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Heat Transfer Engineering
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Review of the Manufacturing Techniques for Porous
Surfaces Used in Enhanced Pool Boiling
a
Chinmay M. Patil & Satish G. Kandlikar
a
a
Department of Mechanical Engineering , Rochester Institute of Technology , Rochester ,
New York , USA
Accepted author version posted online: 12 Nov 2013.Published online: 23 Dec 2013.
To cite this article: Chinmay M. Patil & Satish G. Kandlikar (2014) Review of the Manufacturing Techniques for Porous Surfaces
Used in Enhanced Pool Boiling, Heat Transfer Engineering, 35:10, 887-902, DOI: 10.1080/01457632.2014.862141
To link to this article: http://dx.doi.org/10.1080/01457632.2014.862141
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Heat Transfer Engineering, 35(10):887–902, 2014
C Taylor and Francis Group, LLC
Copyright ISSN: 0145-7632 print / 1521-0537 online
DOI: 10.1080/01457632.2014.862141
Review of the Manufacturing
Techniques for Porous Surfaces
Used in Enhanced Pool Boiling
Downloaded by [Rochester Institute of Technology] at 13:40 15 April 2014
CHINMAY M. PATIL and SATISH G. KANDLIKAR
Department of Mechanical Engineering, Rochester Institute of Technology, Rochester, New York, USA
Continuous development of high-performance microelectronic chips requires efficient cooling systems to dissipate large
amount of heat produced over a small footprint. Pool boiling is capable of dissipating large heat fluxes while maintaining
low wall superheat and is receiving renewed interest. Porous surfaces have been investigated extensively for pool boiling
enhancement. This paper presents a review of different manufacturing techniques employed to manufacture porous surfaces
in pool boiling application. Different types of surfaces developed using these techniques are reviewed and their pool boiling
performance is discussed.
INTRODUCTION
As a result of widespread introduction of microelectronics
and the ever-increasing demand of functionality and reliability, thermal management is becoming an important problem to
tackle and has attracted special attention in almost every branch
of the industry. For most electronic chips, it is important to
keep the temperatures relatively constant and below 85◦ C [1].
Compared to other conventional methods of heat transfer, pool
boiling offers a much attractive option, as it is able to dissipate
large amount of heat at low wall superheats. Special surfaces
have been developed to further enhance the heat transfer using
pool boiling. These special surfaces are either porous surfaces
or “structured” surfaces having various geometries [2]. Davis
[3] in his review article has described numerous ways of developing porous surfaces and their potential applications. Numerous studies have shown that these surfaces were effective in
decreasing the wall superheat at boiling incipience and enhancing nucleate boiling heat transfer by providing large number
of active nucleation sites within a confined space [1]. Another
way of enhancing heat transfer is using nanoparticles suspended
in nanofluids, although the enhancement mechanism seems to
Address correspondence to Professor Satish G. Kandlikar, Mechanical Engineering Department, Rochester Institute of Technology, Rochester, NY 14623,
USA. E-mail: [email protected]
Color versions of one or more of the figures in the article can be found online
at www.tandfonline.com/uhte.
emerge from the deposition of nanoparticles on the heat transfer surface. Many investigators have studied effects of these
nanofluids [4–14]. This paper mainly deals with manufacturing
techniques for generating porous surfaces and the associated
pool boiling enhancement.
POOL BOILING ENHANCEMENT MECHANISMS WITH
POROUS SURFACES
There are numerous mechanisms suggested by researchers
for boiling through porous surfaces. After analyzing some of
the previous models, Bergles and Chyu [15] proposed a possible mechanism that there is a stable vapor formation occurring
inside the porous media. There are favored vapor escape channels in the porous medium surrounded by a network of channels
supplying liquid. The actual evaporation takes place at the entrance of the liquid channel. Since the porous surface offers
a higher surface area, it results in a low wall superheat. The
cavities inside the porous network act as nucleation sites.
Wang et al. [16] suggested that the porous network has many
cavities that act as nucleation sites. They proposed that once
the bigger bubbles depart, the smaller nucleation cores become
active, producing bubbles over and over again. A schematic of
their model is shown in Figure 1. The higher the nucleation
frequency, the higher the is heat dissipated from the surface.
The bubbles form a large vapor column after departure and
create a turbulent convective flow, further enhancing the heat
transfer rate. The porous structure matrix also promotes upward
887
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C. M. PATIL AND S. G. KANDLIKAR
MANUFACTURING TECHNIQUES FOR POROUS POOL
BOILING SURFACES
Sintering
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Figure 1 Proposed mechanism of boiling in porous media, adapted from
reference [16].
squirt effect. To conclude, a porous medium offers a competent
method of enhancing pool boiling heat transfer.
