Crystallization-Induced Fouling during Boiling - ACS Publications

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Crystallization-Induced Fouling during Boiling: Formation Mechanisms to Mitigation Approaches Susmita Dash, Leonid Rapoport, and Kripa K Varanasi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02936 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 8, 2017

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Crystallization-Induced Fouling during Boiling: Formation Mechanisms to Mitigation Approaches Susmita Dash, Leonid Rapoport, Kripa K. Varanasi Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

ABSTRACT

Boiling is significantly altered by the presence of dissolved salts. In particular, salts whose solubility decreases with temperature have the tendency to crystallize and adhere to the heat transfer surface and adversely affect the thermal performance. Scaling due to the precipitation of such salts poses serious operational and safety challenges in several practical applications including heat exchangers, pipelines, and desalination. Here, we study the effect of dissolved salts on the dynamics of pool boiling and its impact on the heat transfer coefficient and critical heat flux (CHF). We find that even undersaturated conditions can lead to crystallization and scale buildup on boiling surface and dramatically reduce heat transfer performance. For example, the CHF for a salt solution that is 75 percent of the saturation concentration is found to be at least two times lower than that for deionized water. Using simultaneous high-speed optical and infrared imaging, we determine the interdependence between crystallization-induced scale formation and bubble evolution dynamics, including, bubble nucleation, growth, and departure.

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We find that salt crystallizes in a “coffee-ring” pattern due to evaporation at the contact line of the bubble. Based on the role of the microlayer and triple contact line on scale formation, we propose manipulating surface wettability as a means to avoid scale formation and associated reduction in heat transfer coefficient. Surfaces with hybrid wettability are demonstrated as a means to mitigate the reduction in heat transfer coefficient and CHF in the presence of dissolved salts. KEYWORDS: Boiling, crystallization, fouling, scale, coffee-ring effect, critical heat flux, microlayer, hydrophilic, hydrophobic, hybrid wettability INTRODUCTION Boiling is important in various processes ranging from power generation, oil and gas, chemical processes, and desalination [1, 2]. There has been an intense effort towards enhancing boiling performance by engineering surfaces [3-9]. These studies have almost always used deionized water or pure working fluids such as fluorinated liquids to characterize boiling behavior. However, the presence of even small amount of salts (minerals) can have a significant negative impact on the overall performance of heat exchangers, and boiling curves obtained using idealized working liquid are no longer applicable [10]. Crystallization of dissolved minerals in water often leads to scaling (fouling) of heat exchanger surfaces, which adversely affects the thermal performance and power efficiency in industrial applications [10, 11, 12]. The formation of scale in combustion steam generators can lead to rapid increase in wall temperature and pose safety and operational issues in boilers [13]. The efficiency losses associated with fouling of heat exchanger surfaces have been estimated be about 2% of the total world energy production per year [14, 15].

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The issue of scaling is pronounced in the presence of salts such as calcium sulfate, barium sulfate and calcium carbonate that have an inverse solubility (the solubility decreases with increase in temperature) [16]. Owing to the low thermal conductivity, the precipitated scale layer acts as a thermal barrier and causes severe deterioration in heat transfer, which results in the loss in energy efficiency, overheating of boiler tube material, and even tube failure [17]. Due to the detrimental effects of scale, the quality of feed water into the boilers is stringently controlled. Even with the controlled water chemistry, scaling almost always occurs. Existing solutions for scale mitigation rely on mechanical or chemical techniques [13]. The mechanical techniques are based on physical removal while the chemical techniques rely on additives that can shift the precipitation equilibrium curve or act as inhibitors that slow down the kinetics of scaling. These approaches are expensive, energy intensive or environmentally unfriendly, and are often insufficient to remove the scale layer [17]. Furthermore, these approaches require a shut down of the equipment to either treat or to replace the fouled components [17]. In order to devise solutions for efficient scale mitigation under boiling, it is important to understand how scale forms during boiling. Most studies on the mechanism of scale formation have focused on slow evaporation, where precipitation occurs as the bulk fluid becomes supersaturated [18, 19]. However, in the case of boiling, precipitation occurs even when the bulk fluid is undersaturated. There are limited studies on scale formation during boiling and its impact on heat transfer [10, 11, 14]. The reduction in the heat transfer coefficient in the presence of salts has been primarily attributed to the reduction in bubble nucleation as a result of change in surface tension due to dissolved salts [11]. A few studies have proposed a reduction in adhesion of scale by means of altering the surface energy of the boiling surface using ion-implantation and hydrophobic coatings [20, 21]. While some of these results are promising, a fundamental


