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Nanostructure-Supported Evaporation underneath a Growing Bubble Shakerur Ridwan, and Matthew McCarthy ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21260 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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ACS Applied Materials & Interfaces

NANOSTRUCTURE-SUPPORTED EVAPORATION UNDERNEATH A

GROWING BUBBLE

Shakerur Ridwan and Matthew McCarthy* Department of Mechanical Engineering and Mechanics, Drexel University 3141 Chestnut St. Philadelphia, PA, 19063, USA *Corresponding author: [email protected]

Keywords: Heat Transfer, Nanostructured Coatings, Boiling, Evaporation, IR Thermography

Abstract High porosity nanostructured coatings have been extensively studied for their use in enhancing liquid-to-vapor phase change due to their ability to wick liquids laterally across surfaces during boiling. While the effect of these coatings on the maximum heat transfer rate achievable (the critical heat flux) is now well understood, the impact on boiling efficiency (the heat transfer coefficient) is less clear. In this work a novel experimental apparatus is used to

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take heat transfer measurements beneath growing and departing bubbles on nanostructured surfaces. By independently tuning the surface heat flux and bubble departure time, IR thermography is used to directly visualize and characterize surface superheat, heat flux, and heat transfer coefficient during the highly transient bubble ebullition cycle. It is shown that while flat surfaces exhibit large spatial and temporal variations in surface temperature and heat flux, the nanostructured coatings produce a uniform temperature profile with enhanced heat transfer due to evaporation from the nanostructure-supported liquid films beneath the bubble. This work demonstrates the relative importance of advancing and receding contact lines, as well as the quenching process, on the overall thermal performance of structured and non-structured surfaces. It is seen that the combined effects produce a net increase in heat transfer coefficient of over 30%, averaged over the entire ebullition cycle and throughout the entire area of influence. Additionally, the impact of viscous resistance and the importance of nanostructure dry out has been studied by tuning the ebullition cycle time to create dry spots. This work shows for the first time the


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role of nanostructured coatings and thin-film evaporation during nucleate boiling, and

it

provides

a

framework

to

understand

the

complicated

nature

of

nanostructured boiling across all portions of the boiling curve from nucleation to critical heat flux.



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Introduction Boiling heat transfer is used in a wide variety of commercial and industrial applications including HVAC1, power generation2-3, desalination4 and chemical processing5 . During boiling, heat is transferred from the surface to the fluid via several different phenomena, including microlayer evaporation, macro-convection, and transient conduction6-10. The complicated nature of the boiling process has driven numerous scientific studies, each focusing on different mechanisms and elements of the bubble ebullition cycle. Several phenomena have been shown to affect boiling including nucleation site density, bubble departure diameter and frequency, contact line dynamics, hydrodynamic interactions, and surface wetting 6,

10-12.

Additionally, it has been shown that the surface finish and other

interfacial properties play a crucial role in the heat transfer coefficient (HTC) during boiling, and the critical heat flux (CHF)

12-13.

Research in this area,

however, has been hampered due to the short time scales and small length scales of the phenomena being studied.


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Various techniques have been used to enhance boiling heat transfer, including the incorporation of engineered surface structures to increase surface wettability and nucleation site density, which dates back to the 1940’s

12-14.

With the advent of modern micro and nano-fabrication techniques, many researchers have shown substantial increases in heat transfer rate, as well as the ability to tune several boiling processes

15-20.

Flat surfaces with mixed

hydrophilic-hydrophobic regions have been used to control nucleation sites and wettability leading to increases in CHF and HTC

21.

Millimeter-scale structures

with contoured fins have been used to leverage evaporation momentum to create separate liquid and vapor pathways22. Structured surfaces comprised of nanostructures15,

19,

microstructures18,

20,

and hierarchical structures17 have all been shown to enhance CHF. Several types of nanostructure coated surface including nanowires15-16, 24,

23,

and flower like structures25 have been shown to increase CHF

nanorods19,

23-

15-16, 19-20, 23-25

during boiling. While the enhancement of CHF using nanostructures is now well established19-20, the impact of nanostructured surfaces on HTC across the entire



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boiling curve is less clear. While nanostructures have been found to both increase and decrease HTC15-16,

19-20, 23-25, 26,

the exact mechanisms behind

these variations in performance have not yet been sufficiently explained. While increasing nucleation site density using nanostructures has been proposed as an enhancement mechanism15,

23,

other work has shown that nanostructures

can also reduce nucleation site density

26.

