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Nanoscale and Microscale
Thermophysical Engineering
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Boiling Augmentation with Micro/
Nanostructured Surfaces: Current Status
and Research Outlook
a
b
c
d
Sushil Bhavnani , Vinod Narayanan , Weilin Qu , Michael Jensen ,
e
f
g
Satish Kandlikar , Jungho Kim & John Thome
a
Department of Mechanical Engineering, Auburn University, Auburn,
Alabama
b
School of Mechanical, Industrial, and Manufacturing Engineering,
Oregon State University, Corvallis, Oregon
c
Department of Mechanical Engineering, University of Hawaii at
Manoa, Honolulu, Hawaii
d
Department of Mechanical, Aerospace, and Nuclear Engineering,
Rensselaer Polytechnic Institute, Troy, New York
e
Department of Mechanical Engineering, Rochester Institute of
Technology, Rochester, New York
f
Department of Mechanical Engineering, University of Maryland,
College Park, Maryland
g
École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
Accepted author version posted online: 13 May 2014.Published
online: 23 Jul 2014.
To cite this article: Sushil Bhavnani, Vinod Narayanan, Weilin Qu, Michael Jensen, Satish Kandlikar,
Jungho Kim & John Thome (2014) Boiling Augmentation with Micro/Nanostructured Surfaces: Current
Status and Research Outlook, Nanoscale and Microscale Thermophysical Engineering, 18:3, 197-222,
DOI: 10.1080/15567265.2014.923074
To link to this article: http://dx.doi.org/10.1080/15567265.2014.923074
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Nanoscale and Microscale Thermophysical Engineering, 18: 197–222, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1556-7265 print / 1556-7273 online
DOI: 10.1080/15567265.2014.923074
Downloaded by [Rochester Institute of Technology] at 19:16 20 September 2014
BOILING AUGMENTATION WITH
MICRO/NANOSTRUCTURED SURFACES:
CURRENT STATUS AND RESEARCH OUTLOOK
Sushil Bhavnani1 , Vinod Narayanan2 , Weilin Qu3 ,
Michael Jensen4 , Satish Kandlikar5 , Jungho Kim6 ,
and John Thome7
1
Department of Mechanical Engineering, Auburn University, Auburn, Alabama
School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State
University, Corvallis, Oregon
3
Department of Mechanical Engineering, University of Hawaii at Manoa,
Honolulu, Hawaii
4
Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer
Polytechnic Institute, Troy, New York
5
Department of Mechanical Engineering, Rochester Institute of Technology,
Rochester, New York
6
Department of Mechanical Engineering, University of Maryland, College Park,
Maryland
7
École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
2
Advances in the development of micro- and nanostructured surfaces have enabled tremendous progress in delineation of mechanisms in boiling heat transfer and have propelled the
rapid enhancement of heat transfer rates. This area of research is poised to make great strides
toward tailoring surface features to produce dramatically improved thermal performance.
A workshop was held in April 2013 to provide a review of the current state-of-the-art and
to develop near-term and long-term goals for the boiling augmentation community. A brief
historical perspective and primary findings are presented in this article. Though impressive gains have been made in enhancement of boiling heat transport, there still remain
several unknowns such as the mechanisms that affect critical heat flux and optimization
of surfaces for boiling heat transport. The promise of improved spatial resolution of optical techniques should improve knowledge of near-surface mechanisms. Standardization of
experimental test sections and procedures has emerged as a critical issue that needs to be
addressed immediately.
KEY WORDS: boiling, micro- and nano-structured surfaces, research outlook article
HISTORICAL PERSPECTIVE
Boiling heat transfer is ubiquitous in many commercial and industrial processes.
Since the first quantitative studies of boiling were initiated, there has been a continuous
effort to improve boiling processes, whether in pool or flow boiling. The studies have taken
Manuscript received 18 December 2013; accepted 6 May 2014.
Address correspondence to Sushil Bhavnani, Department of Mechanical Engineering, 1418 Wiggins Hall,
Auburn University, Auburn, AL 36849. E-mail: [email protected]
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/umte.
197
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S. BHAVNANI ET AL.
two tracks: one focused on the fundamental governing processes and the other focused on
the practical application of boiling. Looking back (albeit quite briefly) at some of the investigations, we can quickly see that there are two main characteristics that affect and that can
be manipulated to improve boiling behavior.
Though boiling heat transfer has been used since time immemorial in food preparation, distilling of spirits, etc., we should start a discussion of boiling heat transfer with
perhaps what was probably the first systematic study of boiling in 1756 when Johann
Leidenfrost reported on what is now called the “Leidenfrost phenomenon,” the formation
of an insulating vapor layer between a drop and a heated surface. He had a practical application for studying this boiling process, and that was to determine whether or not his wine
had been watered down. It was a qualitative study (no measurements were made; only
visual observations were conducted of the change in the boiling process when more or less
water was mixed with the wine), but the experiment probably included much testing of the
wine–water mixtures on the stove and personally.
One of the first quantitative, systematic studies of boiling was described in the pioneering 1934 paper by Shiro Nukiyama [1] entitled “The Maximum and Minimum Values
of the Heat Q Transmitted from Metal to Boiling Water under Atmospheric Pressure.” This
paper clarified and provided an overview of the pool boiling phenomena in the form of the
Nukiyama (boiling) curve (Figure 1). What is noteworthy in these two studies is that they
were overviews/high-level investigations to understand basic trends in the boiling process,
one focused on single-phase convection through fully developed nucleate boiling through
the critical heat flux (CHF) condition to film boiling and the other focused exclusively on
film boiling. But though these were on plane surfaces, even at this early date there was
research underway to improve the boiling characteristics.
Probably the first enhanced boiling heat transfer study was published in 1931 by
Jakob and Fritz [2], who studied the effects of surface finish on boiling in water.
Figure 1 The original Nukiyama (boiling) curve [1]. © Elsevier. Reproduced by permission of Elsevier.
Permission to reproduce must be obtained from the rightsholder.
