DROPWISE CONDENSATION

Transcription

1 DROPWISE CONDENSATION Davide Del Col Università di Padova Dipartimento di Ingegneria Industriale Via Venezia, Padova

2 Outline Surface wettability Dropwise condensation over hydrophobic surfaces: experiments and modeling Dropwise condensation over super-hydrophobic surfaces

3 Wettability Hydrophobicity: solid surface energy vs liquid surface tension Static Angles Θ = Static Contact Angle Θ a = Advancing Contact Angle Gas Θ r = Receding Contact Angle Gas θ Liquid θ ΔΘ = Θ a Θ r = Contact Angle Hysteresis Liquid Solid Solid Dynamic Angles θ > 90 the surface is defined hydrophobic θ a > 150 Δθ < 10 the surface is defined superhydrophobic

4 Wettability A flat surface is used to define the equilibrium contact angle. The equilibrium contact angle for a droplet is the result of the force balance at the three-phase contact line (Young, 1805). Gas γ lg Young model: Liquid γ ls θ γ gs γ gs γ lg cos θ Young γ ls = 0 Solid

5 Wettability Wenzel state Wenzel extended the wetting analysis to rough and porous surfaces. For a surface with roughness r defined by the ratio of the total surface area to the projected area, Wenzel (1936) showed that the apparent contact angle in the Wenzel state θ W is defined by: Wenzel model: cosθ W = r cosθ Young, r = real surface projected surface The Wenzel state is typically less desired due to the higher adhesion associated with this wetting state

6 Wettability Cassie and Baxter state Where the droplet rests on the tips of the roughness, Cassie and Baxter (1944) showed that the apparent contact angle in the Cassie state θ CB is defined by: Cassie-Baxter model: cosθ CB = f cosθ Young f = fraction area where the solid fraction f is the ratio of the structure or roughness area contacting the droplet to the projected area. It is important to note that in the case of condensation the nucleation of droplets can initiate within the roughness, which may render previous equations non applicable.

7 Condensation In typical industrial systems, the vapor condenses on a surface forming a liquid film (filmwise condensation). In fact, the majority of industrial heat exchangers materials (copper, aluminum, stainless steel ) present high surface energy. During filmwise condensation, the heat transfer performance is limited by the thickness of the liquid film. The thermal resistance is mainly due to the heat conduction though the liquid film. The heat transfer coefficient can be enhanced or thinning the liquid film or promoting a different type of condensation mechanism: dropwise condensation.

8 Condensation Filmwise Condensation Dropwise Condensation

9 Dropwise condensation Dropwise condensation (DWC) is a nucleation phenomenon. It is similar to nucleate boiling except that the active nucleation sites are much smaller and the nucleation site density is much larger. Dropwise condensation, first reported by Schmidt et al. (1930), occurs when steam and a few other relatively high surface tension fluids condense on surfaces which are not wetted by the condensate.

10 Dropwise condensation: history of research (Rose, 2002)

11 Dropwise condensation On low surface energy materials, the vapor can condense forming discrete liquid droplets. During DWC, droplets roll off clearing the surface for renucleation. The droplets roll off can be induced by gravity, vapor shear stress, presence of a wettability gradient on the surface Since most of the materials used in heat exchangers present high surface energy, DWC can be achieved functionalizing the surface with a hydrophobic coating. Compared to filmwise condensation, DWC allows heat transfer coefficients one order of magnitude higher.

12 Dropwise condensation phenomenon Phenomenon: 1. Nucleation radius (r min ) 2. Coalescence radius (r e ) 3. Departing radius (r max )

13 Dropwise condensation DWC has been a topic of interest for the past eight decades. The works focused on the realization of nonwetting surfaces obtained by applying different coatings. However, the realization of robust coating is still a challenge and further research on this topic is required. Recent advancements in nanofabrication and material science have given further impulse to this field.

14 Dropwise condensation The condensation heat transfer coefficient (HTC) α is the ratio between the heat flux q and the saturation-to-wall temperature difference: T sat q' T wall Three properties affect the condensation heat transfer coefficient: - Nucleation density; - Advancing/receding contact angle; - Droplet departure radius.

15 Dropwise condensation The HTC increases with nucleation density. In fact small droplets have low conduction thermal resistance. High apparent contact angle can lead to an increase in conduction resistance due to the reduced size of the droplet base. Low contact angle hysteresis promote droplet shedding. Larger droplet departure size reduce the HTC because larger droplets have high thermal resistance.