This review paper deals with porous media and different
manufacturing techniques used to manufacture porous surfaces
that are suitable in pool boiling applications and provide a high
degree of heat transfer enhancement.
DESIRABLE FEATURES OF POROUS MEDIA
For being effective in boiling application, the porous media
should be insoluble in the working fluid. It should be physically
and chemically stable and should not be affected by elevated
temperatures. It should be thermally conductive in order to allow heat transfer through the interconnected porous network
[17]. If created using particles, the thickness of the porous surface should not be more than four times the mean particle diameter [18]. Chien and Chang [19] conducted experiments and
determined that the optimal ratio of layer thickness to particle
size is about 3.85. Poniewski and Thome [20] suggested that
the maximum coating thickness should not exceed 1.5–3 mm,
which is suitable for low heat transfer performance applications.
Increased thickness of porous layer would increase maximum
heat flux, but extra thickness adds thermal resistance, increasing
wall superheat [21]. Hence, an optimal thickness is important.
Sintering is a method of producing metal components from
powdered metal particles by fusion of the particles upon application of heat. It involves two main steps: compacting and fusing.
Compacting involves application of pressure on the powder to
create loose bonds. This is further heated in a furnace, fusing
the particles together. On further heating, the pore sizes reduce,
developing grain boundaries. Smaller particles have more surface energy than larger particles, and hence they sinter quickly
as compared to larger particles [22].
Hanlon and Ma [23] fabricated a porous medium from a
100 mesh (149 μm) 99.9% pure copper particles sintered at
840–900◦ C for 12–45 min. They gradually decreased thickness
of the porous layer to study the effect of thickness on boiling
performance. Based on their experimental work, they concluded
that only the top surface of the wick plays an important role in
heat transfer enhancement. Also, the heat transfer was enhanced
by decreasing the particle size. They also observed that dryout
heat flux depends significantly on the wick (layer) thickness.
Li et al. [24] fabricated surfaces of single- and multilayered
sintered isotropic copper mesh as shown in Figure 2. The fabrication was done in three steps. First, the layers of copper mesh
were sintered together and cut to required dimensions, and then
sintered to the heater block. Sintering of copper mesh was done
at 1030◦ C in 75% nitrogen and 25% hydrogen gas for 2 hr.
Li and Peterson [25] analyzed these samples and found out
that heat transfer performance of the wick increases with an
increase in wick thickness and a reduction in mean pore size.
Thus, for a given value of thickness and porosity, the wall superheat increases with increase in mesh size. Also, they found
that beyond a porosity of approximately 50%, the critical heat
flux drops. They obtained a maximum overall heat transfer coefficient of 117.3 kW/m2-K for a relatively coarse mesh with
distilled water.
Weibel et al. [21] sintered different sizes of copper particles
at their natural packing density using a high-temperature mold
Figure 2 (a) Side view of sintered isentropic mesh. (b) Top view of sintered isentropic copper mesh. © [ASME]. Reproduced by permission of ASME.
Permission to reuse must be obtained from the rightsholder.
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vol. 35 no. 10 2014
C. M. PATIL AND S. G. KANDLIKAR
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Figure 3 (a) Modulated surface. (b) SEM image of surface by second method. © [IOP]. Reproduced by permission of IOP. Permission to reuse must be obtained
from the rightsholder.
having a uniform thickness. The porosities of the samples were
about 65%. For 900-μm-thick samples of sintered copper surfaces, the highest heat flux observed was 596.5 kW/m2 with
water at wall superheat of about 26◦ C. They found that a minimum thermal resistance is exhibited by intermediate particle
sizes (106 to 150 μm), which are optimal size for design.
Cora et al. [26] manufactured microscale, porous surfaces
with modulations with copper powder (100 μm) using two different methods. In the first method, copper powder was compacted under a pressure of 15–50 MPa and a temperature of
350–500◦ C and then sintered at around 900◦ C for 1 h under
controlled atmosphere in a tube furnace. In the second method,
they first sintered the copper, and then compacted it at different temperatures. Surface modulations are shown in Figure 3.
They observed that the porosity decreased with an increase in
compaction pressure. They could generate surfaces with porosities ranging from 15 to 40%. Pool boiling experiments showed
that the critical heat flux was approximately 815 kW/m2 at wall
superheat of 20◦ C for n-pentane at atmospheric pressure. The
pool boiling curve showed no signs of temperature overshoot.