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Sandblasting and machined grooves on a tube showed some enhancement of the heat transfer coefficient, but the improvements dissipated after a short time. However, before the
effects of aging were felt, the enhancement in the heat transfer coefficient was significant,
as shown in Figure 2. Between 1954 and 1962 several other studies were conducted on
heaters whose surface geometries had been modified, such as the one by Berenson [3] in
1962, which showed a sixfold increase in the boiling heat transfer coefficient for n-pentane
on a copper surface. In 1959, Clark et al. [4] identified naturally occurring pits and scratches
as active boiling sites for pentane. This was a fundamental advance in the field that eventually gave rise to research on artificially formed nucleation sites and the influence of cavity
shape on boiling.
These experimental investigations demonstrated the influence of surface geometry
on the boiling process, and during this same time period, Griffith and Wallis [5], Bankoff
[6, 7], and others theoretically investigated the effects of cavity shape on boiling inception
and stability as a complement to the experimental studies. In the late 1950s, Bankoff [8]
(Figure 3) worked on the idea that a preexisting vapor or gas nucleus must be trapped in
a cavity for a site to become active. In 1960, Griffith and Wallis [5] demonstrated that
the mouth diameter of a cavity determined the superheat needed to initiate boiling and
Figure 2 Effect of various surface finishes and time on boiling curve [2] (from Bergles [96]). © Elsevier.
Reproduced by permission of Elsevier. Permission to reproduce must be obtained from the rightsholder.
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Figure 3 Advance of a liquid film over a gas-filled pore; θ is the cone angle and β is the contact angle [8] (from
Thome [13]). © American Institute of Chemical Engineers. Reproduced by permission of American Institute of
Chemical Engineers. Permission to reproduce must be obtained from the rightsholder.
the cavity shape determined its stability. These types of studies led to investigations with
artificial reentrant cavities, such as the study by Marto and Rohsenow in sodium [9]; as can
be seen in Figure 4, the performance is much better than other types of enhancement in this
study. Also, note that the dimensions of these cavities are in the 100s of micrometers.
Building on these investigations and the insight into the governing processes, various surface modifications have been made to improve the boiling heat transfer process, and
commercial products proliferated. Efforts were made to affect surface chemistry, increase
surface area, create reentrant type cavities, and go toward smaller feature sizes. For example, the high flux surface (e.g., Gottzmann et al. [10]) was created from sintering small
metal particles onto the surface of a tube. The particles were on the order of 44–1,000 µm.
Significant enhancement in the boiling heat transfer coefficient (factor of 20) was realized.
In 1957, Bankoff analytically showed that surface chemistry affected the boiling process
[6]. His analysis indicated that the energy required to create a vapor nucleus on a planar
solid surface in a liquid was less than that required for homogenous nucleation in the liquid
phase alone. Contact angle and surface tension were determined to be important parameters.
Building on this knowledge, Young and Hummel [11] used a nonwetting material (Teflon)
on a smooth stainless steel surface, which resulted in different surface energy effects than
the native metal. They used four surfaces that combined pits and Teflon. The heat transfer
coefficient increased by a factor of 10, and nucleation could be supported at much lower
wall superheats than on the bare smooth surface.
Parallel advances were also being made in flow boiling, though at a different level of
understanding than with pool boiling. John Chen [12], for example, published his seminal
paper with his correlation on convective boiling that posited that nucleation was suppressed
due to convection. Likewise, efforts were underway to enhance the hA product for boiling,
generally through the addition of internal fins, corrugating the tubes, or structuring the
outside of the tubes with fins, reentrant cavities, etc. (see, for example, Thome [13] and
Webb [14]).
The early research described above helped point the way for further study of boiling
to understand the governing processes (and the influencing factors) and how improvements
in boiling could be attained. From this and subsequent research on boiling, we learned
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Figure 4 (a) Heat flux vs. wall superheat for several surface treatments on sodium boiling. (b) Cross section
of doubly reentrant cavity [9] (from Webb [14]). © American Society of Mechanical Engineers. Reproduced by
permission of American Society of Mechanical Engineers. Permission to reproduce must be obtained from the
rightsholder.
that several important quantities, most notably, surface topography, surface/system size,
and surface chemistry, influence the process. Likewise, we have seen that complementary
approaches to investigations—experimental investigations and analytic and/or numerical
studies—are needed to gain more knowledge to advance the field. We can also see that
the famous quote by Issac Newton—“If I have seen further, it is by standing on the
shoulders of giants”—is very relevant to progress in our field. The sections that follow
provide information and ideas about where we now stand in our understanding of the boiling processes and how we can use this knowledge to improve the boiling processes in
applications.
CURRENT STATE-OF-THE-ART
The last few decades have seen rapid improvement in the understanding of the
complex fluid dynamics and heat transfer phenomena that characterize boiling. Many
breakthroughs have resulted from improvements in visualization techniques. Improved
computational methods have also enabled the development of better modeling. Recent
developments are highlighted in this section.
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Pool Boiling
Bubbles nucleate on a heated surface when the wall superheat is large enough to cause
an embryo of gas or vapor trapped in cavities on the surface to overcome the surface tension
forces. However, there is very little understanding regarding how nucleation occurs at the
liquid–vapor interface, especially for fluids with very low contact angles. For example,
surfaces with identical value of surface roughness can result in different vapor/gas trapping
capability. The established principles described in this section provide a basic mechanistic
understanding.
Heat Transfer in the Isolated Bubble Regime A review of the experimental,
numerical, and analytical work on single bubble heat transfer by Kim [15] indicated that
transient conduction and microconvection are the dominant heat transfer mechanisms for
FC-72 bubbles. The experimental work performed using a wide array of techniques by
independent researchers found that the dominant mechanism by which heat is transferred
by isolated bubbles is through transient conduction and/or microconvection. Heat transfer
through microlayer evaporation and contact line heat transfer did not account for more than
approximately 25% of the overall heat transfer and often substantially less.
It was also found that bubble heat transfer models based on microlayer evaporation,
transient conduction, contact line heat transfer, and enhanced convection were inconsistent with the experimentally observed heat transfer signatures. Contrary to what would be
expected from the transient conduction model of Mikic and Rohsenhow [16], large heat
transfer by transient conduction was observed during the wall rewetting process before the
bubble actually departed, not only during regrowth of the superheated liquid layer after
bubble departure. The heat transfer was also not spatially uniform as their model suggests,
and the area of influence of the departing bubble was much smaller than twice the departure
diameter assumed in the model.