16 Surface functionalization Since most industrial metals, such as aluminum, copper, titanium and stainless steel have high surface energy, surface functionalization via a coating that can reduce surface energy is typically required to obtain DWC. Furthermore, the micro- and nanostructured surfaces also need to be coated to impart low surface energy in order to take advantage of the benefits offered by the Cassie state. This is a particularly active area of research, as no solution has yet been proposed which satisfactorily addresses durability, cost and performance.

17 Surface functionalization Several approaches to obtaining suitably low surface energies are presented. Self-assembled monolayers (SAMs) result from spontaneous physi- or chemisorption of a thin molecular film ( 1 nm) comprised of individual molecules on the condensing surface. These molecules have hydrophobic tails pointing away from the surface that interact with the condensate and ligand heads that bind to the surface. This functionalization method does not introduce a significant thermal resistance; however, durability is a primary concern. Exemples are silicon-based ligand silanes.

18 Surface functionalization Polymer coatings such as polytetrafluoroethylene (PTFE) and silicones have been used as a functional coating to promote DWC. However, the required coating thickness to realize satisfactory durability results in an added thermal resistance which offsets the heat transfer improvement due to DWC. Ion implantation promotes DWC through carbon, nitrogen and oxygen ion implantation in copper, aluminum, titanium and steel surfaces. Noble metals applied as a thin coating are a robust approach to achieve DWC. For example, gold, while intrinsically hydrophilic, rapidly adsorbs hydrocarbons from air resulting in increased contact angle and DWC.

19 Surface functionalization Rare earth oxides (REOs) have recently been demonstrated experimentally to be hydrophobic and promote DWC of steam. The low cost of REOs relative to noble metals and high resistance to physical wear offer promise as a potential candidate for surface coatings, but REOs have relatively low thermal conductivity

20 HTC measurement during dropwise condensation over hydrophobic surfaces

21 Experimental measurement technique WATER OUT VAPOR IN y HTC = q T WATER q z VAPOR q = m cool c cool T cool A WATER IN q = λ ΔT Δy q z q y ~ 1% VAPOR OUT Del Col et al., 2017

22 Modeling of DWC on flat surfaces

23 Theory of DWC A theory of heat transfer by dropwise condensation determines the heat transfer through a drop of given size and combines this with an expression for the distribution of drop sizes to obtain the mean surface heat flux. Drops range in size from the smallest on which condensation can take place, primary drops, which form at nucleation sites, to the largest to which drops grow before the region is swept by a falling drop. The drops range in size from nanometer to millimeter scale. Condensation on the smallest drops is inhibited by the surface curvature-surface tension effect which necessitates cooling of the vapor below its normal saturation temperature.

24 Theory of DWC The primary drops are closely packed (nucleation site densities exceed per cm 2 ) and coalescences rapidly lead to larger drops. New primary drops form in the spaces that are vacated by coalescences. Those drops somewhat larger than primary drops, where the curvature effect becomes small, experience intense condensation rates and the temperature drop at the vaporcondensate interface is important. For the largest drops the dominant thermal resistance is that due to conduction in the drop. A complete theory must account for all drop sizes.

25 Theory of DWC Drops continue to grow by coalescence and condensation until a region of the surface is swept by a falling droplet. Many thousands of coalescences take place during the formation of the largest drops. In the models here proposed, the detail of the drop growth process is disregarded and an effective, steady, mean size distribution function is used.

26 Dropwise condensation phenomenon T vapor T wall Khandekar et al., 2014 φ ve = φ le r min = 2σTv liquid h lv (T vapor T wall g r e = 1 4N s 1 μm Nucleation sites (N S )

27 Dropwise condensation phenomenon T wall Rose, 2002 T vapor r max = K 3 σ ρg 1 2 g F adesion = F gravity r max = 6 cos(θ r cos( θ a )) sin θ π(2 3 cos θ + cos 2 θ γ ρ l g Abu-Orabi, 1998

28 Dropwise condensation model history 1930 Schmidt et al Le Fevre & Rose 1998 Abu-Orabi 2011 Kim et al. Models on hydrophobic surfaces: surface roughness is not considered

29 Le Fevre & Rose model Schmidt et al. Le Fevre & Rose 1998 Abu-Orabi 2011 Kim et al. Drop curvature resistance q = r maxqd (r) N r dr r min Q d (r) = r K 1 + K λ 2 l ΔT 2σT SAT r ρ l h lv T SAT γ + 1 h 2 lv ρ l γ 1 RT SAT 2π 0.5 N r = 1 3πr 2 r max r r max 2 3 Resistance through the drop Vapor-liquid interface resistance Rose, 2002

30 Le Fevre & Rose model Heat transfer through a drop of given size Conduction in a drop. The surface and the base of the drop have non-uniform temperatures which are equal at the perimeter of the base. An effective average temperature between the curved and plane surfaces is used in the model. Surface curvature effect. Surface curvature introduces an effective resistance to heat transfer which is significant for very small drops. In order for condensation to occur, the vapor adjacent to the drop surface must be subcooled.