Min et al. [27] created two-dimensional (2D) and threedimensional (3D) modulated porous coatings manufactured using hot powder compaction technique as shown in Figure 4.
They pressurized the powder under 15–50 MPa pressure and
350–500◦ C temperature to create the desired 2D and 3D coatings in a punch and die assembly, and sintered at 900◦ C for 1 h,
as shown in Figure 4. In their work, they determined that the 2D
structure created under 25 MPa pressure, 350◦ C temperature,
with a length of 1.8 mm length, a width of 1 mm width, and
1 mm peak to peak distance was the best performing surface.
Figure 4 SEM of (a) 2D and (b) 3D modulated surfaces. © [Elsevier]. Reproduced by permission of Elsevier. Permission to reuse must be obtained from
the rightsholder.
heat transfer engineering
Wang et al. [16] kept the base material of the porous surface as carbon steel and sintered 0.25 mm layer of bronze (Sn
9–11%). The initial bonding was done with polystyrene and
dimethyl benzene. Initially the rust was cleaned by milling and
acid washing, and the bronze was then bonded by spraying it
on the surface. It was then sintered in a hydrogen atmosphere
at about 400◦ C for 30 min to vaporize the bonding material and
then at an elevated temperature of 760◦ C for 2 h. The porosity
obtained by this technique was about 60% and the mean pore
size was 40 μm. The optimal thickness of the porous layer was
about 0.25 mm. They found that the heat transfer through the
porous tube was about 5 to 8 times more than the smooth tube.
The boiling results showed that the heat transfer coefficient and
the maximum heat flux of the samples were increased by 8 to
14 times and 5 to 8 times, respectively. In their study, they found
that higher porosity and thickness gave better results.
Bergles and Chyu [15] used proprietary brazed surfaces to get
a 250% enhancement with water. They investigated the mechanism and concluded that vaporization occurred within the porous
matrix and the vapor bubbles were forced out. Pool boiling testing with water showed a 250% enhancement in pool boiling heat
transfer coefficient. With Freon, the heat transfer coefficient increased by 400–800%.
In the following some additional methods are reported that
can be employed to create porous surfaces for enhanced boiling
heat transfer.
Leong et al. [28] mixed copper powder (63 μm average size)
with Emultex D64 (water based binder) to create a paste, which
consisted of 38% binder and 62% copper. It was then compacted under a pressure of 1962 kPa. This compacted specimen
was heated at 140◦ C for 24 h to remove water, and 250◦ C for
half an hour to get rid of the latex composition of the binder.
Two different samples were sintered at 800◦ C and 1000◦ C at
a ramp rate of 60◦ C/h. The cycle is shown in Figure 5. There
was a variation in the porosity for both the temperatures. Also,
porosity decreased with the increased sintering time. For higher
sintering temperatures, the pore sizes are in a higher range of
30–100 μm. At 1000◦ C, a higher percentage of shrinkage of
copper is observed.
Zhao et al. [29] performed experiments using copper particles (<75 μm diameter) and carbonate (53–1500 μm diameter)
powder. The powder was mixed (0.15 to 0.5 by volume) with
small quantities of ethanol (1% volume of the powder mixture),
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C. M. PATIL AND S. G. KANDLIKAR
porosity. But when the naphthalene content was increased, the
porosity also increased. Further, the porosity decreased with an
increase in the sintering time for the same proportion. Regarding
the effect of the compaction pressure, they observed that higher
the compaction pressure, the lower was the porosity. Based on
their experiments, the following correlation was derived:
P = 64.24 − 2.59 × 10−2 X1 + 1.59 × 10−2 X2 − 6.13
× 10−3 X3 + 30.57 × 10−2 X4 − 5.6 × 10−5 X1 X2 + 8.8
Downloaded by [Rochester Institute of Technology] at 13:40 15 April 2014
× 10−6 X1 X3 − 7.4 × 10−5 X2 X3 + 1.52 × 10−3 X1 X4
Figure 5 Sintering cycle used by Leong et al. for 1000◦ C, adapted from
reference [28].
which acted as binder, and compacted under a pressure of 200
MPa in a hydraulic press. Figure 6 shows three different paths
studied by them. In route A, very little shrinkage was observed
since the carbonate remained solid after sintering, holding the
powder in its place. The carbonate was dissolved in running water. For route B, some shrinkage was observed during carbonate
removal phase when the temperature was elevated to 950◦ C. In
route C, weak bonds were formed and sometimes the structure
collapsed due to large cavities that were formed, since carbonate
decomposes at such high temperatures during sintering.