Nucleation site density, however, is

not the sole factor dictating HTC. While previous work has shown that lateral wicking through nanostructures increases CHF15,

20, 27,

the impact of surface

wicking on HTC has not been established. Understanding the role of nanostructure-supported liquid films underneath growing

bubbles

requires

the

direct

characterization

of

local

surface

temperatures and heat fluxes during bubble ebullition. The spatial and temporal changes in surface temperature and heat flux during nucleate boiling create complex variations in local HTC across the surface. These quantities can not be measured using standard pool boiling apparatuses focused on measuring average surface temperature, heat flux, and HTC. IR thermography, however,


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has

been

used

to

successfully

collect

qualitative

information during phase change processes8-11,

and

28-34.

quantitative

local

Several types of IR

characterization techniques have been used to study boiling, including the use of thin film heaters fabricated directly onto a substrate8-9, itself as a substrate10-11,

31,

28-29,

using the heater

and the use of thin foils as the heater30,

34.

The

work of Theofanous et al. showed the formation of a hot spot underneath growing bubbles on flat surfaces

28-29.

It was shown that as the surface

approached CHF, these hot spots grew irreversibly. Gerardi et al.8 argued that heat transfer due to thermal boundary layer re-formation over a dry spot (also known as quenching) is the dominant heat transfer mode during boiling on a flat surface. Jung et al. showed high heat fluxes in the region of an advancing meniscus, but also reported that single-phase heat transfer throughout the wetted surface area was dominant10. While IR thermography has been used to overcome many challenges and provide local information about these processes, the role of surface structures is still not well understood. This is due, in part, to the fact that incorporating



nanostructure

fabrication

processes

with

IR

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visualization

techniques

is

challenging. Several studies have, however, used IR thermography to study boiling on structured surfaces. Dhillon et al.35 used an IR thermography technique paired with optical visualization and showed the formation of dry spots occurring near CHF. This approach, however, was unable to characterize the temperature and heat flux at the liquid-solid interface. Park et at.36 measured wetted area and contact line density using the DEPIcT technique to correlate with HTC and CHF during boiling on structured surface without local information of heat transfer. Kim et al.

37

observed the bubble departure

diameter, frequency during boiling on structured surface and found that dry spots can be formed underneath a bubble. Fischer et al.38 characterize a stationary and moving meniscus on nano-textured heater surfaces using IR to measure local thermal parameters. Gibbons et al.

39

studied the quasi-steady

evaporation of a droplet on a superhydrophobic surface, and showed higher heat fluxes near the three-phase contact line, while Adera et al. and Antao et

al.

40-41

successfully showed the dry out process and evolution of thin liquid film


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on microstructured surface. The current study adds to this body of work by providing a direct characterization of the nanostructure-supported evaporation underneath growing and departing bubbles during the transient ebullition cycle.

Results and Discussion A custom-built experimental apparatus and novel testing procedure were used to characterize nanostructure-supported evaporation by artificially extending the ebullition cycle time of a growing bubble. The resulting ebullition cycle can be tuned and slowed down to be compatible with standard IR thermography set-ups capturing images at a frame rate of 25 Hz using a long wavelength IR camera (LWIR). This flexibility has allowed for novel measurements to be taken with standard laboratory equipment available to most researchers. Using the apparatus, the heat transfer performance of nanostructured surfaces was characterized and compared with flat surfaces at identical thermal loadings.



A schematic of the experimental apparatus is shown in the Figure 1. A water bath is maintained at saturation conditions atop an electrically powered thin foil heater. A tube inserted in the bath is used to control the upward vapor escape rate, while the bath temperature is monitored using a submerged thermocouple probe.

An IR camera is paired with a gold mirror to image

backside foil temperature during testing. The topside of the foil is decorated with CuO nanostructure to characterize the impact of nanostructure-supported thin liquid films underneath the growing bubbles. An insulation layer of SU-8 was placed between the CuO nanostructures and the stainless steel foil for electrical isolation ensuring a uniform heater resistance. Complete details of the heater assembly fabrication process are provided in the methods section below, as well as the supplemental information. Additionally, flat heater surfaces (without CuO nanostructures) were also fabricated to compare against the performance of the nanostructured surfaces.


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Figure 1. Experimental setup and nanostructured surfaces. (a) Schematic representation of the experimental apparatus used in the current work, showing a small water bath maintained at saturation conditions with a thin foil heater used to drive bubble growth. A tube is inserted in the bath to provide a controlled

rate

temperature.

(b)

of

vapor Optical

escape, images

while of

the

an

IR

camera

apparatus,

the

records

surface

heater

sealing

mechanism, and the nanostructured CuO fabrication process. (c) SEM image of the CuO nanostructures fabricated onto the thin foil heater. (d) Schematic of the fabricated heater assembly, showing the nanostructures, an insulating SU-8 polymer layer, a stainless steel heater foil, and a backside coating of high emissivity black paint.



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Using the IR camera, the transient wall temperature distribution, Tw(x,y,t), across the surface was recorded as bubbles go through the ebullition cycle. This temperature distribution is then used to calculate the local transient heat flux, q’’(x,y,t) supplied to the fluid by applying the conservation of energy to each individual pixel. The measurement techniques for temperature and heat flux, including their calibrations and validations, are explained in more detailed in the supplemental information. Briefly, beginning with the heat generated within the stainless steel foil (due to joule heating), and accounting for convective and radiative losses from the backside as well as lateral conduction through the film, the heat flux delivered upward into the liquid can be directly calculated. Knowing the surface temperature distribution and the surface heat flux, the transient HTC across the surface is calculated for each pixel at each time step using Equation 1, where Tsat is the saturation temperature of water bath.