31 Le Fevre & Rose model Heat transfer through a drop of given size Interface temperature drop. Because of molecular kinetics effects during condensation, the interface temperature has to be slightly lower than the saturation temperature in order to achieve a finite rate of condensation. Heat flux at the base of the drop. It can be obtained equating the sum of the three temperature differences to the bulk vapor-surface temperature difference ΔT. Heat flux for the whole surface Knowing the heat exchanged by a single drop and the number of droplets per unit area, the heat flux can be calculated through the operation of integration from the minimum radius to the maximum one.

32 Abu-Orabi model 1930 Schmidt et al Le Fevre & Rose Abu-Orabi 2011 Kim et al. q = r e Qd r n r dr + r min r e r maxqd r N r dr Coating resistance Drop curvature resistance 1 r N r = Q d r = 4πr2 1 r min 3πr 2 r ΔT max r δ + r + 2 λ coat λ l h Vapor-liquid n r e = N(r e ) i interface resistance Resistance through the drop r max 2 3 r(r e r min )(A 2 r + A 3 ) n r = N(r e ) e r e (r r min )(A 2 r e + A 3 ) B 1+B 2

33 Kim et al. model 1930 Schmidt et al Le Fevre & Rose Abu-Orabi Kim et al. q = r e Qd r n r dr + r min r e r maxqd r N r dr Drop curvature resistance q d (r) = ΔTπr 2 1 r min r δ λ coat sin θ 2 + rθ 4λ l sin θ + 1 2h i (1 cos θ) Coating resistance Resistance through the drop Vapor-liquid interface resistance

34 Dropwise condensation model comparison Variable Value Le Fevre Abu- Kim et al. & Rose Orabi tsat [ C] 108 X X X T [ C] 5 X X X δp [μm] 0.2 X X λp [W m -1 K -1 ] 0.2 X X α [-] 1 X X NS [m -2 ] X X θ [ ] 90 X θa [ ] 88.6 X θr [ ] 63.4 X LeFevre & Rose : 2 3 Abu-Orabi : r λ l Kim et al. : rθ 4λ l sin θ r λ l

35 Dropwise condensation over superhydrophobic surfaces

36 Superhydrophobic surfaces Superhydrophobic surfaces allow nearly spherical water droplets to sit on them with high mobility. Superhydrophobic surfaces can promote dropwise condensation and favor droplets shedding (reducing the droplet departure diameter). The high contact angle of droplets can increase the heat transfer resistance (small portion of the droplet in contact with the wall). Such surfaces can become flooded at low values of heat flux.

37 Superhydrophobic surfaces Superhydrophobic (SH) surfaces can be obtained combining surface roughness and low surface energy materials or coatings. SH surfaces can have apparent contact angles greater than 150 and contact angle hysteresis near to 0.

38 Superhydrophobicity Hydrophilic surfaces: Θ eq < 90 Hydrophobic surfaces: Θ eq > 90 Superhydrophobic surfaces: Θ eq > 150 To promote high droplet mobility the surfaces must present a Cassie-Bexter wetting regime, with very low hysteresis. High hysteresis means high adhesion of the drop on the substrate, which decreases the effective superhydrophobic properties even with very high Θ eq During pure steam condensation on superhydrophobic nanostructured surfaces vapor could condense between the surface textures, leading to Wenzel wetting regime of the droplets. In this case, droplet mobility can be favored by vapor velocity. Superhydrophobicity can be obtained combining: micro-/nano- superficial roughness low surface energy

39 Superhydrophobic surfaces During condensation, droplet nucleation can initiate within the surface roughness and the droplet can undergo a nonequilibrium wetting process where Wenzel and Cassie- Baxter equations for which equilibrium is assumed, may not apply. Studies on structured superhydrophobic surfaces have demonstrated that, during condensation, highly adhered Wenzel droplets form that are distinct from the mobile Cassie droplets formed upon fluid deposition with a syringe.

40 Superhydrophobic surfaces Miljkovic et al., 2013 have shown that three different droplet morphologies exist during condensation: Wenzel (W), partially wetting (PW) and suspended (S). Both S and PW droplets are highly mobile relative to W droplets and, as such, are favorable due to their increased ability to depart form the surface.