Although the formation of a microlayer between the bubble and the heated wall was
observed in many cases and was accompanied by large heat transfer rates, the duration was
not long enough or large enough to account for the physical size of the bubble, indicating
that the bubble gained the majority of its energy for growth through the bubble cap and
not from processes at the wall. The heat transfer during the liquid rewetting process during
bubble departure was observed to be significantly larger in many cases than during bubble
growth times.
The contact line heat transfer model was lacking for similar reasons. Very high heat
transfer rates at the three-phase contact line would have been expected as the microlayer
dried out, but higher local contact line heat transfer was observed during rewetting of the
dry area during bubble departure.
Most of the above work was performed using refrigerants similar to FC-72.
Additional measurements are needed using a variety of fluids including water under saturated and subcooled conditions to verify the mechanisms. Recent measurements with water
are beginning to appear (e.g., Jung and Kim [17] and Gerardi et al. [18]). Predictive models
that account for the observed mechanisms are also needed.
Critical Heat Flux Numerous empirical correlations and models for CHF have
been proposed over the years. The hydrodynamic instability model was originally proposed
by Kutateladze [19] and refined by Zuber [20]. The surface was assumed to be covered by
an array of escaping vapor jets whose spacing was given by the wavelength that amplifies
most rapidly. The vapor–liquid interface was assumed to become Helmholtz unstable if the
velocity difference between the vapor and the surrounding liquid exceeded a critical value,
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causing vapor to blanket the surface. This model has been quite popular because it gives
reasonable predictions for large, upward-facing heaters. However, it does not contain any
information about the affinity of the liquid for the surface and cannot predict the effects of
changing contact angle on CHF.
The macrolayer model proposed by Haramura and Katto [21] focused on the behavior
of the thin liquid layer (the macrolayer) trapped between the wall and a large, coalesced
bubble that formed from the merger of vapor generated at numerous nucleation sites. The
model assumed that CHF occurs if the hovering time of the coalesced bubble was longer
than the time it takes for the macrolayer to evaporate. Bang et al. [22] directly observed
such a liquid layer structure under a massive vapor clot. Ono and Sakashita [23] measured
liquid–vapor structures close to the heating surface using a conductance probe, and Ahn
and Kim [24] confirmed the existence of the macrolayer on their small heater. Like the
hydrodynamic instability model, the drawback to this model is that it does not include any
information regarding the fluid–wall interactions. For example, it has been experimentally
demonstrated that CHF decreases to near zero on nonwetting surfaces, a trend that cannot be
predicted using the macrolayer model. It has also been questioned since some researchers
(e.g., Gong et al. [25, 26]) have found that the liquid layer could be continually replenished
through a network of liquid channels underneath the coalesced bubble.
Guan et al. [27] developed a model based on the assumption that CHF occurs when
the vapor momentum is sufficiently large to lift the macrolayer from the surface. They
validated their model using various fluids and pressures and found agreement within about
20%.
Nishio and Tanaka [28] used a total internal reflection technique to directly observe
the liquid distribution during boiling. The wetted area fraction decreased monotonically
with wall superheat, and no correlation with wall heat flux was observed. The contact line
length, however, increased as the wall superheat increased, peaked at CHF, then decreased
for temperatures above T CHF . It is unknown at this time whether heat is transferred at the
contact line by the thin-film heat transfer mechanisms or by an alternate mechanism such
as transient conduction into the liquid as it moves over the surface, as proposed for pool
boiling by Demiray and Kim [29].
Enhancement of Pool Boiling In early years, pool boiling enhancement efforts
focused on increasing the number of nucleation sites and providing extended surface area.
McGillis et al. [30] experimented with short fin structures and realized that shorter fins
had great potential for increasing heat transfer. However, the limitations offered by simply increasing the surface area became apparent as the fin resistance becomes a severe
impediment at higher heat fluxes.
Surface manipulation by changing the nanoscale structure and surface energy is seen
as another pathway for pool boiling enhancement. Chen et al. [31] showed the potential
offered by nanowires on silicon substrates to enhance pool boiling heat transfer. The effect
of nanowire height was studied by Yao et al. [32], who showed that increasing the nanowire
height increased the heat transfer performance with water on silicon substrates. The effect
of contact angle on pool boiling heat transfer and especially on increasing the CHF was
well recognized [33]. Betz et al. [34] demonstrated that smooth and flat surfaces with combined hydrophilic and hydrophobic patterns were able to enhance CHF by 65% and heat
transfer coefficients by 100% with water on oxidized silicon substrates. For one of their
best performing geometries, they obtained a heat flux of 190 W/cm2 with a heat transfer
coefficient of up to 85,000 W/m2 •K.
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Li et al. [35] showed that nanostructured Cu surfaces significantly enhanced pool
boiling heat transfer at low superheated temperatures compared to plain Cu surfaces. The
enhancement effect was attributed to the nanobubbles, which percolated through the interconnected network of nanopores and enabled stable nucleation of bubbles at microscale
cavities (defects) on the film surface. Weibel et al. [36] demonstrated that surface modifications through carbon nanotube coating and micropatterning techniques could provide
significant enhancements of pool boiling heat transfer, reducing the surface superheat up
to 72% compared to baseline tests and eliminating disadvantageous temperature overshoot
at incipience. Such enhancements were attributed to the formation of nanoporous cavities that increased nucleation site density and high-permeability vents through which vapor
could readily depart the surface under vigorous boiling conditions. Dai et al. [37] synthesized hydrophobic–hydrophilic composite interfaces by introducing hydrophilic functional
groups on the pristine multiwall carbon nanotubes. CHF at 210 W/cm2 and corresponding heat transfer coefficient of 168,000 W/m2 •K with water were achieved by the ideal
cavities created on the surfaces. Li and Peterson [38] experimentally studied pool boiling
behavior on horizontal highly conductive microporous coated surfaces. CHF at 360 W/cm2
and corresponding heat transfer coefficient of 57,000 W/m2 •K with water were achieved.
The effects of the geometric dimensions (i.e., coating thickness, volumetric porosity, and
pore size, as well as the surface conditions of the porous coatings) on boiling heat transfer
enhancement were examined.
