A. S. Moita and A. L. Moreira

1. The Combined Effects of Surface Topography and Heat Transfer on the Droplet/Wall Interaction Mechanisms A. S. Moita and A. L. Moreira Instituto Superior Técnico, Department of Mechanical Engineering Lisbon - PORTUGAL

2. Context and Motivation • Cooling processes • Medical applications • Cooling of electronic chips • Agriculture • Fire protection systems Bergeron, V., Quéré, D., “Water droplets make an impact”, physicsweb, published online, 2001 (www.physicsweb.org).

3. Context and Motivation • Internal combustion engines Keoderitz et al., “Physical Mechanisms Involved in Fuel Atomization From Intake Valve and Port Surfaces”, 8th ICLASS, 2000. www.mitsubishi.com • The fuel film formed, at cold start conditions is an important source of engine-out emissions (80-90% of the total HC emissions occur in this period) and loss of performance.

4. Context and Motivation Spray/wall interactions Droplet/wall interactions

5. TLeidenfrost TNukiyama TSaturation Wall Heat Flux ,with Surface Temperature Droplet/Wall Interactions Dimensionless numbers: and additionally: scaling heat transfer and droplet evaporation characteristic times: Film evaporation Nucleate boiling Transition Film boiling Spread Disintegration Rebound Rebound Disintegration Mechanisms

6. Objectives This is an experimental study to characterize the effect of the nature of the surface on the dynamic behaviour of individual fuel droplets impacting onto hot surfaces, mainly concerning: • an analysis to deepen the study of the disintegration mechanisms (e.g. crown morphology, temporal evolution and secondary droplets characteristics); • characterize the disintegration mechanisms occurring in the various boiling regimes of fuel droplets impacting onto heated surfaces; • to provide a wider comprehension of the phenomena which can be a useful information to establish the basic conditions for more practical applications (e.g. effects of the topography of surfaces impacted by fuel sprays on the process of fuel-air mixing in internal combustion engines).

7. Experimental Set-Up Function Generator Laser Photodiode High-speed Camera Temperature Controller

8. Dimensionless numbers Range 400 - 16000 Reynolds Weber 8 - 2100 Ohnesorge 1.8E-4 - 1.0E-1 Working Conditions Parameters fs=0º Impact angle 2.2 mm mm mm < < < < Droplet initial diameter (D0) D 3.1mm 0 0 0 Droplet Ambient temperature Temperatures Target (Ts) Ambient temperature < Ts< 310ºC Pressure Atmospheric pressure 10m 10m 10m m < m < m < < 1940 < 1940 < 1940 m m m m m m H H H Impact velocity Impact velocity [ [ ( ( )] )] U U = = f f H H 0 0 0.44ms 0.44ms < < < 5.1ms < 5.1ms U U - - 1 1 - - 1 1 o o Water Water , ethanol, diesel oil, biodiesel , ethanol, diesel oil, biodiesel Working fluids

9. Target Surfaces Characterization of surface topography • Surface topography of the target plates was characterized by Ra, Rze lr, determined from the profiles obtained using an optical and a mechanical profile meters. • The optical profile meter allows to characterize rough structures between0.02mm and 600mm, while the mechanical profile meter allows to characterize roughness amplitudes up to 250mm.

10. z Z3 Z4 Z2 z Z1 Z5 Ra le Z Z Z Z Z lm lm 5 2 4 3 1 R a lm = 5le lm 1 1 ò ) ( , = , R , = , z dx z lm 5 0 Target Surfaces Characterization of surface topography Ra Rz Mean Roughness Mean peak-to valley Roughness (BS1134) (DIN4768)

11. Target Surfaces Measurement of contact angle (q) Micro-syringe Adapted from: Serro, Ana. P.,“Biomineralização de materiais de implante: estudos de molhabilidade”, Dissertação para obtenção do Grau de Doutor em Engenharia Química, Universidade Técnica de Lisboa, Instituto Superior Técnico, 2001. Light source Camera Microscope • qwas measured forall pairs liquid-target surface,inside a thermostatted ambient chamber (Ramé-Hart Inc., USA, model 100-07-00). • At least8 measurementswere takenfor each pair liquid-surface to obtain average values.

12. Target Surfaces Ra [mm 10%] Rz [mm] lr [mm] q [Degrees] Surface  0 38.5 38.5  0  0  0 Smooth Glass - Random profile 90.0 2.0 12.0 - 57.9 2.7 18.0 71.4  0  0  0 Smooth - 72.1 1.0 7.0 Perspex Random profile - 90.5 12.0 1.7 - 88.3 13.8 1.9 - 1.0 75.3 13.76 Random profile - 72.3 1.52 14.1 93.6 - 19.5 3.0 Aluminium - 79.0 19.6 2.22 115.5 15.0 210 Regular profile 85.0 184.0 530 101.5 41.7 - 430 180.2 41.7 - 0.543 6.0 90.8 Random profile - 96.5 1.3 9.0 - 93.2 3.5 23.0 Copper - 16.8 210 80.0 Regular profile 46.46 169.7 530 121.5 - 142.35 39.2 430 - 0.524 9.0 93.5 Random profile - 14.5 88.0 1.287 Stainless steel - 94.8 3.8 22.0 - 81.5 17.1 210 Regular profile - 42.0 175.3 530

13. Disintegration Disintegration Mechanisms • Film evaporation(Ts<Tsat) • Nucleate boiling (Tsat<Ts<TNukiyama) • Transition (TNukiyama<Ts<TLeidenfrost) • Film boiling (Ts>TLeidenfrost)

14. Disintegration Disintegration Mechanisms • Film evaporation(Ts<Tsat) • Nucleate boiling (Tsat<Ts<TNukiyama) • Transition (TNukiyama<Ts<TLeidenfrost) • Film boiling (Ts>TLeidenfrost)

15. Disintegration Deposition and spread Disintegration limits Disintegration Mechanisms Generation of secondary droplets Generation of a film liquid

16. Disintegration Mechanisms Disintegration limits Critical Weber number for “prompt” splash, as a function of dimensionless roughness (Ra/R0).