ℎ(𝑥,𝑦,𝑡) =

𝑞′′(𝑥,𝑦,𝑡) 𝑇𝑤(𝑥,𝑦,𝑡) ― 𝑇𝑠𝑎𝑡


(1)

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During testing a low heat flux (~ 0.4 W/cm2) was delivered from the foil to the bath, ensuring that no nucleation occurs on the surface. The tube inserted in the bath (Figure 1) is used to position a liquid-vapor interface in close proximity to the surface. The heat from the foil drives evaporation from this liquid-vapor-interface causing it to first bend downward and then contact the heater surface. This process is shown schematically in Figure 2a. The bubble continues to grow due to evaporation, creating a three-phase contact line that spreads outward until buoyancy effects become dominant and the bubble releases from the surface (outside of the tube). By controlling the vapor release rate through the tube using the integrated valve, the growth rate of the bubble can be independently tuned relative to the applied heat flux. It is this novel feature of the experiment that allows for the artificial slowing down of the normal ebullition cycle.

During true boiling bubble nucleation, growth, and

departure are naturally linked to the applied surface heat flux through a complex series of processes. In this work, a simplified experiment is used to



isolate the role of nanostructure-supported evaporation underneath bubbles leaving many of these complications to be addressed in future work.

Figure 2. IR thermography results for bubble ebullition on a flat surface. (a) Schematic representation of the bubble ebullition cycle controlled using the custom-built experimental apparatus. IR thermography results showing the spatially resolved (b) superheat temperature, Tw(x,y,t) - Tsat, (c) the local heat flux distribution, and (d) the resulting heat transfer coefficient for a flat surface undergoing a single ebullition cycle.


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Figure 3. IR thermography results for bubble ebullition on a nanostructured surface. (a) Schematic representation of the bubble ebullition cycle controlled using the custom-built experimental apparatus. IR thermography results showing the spatially resolved (b) superheat temperature, Tw(x,y,t) - Tsat, (c) the local heat flux distribution, and (d) the resulting heat transfer coefficient for a nanostructured surface undergoing a single ebullition cycle.


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Spatially Resolved Heat Transfer Measurements Local temperature, heat flux, and heat transfer coefficient (HTC) were measured on a bare flat surface (Figure 2) as well as a nanostructured surface (Figure 3). See the supporting information for a set of movies showing the transient visualization of these three parameters over several ebullition cycles. Figure 2 shows the performance of the flat surface for one representative ebullition cycle. Initially (t = 0.0 s), the LV interface is held by the tube at a distance of ~ 3 mm from the surface. As the applied heat drives evaporation upward through the tube, the valve creates backpressure which causes the LV interface to bend downward and eventually touch the surface (t = 0.55 s).

The

bubble then grows along the surface, creating an outwardly propagating threephase contact line (t = 0.55 s – 1.83 s). Finally, the bubble grows too large to be held in place and buoyancy forces lift it from the surface and it travels out and upward around the outside of the tube. For this experiment, the valve was tuned to achieve an ebullition cycle time of approximately Δtcycle = 2 s.



Figure 2(b – d) shows the evolution of surface superheat, heat flux, and HTC showing behaviors consistent with the literature and as would be expected for flat surfaces. As the bubble makes initial contact (t = 0.55 s) the surface temperature first decreases, while the heat flux and HTC increase at the center point due to the effects of evaporation. As the three phase contact line propagates outward, a dry spot forms underneath the bubble (t = 1.83 s). Because there is no liquid to absorb heat via phase change, there is a characteristically high surface temperature, a low heat flux, and resulting low HTC within the dry region.

A ring-shaped high heat flux region is seen to

surround the dry spot. This ring-shaped region represents the three-phase contact line region where microlayer evaporation is occurring. Outside of this region single-phase convection shows moderate values of HTC through the process. Throughout the test, the HTC near the three-phase contact line remains higher than that seen in any other region. A similar dry spot region and behavior was also observed in the work by the Theofanous et al.28-29. The last stage of the ebullition cycle is bubble departure and quenching. During


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quenching the superheated dry region on the flat surface is quickly flooded by saturated liquid as the bubble departs from the surface. A high heat flux and a high HTC were observed throughout this inner region during the fast quenching process, consistent with previous work8,

10.