Nam et al. [39] studied the effect of surface wettability on the bubble dynamics of
an isolated bubble. Numerical analysis using a level-set method complemented the experimental work in water and FC-72 to conclude that bubble departure and frequency on a
superhydrophilic nanostructured silicon dioxide surface with a contact angle of 7.5◦ in
water were much smaller than those on a naturally oxidized silicon surface with a contact
angle of 44◦ in water.
Chu et al. [40] developed roughness-augmented hydrophilic surfaces by fabricating
10- to 20-µm-diameter pillars with the spacing varying from 5 to 10 µm. They extended the
CHF model by Kandlikar [33] and showed that the enhancements were attributable to the
localized changes in the contact line over the roughness introduced by the microstructures.
The boiling performance for a configuration with 20-µm fin height, 5-µm diameter and
spacing, and a roughness of 5.94 µm resulted in a maximum heat flux of 208 W/cm2
at a wall superheat of 39◦ C. O’Hanley et al. [41] conducted a systematic study to reveal
the separate effects due to wettability, porosity, and roughness changes introduced by the
nanostructures. Chu et al. [42] further studied the effect of hierarchical surfaces with roughness factors of up to 13.3 and found that roughness and wettability play crucial roles in
enhancing heat transfer and CHF. Figure 5 shows their pool boiling performance in water
with several different fin geometry parameters.
At high flux values, the critical heat flux and mitigation of the onset of critical heat
flux are of greater concern. It has been suggested that this be approached as a rewetting problem, rather than the conventional dry-out paradigm. Vapor production at the
surface needs to be effectively channeled away from the surface to permit rewetting.
Nanostructured surfaces could play a vital role in promoting effective wetting of the surface
via improved contact line dynamics, as well as effective removal of vapor, during near-CHF
conditions. Some researchers (e.g., Sefiane et al. [43] and Nikolayev and Beysens [44])
explained the process of dewetting through a contact line instability due to a vapor recoil
force. CHF was assumed to occur when the unbalanced forces acting at the contact line
resulted in spreading of vapor on the surface at the receding contact angle. Kandlikar [33]
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Figure 5 Pool boiling performance of water over silicon microstructures, Sm = smooth surface, and other hierarchically enhanced geometries with roughness factor, r, defined as the product of roughness factors at micro- and
nanoscale, from 1 (plain surface) to 13.3. Adapted from Chu et al. [42].
derived a CHF relation that included the effect of receding contact angle and orientation of
the surface relative to the gravity vector. Janecek and Nikolayev [45] found a weak effect
of the surface forces on the apparent contact angle, and Raj et al. [46] and Ajaev et al. [47]
showed that the recoil force does not influence the apparent receding contact angle.
Experiments conducted on silicon and silicon dioxide nanostructured surfaces in the
form of repeated ridges (Zou and Maroo [48]) showed an increase in CHF in water of
up to 125% with only 40% increases in surface area. The increase was attributed to the
breakdown of the nonevaporating film in the central region under a growing vapor bubble.
The minimum ridge height that promoted CHF enhancement was determined to be 450 nm
(900 nm for SiO2 ).
Several researchers attempted to explain the process of dewetting through liquid film
dynamics, because it is believed that the behavior of the near-wall liquid layer plays a
key role in boiling heat transfer and CHF. Theofanous et al. [49, 50] used a high-speed
infrared camera to directly observe the liquid distribution and temperature during pool boiling of water on borosilicate glass and sapphire substrates coated with titanium film heaters.
At high heat fluxes, both reversible and irreversible dry spots were observed. They found
that the growth of irreversible dry spots leads to burnout and suggested that the process of
dewetting in pool boiling is controlled by the microhydrodynamics of the near-wall liquid
layer. Gong et al. [25, 26] investigated the evaporation of thin liquid films; they observed the
formation and expansion of dry spots below a critical thickness and developed a model to
predict the critical thickness of the film rupture based on lubrication theory. Rednikov and
Colinet [51] showed numerically that the wetting film ruptures when the imposed superheating exceeds a critical value. Chu et al. [52] used a total internal reflection technique
to observe the liquid distribution during boiling of water on a sapphire substrate with an
Indium-tin oxide (ITO) heater layer using a high-speed camera. They found that the formation of irreversible dry patches due to bubble coalescence leads to CHF. Chung and No
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[53] presented a model where the overall heat flux from nucleate boiling to CHF can be
represented as a “nucleate boiling heat flux” (computed by multiplying the heat transferred
at a single bubble site by the nucleation site density) weighted on the “nucleate boiling
fraction” (the ratio of the number of isolated bubbles contributing to pure nucleate boiling
to the number of bubbles which may exist in a given area). Using correlations to obtain the
coalescence rate and the dry area fraction, they were able to obtain curves that agreed well
with experimental data.
Gerardi et al. [54] studied the effect of water-based nanofluids with diamond and silica particles on CHF using a high-speed IR camera on a sapphire substrate with ITO film
heaters. They found that the nanofluid caused a reduction in the frequency of the bubble
departure and nucleation site density for a given wall superheat. An increase in the surface wettability due to the porous nanoparticle layer on the heater surface resulted in CHF
enhancement.
A note on the comparison of various enhancement techniques in pool boiling is warranted. Ultimately, the performance of a boiling surface is relevant at the specific operating
conditions. Some researchers use the slope of the boiling curve instead of the heat transfer
coefficient (HTC) while comparing different enhanced surfaces. The slope of the boiling
curve is somewhat irrelevant as the actual value of HTC and the upper limit as defined by
CHF are important criteria. It is therefore recommended that the slope of the boiling curve
near CHF may be used in discussing the CHF mechanism but not for comparing boiling
performance of enhanced surfaces. A plot showing HTC as a heat flux will be very useful
in such comparisons.
Critical Heat Flux in Open Microchannels in Pool Boiling Cooke and
Kandlikar [55, 56] employed an open microchannel geometry in pool boiling and
obtained a maximum CHF of 249 W/cm2 and a corresponding heat transfer coefficient
of 275,000 W/m2 •K with water on copper substrates. They noted that the microchannels
provided efficient liquid pathways to the nucleation sites. Mehta and Kandlikar [57, 58]
employed the open microchannel geometry on cylindrical surfaces in both horizontal and
vertical orientations and observed significant performance enhancements with water over
tubular copper substrates. Yao et al. [32] showed that coating the open microchannels with
silicon nanowires improved the CHF and heat transfer coefficient over plain microchannels
on silicon substrates. They developed a method to fabricate nanowires on orthogonal surfaces of the microchannels. Combining the microstructures and nanostructures clearly
provides a pathway for enhancing the pool boiling performance. Further research on
developing such combined hierarchical structures is warranted.