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understanding of incipience of crystallization under boiling remains elusive [11]. We believe that such insights can lead to better anti-scaling approaches for boiling applications. Here, we study how coupled microscale phenomena of bubble evolution and salt precipitation impacts the macroscale heat transfer performance. We use calcium sulfate solution as a model system since it is one of the commonly found salts in feed water. By using simultaneous highspeed visualization and infrared imaging, we study the coupled dynamics of bubble evolution and precipitation, and explain the precipitation mechanism in terms of the transport phenomena in the liquid microlayer associated with bubble formation. Based on this understanding, we propose tailoring the surface wettability to alter the morphology of the liquid microlayer, to reduce scale formation and enhance boiling performance. EXPERIMENTAL SETUP The boiling test chamber consists of a double walled jacket through which a 1:1 mixture of propylene glycol and water is circulated to maintain the test solution inside the main chamber at saturated boiling conditions of 100 °C and 1 atm. A schematic of the experimental setup is shown in Figure 1a. The test substrate comprises of an IR transparent grade silicon wafer. A layer of titanium (150 nm) is evaporated on top of the wafer, which ensures accurate measurement of the substrate temperature by eliminating the difference in the IR signal due to the difference in the emissivity of water and water vapor [22]. The titanium layer is coated with a layer of silicon nitride (100 nm) for chemical resistance and a layer of silicon dioxide (200 nm) for electrical insulation. A 1 cm x 2 cm indium tin oxide (ITO) layer that is 120 nm in thickness, sputtered on the electrically insulated backside of the 5 cm x 5 cm silicon test substrate, serves as the heater and defines the boiling area (Figure 1a). The sputtering parameters of the ITO layer are selected to ensure semi-transparency of the layer to IR radiation. The ITO heater is resistively 4 ACS Paragon Plus Environment

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heated via silver contact pads using a DC power source. The test solution comprises of an undersaturated solution of calcium sulfate to avoid precipitation in the bulk and on the chamber walls and to restrict crystallization exclusively to the heated area. The salt solution is prepared by dissolving calcium sulfate in deionized water by magnetically stirring the solution for approximately 48 hours. The solution is degassed by boiling in a microwave for ~ 15 minutes, and subsequently left inside the test chamber maintained at 100 °C for an hour, prior to experiments to ensure temperature equilibration. Consistent with previous studies, two different salt concentrations, 0.94 Csat (8.70 mM) and 0.75 Csat (6.97 mM), where Csat is the saturation concentration at 100 °C, are used for the experiments [10, 14]. To eliminate loss of water vaporinduced change in salt concentration during boiling, chilled water is recirculated through a reflux condenser to condense the water vapor back into the boiling chamber. A high-speed optical camera is used to visualize the boiling behavior at 1000-2500 fps from the top of the substrate and a high-speed infrared camera is used to image the temperature profile of the boiling area at 1250 fps from the bottom (Figure 1a). An in-situ calibration of the IR camera is used to correlate the infrared counts with the absolute temperature; the details of the calibration method are provided in the Supporting Information. Figure 1b and Figure 1c show representative optical and infrared images, with the temperature contour lines superimposed on the latter, of boiling DI water and salt solution, respectively, at a heat flux of 25 W/cm2.


