toowoomba, Qld days 2- 8 days 3- 6 days 4- 7 days 5- 7 days 38% 17% 10% 38% 40% Capella Qld days 2- 8 days 3- 7 days 4- 8 days 0% Cubby Station, Dirranbandi, Qld days days 3- 8 days 31% 27% 0%

5 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

6 1.METEOROLOGICAL DRIVERS OF EVAP LOSS AT MACRO-SCALE (Hancock et al. 2011)

7 2.PLUS DRIVERS AT MONO-MOLECULAR SCALE (Hancock et al. 2011)

8 Damping capillary waves reduces wind shear R G reduced and eddies (Rayleigh-Benard convection) R L 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 R G, R I & R L

9 Cold surface film –thermally unstable, strong eddies reduce R L. Warm surface film –thermally stable, no eddies increase R L. 4.IMPACT OF MICROMETEOROLOGY ON RESISTANCE TO EVAPORATIVE LOSS Liquid thermal boundary layer (LTBL) (Figure 7.1 Davies and Rideal 1963) GAS PHASE

10 5.METEOROLOGICAL DRIVES AT THE MACRO SCALE Q* radiation flux Q E turbulent latent heat flux Q H sensible heat flux (Δ heat storage) (Δ water current heat transfer) (Fig 3.14 Oke 1987)

11 6.METEOROLOGICAL DRIVERS AT THE MICRO SCALE If θ a – θ w > 0 induces a cold surface film (θ 0 –θ w <0), small R L induces evap loss. Increasing wind speed to 1.5 m sec -1 increases the cold surface film, reducing R G & R L, increasing evap loss. 1= reservoir 2 = 0 ms -1 wind 3 = 0.5 ms -1 wind 4 = 1.5 ms -1 wind surface – subsurface  C Air – subsurface  C Fig 2.5, Gladyshev (2002)

12 6.METEOROLOGICAL DRIVERS AT THE MICRO SCALE concluded: Air–subsurface water (θ a – θ w ) is a surrogate of Q H Surface–subsurface water (θ 0 – θ w ) is a surrogate of Liquid Thermal Boundary Layer resistance (θ 0 – θ w ) <0 = cold surface film (thin LTBL, < R L ) (θ 0 – θ w ) >0 = warm surface film (thick LTBL, > R L )

13 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

14 INSTRUMENTATION ABOVE AND UNDER PHYSICAL COVERS NOT TO SCALE

15 EFFECT OF PHYSICAL COVERS ON EVAPORATIVE LOSS: Black cover >> effective in reducing evap loss but ……. ADDING C 18 OH MONOLAYER NO IMPACT Black

16 DIURNAL ENERGY BALANCE FOR SHALLOW WATER (Fig 3.15 Oke 1987) Shallow water Japan (Q G is soil heat flux)

17 IMPACT OF COVERS ON Q H UNDER LOW WIND (<6m sec -1 ) Black cover absorbs & re- radiates heat (>> Q H ?) White cover reflects heat (< < Q H ?) Black

18 IM PACT OF MONOLAYER ON Q E\ &/OR Q H (hourly data, for 3 days wind <6 m s -1 ) NO EVIDENCE OF ADDITIONAL EVAPORATION REDUCTION. Black cover White cover (θ a – θ w ) (θ 0 – θ w )

19 IMPACT OF COVERS ON LIQUID THERMAL BOUNDARY LAYER AT THE MACRO SCALE Black cover water thermal gradient White cover water iso- thermal

20 IMPACT OF COVERS ON LIQUID THERMAL BOUNDARY LAYER concluded Warm surface film (thick LTBL) Cold surface film (thin LTBL) black

21 CONCLUSIONS 1.Monomolecular films increase R in liquid thermal (R L ) & gaseous (R G ) boundary layers 2.Calm conditions with thermally stable LTBL (warm surface film), R L > R monolayer (no effect) 3.Light wind, thermally unstable LTBL, R L < R monolayer (water savings) 4.Hourly analysis is ESSENTIAL to interpret R and drivers of evaporation (Q H, Q E, Q*)

22 THANK YOU


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