A pronounced difference was observed for the bubble ebullition cycle on a nanostructured surface (Figure 3), as compared to a flat surface (Figure 2). When the bubble first touches the heater surface, the nanostructure coated surface behaves in the similar way to that of the flat surface, exhibiting an increase in heat flux and HTC at the center point (t = 0.3 s). But as the bubble grows outward along the surface, a distinguishable difference can be seen for the nanostructured surface. No hot spot was observed underneath the bubble as was seen on the flat surface. This is due to the ability of the nanostructures to remain wetted by drawing liquid inward underneath the bubble. On the contrary, the temperature of the surface underneath the bubble decreases as the liquid being wicked inward undergoes evaporation across the nanostructure-supported LV interface beneath the bubble. This results in lower



surface temperatures, and higher heat transfer rates and HTCs during the bubble growth phase (t = 0.3 - 1.85 s). It can be seen that for flat surfaces (Figure 2) the high HTC region has an annular shape and occurs along the three-phase contact line, with a low HTC in the inner dry region. However, for nanostructured surfaces the entire region under the bubble exhibits a large HTC consistent with nanostructure-supported evaporation from the “apparent” dry area (t = 1.87 s). The height of the CuO nanostructures is estimated to be ~1 µm, which is capable of supporting a liquid film of low thermal resistance underneath the bubble throughout the entire ebullition cycle. After the bubble departs (t = 1.97 s), there is no characteristic quenching observed, as was clearly seen for the flat surface. By maintaining a wetted state underneath the bubble for the entire ebullition cycle, the surface temperature does not increase during bubble growth and therefore the returning liquid does not quench the surface. The bubble ebullition cycles for flat and nanostructured surfaces can be further compared by examining the heat flux and HTC across the center line of the 2D plots, as shown in Figure 4 (flat surfaces) and Figure 5


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(nanostructured surfaces). The transient change of local heat flux and HTC along the centerline is shown in Figures 4 and 5 for various time steps over the ~ 2 s ebullition cycle. It is shown that, on the flat surface (Figure 4), the heat flux during quenching (t = 1.96 s) is much higher (~18 kW/m2) than during any other stage of the ebullition cycle. The resulting HTC value was ~2,500 W/m2K during the quenching process. Immediately prior to quenching (t = 1.49 s) the heat flux underneath the bubble is essentially zero due to the hightemperature dry spot. This shows the importance of the local increase in surface temperature (due to the dry spot) and the resulting quenching process on the overall heat transfer rate from a surface during boiling. Conversely, the nanostructured surface (Figure 5) maintains a nominally constant heat flux throughout the ebullition cycle. This fact, paired with the decrease in surface temperature shown in Figure 3b, results in HTC values of ~2,500 W/m2K during the entire bubble growth phase. This is a stark contrast to the behavior on flat surfaces, which showed a spike in HTC up to ~2,500



W/m2K only during the fast quenching process after bubble departure, as seen in Figure 4b.


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Figure

4.

Spatial

and

temporal

Figure

5.

Spatial

and

temporal

variation along the centerline of a flat

variation along the centerline of a

surface, showing (a) local heat flux,

nanostructured surface, showing (a)

and (b) heat transfer coefficient.

local heat flux, and (b) heat transfer coefficient.




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Bubble Ebullition and Quenching The importance of nanostructure-supported evaporation on the heat transfer characteristics underneath a growing and departing bubble can be further shown by examining the center point data (Figure 6), advancing and receding meniscus effects (Figure 7), and average values underneath the entire area of influence (Figure 8). Figure 6 shows the transient behavior of the surfaces at the center point of the growing bubble where the bubble initiation, growth, and departure are labeled. The center point data represents the area of one pixel (150µm across) where time has been normalized by the total cycle time to compare flat and nanostructured surfaces. See the supporting information for more details and complete data sets of the cyclical ebullition. Initially, the LV interface is raised above the surface and heat is transferred to the fluid by natural convection alone (t/Δtcycle = 0 – 0.2). As the LV interface contacts the surface (t/Δtcycle = 0.2, “bubble initiation”) surface temperature first decreases, while heat flux and HTC increase for both surfaces. Then the two surfaces behave notably different. The flat surface experiences a large and consistent



increase in surface temperature as the dry spot grows outward, while the temperature on the nanostructured surface continually decreases. This decrease is due to nanostructure-supported evaporation providing a reduced thermal resistance as compared to natural convection. The nanostructured surface exhibits a modest increase in heat flux and a characteristically large and constant HTC throughout the entire bubble growth phase. By comparison the heat transfer and HTC on the flat surface reduce dramatically due to the fact that there is no liquid underneath the bubble to absorb the heat generated by the heater. This pronounced difference in behaviors during bubble growth is a key factor attributed to increased HTC for pool boiling on nanostructured surfaces. When the bubbles depart (t/Δtcycle = 0.75) the flat surfaces experience a large spike in heat flux due to quenching, while the nanostructured surface heat flux remains relatively constant. The HTCs for both surfaces then normalize back to a value consistent with natural convection and the process begins again.


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Figure 6: Temporal variations at the center point of flat and nanostructured surfaces showing (a) wall superheat, (b) heat flux, and (c) HTC during one ebullition cycle.