Kandlikar [59] developed microstructures utilizing evaporation momentum force to
direct bubbles away from the nucleation sites along the tapered substrate surfaces. The
microstructures were designed with a central fin of 200 µm thickness and 400 µm height.
The nucleation occurred preferentially at the corner of the fin and the substrate. The substrate gradually sloped upwards to the same height of the fin. The surrounding valley
around the fin was 1 mm × 0.8 mm, forming a single microstructure feature. Several
such features were embossed on a copper chip. This configuration resulted in separate
flow pathways for liquid and vapor. The high-speed videos showed the trajectories of bubbles nucleated at the corners of a short fin that followed the predicted trajectory. Figure 6
shows the pool boiling performance of two chips using this microstructure. A substantial
enhancement in CHF (300 W/cm2 ) was noted with a corresponding heat transfer coefficient
of 629,000 W/m2 •K over copper substrates with water boiling at atmospheric pressure.
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Figure 6 Pool boiling performance of water over copper substrates with embossed microstructures utilizing evaporation momentum force, chip 1 with eight features, chip 2 with seven embossed features. Substrate 20-mm-square
copper chip with 10-mm-square central heated area [59]. © Applied Physics Letters. Reproduced by permission
of Applied Physics Letters. Permission to reuse must be obtained from the rightsholder.
Incorporating superhydrophilic surfaces along the vapor flow path is expected to provide
further heat transfer enhancement.
Convective Boiling in Microchannels
Single-phase flow in microchannels yields a high heat transfer coefficient due to the
small hydraulic diameters of the channels. Attempts to push performance limits even higher
included the use of traditional macroscale enhancements techniques such as offset strip
fins [60] and pin fins [61]. The heat transfer coefficient in the offset strip fin geometry
was measured to be in excess of 500,000 W/m2 •K [62]. Although the pressure drop was
very high, Colgan et al. [60] employed very short flow lengths of only 2 mm to provide a
practical silicon chip cooler to dissipate over 1,000 W/cm2 with water.
The benefits realized in single-phase flow by reducing channel dimensions led naturally to the exploration of the use of these geometries in concert with the mechanistic
benefits attainable with phase change. Efforts were directed toward understanding the
nature of the flow by mapping flow regimes. A major roadblock encountered was the
existence of unstable flow. Consequently, flow boiling research conducted during the last
decade has been mainly focused on flow mapping and on resolving flow instabilities, as
discussed below.
Flow Patterns Figure 7a shows the experimental results for refrigerant R236fa
taken for a multi-microchannel evaporator with 67 parallel square channels of 100 × 100
µm size and 10 mm in length. Utilizing a pixel-by-pixel calibration procedure for an IR
camera [63, 64], 90 local (width-wise averaged) flow boiling heat transfer coefficients
are captured in one snapshot from inlet to outlet, illustrating directly the strong contrasting trends in the heat transfer due to the bubbly, slug, churn, and annular flow regimes.
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Figure 7 (a) Experimental results for R236fa, mass velocity, Gch = 2,299 kg m−2• s, and qb = 48.6 W/cm2 ,
illustrating significant influence of flow patterns on microchannel flow boiling heat transfer. (b) Flow pattern–
based model that captures flow regime induced trends in heat transfer of R236fa for Gch = 1,525 kg m−2• s and
qb = 35.4 W/cm−2 compared to conventional model (figures adapted from Szczukiewicz et al. [63, 64]).
These results in particular highlight the need for flow pattern based prediction models
with an associated flow pattern map to capture these transitions and heat transfer trends.
In Figure 7b, another set of flow boiling data is compared to two leading microchannel
flow boiling models, one that is flow pattern based and one that is not. Thus, though the
Bertsch et al. [65] method works reasonably well at intermediate conditions in this graph,
it does not capture the minimum at the change of flow pattern (churn flow regime between
the coalescing bubble/slug flow regime with decreasing heat transfer and annular flow with
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BOILING AUGMENTATION
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increasing heat transfer) or the slopes of the changing heat transfer coefficient due to their
diverse underlying flow structures. Instead, a flow pattern-based method proposed by CostaPatry and Thome [66], made up of the three-zone slug flow model of Thome et al. [67] and
the annular flow model of Cioncolini and Thome [68], applied here to the present data, captures quite well the location of the minimum and the trends in this new data of Szczukiewicz
et al. [63, 64]. Thus, interpretation of flow boiling data (and two-phase pressure drops,
etc.) using flow pattern maps and mechanistic models emulating their flow structure is an
important step toward more accurate and reliable prediction methods for general use.
Flow Instabilities and Heat Transfer Performance Another important aspect
to consider while performing such tests and producing local flow boiling heat transfer data
is the stability of the flow. Microchannel flow boiling tends to be much more susceptible to
two-phase flow instabilities (compressible volume and parallel channel instabilities) than
macrochannel flows. In particular, this tends to create back flow of vapor into the inlet
header, and this gives rise to significant channel-to-channel flow maldistribution, which
must be avoided in electronics cooling applications to guarantee reliable cooling. Figure 8
shows some still images from high-speed video and infrared (IR) camera images of stable
and unstable flows. At the bottom, the flow has not been stabilized, and a large amount of
vapor is visible in the inlet header at the left and some channels have only single-phase flow
from inlet to outlet; hence, though measurement of width-averaged heat transfer coefficients
for a line of IR pixels normal to the flow is possible, the resulting value includes some
channels in flow boiling and some channels in liquid flow, giving a value that is test section
dependent (perhaps not even repeatable). Such data do not qualify for inclusion in a heat
transfer database for development of prediction models; that is, no flow boiling model can
capture the effects of such flow maldistribution, and one should not validate or disprove the
accuracy of prediction models with such results. At the top, the flow has been stabilized by
Figure 8 High-speed video and IR camera images of stable and unstable flows.