Ahmed et al. [30] concluded that the factors affecting porosity are sintering temperature, sintering time, pressure of compaction, and amount, type, and size of the foaming agent, if used.
In their experiments, they used copper powder and naphthalene powder. The powders were mixed in different proportions,
compacted in a uniaxial manner under 75–300 MPa compaction
pressure range to get the desired dimensions. The compacted
material was then fired at a ramp rate of 5◦ C/min. Based on
their experiments; they concluded that increasing the sintering
temperature for the same proportion of the mixture decreases the
Figure 6 Different paths followed to sinter the copper adapted from Zhao et al.
[29].
heat transfer engineering
+ 2.52 × 10−3 X2 X4 + 1.91 × 10−3 X3 X4
(1)
where, X1 is temperature (◦ C), X2 is the sintering time (min),
X3 is the compaction pressure (MPa), and X4 is naphthalene
content (percent).
Sintering is the most common technique to produce a metallic
microporous surface. A solution of metal particle and a carrier
like acetone, ink vehicle, and so on can be prepared and deposited using a different means. The mixture can be heated at
different temperatures for the desired times.
Disadvantages of this process are requirement of a highpressure press for compacting and a high-temperature furnace
with vacuum or inert atmosphere to prevent oxidation. Also, the
powders made of micro-/nanoparticles show high reactivity, and
handling them requires specialized equipment.
Electrodeposition
Electrodeposition is a simple way of generating micro-/
nanostructures, involving simple electrochemical process of ion
reduction at the cathode by passing direct current through the
solution (galvanostatic), or holding the substrate (cathode) at a
potential where reduction occurs (potentiostatic).
Shin and Liu [31] created a bath of different combinations of
CuSO4 (0.2–0.8 M), H2 SO4 (0.1–1.5 M), CH3 COOH (0.03–0.5
M), and HCl (1–50 nM). A high current density of 3 A/cm2
was applied for different times. It was observed that changing
the concentration of sulfuric acid affected pore size and wall
thickness, with a conclusion that the evolved hydrogen bubbles
participated in the electrodeposition process. Larger sizes of the
pores were due to coalescence of the bubbles. Acetic acid acts as
a bubble stabilizer, preventing coalescence and giving a better
control over pore sizes. A very small quantity of hydrochloric
acid was added to control the branch sizes and accelerate the
process. This is a promising technique for creating microporous
surfaces for pool boiling applications.
Albertson [32] in his work showed that applying a high current density gives rise to dendrites and nodules forming the
porous structures. He employed a multistep electrodeposition
process to create the porous network with a bath comprised of
sulfuric acid and copper sulfate. Initially he used a high current
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C. M. PATIL AND S. G. KANDLIKAR
density to generate the nodes and valleys, which was followed
by a lower current density for a long duration so that electrodeposition could take place.
Kim [33] prepared a bath of cupric sulfate (200 g/L), sulfuric acid (70 g/L), and 2% hydrochloric acid (10 mL/L). The
substrate was prewashed in 2% hydrochloric acid and was connected to the working electrode. A magnetic stirrer was used
to maintain uniform distribution of the coating surface. High
current densities of 0.166, 0.25, 0.33, 0.5, 1, and 1.2 A/cm2
were applied for the first 20–30 sec, followed by a uniform
low current density of 0.05 A/cm2 for 80 min. After the process, the substrate was rinsed in distilled water and dried with
compressed air. For lower initial current densities, small cavities were observed. Using a current density of 0.25 A/cm2, it
was seen that mounds were formed, and many cavities were observed at the area where those hills were connected. For current
densities of 0.33 A/cm2 and 0.5 A/cm2, numerous tiny particle
shapes were observed on the surface. For higher current densities, honeycomb-like structures with microporous cavities were
observed. When tested with water, R-123, and FC-72 at atmospheric pressure, the coating with 0.5 A/cm2 performed the best,
showing about 250–700% enhancement in nucleate boiling and
about 50–60% enhancement in the critical heat flux. It was observed that R-123 and FC-72 showed a temperature overshoot,
but water did not display the same. Figure 7 shows the pool
boiling curves of these surfaces with water.