During bubble growth a receding meniscus forms as the three-phase contact line propagates outward. Similarly, an advancing meniscus forms after bubble departure when the liquid returns to the surface. The role of advancing and receding menisci on overall heat transfer is shown in Figure 7. Data from several discrete points, spanning from the center of the bubble to outside of the three-phase contact line, are plotted on top of each other for both flat surfaces and nanostructured surfaces. Figure 7 shows the heat flux as well as the heat transfer coefficient varying in space and time over one ebullition cycle for both surfaces tested. The behaviors shown for the flat surfaces (Figure 7a,b) are qualitatively and quantitatively consistent with the open literature as well as the general understanding in the field. As the bubble grows outward a receding meniscus forms, producing a modest increase in heat flux near the three-phase contact line region. Inside of the three phase contact line low heat flux is observed due to the dry spot. After bubble departure the liquid returns quickly to the superheated dry surface producing the quenching effect, which yields a substantially larger increases in heat flux as compared to the receding


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meniscus. Outside of the three-phase contact line, very little variation in heat flux is seen. Examination of the HTC values for both receding and advancing menisci shows a comparable value (~ 2,500 W/m2K) for both. The

nanostructured

surface,

however,

does

not

exhibit

these

typical

behaviors. As can be seen in Figure 7c-d, the impact of moving contact lines is much less evident, for either advancing or receding menisci. As the bubble grows outward, more and more of the nanostructure-supported LV interface is exposed resulting in a uniform and consistent HTC associated with evaporation from the thin liquid film. While a small increase in heat flux is seen during the receding phase, these results show that nanostructured surfaces do not exhibit the temporal variations in wetted state and surface temperature that drive local spikes in heat flux and HTC as seen on flat surfaces.



Figure 7: The role of advancing and receding contact lines on heat transfer, showing (a,c) heat flux, and (b,d) heat transfer coefficient for a (a,b) flat and a (c,d) nanostructured surfaces over one ebullition cycle.


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The result is a more isothermal surface with consistently high HTC throughout the apparent dry area underneath a bubble. In the previous work by Fischer et

al.

38,

a high heat flux region was observed at the advancing meniscus of a

nanostructured surface. But in that work the meniscus was advancing over a truly dry area; in this work the nanostructures remain wetted at all time creating an “apparent” dry area with low surface temperature. The cumulative effect of nanostructured coatings underneath a growing and departing bubble can be understood by examning the transient variations in the total heat transfer rate from the surface, as well as the spatially and temporally averaged HTC during a single ebulition cycle.

To examine these parameters,

the heat flux and HTC data have been averaged over a representative area of influence under the tube (Figure 8a). The area includes part of the outer natural convection region, the entire three-phase contact line region, and the entire apparent dry region. The time dependent heat transfer rate, Q(t), is calcualted by integrating the heat flux over the area of influence, and the time



Page 32 of 63

averaged heat transfer rate, 𝑄, is calcualted by integrating the heat transfer rate over the ebulition cycle, where 𝑄(𝑡) =

∫

𝑞′′(𝑥,𝑦,𝑡)𝑑𝐴 𝐴𝑡𝑢𝑏𝑒

1 𝑄= Δ𝑡

∫

(2)

.

∆𝑡

𝑄(𝑡)𝑑𝑡 0

(3)

Similarly the area-averaged HTC, ℎ , and the time-averaged HTC, ℎ, are caluclated as

ℎ(𝑡) =

1 A𝑡𝑢𝑏𝑒

∫

1 ℎ= Δ𝑡

ℎ(𝑥,𝑦,𝑡)𝑑𝐴 𝐴𝑡𝑢𝑏𝑒

∫

∆𝑡

ℎ(𝑡)𝑑𝑡 0


(4)

(5)

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Figure 8: Nano-enhancement affect over the 9.2 mm diameter area of influence showing (a) normalized heat transfer rate, and (b) average heat transfer coefficient for flat and nanostructured surfaces over one representative ebullition cycle.



Figure 8a

shows the plot of normalized heat transfer rate (𝑄/𝑄) realtive

to normalized time (t/Δtcycle). As can be seen, prior to bubble departure the flat surface exhibits a decreasing heat transfer rate due to the growing dry spot underneath the bubble, while the nanostructured surface maintains a relatively constant heat transfer rate due to the wetted nanostructures. The surface temperature rises on the flat surface, and then when the bubble finally departs a quenching event takes place resulting in a large spike in heat trasnfer. For the nanostructured surfaces this behavior is not seen. Wicking through the nanostructures forms a thin liquid film on the surface. This provides low thermal resistance through evaporation, which keeps the local surface temperature fairly uniform, with no hot/dry spot. As the surface is already wet and at a low temperature, quenching is absent. While both surfaces are dissipating the same amount of heat over one cycle, the flat surfaces undergo drastic and cyclical variations in temperature and heat flux. This is not occuring for nanostructured surfaces.