41 Superhydrophobic surfaces Both PW and S droplets are highly mobile compared to W droplets. However, growth prior to departure also affects heat transfer. In fact, in certain cases, surface structuring can degrade heat transfer performance. A recent study demonstrated for a specific geometry that the growth rate and individual droplet heat transfer of PW droplets are higher than those of S droplets. This difference is due to the composite vapor solid interface beneath S droplets, where the vapor provides a significant thermal resistance to droplet growth. Structure design needs to be considered for maintaining easy droplet removal while simultaneously avoiding the thermal resistance of vapor beneath droplets.

42 Superhydrophobic surfaces: jumping droplets Superhydrophobic surfaces have potential to enhance condensation performance by reducing droplet departure size and enabling faster clearing of the surface for re-nucleation. This is possible due to the low contact angle hysteresis, which allows less pinning force to hold the droplet in place against the body force due to gravity. Smaller droplet departure sizes than those observed during DWC on a flat surface are expected. When a structured surface is suitably designed, coalescenceinduced jumping condensation occurs and departure radii are orders of magnitude smaller.

43 Superhydrophobic surfaces: jumping droplets Jumping occurs due to a release of surface energy upon coalescence of two droplets, some of which is converted to kinetic energy manifested as the motion of the merged droplet perpendicular to the condensing surface. Jumping condensation offers an alternative method for transportation of condensate in phase-change systems. The key limitation for jumping condensation occurs when the nucleation density becomes too high and the spacing between droplets is reduced, at which point the surface is flooded and droplet jumping cannot be sustained. In this case, discrete droplets form which are highly adhered to the surface, and heat transfer performance is worse than for DWC.

44 Surface treatments In the recent years several methods have been proposed to obtained a desired superficial roughness, such as micromachining, micro contact printing, deep radiative ion etching and chemical etching. The chemical etching is probably the most common procedure and consists in a controlled corrosion process of the substrate, immersing it into an appropriate solution (NaOH, HCl, FeCl 3, ). To decrease the surface energy usually the substrate is coated with a very thin layer of a proper material, such as organic substances, polymers and noble metals. These can be deposited over the surface either by physical or chemical deposition processes. The literature demonstrates that properly combining surface etching and functionalization it is possible to obtain surfaces having static contact angles > 160 and contact angles hysteresis < 5.

45 Superhydrophobic sample preparation (copper surface) TWO MAIN STEPS Initially the copper substrate is mechanically polished and finely cleaned 1. CHEMICAL ETCHING. Dipping the sample in a mixture solution of 2.5 M L -1 NaOH and 0.1 M L -1 (NH 4 )S 2 O 4 for 12 min. WIRE-LIKE NANOSTRUCTURES ON THE SURFACE At this stage, the surface is superhydrophilic. the sample is rinsed with DI water and dried in a N 2 stream 2. SURFACE FUNCTIONALISATION. Immersing the sample, at room temperature, in an ethanol solution of 1 mm L -1 1H,1H,2H,2Hpersluorodecanethiol for 30 minutes followed by immersing it in ethanol for 1 hour. SURFACE ENERGY CHANGE

46 Superhydrophobic sample UNTREATED SAMPLES: Θ eq = 86 ± 2 (HYDROPHILIC) TREATED SAMPLES: Θ eq = 159 ± 2 (SUPERHYDROPHOBIC)

47 Experimental apparatus Thermosyphon test rig Filled with DI water NCGs presence is avoided Overpressure during the night p = 1.43 bar (T sat = 110 C)

48 Test section Water inlet Water outlet Rectangular Teflon minichannel (D h = 3.6 mm) Front glass (heated) Cylindrical copper specimens (D = 20 mm) Water cooling system on the back side (finned copper plate)

49 Data reduction Specimens are fitted with 5 T-type thermocouples. T wall is evaluated from the acquired temperature profile. q = -k dt/dz h = q / (T sat T wall )

50 q [kw m -2 ] h [kw m -2 K -1 ] Experimental results: effect of vapor velocity m s^-1 12 m s^ m s^-1 12 m s^ m s^ m s^ T sat - T wall [K] T sat - T wall [K] Heat flux increases when increasing temperature difference and vapor velocity Heat transfer coefficient depends only on vapor velocity Increasing v vapor leads to a reduction of droplet departing size, thus leading to high mobility and high rate of thermal transport

51 Flow visualization ΔT = 3.5 K v steam = 18 m s -1 ΔT = 3.5 K Increasing the vapor velocity the size of departing droplets decreases and the frequency increases. Reducing the droplets departing diameter leads to thermal performance enhancement, due to the lower thermal resistance of the smaller drops.