El-Genk and Ali [34] used a solution of 0.8 M CuSO4 and 1.5
M H2 SO4 using the dual current technique. They applied a high
current density of 3 A/cm2 for 15 to 44 sec and then a low current
Figure 7 Pool boiling curve of electrodeposited surfaces with water, adapted
from reference [33]. © [Joo-Han Kim]. Reproduced by permission of Joo-Han
Kim. Permission to reuse must be obtained from the rightsholder.
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891
Figure 8 SEM images of the microstructures of two different samples obtained, showing repeatability. © [Elsevier]. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.
density for a much longer time. In the first few seconds, there was
a concurrent evolution of hydrogen gas along with the deposition
of copper, creating a layer with a much higher porosity. During
the period of lower current density, there was no hydrogen gas
evolution, thus reducing the porosity of the deposition. Open
dish-like structures were observed, as shown in Figure 8. When
testing with PF-5060, they observed that the 171-μm-thick layer
yielded a critical heat flux of 27.8 W/cm2 at a wall superheat
of 2.16 K, and a heat transfer coefficient of 13.5 W/cm2-K at
wall superheat of 2 K as shown in Figure 9. Porosity of this
surface was estimated to be 76.8%. No temperature overshoot
was observed. They also concluded that there is an optimal
value of thickness and porosity, before and after which the curve
drifted toward the right, indicating performance deterioration.
Yao [35] deposited copper nanowires on flat silver-coated silicon, gold, and copper substrates. The substrates were cleaned
with acetone, isopropanol, and deionized water and then dried
at room temperature. Anodized aluminum oxide template was
used to assist the growth of nanowires. Platinum wire gauze was
used as a counterelectrode. A bath was made of 0.1 M CuSO4
without any additives, and 0.9 V was applied for 900 sec for
all three substrates. The samples were then dipped in NaOH
Figure 9 Comparison of boiling of PF-5060 on copper microporous substrate,
adapted from reference [34].
vol. 35 no. 10 2014
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C. M. PATIL AND S. G. KANDLIKAR
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Figure 10 SEM images of (a) AAO template (b) Copper nanowires grown. ©
[The Institution of Engineering and Technology]. Reproduced by permission
of The Institution of Engineering and Technology. Permission to reuse must be
obtained from the rightsholder.
solution to remove the AAO template, and then rinsed with
water and vacuum dried at room temperature. Scanning electron micrographs (SEMs) of these microstructures are shown
in Figure 10. Figure 11 shows pool boiling performance of this
surface with water. A maximum heat flux of 160 W/cm2 was
dissipated at a wall superheat of 11.2 K. The nanowires were
reproducible. After testing they observed that there was no peeling or aging, displaying that the nanowires possess high thermal
and mechanical properties.
Lee et al. [36] created nanoporous surfaces on aluminum
using a two-step process consisting of electropolishing and anodizing. In the electropolishing step, aluminum was the anode
and platinum was cathode. The electrodes were immersed in a
bath comprising of perchloric acid (80 mL), ethanol (900 mL),
and pure water (20 mL) at 0◦ C for 240 sec, while a voltage
of 21 V was applied across the electrodes. The polished and
smooth surface was then cleaned with pure water and stored in
a dry place. In the anodizing process, the cleaned and polished
aluminum was connected to the anode and platinum was used
as a cathode. It was dipped in 0.3 M oxalic acid at 0◦ C temperature and a 40-V voltage was applied for 10 min. This created a
thin oxide layer protecting the surface from application of high
potential. The voltage was then increased to 70 V at the rate of
1 V per second and was kept constant for 2 h. The surface was
then removed from the bath and cleaned with water and acetone.
SEM of this surface is shown in Figure 12. The size of the pores
Figure 11 Pool boiling curve of water on copper surfaces with copper
nanowires, adapted from reference [35].
heat transfer engineering
Figure 12 SEM of nanoporous surface created by Lee et al. © [Elsevier].
Reproduced by permission of Elsevier. Permission to reuse must be obtained
from the rightsholder.
was in the range of 50–70 nm. From their pool boiling curve it
was seen that the dissipated heat flux was around 85 kW/m2 at
a wall superheat of 9 K.
Ahn et al. [37] prepared enhanced surfaces using an anodizing technique. Rectangular zircolay-4 plates (20 mm × 25 mm ×
0.7 mm) were polished by #1200 silicon carbide abrasive, creating a uniform polished surface. This was cleaned in a 1:1
mixture of acetone and methanol in an ultrasonic bath, followed
by rinsing in deionized water and drying completely. The dried
samples were immersed in an anodizing bath containing 0.5%
by weight of hydrofluoric acid solution and anodized at 20 V
DC for 0–600 sec at 10◦ C. The anodized samples were heated
in a furnace at 300◦ C for 6 h to remove any fluoride residue.
