The effects of hydrophilic/hydrophobic surface patterning on critical heat flux (CHF) and heat transfer coefficient (HTC) were studied using custom-engineered testing surfaces. Patterning was created over a sapphire substrate and tested in a pool boiling facility in MITs Reactor Hydraulics Laboratory. The hydrophilic and hydrophobic matrices were created using layer by layer deposition of 50 nm thick SiO2 nanoparticles and monolayer thickness fluorosilane, respectively. Ultraviolet ozone patterning was then used with chrome-printed masks to create the desired geometric features. Hexagon, ring, star, and mixed patterns were tested to determine their abilities to affect CHF and HTC through prevention of bubble pinning at high heat fluxes. During testing, an infrared camera was used to measure the surface temperature distribution as well as locate nucleation sites for data analysis. It was found that CHF values were enhanced over the bare sapphire values by approximately 90% for hexagons, 60% for stars, 65% for rings, and 50% for mixed patterns. Contrary to expectations, patterning did not seem to affect the HTC values significantly. Although patterning did improve CHF performance over bare heaters, both CHF and HTC were found to be statistically similar to those for unpatterned, uniformly hydrophilic surfaces. Copyright © 2013 by ASME

Buongiorno, Jacopo

Coyle, Carolyn P.

McKrell, Thomas J

O'Hanley, Harrison F

Phillips, Bren Andrew

McKrell, Thomas J.