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Figure 1(a) Boiling experimental setup showing schematic of the specifically fabricated test substrate and arrangement of the high-speed camera and IR camera. Representative optical and infrared images of bubble dynamics during pool boiling of (b) DI water (c) 0.94 Csat salt solution at a heat flux of 25 W/cm2. The scale bar represents 5 mm. RESULTS AND DISCUSSION For the CHF studies, the applied heat flux is increased in steps of 5 W/cm2 every 5 minutes (defining ramp rate) till a sudden increase in substrate temperature that is often accompanied with burnout of the sample. For DI water, CHF conditions are reached when the vapor film, which acts as an insulating layer, covers the entire heat transfer surface and leads to a sudden increase in the temperature of the substrate. In the case of the salt solution, the crystalized scale layer on the substrate acts as insulation and has a similar effect to that of a vapor film leading to CHF conditions. It should be noted that for salt solution, the ramp rate of applied heat flux determines the extent of salt crystallization on the substrate, which in turn affects the CHF. Figure 2 shows the plot of heat flux versus wall superheat corresponding to DI water and salt solutions. The data points are an average of two to three independent experiments and error bars denote standard error. The CHF for DI water was measured to be 115 ± 9 W/cm2, consistent with 6 ACS Paragon Plus Environment

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previous studies [6]. In the presence of dissolved salts, the boiling performance deteriorates, resulting in a significant increase in substrate temperature with increasing heat flux. For instance, at a heat flux of 15 W/cm2, the heat transfer coefficient, given by the ratio of surface heat flux to wall superheat, reduces from ~ 0.83 W/cm2K for DI water to 0.64 W/cm2K and 0.48 W/cm2K for 0.75 Csat and 0.94 Csat salt solution, respectively. At the salt concentration of 0.94 Csat, the CHF reduces drastically by three folds to 32 ± 2 W/cm2. Even at a lower salt concentration of 0.75 Csat, the CHF still remains low at 43 ± 3 W/cm2. These results indicate the adverse effects of the presence of dissolved salts on boiling performance.

Figure 2. Boiling curve for DI water and salt solutions of two concentrations corresponding to a ramp rate of 5 W/cm2 per 5 mins. The critical heat flux is significantly reduced in the presence of salts. The data points are connected by dashed lines as a guide to the eye.


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The reduction in the boiling heat transfer coefficient in the presence of dissolved salts can be explained by considering the bubble dynamics and evolution of the crystallization-induced scale layer. The number of bubble nucleation sites reduces drastically in the presence of salts (Figure 1b,c). For instance, at a heat flux of 25 W/cm2, the number of nucleation sites, indicated by circular darker areas on the infrared image (Figure 1c), reduces by 5 times in the salt solution compared to DI water. Since the heat transfer coefficient is directly related to the number of nucleation sites, the boiling performance significantly reduces and there is a steep increase in the substrate temperature. Owing to the inverse solubility of calcium sulfate with temperature, salt precipitates on the hot substrate. These crystal deposits pin the contact line and delay bubble departure, reducing the bubble departure frequency by a factor of ten and increasing the bubble size by a factor of two (Figure 3a). The reduced bubble departure frequency leads to further increase in substrate temperature compared to that of DI water (Figure 3a). In addition to influencing the bubble dynamics, the calcium sulfate scale deposit increases the thermal resistance of the substrate due to its low thermal conductivity (~ 1 W/mK). For example, at a heat flux of 20 W/cm2, a 100 µm thick scale layer can result in a temperature drop of 20 °C across its thickness. The lower temperature atop the fouling layer restricts boiling, causing the substrate to significantly overheat (Figure 3a). The scale layer also increases the thermal mass of the substrate resulting in longer heating timescale (~ 400 ms) before a bubble nucleates when compared to that of DI water (~ 40 ms) as shown in Figure 3a. Figure 3b shows the spatial evolution of temperature along the centerline of the bubble basal area for salt solution and DI water. The locations of the lowest temperature, denoted by arrows, along each temperature curve correspond to the triple contact line where maximum evaporation occurs. The presence of scale in the case of the salt solution restricts the lateral spreading of the 8 ACS Paragon Plus Environment

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bubble-formation-induced evaporative cooling on the substrate because the heat has to be transferred along the thickness of the scale layer. This localization of cooling at the triple contact line results in insufficient cooling at the center of the bubble leading to large temperature gradients along the centerline. For example, the temperature differential between the center of the bubble and the triple contact line is 2 °C/mm for salt solution as opposed to 0.7 °C/mm for DI water at a time instant corresponding to one-fourth the bubble residence time (Figure 3b).