In their study they observed that the contact angle of the surface changed, and a reduction in contact angle resulted in an
increase in critical heat flux. At contact angles lower than 10◦ ,
due to increased spreadibility from the micro-/nanostructures,
the critical heat flux was beyond the predicted values by the
available theoretical models [38–40].
Advanced Techniques
Anderson and Mudawar [41] compared three test surfaces of
copper. The first surface was a highly polished surface prepared
by buffing with a 12,000 grit lapping compound. The second
surface was created by using the polished surface as earlier
and roughening it with 600 grit silicon wet/dry sandpaper. The
third surface was prepared by vapor blasting water-based slurry
of 12,000 grit silica particles on the polished surface using compressed air. Vapor blasting created a homogeneous structure
with an effective pore size of 15 μm. When tested with FC-87,
it was observed that the roughened surface shifted the pool boiling curve to the left, indicating a performance improvement.
The dendritic surface prepared by the vapor deposition technique performed better than the polished and sanded surfaces.
A scanning electron micrograph (SEM) of the vapor deposited
surface is shown in Figure 13. All three surfaces had critical
heat fluxes in the range of 19.5–20.5 W/cm2.
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C. M. PATIL AND S. G. KANDLIKAR
Figure 13 SEM of vapor blasted surface created by Anderson and Mudawar.
© [ASME]. Reproduced by permission of ASME. Permission to reuse must be
obtained from the rightsholder.
You et al. [42] used a mixture of fine powder (0.3–3 μm) and
water, placed in a nebulizer, sprayed vertically downward with
compressed air and then dried by mixing clean air with it. The
nominal velocity of flow was 1 m/s. Impinging time and distance
between the jet exit and surface determine the attachment area
and thickness of the surface. The particles were attached to the
substrate by intermolecular attraction or glue. Figure 14 shows
a schematic of their particle deposition system. They tested the
surface on their pool boiling setup with FC-72. A maximum
heat flux of 28.5 W/cm2 was observed at a wall superheat of
15◦ C. The pool boiling curve displayed temperature overshoot
at incipience.
Golobic and Ferjancic [43] created similar surfaces using different proportions of metals and metal oxides and an adhesive
material. Coatings of different thickness were produced by using
a commercial spray paint gun similar to the work already described, and the coating was sprayed on a ribbon heater. In their
investigation, Golobic and Ferjancic observed that increasing
the proportion of metal and metal oxides enhances the critical heat flux. When tested with FC-72, they observed a 130%
enhancement in critical heat flux.
Figure 14 Particle deposition system used by You et al., adapted from reference [42].
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893
Memory et al. [44] fabricated a porous surface by spraying
a coating of binder, metallic powder and brazing powder. This
was then heated to melt the brazing powder, leaving a porous
surface. From their experiments with R-114 and its different
combinations with oil, they concluded that the reentrant cavity
tubes provide largest enhancement due to increased nucleation
sites and better pumping. For R-114 and oil mixture, there is a
drop in performance, since oil tended to clog the pore structure.
You and Chang [18] used copper powder (1–50 μm) with
a binder (Devcon Brushable Ceramic, Omegabond 101 or Devcon Titanium putty) and methylethyl ketone (10 mL) as a
carrier liquid. The mixture was applied on the surface with a
spray gun using compressed air. To cure the coating, the surface
was heated in an oven at 150◦ C for about 1 h. They studied
the effects of the binder based coating on the copper surface
in FC-72 atmosphere. There was a 30% enhancement in heat
transfer coefficient and 100% enhancement in critical heat flux.
It also showed an 80–90% reduction in boiling incipience. This
performance enhancement was due to an increase in the nucleation sites due to the porous surface. The critical heat flux for
the copper sample was attained at a heat flux of 26.8 W/cm2.
Changing the binder had no significant effect on the pool boiling
curve as seen from Figure 15.
Kim [33] designed a technique using nickel powder (75 μm
and 120 μm) mixed with solder paste and a carrier solvent.
Nickel powder and solder were mixed in a ratio of 2 to 1 by
weight, and further thinned by isopropyl alcohol. The mixture
was applied by using a paintbrush. The sample was heated to
100◦ C to vaporize the alcohol, further heated to 250◦ C to melt
the solder paste, and then allowed to cool to the room temperature. The flux was removed with a mixture of acetone and 2%
hydrochloric acid. A coating with 44–150 μm powder surface
(Figure 16) performed best, showing 40–60% enhancement in
critical heat flux when tested with R-123, FC-72, methanol and
water, with a highest critical heat flux of 200 W/cm2 at wall
superheat of about 17◦ C with water.