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micro-orifices at the entrance of each channel, and one can see a nearly uniform front of
onset of boiling about one-third of the distance along the channel; hence, in this case, heat
transfer data after all channels are in the boiling model for this uniform flow will provide
valuable flow boiling heat transfer data. (The video imagery is at http://ltcm.epfl.ch/page54040-en.html.)
Recent efforts on reduction of flow instabilities have focused on mitigation using inlet
restrictors and artificial nucleation sites [69–72]. A detailed discussion of the instabilities
and methods to overcome them has been provided by Peles [73].
A number of innovative techniques have been proposed and tested to further enhance
flow boiling heat transfer in microchannels, including passively inducing self-sustained
oscillations [74, 75] and superhydrophilic Si nanowire inner walls [76–80]. These studies
show promising results.
Though heat transfer coefficients of over 500,000 W/m2 •K have been reported for
single-phase flow in microchannels and also during pool boiling, the highest values reported
during flow boiling in microchannels are limited to around 80,000 W/m2 •K [71, 81]. The
associated pressure drop is over 50 kPa with water for a flow length of 10 mm. Such high
pressure drops are undesirable for safe operation of IC chips.
Mukherjee and Kandlikar [82] proposed a different configuration with tapered
microchannels in which the flow cross-sectional area increased in the flow direction. Lu
and Pan [83] and Balasubramaniam et al. [84] obtained experimental data for flow boiling
in tapered microchannels and observed highly stable flow.
Although flow boiling instabilities have been well understood and some resolution is
now possible, low CHF, low heat transfer coefficient, and high pressure drop still remain
a concern. A new configuration of open microchannels with manifold (OMM) utilizing a
uniform or tapered gap over the microchannels was introduced by Kandlikar et al. [85].
The flow configuration is illustrated in Figure 9. The nucleating bubbles rise over the
microchannels and flow in the expanding gap, while liquid flows in the microchannels.
Providing separate pathways for liquid and vapor and increasing the flow cross-sectional
area along the flow direction in the tapered manifolds provide both stable flow and a
high CHF. The pressure drop is reduced dramatically because of the expanding geometry. Figure 10 shows the boiling curve for a uniform gap configuration. It is seen that a
maximum heat flux of 506 W/cm2 could be dissipated with water as the test fluid without
reaching the CHF. Similar heat transfer results were obtained with tapered manifolds, but
the pressure drop was reduced from over 50 kPa for the uniform manifolds to less than 5 kPa
for the tapered microchannels at the highest heat flux tested. A maximum heat transfer coefficient value of 290,000 W/m2 •K is obtained at an intermediate heat flux of 319 W/cm2 .
This value is significantly higher than 80,000 W/m2 •K reported for plain microchannels as
discussed earlier.
The OMM configuration provides not only a high heat transfer coefficient but
extremely low pressure drops. Further research on this configuration is warranted to understand the underlying heat transfer mechanism. Some of the areas where further research is
warranted are as follows:
• Optimization of the microchannel and taper geometry
• Integration of superhydrophilic surfaces to further enhance CHF
• Development of heat transfer models to aid in understanding the underlying mechanism
and further improve performance
• Extension of the investigation to newer refrigerants and electronics cooling fluids
• Implementation in a practical integrated circut (IC) chip cooler design
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Figure 9 OMM design to provide highly enhanced heat transfer and low pressure drop [85]. © American Society
of Mechanical Engineers. Reproduced by permission of American Society of Mechanical Engineers. Permission
to reproduce must be obtained from the rightsholder.
Figure 10 Flow boiling performance of water in OMM configuration over a microelectronic chip using a uniform
manifold and a gap of 0.127 mm over the microchannels [85]. © American Society of Mechanical Engineers.
Reproduced by permission of American Society of Mechanical Engineers. Permission to reproduce must be
obtained from the rightsholder.
Computational Modeling of Microchannel Flow For detailed analysis of
two-phase flows and evaporation in microchannels, numerical simulation capabilities are
progressing rapidly and are becoming an important research tool. These codes, when
validated against benchmarks and experimental data, can provide valuable insights into
the phenomena not obtainable experimentally. Figure 11 for instance, shows an adiabatic
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Figure 11 Numerical simulation of three bubbles in a microchannel illustrating the velocities (top) and the mesh
(below) during the coalescence of two of the bubbles (from Anjos et al. [86]) for R1234ze flowing in a 100-µm
channel at a mean velocity of 0.611 m/s.
2D simulation of three elongated bubbles in a microchannel (the video can be seen at
http://ltcm.epfl.ch/page-70192-en.html). The governing equations in this in-house simulation code were developed in the generalized Arbitrary Lagrangian-Eulerian framework
through the finite element method, as described in Anjos et al. [86]. The vapor and liquid
velocity profiles are shown at the top where the highest horizontal speed is represented
by the red color and the lowest is represented by the blue color. The mesh used in this
simulation is shown below, where the red and blue triangles represent the vapor and liquid phase, respectively. The three bubbles are initially distributed along the channel and
during the simulation, the last two bubbles coalesce and generate one elongated bubble,
thus reproducing a typical phenomenon found in microchannels, where the velocities in
the liquid slug between successive bubbles and in the thin liquid film region between the
wall and bubble interface are difficult to be quantified experimentally. As flow patterns,
their transitions, and mechanistic models that represent the two-phase flow structure and
its dynamics are the best way forward in better physical understanding and the prediction of microchannel heat transfer, numerical modeling is now able to play a strategic
role.
As another example, Magnini et al. [87] developed an in-house user-defined function code for a popular commercial code and used it to numerically simulate multiple
elongated bubbles evaporating in microchannels. Based on their numerical heat transfer
results, they proposed a mechanistic model for the single-phase heat transfer occurring
between two successive bubbles including the flow recirculation effects. Figure 12 depicts
one such simulation and the model. At the top, the numerical simulation shows the streamlines of the relative velocity field in the liquid between the tail of one bubble at the
right and the nose of the trailing bubble at the left, where the velocities have been normalized relative to the mean velocity of the flow to illustrate the recirculation going on
between the bubbles. The flow of the vapor in the bubbles is also shown while the capillary waves near the end of the tail of the lead bubble can be seen, which are observed in
experimental videos in many studies. In the lower diagram, the schematic of their heat
transfer model for the recirculating zone is depicted; it captures the local liquid-phase
heat transfer as a function of the flow parameters in terms of the Peclet number and
slug length–to–channel diameter ratio. Hence, one can see the emerging importance of
numerical modeling of evaporation in microchannels as a powerful research tool to further the scientific knowledge and understanding when such results are very difficult to
extract experimentally, and such results can be used to make it easy to apply mechanistic
models.