52 q [kw m -2 ] Experimental results: Lifetime test For the first five days no sign of degradation was found Day-1 Day-2 Day-3 Day-4 Day-5 Day T sat - T wall [K] At the same subcooling, heat flux is reduced by 35% after six working days After six days steam condenses over the specimen no more in dropwise mode, but changes to filmwise v vapor = 12 m s -1

53 Lifetime test Day 1 Day 6 Performances decrease is due to the deterioration of the hydrophobic monolayer and of the surface morphology.

54 q [kw m -2 ] Experimental results: Comparison against non-treated surface On the naturally oxidized polished copper sample FWC occurs While the superhydrophobicity is sustained, the treated surface performs clearly better than the untreated one for ΔT > 5 K DWC Superhydrophobic Day-1 FWC Superhydrophobic Day-6 FWC Oxidized T sat - T wall [K] After 6 consecutive working days, thermal performance of the naturally oxidized surface becomes higher v vapor = 12 m s -1

55 Concluding remarks DWC enhances heat transfer performance as compared to FWC, because of absence of liquid film and droplet mobility. DWC mode is promoted by using hydrophobic or superhydrophobic surfaces. The challenge is to realize robust surface treatments that are able to promote DWC. The performance of superhydrophobic surfaces can be limited by flooding.

56 References DWC Abu-Orabi M., Modeling of heat transfer in dropwise condensation, Int. J. Heat Mass Transf., Vol. 41, pp , Bisetto A., Bortolin S., Del Col D., Experimental analysis of steam condensation over conventional and superhydrophilic vertical surfaces, Experimental Thermal and Fluid Science, Vol. 68, pp , Bisetto A., Bortolin S., Martucci A., Del Col D., Condensation of steam over nanoengineered surfaces, 9th International Conference on Boiling and Condensation Heat Transfer, Boulder, Colorado, April 26-30, Del Col D., Parin R., Bisetto A., Bortolin S., Martucci A., Film condensation of steam flowing on a hydrophobic surface, Int. J. Heat Mass Transf., Vol. 107, pp , Khandekar S., Muralidhar K., Dropwise Condensation on Inclined Textured Surfaces, Springer, Kim S., Kim K.J., Dropwise Condensation Modeling Suitable for Superhydrophobic Surfaces, J. Heat Transfer., Vol. 133, 81502, Miljkovic N., Enright R., Nam Y., Lopez K., Dou N., Sack J., Wang E.N., Jumping-dropletenhanced condensation on scalable superhydrophobicnanostructured surfaces, Nano Lett., Vol. 13(1), pp , Miljkovic N., Enright R., Wang E.N., Modeling and optimization of superhydrophobic condensation, J. Heat Transfer, 135(11), , Miljkovic, N., Preston, D.J., Enright, R., Adera, S., Nam, Y. and Wang, E.N., Jumping droplet dynamics on scalable nanostructured superhydrophobic surfaces, J. Heat Transfer, Vol. 135(8), , 2013.

57 References DWC Miljkovic N., Preston D.J., Wang E.N., Recent Developments in Altered Wettability for Enhancing Condensation, in: J.R. Thome and J. Kim (Eds.), Encyclopedia of Two-Phase Heat Transfer and Flow II - Special Topics and Applications, Volume 3: Special Topics in Condensation, pp., , World Scientific Publishing Co., Singapore, Parin R., Del Col D., Bortolin S., Martucci A., Dropwise condensation over superhydrophobic aluminium surfaces, Journal of Physics: Conference Series, Vol. 745, , Parin R., Penazzato A., Bortolin S., Del Col D., Modeling of dropwise condensation on flat surfaces, 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Rose J.W., Theory of Dropwise Condensation, in: J.R. Thome and J. Kim (Eds.), Encyclopedia of Two-Phase Heat Transfer and Flow II - Special Topics and Applications, Volume 3: Special Topics in Condensation, pp., 1-13, World Scientific Publishing Co., Singapore, Rose J.W., Dropwise condensation theory and experiment: a review, Proc. Inst. Mech. Eng. Part A : J. Power Energy, Vol. 216, pp , Schmidt E., Schurig W., Sellschopp W, Versuche Über Die Kondensation von Wasserdampf in Film- Und Tropfenform, Forsch. im Ingenieurwes, Vol.1 (2), pp , Torresin D., Tiwari M.K., Del Col D., Poulikakos, D., Flow Condensation on Copper-Based Nanotextured Superhydrophobic Surfaces, Langmuir, Vol. 29 (2), pp , 2013.

58 Thanks for your attention! Davide Del Col University of Padova Department of Industrial Engineering