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In Figure 8b, it can be seen that the area averaged HTC increases steadily during bubble growth, due to nanostructure-supported evaporation, while the flat surfaces display the characteristics consistent with receeding/advancing menisci and the quenching phenomnea. Additionally, it is shown that the resulting time-aveaged heat transfer coefficient for the nanostructured surface is approximately 30% larger than that of the flat surface, consistent with findings in the literature15,

19.

This net positive affect is attributed directly to the impact

of nanostructure-supported evaopration and its ability to maintin consistent surface temperatures and heat fluxes during the bubble ebulition cycle. While the effects of these enhancements have been measured using tradtional pool boiling set-ups, the novel IR thermography set-up used in this work has allowed the exact mechanisms behind these enhancments to be elucidated and directly visualized.

Nanostructure Dry Out All the enhancements demonstrated in this work come from the ability of the nanostructured coating to maintain a wetted thin film of liquid beneath a



growing

bubble.

If

during

the

growth

and

Page 36 of 63

departure

of

a

bubble,

the

nanostructured coating is not capable of maintaining a wetted state, these enhancements will disappear. Using the experimental technique demonstrated here, dry out of the nanostructures can be initiated by simply adjusting the vapor escape valve to extend the bubble ebullition cycle time. For the testing conditions used here, dry out was observed when the ebullition cycles were extended longer than three seconds. The liquid layer underneath a growing bubble forms due to capillary action of the nanostructures drawing liquid from outside of the three-phase contact line. However, during evaporation for larger ebullition cycles, viscous resistance in the nanostructured coating cannot be balanced by capillary pressure and a dry spot will form. Figure 9 shows experimental results for a bubble ebullition cycle of over four seconds, where a clear dry spot forms around t = 3.6 s. To

analyze

the

competition

between

capillary

pressure

and

viscous

resistance the Navier Stokes equation was modified with Darcy’s law to find the liquid pressure within the porous nanostructures


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𝑑𝑉𝑟 𝑑𝑝 𝜇 (𝑟,𝑡) = 𝑉𝑟 ― 𝜌𝑙 𝑑𝑟 𝐾 𝑑𝑡

(6)

where, radial fluid velocity is calculated based on the evaporative mass flux as

𝑉𝑟(𝑟,𝑡) = ―

𝑞′′𝑟(𝑡) 2𝜌𝑙𝐻ℎ𝑓𝑔

(7)

As can be seen the viscous pressure drop depends on the permeability of the nanostructures, K, and liquid film thickness, H. The permeability of the fabricated

nanostructured

coating

was

measured

experimentally

using

an

alternate testing set-up described in more detail in

Figure 9: Experimental observation of nanostructure dry out during bubble ebullition, showing (a) a representative schematic of dry out where a true dry spot forms within the nanostructures, and (b) IR thermography results for HTC showing a dry out event for an extended ebullition cycle.



Page 38 of 63

the supporting material. Briefly, a nanostructure-supported film was formed on a surface and liquid was supplied to the film via a small tube and a microsyringe pump. The liquid water was continuously supplied, and wicked radially outward. The surface was heated from below to drive evaporation. The size of the apparent wetted area, 𝑟1, and the true wetted region, 𝑟2, were measured using optical and IR cameras. Applying Darcy’s law and conservation of mass at the steady state condition, the permeability of the nanostructure is estimated as

( )(

𝑟2 𝜇 𝐾= ― 𝑉𝑡 [ ln 2𝜋𝐻 ∆𝑃 𝑟1

1+

𝑟12

)

𝑟22 ― 𝑟12

―

1 ] 2

(8)

where 𝑉𝑡 is the flow rate supplied from the syringe pump. The viscous pressure drop, ΔP, in the wicking experiment can be equated with capillary pressure as given by


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∆𝑃 = 𝑃𝑐 = ―

𝜎𝑙𝑣 𝜒𝐻

[(𝑟𝑔 ― ∅)𝑐𝑜𝑠𝜃 ― (1 ― ∅)]

(9)

SEM images was analyzed to estimate the roughness factor, rg, solid fraction ∅,

and porosity 𝜒. Using this approach, the permeability of the CuO

nanostructures was estimated to be K = 5.38x10-15 m2. Knowing this, Equations 6-7 were then solved iteratively to account for the reduction in fluid film height,

H, due to meniscus curvature. Experimentally measured values of the size of the nanostructure-supported region and heat flux were used as inputs to the equations. See supporting information for more details.



Figure 10: Validation of prediction for dry out event, showing (a) the evolution of absolute liquid pressure in the nanostructure supported region before dry out, and (b) the pressure difference across the nanostructure-supported LV interface at the center point as compared with the predicted capillary pressure.