Figure 3. (a) Frequency of bubble formation represented by the variation of the local temperature at substrate underneath an individual bubble at a heat flux of 20 W/cm2, and (b) Temperature along the contact diameter of the bubble for DI water (blue) and 0.94 Csat salt solution (black dashed) at time instants corresponding to t = 0 ms, 1/4th and 3/4th the bubble residence time at a heat flux of 20 W/cm2. The arrows denote the location of the triple contact line. To address the issue of scale buildup during boiling, it is important to understand its incipience mechanism. The driving force for crystallization is the increase in supersaturation at the heated substrate due to the reduced solubility of calcium sulfate with increasing temperatures. At low heat flux, as the bubble grows and departs from the surface, salt crystallizes in a “coffee-ring” 9 ACS Paragon Plus Environment

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pattern under the bubble (Figure 4a) [23]. The ring shaped scale can be rationalized in terms of the transport mechanism in the liquid microlayer beneath the bubble as it grows. The microlayer beneath a growing bubble is well established on a hydrophilic surface and is attributed to the lubrication layer attached to the surface [22, 24]. The maximum heat transfer during boiling occurs due to the evaporation of the liquid microlayer associated with bubble nucleation [25]. The thickness of this microlayer has been experimentally shown to be on the order a few microns [22, 25]. Assuming that the mass transfer to form the vapor bubble is only from the microlayer, the volume of microlayer that has evaporated to form a bubble of volume Vb is given as = ⁄ , where and are the density of liquid and vapor, respectively. The evaporation or the bubble formation timescale (τe), given by ~ ℎ ⁄ " , is on the order of tens of milliseconds [6]. Here, R is the radius of the bubble prior to departure, hfg is the latent heat of vaporization, and q” is the applied heat flux. The diffusion timescale of the SO ions in the microlayer ⁄ ~ 300 s, where D is the diffusion coefficient of the ion in water (= 3 x 10-9 m2/s [26]), and L is the length of the microlayer beneath the bubble (~ 1 mm) [22] (Figure 4a), is significantly larger than the bubble departure timescale (~ 100 ms, see Figure 3a). Thus, it can be assumed that the ions in the microlayer remain trapped during bubble growth and the evaporation of water causes the microlayer to become increasingly supersaturated (Figure 4a). The local ion concentration

in

the

microlayer

( ,

therefore,

increases

as

(

= 1 − 1⁄_# $% ⁄%&$Δ&$ , where is the bulk salt concentration, _# is the initial volume of the microlayer, % ⁄%& ~ ) " * ℎ is the rate of evaporation, and ∆t denotes the instantaneous time difference from the instant of formation of the initial microlayer. When the supersaturation ⁄+ , in the microlayer (+ , is the saturation concentration corresponding 10 ACS Paragon Plus Environment

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to the substrate temperature) exceeds critical supersaturation crystallization occurs at the contact line resulting in a coffee-ring-shaped scale deposit. With increase in heat flux, the surface temperature increases leading to a reduction in + , , which in turn leads to faster supersaturation of the microlayer which causes early precipitation leading to a thicker deposit (Figure 4b). At even higher heat fluxes (> 25 W/cm2 for 0.75 Csat concentration), a uniform scale covers the entire bubble basal area.