Mori and Okuyama [45] studied a commercially available
honeycomb plate used for purifying gases in exhaust of the
internal combustion engine as shown in Figure 17. They maintained the wall thickness at 0.4 mm, the escape channel width
1.3 mm, diameter of the plate 30 mm, and the aperture rate 0.55.
These wicks were attached to a boiling surface by pushing them
against the surface of stainless steel wire mesh without thermal
grease. Porosity of the test surface was 24.8% and the average
pore radius was 0.037 μm. The surface was tested with water
(Figure 18), and a critical heat flux of 2.51 MW/m2 was obtained
at wall superheat of 50◦ C.
Tang et al. [46] devised a two-step procedure to create a
porous surface. In the first step, the copper substrate was cleaned,
degreased, and dipped in a flux solution (180g/L ZnCl2 , 120 g/L
NH4 Cl) in that order, and was further dipped in a zinc bath at
540◦ C for about 2 min. The specimen was then cooled in running
water after removal from the bath. A copper–zinc layer was
formed on the surface of the specimen. In the second step of this
process, the removal of zinc was accomplished by immersing
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Figure 17 Honeycomb plate dimensions. © [Elsevier]. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.
Figure 15 Pool boiling curve with FC 72 and different binders, adapted from
reference [18]. © [Elsevier]. Reproduced by permission of Elsevier. Permission
to reuse must be obtained from the rightsholder.
the specimen in 5% by weight of sodium hydroxide solution for
48 h at 25◦ C, and further drying in nitrogen gas. SEM of this
surface is shown in Figure 19. About 20–200 nm size pores were
observed after this experiment. Measuring from 20 different
locations on the surface, an average pore size of 115 nm was
obtained. Porosity was estimated to be between 33 and 44%. The
Figure 16 Microscopic image of −100+325 mesh soldered copper powder.
©[Joo-Han Kim]. Reproduced by permission of Joo-Han Kim. Permission to
reuse must be obtained from the rightsholder.
heat transfer engineering
surface showed about 66.33% reduction in the wall superheat at
a heat flux of 20 kW/m2.
Im et al. [47] employed a chemical bath deposition technique to deposit flowerlike CuO microstructures on flat and
microgrooved copper substrates. A solution consisting of 1.5 M
NaOH and 0.1 M (NH4 )2 S2 O8 was prepared, and the copper
substrate was immersed in this bath for an hour and was cleaned
with deionized water and then dried with air after removal. The
thickness of the deposit was about 3–4 μm. SEMs of the flowerlike microstructures are shown in Figure 20. The surface showed
an increase in critical heat flux by 58% for the flat substrate, and
30% for the microgrooved substrate when tested with PF-5060
as shown in Figure 21.
Yamamura et al. [48] created porous copper surfaces by
unidirectional solidification under pressurized hydrogen atmosphere. Copper that was 99.9% pure was induction melted in a
furnace and a mixture of hydrogen and argon gas was introduced
in the furnace. Partial pressures of the gases were controlled in
the range of 0–1 MPa and held at 1250◦ C for 30 min, so that
hydrogen gas was dissolved in the molten metal. This was then
poured in a mold cooled by water from the lower side. These
surfaces have not yet been studied for pool boiling.
Figure 18 Pool boiling curve of surfaces developed by Mori and Okuyama at
different heights, adapted from reference [45].
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Figure 19 SEM Image of the de-alloyed surface. © [Elsevier]. Reproduced
by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.
Forrest et al. [49] prepared a thin film coatings using a technique designed by Bravo et al. [50] on nickel wires. The process
involved immersion of the surface in a positively charged solution for 10 min, followed by rinsing three times in deionized water. This was further immersed in a negatively charged solution
for 10 min, followed by rinsing three times. This process was
repeated multiple times to prepare a multilayered thin coating
for enhancing pool boiling heat transfer. The first solution was
prepared using sodium polystyrene sulfonate (70,000 g/mole)
and polyallylamine hydrochloride (70,000 g/mole) and titrated
to a pH of 4.00. The second solution was prepared using similar chemicals, but titrated to a pH of 9.00. Silica particles of
sizes 24 nm and 50 nm were added to the solution (0.03%
by weight). The solutions were titrated by adding either 1 M
aqueous hydrochloric acid or 1 M aqueous sodium hydroxide;
all the aqueous solutions were prepared to 10 mM concentrations. The samples were allowed to dry overnight. Figure 22
illustrates the process followed.