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Figure 10a shows the results of the wicking-evaporation analysis where the pressure profile within the nanostructured film is plotted for various time steps. Over the course of the ebullition cycle, the absolute liquid pressure becomes negative within the nanostructured film. Similar negative pressures were found in previous nano-scale thin film evaporation experiments

42,

where the liquid is

thermodynamically metastable with respect to the vapor phase

43.

During this

process the liquid within the nanostructures can be in tension, which makes it susceptible to cavitation. However, the confinement of the nanostructuresupported film effectively prevents cavitation44-45. Figure 10b shows a plot of the pressure difference across the liquid film at the center point of the growing bubble. As the bubble grows and viscous effect becomes dominant, this pressure difference will eventually violate the maximum capillary pressure that the nanostructures can support without collapsing. Using the estimated capillary pressure of 198 kPa, this analysis suggests that dry out would first begin at ~ 2.7 seconds for the experiment shown in Figure 9. The IR thermography results show the first visual signs of a dry spot occur around ~3.6 s. The discrepancy



Page 42 of 63

between predicted and observed times can be easily attributed to a variety of simplifying assumptions made, as well as the delay between the true onset of dry

out

(predicted

analytically)

and

the

first

visible

dry

spot

(observed

experimentally). Nevertheless, this analysis is deemed sufficiently accurate for the purposes of this study. Namely, to explain the phenomena leading to nanostructure dry out during bubble growth. After the onset of dry out, the dry region gradually increases during bubble growth up to ~4.92 s, and then the bubble departs.

As would be expected,

the dry spot exhibits an increase in surface temperature and the characteristic quenching behavior after bubble departure (Figure 9b, t = 4.96 s), as would traditionally be observed for flat surfaces. A similar behavior was also seen for advancing three-phase contact lines in prior publications further

research

is

warranted

to

study

the

nanostructured surfaces during boiling near CHF.

Conclusions


impact

38.

of

This suggests quenching

on

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In this work a custom-built experimental apparatus was developed and used to collect novel heat transfer measurements from nanostructured surfaces beneath growing and departing bubbles. By incorporating a 1 µm thick coating of nanostructured copper oxide onto a thin foil heater assembly, direct measurements of surface temperature, heat flux, and heat transfer coefficient at the true solid-liquid interface were collected. Using an integrated vapor release valve and a traditional IR thermography technique, the bubble ebullition cycle has been tuned independently of the applied heat flux. This allows for direct visualization of a variety of important phenomena at a reduced ebullition cycle time. Using this approach it has been shown that flat surfaces exhibit large spatial and temporal variations in surface temperature and heat flux. This is due to the formation of dry spots, the effects of advancing and receding menisci, and the quenching process observed at bubble departure. These results were generally consistent with the understanding of heat transfer in the microlayer region near the three-phase contact line. More importantly, this work shows that these behaviors are largely absent during a bubble ebullition on



Page 44 of 63

nanostructured surfaces. It was shown that the nanostructured coatings create an “apparent” dry area underneath the bubbles, consisting of a nanostructuresupported thin liquid film. This film wicks liquid from outside of the apparent three-phase contact line and drives evaporation underneath the bubble during the entire ebullition cycle. This process results in substantial decreases in surface temperature and corresponding increases in heat flux and HTC throughout the apparent dry area during the entire growth phase. This is distinctly different than the behavior shown for flat surfaces. The spatially- and temporally-averaged HTC showed an increase of 30% due to this combined nano-enhancement effect. Additionally, while quenching was found to be an important

part

of

the

ebullition

inconsequential

to

heat

transfer

cycle on

for

flat

surfaces,

nanostructured

it

surfaces.

was

largely

Finally,

the

eventual dry out of the nanostructured coatings was also studied by extending the ebullition cycle time using the integrated vapor escape valve. For large ebullition cycles, viscous resistance through the nanostructures impeded surface wicking leading to a “true” dry area within the “apparent” dry area. For this


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case, both nanostructure-supported evaporation and surface quenching were observed, demonstrating the more complex thermal-fluid processes occurring near critical heat flux on nanostructured surfaces.



Methods Fabrication techniques: A 25 µm stainless steel foil was used as a resistive heater.

A 2 µm SU8 layer, which works as an insulation layer, was spin

coated on top of the steel foil. A 1 µm layer copper was vacuum deposited on the SU8 layer. Using hydrothermal oxidation process the copper layer was turned into CuO nanostructure by submerging the foil assembly in a bath of alkaline solution (NaClO2, NaOH, Na3PO412H2O, H2O)46 for 8 minutes at a temperature of 96°C. The SEM image in Figure 1c shows the blade shaped structure of the CuO nanostructure. A black paint coating was sprayed on the back side of the steel foil. Thickness of the black paint was 20 µm based on the measurement with a microscope.