Figure 4. (a) Schematic of factors affecting scale formation during bubble evolution. The inset on the right shows the transport mechanism in the liquid microlayer. (b) Representative images of bubble departure as the heat flux is increased for the salt solution of 0.75 Csat concentration; the thickness of the scale ring increases with increasing heat flux. One way to avoid scale formation is to eliminate the ionic supersaturation caused by the evaporation in the microlayer. To achieve this, we fundamentally alter the microlayer and bubble geometry by rendering the surface hydrophobic. Hydrophobicity is incorporated by coating the boiling surface with Octadecyltrichlorosilane (OTS). The advancing contact angle of a water 11 ACS Paragon Plus Environment

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drop on the surface is ~ 110 deg while the receding contact angle is ~ 100 deg. As shown in Figure 5a, the bubble geometry on the hydrophobic surface is different than a conventional hydrophilic surface: the vapor-liquid interface at the contact line is concave on the hydrophobic surface in contrast to the convex shape on a hydrophilic surface (Figure 5a) [27]. The high receding contact angle ensures an absence of the liquid microlayer in the case of the hydrophobic surface (Figure 5a and see IR images in Supporting Information). As a result, bubble formation on a hydrophobic surface is not accompanied by scale formation further confirming the absence of microlayer. Recent studies using Lattice Boltzmann simulations have also shown the absence of microlayer during boiling on a hydrophobic surface [28]. The maximum evaporation occurs via heat transfer in the thermal boundary layer of the heated surface and subsequent growth is aided by the coalescence of multiple bubbles. The geometry of the triple contact line leads to an absence of evaporation-induced supersaturation near the contact line, thus preventing crystallization. The absence of scale during boiling on a hydrophobic surface results in a sustained near-uniform boiling performance at a given surface heat flux over the experimental test duration (90 mins). At a constant heat flux of 20 W/cm2, the initial heat transfer coefficient of the hydrophobic surface is higher than that of the unscaled hydrophilic surface due to higher bubble nucleation density. While boiling heat transfer on a hydrophilic surface decreases by 50% within 60 minutes due to scale formation, the heat transfer coefficient on hydrophobic surface remains relatively unchanged over the same duration (Figure 5b). At higher heat fluxes the surface temperature can exceed a critical value leading to supersaturation conditions near the surface, which can result in crystallization at the contact line. Here we find that crystallization is initiated when heat flux exceeds 40 W/cm2, and the surface temperature is greater than 130 °C. The corresponding supersaturation ⁄+ , near the surface


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exceeds 1.5 leading to crystallization on the heat transfer surface independent of the bubble evolution. While hydrophobic surfaces are known to increase the energy barrier for crystallization [14, 19, 21], we believe that the antiscaling property of hydrophobic surfaces in boiling can be attributed to the absence of microlayer. Unlike hydrophilic surfaces, the CHF on hydrophobic surface remains relatively unaltered (~ 40 - 55 W/cm2, see Supporting Information) going from DI water to salt solutions. This is because the CHF limitation on hydrophobic surfaces arises from the affinity of vapor films rather than scaling as in the case of hydrophilic surfaces.

Figure 5. (a) Dependence of bubble morphology on wettability of surface. (b) Temporal evolution of heat transfer coefficient on a hydrophilic and hydrophobic surface for a 0.94 Csat salt solution at constant heat flux of 20 W/cm2; insets show scaling on the two surfaces with different wettability. While hydrophobic surfaces provide sustained heat transfer performance at lower heat fluxes due to higher nucleation sites and ability to resist scale formation, the CHF remains low due to 13 ACS Paragon Plus Environment

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vapor coverage (Figure 6). To enhance CHF while controlling the extent of vapor affinity and scaling we propose to pattern the hydrophilic surface with disconnected hydrophobic regions (Figure 6 inset, patterning approach included in Supporting Information). This hybrid surface aids bubble nucleation, avoids vapor film coverage, and restricts scaling to the hydrophilic regions. Such hybrid surfaces (also termed as biphilic surfaces) have been shown to enhance heat transfer coefficients and critical heat flux during boiling of DI water [27, 29]. The CHF experiments are conducted using a 0.75 Csat salt solution. We find that the hybrid surface outperforms the hydrophilic and hydrophobic surfaces both in terms of heat transfer coefficient and CHF (Figure 6). The higher heat transfer coefficient results from the hydrophobic islands contributing to an increased number of bubble nucleation sites that lead to smaller bubbles and higher departure frequency (inset of Figure 6). The lower extent of scale formation that is restricted to the hydrophilic areas in conjunction with the aforementioned bubble dynamics results in higher CHF. The CHF on the hybrid surface is ~ 100 W/cm2 as opposed to ~ 43 W/cm2 on a hydrophilic surface, and ~ 50 W/cm2 on a hydrophobic surface under identical experimental conditions. Thus, we are able to mitigate the adverse effect of dissolved salts through hybrid surfaces.


