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Towards Prediction of Artificial Monolayer Performance for Water Conservation

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

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Towards Prediction of Artificial Monolayer Performance for Water Conservation

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  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)

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

  7. PLUSDRIVERS AT MONO-MOLECULAR SCALE (Hancock et al. 2011)

  8. IMPACT OF ARTIFICIAL MONOLAYER AT MONO-MOLECULAR SCALE (Figure 7.1 Davies and Rideal 1963) GAS PHASE Liquid thermal boundary layer (LTBL) • Damping capillary waves reduces wind shear RGreduced and eddies (Rayleigh-Benard convection) RLreduced. A condensed monolayer increases RG , RI & RL

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

  10. METEOROLOGICAL DRIVES AT THE MACRO SCALE Q* radiation flux QEturbulentlatent heat flux QHsensible heat flux (Δ heat storage) (Δ water current heat transfer) (Fig 3.14 Oke 1987)

  11. METEOROLOGICAL DRIVERS AT THE MICRO SCALE Ifθa – 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, reducingRG & RL, increasing evap loss. surface – subsurface C Air – subsurface C 1= reservoir 2 = 0 ms-1 wind 3 = 0.5 ms-1 wind 4 = 1.5 ms-1 wind Fig 2.5, Gladyshev (2002)

  12. 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)

  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. Black EFFECT OF PHYSICAL COVERS ON EVAPORATIVE LOSS: Black cover >> effective in reducing evap loss but ……. ADDING C18OH MONOLAYER NO IMPACT

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

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

  18. Black cover IM PACT OF MONOLAYER ON QE\&/OR QH (hourly data, for 3 days wind <6 m s-1) NO EVIDENCE OF ADDITIONAL EVAPORATION REDUCTION. (θ0 – θw) White cover (θ0 – θw) (θa – θw)

  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) black Cold surface film (thin LTBL)

  21. CONCLUSIONS • Monomolecular films increase R in liquid thermal (RL) & gaseous (RG) boundary layers • Calm conditions with thermally stable LTBL (warm surface film), RL> Rmonolayer(no effect) • Light wind, thermally unstable LTBL, RL< Rmonolayer(water savings) • Hourly analysis is ESSENTIAL to interpret R and drivers of evaporation (QH, QE, Q*)

  22. THANK YOU

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