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Figure 12 Streamlines of a microchannel slug flow (top) and slug flow heat transfer model (bottom) from Magnini
et al. [87] (λt is the liquid thermal conductivity, δ is the liquid film thickness, and δ s is the liquid slug thickness).
Boiling-Induced Pumping Using Micro/Nanostructured Surfaces
The concept of asymmetry in thermophysical properties or geometry has been used
to transport phase-change flows. Mukherjee and Mudawar [88] experimentally studied a
gravity-driven pumpless closed-loop phase change system with no surface enhancements,
minichannel surface, and microchannel surface enhancements to the boiler section. The
system consisted of a vertically oriented closed loop containing a boiler positioned on one
of the vertical legs (rising tube) and a condenser located on the horizontal leg. The presence
of bubbles in the rising tube leaving the boiler reduced the density in the tube, thus creating
a hydrostatic pressure difference between the rising tube and the cold return vertical leg
on the opposite side of the loop. Geng et al. [89] exploited asymmetry in surface tension
forces created due to an expansion in the geometry of the microchannel to cause preferential
motion of liquid during phase change.
Linke et al. [90] discovered a novel method to move liquid droplets in the Leidenfrost
regime using asymmetrically patterned surface structures. These structures, in the form of
a 60◦ /30◦ millimeter-sized ratchets caused liquid droplets to be propelled at speeds on the
order of 5 cm/s. The net viscous drag in the vapor layer between the liquid droplet and
the ratcheted surface was hypothesized to be the driving force for the droplet motion. This
hypothesis was later confirmed by Lagubeau et al. [91]. Building upon the concept of Linke
et al. [90], Thiagarajan et al. [92] and Kapsenberg et al. [93] introduced a second level of
asymmetry in the form of reentrant cavities located on one face of the ratchets. Bubbles
emanating from such cavities were observed to grow and depart in a direction perpendicular
to the slope of the ratchet. Tests were conducted under both 1-g and 0-g conditions in FC72 and water. Liquid velocities on the order of 30 mm/s parallel to the heated structured
surface were observed.
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ONGOING EFFORTS AND RESEARCH NEEDS
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A workshop sponsored by the Office of Naval Research (ONR), National Science
Foundation (NSF), Advanced Research Projects Agency-Energy (ARPA-E), and Defense
Advanced Research Projects Agency (DARPA) was held in April 2013 at MIT to provide
a review of the current state-of-the-art and to develop near-term and long-term goals for
the boiling augmentation community. Based on the overview of the state-of-the-art detailed
in the previous section, and recognizing the exciting new capabilities that have recently
become available in both the experimental and computational domains, the workshop participants concluded that several goals are within reach. These goals can be organized into
three topical areas:
• Micro/nanostructure optimization for pool boiling and flow boiling
• Measurement/experimentation challenges
• Practical implementation
Micro/Nanostructure Optimization for Pool Boiling and Flow Boiling
The questions posed to the workshop attendees were the following:
• How small can microchannels become before they become too small for bubble
nucleation; for CHF; for pressure loss (flow boiling)?
• What is the characteristic dimension in micro/nanostructured surfaces on which the
boiling heat transfer scales?
• Could different geometries be used to enhance the heat transfer coefficient as well as raise
CHF to higher values?
• Would some type of surface modification be applicable for both pool and flow boiling?
• Do roughness and porosity play the same role in enhancing CHF? If so, what is the
appropriate unit of roughness/porosity for a fair comparison? Can capillary wicking or
“wickability” be used as a measure of the effect of roughness/porosity in enhancing
CHF? If so, how should the wickability be measured?
The panelists concluded that it may not be possible to develop a single optimal boiling micro/nanostructure. There are several issues that need to be addressed depending on
boiling mode and regime and on the particular goal that needs to be accomplished. At low
heat flux levels, the primary issue might be reliable boiling incipience. The use of dendritic structures to overcome the overshoot problem associated with boiling incipience has
been studied. Test structures that can help isolate geometric variables can be utilized to
reduce the current dependence on empiricism, particularly for parameters like nucleation
site density.
As indicated in the previous section, the use of hierarchical (micro/nano) structures
for optimizing boiling is an option for creating optimized surfaces for boiling. There is
need for delineation of the boiling mechanisms such as microconvection and contact line
dynamics to develop models for such hierarchically structured surfaces.
An optimal surface microstructure might be relevant only at low void fractions in flow
boiling. In annular flow, the importance of surface microstructure diminishes greatly. The
onset of dry-out in flow boiling is governed by the hydrodynamics of the flow. Entrained
droplets could lead to premature dry-out and lead to as much as a 50% reduction in CHF.
There was some debate on whether the annular flow model accurately captured the heat
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transfer. One panelist felt that the annular flow model only represented low heat flux conditions but was inaccurate for higher heat fluxes. It was felt that independently verified and
replicated experiments were still the most reliable way to characterize flow boiling and
develop semi-empirical mechanistic models.
An important topic for future research in the area of boiling on hierarchically structured surfaces lies in the development and integration of mechanistic models into larger
scale models. Toward this end, mechanistic models and numerical simulations will need to
be benchmarked against experimental data/visualization.
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Measurement/Experimentation Challenges
Questions posed to workshop attendees included the following:
• What are the major measurement issues—heat loss, conjugate heat transfer effects, estimation of heat flux, and heat transfer coefficient (area to use for fair comparison between
a flat surface and micro/nanostructured surface)?
• What are the local measurement challenges? With advances in micro/nanofabrication,
it has become possible to create unique microstructure and nanostructured surfaces.
In order to validate in detail with emerging modeling and numerical simulations of
boiling process, what are the measurements needed—flow visualization, conjugate heat
transfer effects, microlayer thickness, dynamic contact angle, microthermometry (both
surface and fluid), local heat flux, nonequilibrium behavior, droplets entrainment, and
deposition?