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Figure 6. Comparison of boiling curves for a smooth hydrophilic surface, hydrophobic surface, and a hybrid surface for a 0.75 Csat salt solution. The insets show the representative visual and IR images of boiling corresponding to a surface heat flux of 20 W/cm2. The contour line plot on the IR images represents absolute surface temperature. The inset in the top right corner shows the hybrid surface, which comprises square hydrophobic islands with a width of 100 µm and pitch of 400 µm on a hydrophilic surface. CONCLUSIONS In summary, we show that the heat transfer performance during pool boiling is severely degraded by the presence of dissolved salts as crystallized scale increases the thermal resistance and dramatically alters the bubble evolution. We explain the mechanism of scale formation by considering the transport in the liquid microlayer underneath the bubble. Tailoring surface wettability is proposed as a means to manipulate the microlayer and avoid scale formation for 15 ACS Paragon Plus Environment

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sustained thermal performance. We demonstrate that hybrid hydrophilic-hydrophobic surfaces can mitigate the heat transfer deterioration caused by dissolved salts. Further studies on the optimization of the hydrophobic-hydrophilic patterns to minimize scale formation can maximize boiling performance in many industrial applications. SUPPORTING INFORMATION Additional details regarding infrared calibration, surface tension measurement, boiling curve on hydrophobic surfaces, and fabrication procedure of hybrid surface (pdf) Boiling on a smooth hydrophilic surface at a heat flux of 10 W/cm2 (avi) A side-by-side comparison of the boiling behavior of DI water and 0.97 Csat calcium sulfate solution on a smooth hydrophilic surface at a heat flux of 25 W/cm2 (avi) IR movies of boiling behavior of DI water and salt solution on a smooth hydrophilic surface at a heat flux of 15 W/cm2 (avi) A comparison of infrared videos of boiling DI water on a hydrophilic and hydrophobic surface at a heat flux of 15 W/cm2 (avi) AUTHOR INFORMATION Corresponding Author Email: [email protected] ACKNOWLEDGEMENT We acknowledge funding from Chevron Corporation. We thank Dr. Navdeep S Dhillon for his help with building the experimental setup. The sample fabrication for this work was performed at the MIT Microsystems Technology Laboratory (MTL). 16 ACS Paragon Plus Environment

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[9] Kandlikar, S. G. Enhanced Macroconvection Mechanism With Separate Liquid–Vapor Pathways to Improve Pool Boiling Performance. J. Heat Transf. 2017, 139 (5), 051501-05150111. [10] Jamialahmadi, M.; Helalizadeh, A.; Müller-Steinhagen, H. Pool Boiling Heat Transfer to Electrolyte Solutions. Int. J. Heat Mass Transf. 2004, 47 (4), 729–742. [11] Jamialahmadi, M.; Blöchl, R.; Müller-Steinhagen, H. Bubble Dynamics and Scale Formation during Boiling of Aqueous Calcium Sulphate Solutions. Chem. Eng. Process. Process Intensif. 1989, 26 (1), 15–26. [12] Garrett-Price, B. A.; Smith, S. A.; Watts, R. L. Industrial Fouling: Problem Characterization, Economic Assessment, and Review of Prevention, Mitigation, and Accommodation Techniques; PNL-4883; Pacific Northwest Lab., Richland, WA (USA), 1984. [13] Bott, T. R. Fouling of Heat Exchangers; Elsevier, 1995. [14] Malayeri, M. R.; Al-Janabi, A.; Müller-Steinhagen, H. Application of Nano-Modified Surfaces for Fouling Mitigation. Int. J. Energy Res. 2009, 33 (13), 1101–1113. [15] Garrett-Price, B. Fouling of Heat Exchangers: Characteristics, Costs, Prevention, Control and Removal; Noyes Publications, 1985. [16] Azimi, G.; Papangelakis, V. G.; Dutrizac, J. E. Modelling of Calcium Sulphate Solubility in Concentrated Multi-Component Sulphate Solutions. Fluid Phase Equilibria 2007, 260 (2), 300–315.