Experimental procedure: Degassed deionized water is used as the working fluid for all tests. Saturation conditions were maintained in the bath at atmospheric pressure using both external bath heater and the foil heater. The fluid temperature was maintained at 100°C, and the heater temperature was


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maintained at ~103°C by finely tuning the DC power applied to the foil. An IR camera reads the surface temperature, while a thermocouple reads the bath temperature. A low heat flux (~4,000 W/m2) was applied during all experiments to prevent nucleation on the heater surface. A tube was inserted in the bath, which provided a LV interface as to not require active surface nucleation. The bubble grows and touches the heater surface as shown in the schematic of the Figure 2. The gap between the tube and heater surface was kept at ~ 3 mm. Bubble eventually left surface due to the buoyancy effect, following the path of least resistance to escape. Bubble forms, grows and departs cyclically. The ebullition cycle of bubbles was tuned using the integrated vapor escape valve, slowing the process down to a few seconds to be compatible with the IR camera frames rates. Supporting information Detailed

description

of

experimental

setup,

measurement

technique

validation, nanostructure dry out analysis (PDF) Surface temperature distribution during ebullition cycles on a flat surface (MPG) Local heat flux distribution during ebullition cycles on a flat surface (MPG)


and


Local heat transfer coefficient distribution during ebullition cycles on a flat surface (MPG) Surface temperature distribution during ebullition cycles on a nanostructure coated surface (MPG) Local heat flux distribution during ebullition cycles on a nanostructure coated surface (MPG) Local heat transfer coefficient distribution during ebullition cycles on a nanostructure coated surface (MPG) Local heat transfer coefficient distribution during dry out event on a nanostructure coated surface (MPG)

Acknowledgements This work was supported by the National Science Foundation under grant number 1454407.

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Figure 1. Experimental setup and nanostructured surfaces. (a) Schematic representation of the experimental apparatus used in the current work, showing a small water bath maintained at saturation conditions with a thin foil heater used to drive bubble growth. A tube is inserted in the bath to provide a controlled rate of vapor escape, while an IR camera records surface temperature. (b) Optical images of the apparatus, the heater sealing mechanism, and the nanostructured CuO fabrication process. (c) SEM image of the CuO nanostructures fabricated onto the thin foil heater. (d) Schematic of the fabricated heater assembly, showing the nanostructures, an insulating SU-8 polymer layer, a stainless steel heater foil, and a backside coating of high emissivity black paint. 578x313mm (72 x 72 DPI)



Figure 2. IR thermography results for bubble ebullition on a flat surface. (a) Schematic representation of the bubble ebullition cycle controlled using the custom-built experimental apparatus. IR thermography results showing the spatially resolved (b) superheat temperature, Tw(x,y,t) - Tsat, (c) the local heat flux distribution, and (d) the resulting heat transfer coefficient for a flat surface undergoing a single ebullition cycle. 539x367mm (72 x 72 DPI)


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Figure 3. IR thermography results for bubble ebullition on a nanostructured surface. (a) Schematic representation of the bubble ebullition cycle controlled using the custom-built experimental apparatus. IR thermography results showing the spatially resolved (b) superheat temperature, Tw(x,y,t) - Tsat, (c) the local heat flux distribution, and (d) the resulting heat transfer coefficient for a nanostructured surface undergoing a single ebullition cycle. 539x371mm (72 x 72 DPI)



Figure 4. Spatial and temporal variation along the center line of a flat surface, showing (a) local heat flux, and (b) heat transfer coefficient. 213x336mm (72 x 72 DPI)


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Figure 5. Spatial and temporal variation along the center line of a nanostructured surface, showing (a) local heat flux, and (b) heat transfer coefficient. 213x333mm (72 x 72 DPI)



Figure 6: Temporal variations at the center point of flat and nanostructured surfaces showing (a) wall superheat, (b) heat flux, and (c) HTC during one ebullition cycle. 227x367mm (72 x 72 DPI)


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Figure 7: The role of advancing and receding contact lines on heat transfer, showing (a,c) heat flux, and (b,d) heat transfer coefficient for a (a,b) flat and a (c,d) nanostructured surfaces over one ebullition cycle. 435x389mm (72 x 72 DPI)



Figure 8: Nano-enhancement affect over the 9.2 mm diameter area of influence showing (a) normalized heat transfer rate, and (b) average heat transfer coefficient for flat and nanostructured surfaces over one representative ebullition cycle. 224x392mm (72 x 72 DPI)


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Figure 9: Experimental observation of nanostructure dry out during bubble ebullition, showing (a) a representative schematic of dry out where a true dry spot forms within the nanostructures, and (b) IR thermography results for HTC showing a dry out event for an extended ebullition cycle. 554x212mm (72 x 72 DPI)



Figure 10: Validation of prediction for dry out event, showing (a) the evolution of absolute liquid pressure in the nanostructure supported region before dry out, and (b) the pressure difference across the nanostructure-supported LV interface at the center point as compared with the predicted capillary pressure. 232x353mm (72 x 72 DPI)


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Table of Contents Graphic 371x225mm (72 x 72 DPI)


Nanostructure-Supported Evaporation underneath ... - ACS Publications

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