DSpace@MIT

EFFECTS OF HYDROPHOBIC SURFACE PATTERNING ON BOILING HEATTRANSFER AND CRITICAL HEAT FLUX OF WATER AT ATMOSPHERIC PRESSURECarolyn Coyle⇤Dept. of Mechanical EngineeringMassachusetts Institute of TechnologyBoston, Massachusetts, 02139Harry O’HanleyDept. of Nuclear EngineeringMassachusetts Institute of TechnologyBoston, Massachusetts, 02139Bren PhillipsDept. of Nuclear EngineeringMassachusetts Institute of TechnologyBoston, Massachusetts, 02139Jacopo BuongiornoDept. of Nuclear EngineeringMassachusetts Institute of TechnologyBoston, Massachusetts, 02139Thomas McKrellDept. of Nuclear EngineeringMassachusetts Institute of TechnologyBoston, Massachusetts, 02139ABSTRACTThe effects of hydrophilic/hydrophobic surface patterning oncritical heat flux (CHF) and heat transfer coefficient (HTC) werestudied using custom-engineered testing surfaces. Patterningwas created over a sapphire substrate and tested in a pool boilingfacility in MITs Reactor Hydraulics Laboratory. The hydrophilicand hydrophobic matrices were created using layer by layer de-position of 50 nm thick SiO2 nanoparticles and monolayer thick-ness fluorosilane, respectively. Ultraviolet ozone patterning wasthen used with chrome-printed masks to create the desired ge-ometric features. Hexagon, ring, star, and mixed patterns weretested to determine their abilities to affect CHF and HTC throughprevention of bubble pinning at high heat fluxes. During testing,an infrared camera was used to measure the surface temperaturedistribution as well as locate nucleation sites for data analysis.It was found that CHF values were enhanced over the bare sap-phire values by approximately 90% for hexagons, 60% for stars,65% for rings, and 50% for mixed patterns. Contrary to expec-tations, patterning did not seem to affect the HTC values sig-nificantly. Although patterning did improve CHF performanceover bare heaters, both CHF and HTC were found to be sta-⇤Corresponding author: cpcoyle@mit.edutistically similar to those for unpatterned, uniformly hydrophilicsurfaces.INTRODUCTIONBoiling is a common mechanism of heat transfer in manypower generation systems. The efficiency of the heat transfer ofa system is affected by the surfaces ability to move heat froma source into a working fluid. As a result, it is beneficial to de-sign efficient heat transfer surfaces in the nucleate boiling regimewhere many of these systems operate. Two common figures ofmerit for nucleate boiling heat transfer are the critical heat flux(CHF) and the heat transfer coefficient (HTC). The HTC cap-tures the ability of the fluid to transfer heat efficiently (i.e. with asmall temperature difference), while the Critical heat flux marksthe transition from the nucleate boiling regime to the film boilingregime causing a large reduction in the HTC [1], and thus can beconsidered the upper limit of efficient boiling heat transferWork has been conducted on optimizing the boiling heattransfer properties of surfaces. By altering properties such assurface roughness, wettability, and porosity, substantial enhance-ments in CHF and/or HTC have been recorded [2–4]. The suc-Proceedings of the ASME 2013 Power Conference POWER2013 July 29-August 1, 2013, Boston, Massachusetts, USA POWER2013-981461 Copyright © 2013 by ASMEDownloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 07/16/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-usecess of these methods have been attributed to an increased den-sity of nucleation sites as well as increased rewetting of the heatersurfaces, causing a greater HTC and delay in CHF [2]. In thepast decade the effects of nanofluids on boiling have been inves-tigated. While originally inserted as part of the working fluid,tests that deposit nanoparticles directly on the heater surfacehave similarly resulted in enhancements in CHF and HTC [2–6].Nanoparticle deposits have been found to increase wettabilityand capillarity, allowing for operation in the nucleate boilingregime at higher heat fluxes [7].Most recently, work has begun on creating hydrophilic andhydrophobic networks to test for enhanced properties. Studies ofhydrophilic spots on hydrophobic surfaces (hydrophilic network)and hydrophobic spots on hydrophilic surfaces (hydrophobic net-work) have been examined with circular and hexagonal spot pat-terns [2–4]. In one study, it was found that using hexagonal spotsof diameters between 40% and 60% of a varying pitch between50 and 200-m, CHF and HTC were increased by 65% and 100%respectively over purely hydrophilic surfaces [4].These approaches of creating surface features using nanoflu-ids or hydrophilic and hydrophobic patterning have been suc-cessful in increasing the efficiency of boiling heat transfer. How-ever, while the spot diameter and pitch have been used to exploreenhancements in CHF and HTC, the shape of the spots has notbeen