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[17] Crabtree, M.; Eslinger, D.; Fletcher, P.; Miller, M.; Johnson, A.; King, G. Fighting Scale Removal and Prevention. Oilfield Rev. 2000, 1–16. [18] Azimi, G.; Cui, Y.; Sabanska, A.; Varanasi, K. K. Scale-Resistant Surfaces: Fundamental Studies of the Effect of Surface Energy on Reducing Scale Formation. Appl. Surf. Sci. 2014, 313, 591–599. [19] Subramanyam, S. B.; Azimi, G.; Varanasi, K. K. Designing Lubricant-Impregnated Textured Surfaces to Resist Scale Formation. Adv. Mater. Interfaces 2014, 1 (2), 1300068. [20] Bornhorst, A.; Müller-Steinhagen, H.; Zhao, Q. Reduction of Scale Formation Under Pool Boiling Conditions by Ion Implantation and Magnetron Sputtering on Heat Transfer Surfaces. Heat Transf. Eng. 1999, 20 (2), 6-14. [21] Cai, Y.; Liu, M.; Hui, L. Observations and Mechanism of CaSO4 Fouling on Hydrophobic Surfaces. Ind. Eng. Chem. Res. 2014, 53 (9), 3509–3527. [22] Kim, H.; Buongiorno, J. Detection of Liquid–vapor–solid Triple Contact Line in TwoPhase Heat Transfer Phenomena Using High-Speed Infrared Thermometry. Int. J. Multiph. Flow 2011, 37 (2), 166–172. [23] Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary Flow as the Cause of Ring Stains from Dried Liquid Drops. Nature 1997, 389 (6653), 827-829.


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[24] Cooper, M. G. The Microlayer and Bubble Growth in Nucleate Pool Boiling. Int. J. Heat Mass Transf. 1969, 12 (8), 915–933. [25] Yabuki, T.; Nakabeppu, O. Heat transfer mechanisms in isolated bubble boiling of water observed with MEMS sensor. Int. J. Heat Mass Transf. 2014, 76, 286–297. [26] Mills, R.; Lobo, V. M. M. Self-Diffusion in Electrolyte Solutions: A Critical Examination of Data Compiled from the Literature; Elsevier, 2013. [27] Betz, A. R.; Jenkins, J.; Kim, C.-J. “CJ”; Attinger, D. Boiling Heat Transfer on Superhydrophilic, Superhydrophobic, and Superbiphilic Surfaces. Int. J. Heat Mass Transf. 2013, 57 (2), 733–741. [28] Gong, S.; Cheng, P. Lattice Boltzmann Simulations for Surface Wettability Effects in Saturated Pool Boiling Heat Transfer. Int. J. Heat Mass Transf. 2015, 85, 635–646. [29] Betz, A. R.; Xu, J.; Qiu, H.; Attinger, D. Do Surfaces with Mixed Hydrophilic and Hydrophobic Areas Enhance Pool Boiling? Appl. Phys. Lett. 2010, 97 (14), 141909.


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TOC GRAPHIC 140

DI water Salt solution (0.75 Csat ) Salt solution (0.94 Csat )

10 W/cm 2 120

CHF

100

2

Heat flux (W/cm )

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80 60 CHF

40 CHF

20

5 mm 0

0

10 20 30 40 50 o Wall superheat, Tw - Tsat ( C)

60


Crystallization-Induced Fouling during Boiling - ACS Publications

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