Despite more than a half century of effort, the physical mechanisms triggering CHF
are not yet understood. New experimental methods and models are sorely needed if we
are to fully understand CHF mechanisms. In addition to measurements at the liquid–solid
interface, methods are needed to visualize and measure the turbulence and heat transfer
within the liquid as CHF is approached. This goal has not been achieved using existing
techniques since bubbles obscure and scatter light. Such measurements should help explain
the heat transfer in the liquid-covered areas, can aid in understanding bubble growth, coalescence, and departure processes that result in the formation of dry patches and benchmark
numerical simulations of pool boiling.
One of the reasons is the lack of reliable local information that can enable models to
be tested. Although point measurements or area-averaged measurements have been made,
techniques whereby these quantities can be measured over large areas with higher resolution are generally lacking. Very recently, techniques using IR thermography are beginning
to provide much needed details of CHF mechanisms. Kim et al. [94] were able to measure
the wetted area fraction by using the contrast in IR emissions between the wet and dry areas
under subcooled and saturated conditions using water. They observed dry patches on the
surface that were periodically rewet by liquid below CHF, and irreversible dry spots were
observed at CHF. The wetted area fraction decreased with increasing heat flux and increasing bulk fluid temperature. The contact line length increased with heat flux and decreasing
bulk fluid temperature.
Jung et al. [95] used IR thermography to measure the temperature, heat flux, contact line length density, wetted area fraction, average contact line speed, and dry patch size
along the boiling curve for FC-72 under saturated conditions. The contribution of the heat
transfer at the contact line was negligible. The heat transfer through the liquid-covered
area (q̇l ) was found to increase roughly linearly with wall temperature and likely occurred
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through agitation and mixing of single-phase liquid by bubble growth, coalescence, and
departure processes. The boiling curve could be computed as the heat flux through the liquid area weighted on the wetted area fraction; that is, q̇ = q̇l × WF. CHF was found when
the dry patch size increased faster than the increase in heat transfer through the liquid area,
suggesting that higher CHF can be attained by either increasing the wetted fraction (e.g., by
decreasing the contact angle, modifying the surface to increase capillary forces, or increasing subcooling) or increasing the heat transfer through the liquid area (e.g., by increasing
the surface roughness). Heat transfer from the wall during boiling occurred primarily due to
agitation of superheated single-phase liquid by bubble growth, merger, and departure processes. Bubbles grew by drawing energy from the energy stored in this superheated liquid
layer.
The even smaller scales introduced by the use of micro-nanostructures further exacerbates measurement and experimentation challenges in boiling heat transfer. Current IR
imaging capabilities provide a 2- to 5-µm resolution. There is a need for advancements
in this field to provide submicrometer resolution. The state-of-the-art in particle imaging
velocimetry techniques needs to be improved to allow velocity measurements in the vicinity
of vapor masses. Efforts should also be made to obtain simultaneous temporally and spatially resolved measurements of surface temperature/heat flux on nanostructured surfaces
and bubble dynamics to help the development of computational modeling of boiling.
Benchmarking of Data Industry representatives in attendance suggested that heat
transfer coefficient gains that have currently been realized are sufficient to enable several
additional generations of product development. However, the accuracy of the data generated could be questionable due to the nonstandard sizes used by various investigators.
The role of conduction and conjugate effects becomes more relevant as the microstructure
of the surface is manipulated for improved boiling performance. Conjugate effects serve
to mask the true convective phenomena, thereby preventing the acquisition of fundamental knowledge and accurate estimation of heat transfer. Heater size effects are critical and
need to be studied. It has been suggested that there might be a need to propose a standard test coupon/heater size that should be used to compare the benefits of various surface
micro/nanostructure optimizations. The panel felt the need for better interaction between
the fields of fluid dynamics and heat transfer.
There is a much lower comfort level with reliable knowledge of pressure drop characteristics. As the development of structures transitions from research to production, there
is a need to develop a pressure drop metric. The issue is convoluted further by the absence
of a reliable measure of pressure drop attributable to the enhanced surface alone.
The issue of validation of simulations against benchmark experiments was also
brought forth. The research community needs to come together to identify a set of benchmark experimental results that have been independently verified for use in the development
of improved computational algorithms and software capabilities.
Practical Implementation
Issues presented to workshop attendees included the following:
• Packaging/interconnects (challenges)
• Working fluids (including highly wetting fluids and fluid mixtures)
• Micro/nanostructured surfaces (long-term stability, contaminant/fouling, mechanical
strength, and cycling effects)
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• Manufacturability (large scale) and cost, solid substrates used (silicon, metals), manufacturing methods
• Pressure loss/pumping power for flow boiling
• Single-phase water cooling schemes (competitiveness)
Industry members in attendance felt that the largest barrier to practical implementation of two-phase cooling methods is the lack of a revolutionary breakthrough that would
make two-phase cooling clearly superior to single-phase liquid cooling. The feeling is that
the 100 W/cm2 threshold currently approached by single-phase liquid cooling is good
enough to meet current short-term proposed needs. Recent research push from agencies
such as DARPA in the area of electronic cooling, seeking dependable achievement of flux
targets one order of magnitude higher, is a step in the right direction. High-performance
government (military) systems are likely to host the first implementation platforms that
will eventually drive the cost down toward commercial applications.
An additional demand from industry is that the proposed innovations need to be scalable. It is not sufficient to merely prove the capabilities of nano-microstructured surfaces
in carefully controlled laboratory environments. Effects such as thermal cycling and aging
need to be proven for typical anticipated product lifetimes.
CONCLUSION
As the benefits of micro-nanostructured surfaces start being realized, it is important
to continue to keep track of overarching issues that must be addressed. These include the
need for the surfaces to behave in a predictable fashion with aging effects, if any, following
predicted patterns. Design/analysis rules must be communicated to industrial manufacturing environments in a transparent manner. Limiting the role of instabilities such that they
may be either completely eliminated or restricted to domains outside of those associated
with operating domains would lead to wider implementation. Manufacturing complexity
leading to additional costs must be managed to provide a systemic net positive effect in
mass-produced application areas.
FUNDING
This article is the result of a workshop supported by US Department of Energy
ARPA-E (Grant No. DE-AR0000363), the US National Science Foundation (Grant No.
1261824), and the Office of Naval Research (Grant No. N00014-13-1-0324).
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