exhaustively studied.In this work, hexagonal, star, ring, and mixed shape hy-drophobic networks have been studied in a pool boiling facil-ity, to determine their effects on the enhancements of CHF andHTC. These shapes were chosen to test their abilities to increasebubble nucleation and prevent pinning of the bubble base, thusencouraging bubble departure from the heater surface throughsharp corners and secondary regions.BACKGROUNDPool boiling occurs when the heater surface is submerged ina stagnant body of fluid. In pool boiling there exist various heattransfer regimes. Initially at low heat fluxes, heat transfer is dom-inated by natural convection, seen in Figure 1 in the region fromPoint A to B [2]. As the heat flux increases, the fluid transitionsinto the nucleate boiling regime, seen from Point B to C. Thenucleate boiling regime has a steep slope on the boiling curve,suggesting a high value of the heat transfer coefficient, definedby Newtons law of cooling:q00 = h [Twall Tbulk] (1)where q is the heat flux, h is the heat transfer coefficient (HTC),Twall is the surface temperature, and Tbulk is the working fluidtemperature or, in the case of boiling, Tsat . In the case of pool boiltesting, h will often be temperature dependent [2]. However, at!FIGURE 1. STANDARD POOL BOILING CURVE [2].Point C, critical heat flux (CHF) is achieved and, if the heat fluxcontinues to rise, the fluid moves to Point D in the film boilingregime. In this regime, a thin vapor layer forms on the heatersurface, resulting in greatly increased surface temperatures, oftencausing failure of the heater. As a result, it is crucial to know thevalue of CHF for the given heater in use.Several models for predicting CHF have been postulated,each with their own benefits. Zuber [8] predicts CHF using apurely hydrodynamic instability model, as seen in Eqn. (2). Inthe following equation, q00CHF is defined as the critical heat flux,K is a dimensionless constant dependent on surface geometry,and h fg is the latent heat of vaporization.q00CHF = Krgh f g"gs r f  rg r2g# 14(2)This model, however, fails to account for surface features, whichhave been determined to greatly affect CHF.Other correlations have also been created that account forindividual surface features such as wettability and porosity, butnone have yet been created to predict the effects of spot pattern-ing [9, 10]. While there is not yet a model able to capture allthe physical mechanisms at work in these boiling phenomena,the correlations listed above do provide qualitative trends for thevarious effects of the surface parameters.EXPERIMENTAL METHODSManufacturing Surface PatternsIndium tin oxide-sapphire (ITO) heaters were used as thebase for the heater surfaces, shown in Figure 2. These heaters2 Copyright © 2013 by ASMEDownloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 07/16/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use!FIGURE 2. ITO-SAPPHIRE HEATER SCHEMATIC WITH MATERIALS AND DIMENSIONS IN MM [2].are made of 50.8⇥ 50.8-mm2, square piece of sapphire 0.25-mmthick. A 2-cm wide and 700-nm thick layer of ITO is centeredalong the length of the sapphire and two silver pads 25-µm thickare located on the ends of the ITO. The ITO acts as a resistiveheating element while the silver pads provide attachment pointsfor electrical leads. The following methods describe the processby which the hydrophobic networks were created.Layer by Layer Deposition Layer by layer deposition(LBL) is the process of laying a single layer of nanoparticles ona substrate [11]. A durable LBL coating able to withstand boil-ing temperatures was created by dipping heater surfaces first in apositively charged poly(allylamine) (PAH) solution followed bya negatively charged 50-nm silica (Si02) nanoparticle solution.This process was repeated fifty times to create a hydrophilic bi-layer of 1.4 microns with a contact angle of less than 5 degreeson the heater surface [3].Chemical Vapor Deposition of Fluorosilane To cre-ate a hydrophobic layer, chemical vapor deposition (CVD) wasused. A small volume of liquid fluorosilane was placed in aclosed container with the heater surfaces and baked in an oven at140 C for thirty minutes. This procedure vaporizes the fluorosi-lane and creates an even, hydrophobic layer on the heater surfaceabove the hydrophilic bilayer of static contact angle greater than135 degrees.Ultraviolet Ozone Patterning Ultraviolet Ozone(UVO) patterning was used to create the hydrophilic and hy-drophobic regions. A quartz mask was ordered with a chromiumpattern of the desired hydrophobic regions. The mask wasplaced on the ITO heater and exposed for two hours. During thistime, ultraviolet light passes through the quartz, breaking downthe fluorosilane and exposing the hydrophilic layer while theflurorsilane in contact with the chromium is protected. Thesefour masks