17. Disintegration Mechanisms Disintegration limits Critical Weber number for “splash”, as a function of dimensionless roughness (Ra/R0).

18. H c D l D u Dl – Crown lower externaldiameter Du– Crown upper externaldiameter Hc – Crown height Crown Morphology of Fuel Droplets Temporal evolution

19. Dl [mm] Hc [mm] Growth rate[ms-1] Du [mm] Dl Du 5 – 8 5.9 – 6.4 0.5 – 0.7 4.29 2.5 Ethanol Dieseloil 5 – 7.7 6.5 – 6.8 0.2 – 0.3 3.85 1.5 Biodiesel tends to preclude an evident crown formation Crown Morphology of Fuel Droplets Temporal evolution n • Ra Increase film destabilization promoting crown disruption at earlier stages. Rz

20. 0.3ms 1.0ms 0.0ms 0.5ms 2.0ms 3.0ms 1.5ms 2.5ms 13.5ms 11.0ms Crown Morphology of Fuel Droplets Temporal evolution Temporal evolution of corona splash of an ethanol droplet (D0=2.4mm, U0=3.1ms-1) after impacting onto an aluminum surface (Ra=1.0mm).

21. 0.3ms 1.0ms 0.0ms 0.5ms 2.0ms 3.0ms 1.5ms 2.5ms 13.5ms 11.0ms Crown Morphology of Fuel Droplets Temporal evolution Temporal evolution of corona splash of a diesel oil droplet (D0=2.6mm, U0=3.1ms-1) after impacting onto an aluminium surface (Ra=1.0mm).

22. Secondary Droplet Characteristics • Asliquid viscosity increases: • Nsd • dsd • Crown and ejection angles Droplet size distribution of secondary droplets generated from the impact of diesel oil (D0=2.6mm, U0=3.1ms-1) and ethanol (D0=2.4mm, U0=3.1ms-1) droplets after impacting onto an aluminium surface (Ra=1.0mm).

23. Disintegration Disintegration Mechanisms • Film evaporation(Ts<Tsat) • Nucleate boiling (Tsat<Ts<TNukiyama) • Transition (TNukiyama<Ts<TLeidenfrost) • Film boiling (Ts>TLeidenfrost)

24. 0.5ms 20.5ms 0.5ms 55ms 10.5ms 35.5ms 7.5ms 85.5ms 13.5ms 119ms 20.5ms 119ms 18.5ms 324ms 26.5ms 384ms Morphology Nucleate Boiling: effect of surface topography Ra=0.5mm; Rz=9.0mm Ra=17.1mm; Rz=81.5mm

25. “Pagoda-like” bubbles Secondary Droplet Characteristics Nucleate Boiling: effect of surface topography • dsd • Ra Rz • Nsd with smaller diameters Effect of surface roughness in droplet size distribution of secondary droplets generated from the impact of an ethanol droplet, in the bubble boiling regime (Ts=115ºC).

26. Disintegration Disintegration Mechanisms • Film evaporation(Ts<Tsat) • Nucleate boiling (Tsat<Ts<TNukiyama) • Transition (TNukiyama<Ts<TLeidenfrost) • Film boiling (Ts>TLeidenfrost)

27. Secondary Droplet Characteristics Transition and Film Boiling: effect of surface topography • dsd • Ra Rz • Nsd with smaller diameters Effect of surface roughness in droplet size distribution of secondary droplets generated from the impact of an ethanol droplet, in the transition and film boiling regimes.

28. Summary The work addresses an experimental study of individual fuel droplets impinging onto hot solid surfaces. • Impact at Film evaporation regime: • There are different disintegration mechanisms which were distributed in different “regions” of the plot Wec vs Ra/R0, as a function of the dimensionless roughness. • The wettability is important to understand some disintegration mechanisms, but surface topography may contribute to preclude the formation of the liquid filmsince: • endorses droplet disintegration for smaller U0. • promotes “prompt” splash, occurring in a very small temporal scale (ms);

29. Summary • Using fuels with larger viscosity gives rise to less “efficient” secondary atomization (may be negative in terms of mixture preparation) as they: • have larger Wec, so that are more likely to form an harmful fuel film. • give rise to less but larger secondary droplets, mainly ejected in radial direction. • Impact at Nucleate boiling/ Transition/ Film boiling: • Surfacetopography enhancesnucleate boilingbut decreases secondary atomization efficiency, as the number of secondary droplets decreasesand the diameterincreases, particularly intransition and film boiling.

30. Acknowledgments • National Foundation of Science and Technology, by supporting A. S. Moita with a PhD Fellowship (Ref:SFRH/BD/18250/2004); • Prof. Rogério Colaço, from the Department of Materials Engineering, Instituto Superior Técnico; • Prof. Benilde Saramago and Doctor Ana P. Serro, from the Structural Chemistry Center, Instituto Superior Técnico; • Prof. Olinda Conde, from the Laser Surface Processing Laboratory, Faculdade de Ciências da Universidade de Lisboa.