like the one in Figure 3 were used to create thehexagon, star, ring, and mixed patterned heater surfaces, eachwith a respective 260-µm spot diameter and 1.5-mm pitch.FIGURE 3. MIXED PATTERN QUARTZ MASK WITH 260-µMDIAMETERAND 1.5-MM PITCH PATTERNED CHROMIUMWITHEXPANDED SHAPE VIEW. LEFT TO RIGHT: RINGS, HEXAGONS,STARS. OTHER MASKS ARE SAME LAYOUT WITH A SINGLESHAPE.Apparatus and ProcedureTests were conducted in the pool boiling facility (PBF)shown in Figures 4 and 5. Patterned ITO heater surfaces with3 Copyright © 2013 by ASMEDownloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 07/16/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-useleads were attached to borosilicate tubes, inserted above a squarehole in the center of the inner bath, and sealed using a siliconegel gasket.!FIGURE 4. PBF IN MIT REACTOR HYDRAULICS LABORA-TORY.This inner bath as well as an outer bath were filled withtap water heated to 100 C to create an adiabatic environmentwhile the working fluid was deionized water , which filled theborosilicate tube above the active heater area. The electricalleads from the heater were attached to an Electronic Measure-ments Inc. TCR Power Supply. The heat flux was calculatedusing an Agilent Technologies 34980A data acquisition systemalso connected to the circuit through the power supply. A goldmirror at a 45  angle was then placed under the heater to allow aFLIR Sc6000 IR camera to record the full 2D temperature profileof the bottom of the ITO heater, as sapphire and ITO are trans-parent and opaque to infrared light, respectively.The ITO was imaged using the IR camera at increasing heatfluxes until the point of failure (CHF) where the sapphire sub-strate breaks. Figure 6 below shows two IR image taken, one ofa bare heater without patterning and one of a star pattern wherethe effect of the spot patterning on bubble nucleation is visible.Data ProcessingTo create the boiling curves for the tests, IR camera videosare exported into Flexible Image Transport System (FITS) files.These files are then imported into Matlab where they are ana-lyzed and average ITO surface temperatures are calculated usingexperimentally gathered calibration information. These temper-atures are then used to calculate the average heater surface tem-perature by accounting for conduction through the sapphire.!FIGURE 5. PBF EXPERIMENTAL SETUP [2].(A) $(B) $FIGURE 6. SURFACE TEMPERATURE IMAGES OF (A) BAREAND (B) STARS-3 HEATERS AT 800 AND 1500 KW/M2 HEATFLUXES, RESPECTIVELY (NEAR POINTS OF FAILURE).RESULTSTable 1 shows the CHF results of individual, ITO heater sur-face tests. These CHF values are found from the heat flux atwhich the ITO undergoes material failure.Figure 7(A) shows the boiling curves for each of the heaters,including bare and pure hydrophilic heaters for comparison.These were obtained from the IR camera video taken at eachof the indicated heat fluxes. In the figure, the natural convection4 Copyright © 2013 by ASMEDownloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 07/16/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-useFIGURE 7. (A) INDIVIDUAL BOILING CURVES FOR BARE, PURE HYDROPHILIC, HEXAGON, STAR, RING, AND MIXED PATTERNEDHEATERS. (B) INDIVIDUAL HTC CURVES FOR BARE, PURE HYDROPHILIC, HEXAGON, STAR, RING, AND MIXED PATTERNEDHEATERS. UNCERTAINTIES IN HEAT FLUX RESULT FROM ELECTRONIC MEASUREMENTS INC. TCR POWER SUPPLY WITH REG-ULATION AND STABILITY UNCERTAINTIES OF 0.1% AND 0.05%.TABLE 1. INDIVIDUAL AND AVERAGE CHF VALUES FOR ITOPATTERNED HEATERS.Pattern CHF(kW/m2) Avg. CHF(kW/m2) St. DevHexagons-1 1760Hexagons-2 1412 1691 14.9%Hexagons-3 1903Stars-1 1099Stars-2 1384 1387 20.9%Stars-3 1680Rings-1 1596Rings-2 1289 1452 10.6%Rings-3 1470Mixed-1 1458Mixed-2 1161 1333 11.5%Mixed-3 1380regime (Figure 1, Point A to B) is visible, followed by the nucle-ate boiling regime (Figure 1, Point B to C). Figure 7(B) showsthe HTC curves for each of the heaters, created using a Matlabdata analysis file and Eq. 1.DISCUSSIONThe above results suggest that shape spot patterning doesnot greatly enhance CHF. Using the same experimental setupand ITO heaters, O’Hanley [2] measured average CHF valuesof 873 kW/m2, 1009 kW/m2, and 1617 kW/m2 for uncoated,non-porous hydrophilic, and porous hydrophilic heaters. He alsoconducted experiments using circular hydrophobic network spotpatterning and, for 260-µm diameter and 1.5-mm pitch spots,measured an average CHF value of 1612 kW/m2 [2]. Theseexperiments have supported those results. Shape spot patternheaters have a 50% to 90% enhancement in CHF over uncoatedheaters, but little to no enhancement in CHF over unpatternedhydrophilic heaters. The variations in CHF among the differentpatterns themselves, however, do provide some insight into bub-ble departure from the heater surface. While it was hypothesizedthat sharp corners or rings could prevent bubble base pinningto the surface and thus promote bubble departure and postponeCHF, the results do not support that hypothesis. In fact, the CHFvalue decreases as the shapes increasingly deviate from a purecircle.Patterned HTC values similarly recorded little to no en-hancement over the unpatterned hydrophilic surfaces, but alsolacked enhancements over bare heaters. Bare, unpatterned, andpatterned heaters saw peaks of around 60 kW/m2C with averagesbetween 30-40 kW/m2C. Among the patterned heaters, HTC val-ues were fairly close, though variations even among identical5 Copyright © 2013 by ASMEDownloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 07/16/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-useheater patterns were observed, possibly as an inherently vary-ing feature of porosity. To better understand the nature of thesedifferences, an investigation of the dependence of bubble depar-ture frequency and bubble departure diameter on pattern shapemust be conducted.At this time it can be stated that while hydrophobic spotsdo serve as preferential nucleation sites on the heater surface (asverified directly with the IR images, see Fig. 6(B)), they do notseem to allow for increased rewetting of the heater surface andare therefore not as efficient at removing heat as the unpatternedhydrophilic surface. While studies have shown enhancementin CHF and HTC over purely hydrophilic surfaces [4], poros-ity is perhaps the greater contributor to efficiency in these tests,as shown in the separate-effect study of O’Hanley [2]. Furtherinvestigation into the effects of spot patterning on uncoated ornon-porous heaters may show a greater enhancement in boilingefficiency.CONCLUSIONSA study into the effects of hydrophobic spot patterning ofhexagons, rings, and stars on hydrophilic surface was conductedto determine their effects on pool boiling efficiency, as capturedby CHF and HTC. Special-design surface heaters were manufac-tured with the desired features and tested with water in a poolboiling facility at atmospheric pressure. 2D surface temperaturesimages were recorded using an IR camera.It was found that spot patterning of hydrophobic regions ofall shapes did not enhance CHF and HTC over unpatterned hy-drophilic surfaces. However, CHF was higher for spots of near-circular shape, while HTC remained unaffected.REFERENCES[1] R.A. Knief. Nuclear Engineering: Theory and Technology ofCommercial Nuclear Power. American Nuclear Socity, 2ndedition, 2008.[2] Harry OHanley. Separate effects of surface roughness, wet-tability and porosity on boiling heat transfer and criticalheat flux and optimization of boiling surfaces. Masters the-sis, Department of Nuclear Science and Engineering, Mas-sachusetts Institute of Technology, 2012.[3] Bren Phillips. Nano-engineering the boiling surface for op-timal heat transfer rate and critical heat flux. Masters the-sis, Department of Nuclear Science and Engineering, Mas-sachusetts Institute of Technology, 2011.[4] Xu J. Qiu H. Attinger D Betz, A.R. Do surfaces with mixedhydrophilic and hydrophobic areas enhance pool boiling?Applied Physics Letters, 27:141909, 2010.[5] J. Buongiorno S.J. Kim, I.C. Bang and L.W. Hu. Effectsof nanoparticle deposition on surface wettability influencingboiling heat transfer in nanofluids. Applied Physics Letters,98:071908, 2010.[6] J.B. Kim H.D. Kim and M.H. Kim. Effect of nanoparticleson CHF enhancement in pool boiling of nano-fluids. Int. J.Heat Mass Transfer, 49:5070-5074, 2006.[7] H.D. Kim and M.W. Kim. Effect of nanoparticle depositionon capillary wicking that influences the critical heat flux innanofluids. Applied Physics Letters, 91:014104, 2007.[8] Dweitt Theodore L. Bergman, Frank P. Incropera, David Pand Adrienne S. Lavine. Fundamentals of Heat and MassTransfer 6th ed. John Wiley and Sons, 2007.[9] Y.V. Polezhaev and S.A. Kovalev. Modeling heat transferwith boiling on porous structures. Thermal Engineering,12:617620, 1990.[10] V. Mizo S. Kandlikar and M. Cartwright. Investigation ofbubble departure mechanism in subcooled flow boiling ofwater using high speed photography. In John C. Chen, editor,Convective Flow Boiling. Taylor and Francis Group, 1996.[11] Michael F. Rubner, Daeyeon Lee, and Robert E. Cohen.All-nanoparticle thin-film coatings. Nano Letters of Ameri-can Chemical Society, 6(10):23052312, 2006.6 Copyright © 2013 by ASMEDownloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 07/16/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use

Effects of Hydrophobic Surface Patterning on Boiling Heat Transfer and Critical Heat Flux of Water at Atmospheric Pressure

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Effects of Hydrophobic Surface Patterning on Boiling Heat Transfer and Critical Heat Flux of Water at Atmospheric Pressure

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