Superhydrophillic surfaces were created using the samples prepared by the already-described procedure and calcinating them at 550◦ C for 4 h in a furnace. To prepare a hydrophobic surface, the substrate after calcination was placed
in polytetrafluoroethylene (PTFE) container with an open vial
of 1H,1H,2H,2H-perfluorodecyltriethoxysaline (fluorosaline)
at 140◦ C for 30 min. In these experiments, it was observed
that there was a significant improvement in critical heat flux
on superhydrophilic surface, indicating that the nanopore structures have a role to play in enhancing critical heat flux. The
40-layered calcinated nickel wire performed the best with a
Figure 21 Pool boiling curve with PF-5060 of enhanced structured, adapted
from Im et al. [47].
critical heat flux of 1,583 kW/m2 at a wall superheat of 15◦ C.
Figure 23 shows the pool boiling curve of different samples
prepared by this technique.
Wu et al. [51] deposited TiO2 (and SiO2 ) layers on copper
substrate. The copper surface was first given a mirror finish using
lapping operation, and further cleaned using acetone and then
1% HCl, followed by rinsing in distilled water. One single drop
(0.01 mL) of 1% TiO2 (or SiO2 )–ethanol solution was placed
onto the copper surface and spread across the area. This was
heated to 200◦ C, forcing ethanol to evaporate, leaving behind
a 1-μm-thick coating with microstructures seen in Figure 24.
When tested for pool boiling performance with water and FC-72,
this TiO2 coating enhanced critical heat flux (CHF) by 50.4%
and 38.2% respectively.
Vemuri and Kim [52] created 75-μm-thick nanoporous surfaces using aluminum oxide particles. This porous layer was
glued to an aluminum sheet using a thermal epoxy. The manufacturing technique (not described by the authors) created the
nanoporous surface with pore sizes of 25–250 μm. From their
pool boiling results, they concluded that there was a 30% reduction in incipient superheat as compared to an uncoated aluminum
oxide surface when tested with FC-72. This was primarily due
to increase in the number of nucleation sites and enhancement
in vapor/gas entrapment volume.
Enhanced Surfaces on Silicon Substrates
Figure 20 SEM of flowerlike microstructures. © [Taylor and Francis]. Reproduced by permission of Taylor and Francis. Permission to reuse must be
obtained from the rightsholder.
heat transfer engineering
Yao et al. [53] etched nanowires on microchannels on silicon
substrate. A silicon wafer was first cleaned with piranha solution
containing H2 SO4 , H2 O2 , and HF to clean the substrate from
oxides and organic residues, and then rinsed with deionized
water. The wafer was dehydrated and a 6-μm-thick photoresist
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Figure 22 Illustration of process employed by Forrest et al. © [Elsevier]. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the
rightsholder.
layer was spin coated and cured in the desired pattern using
photolithography. The sample was anisotropically etched using
the plasma etching technique and the remainder photoresist was
removed. After the cleaning procedure, it was immediately immersed in an etching solution containing 4.8 M HF and 0.02 M
AgNO3 for different times, creating vertical nanowires on the
surface displayed in Figure 25.
To create nanowires on sidewalls of the microchannels,
H2 O2 was introduced in the etching solution containing HF
and AgNO3 . Figure 26 shows pool boiling performance of
these surfaces with water. Yao et al. observed that there was
a 42% increase in heat transfer coefficient on microchannels
with nanowires on the sidewalls as compared to the ones with
nanowires only on the base. The microchannels with nanowires
on all sides yielded a heat flux of 165 W/cm2 at wall superheat of 24 K, resulting in 400% improvement over plain silicon
substrate.
Coso et al. [54] created vertical rectangular fin array patterns
on a silicon substrate using plasma etching technique as shown
in Figure 27. The substrate was covered with a photoresist and
the pattern was defined using photolithography. Plasma etching
was employed to create trenches leaving behind the fin array. The
substrate was dipped in a photoresist stripper and then cleaned
with piranha solution and deionized water. A thin layer of silicon
dioxide was deposited using chemical vapor deposition. This
was done to improve wettability of the substrate. In their work
they observed that a maximum of 277 W/cm2 heat flux was
dissipated by the enhanced surfaces with heater area of 1 cm2
and 733.1 ± 103.4 W/cm2 when heated on a small area of
0.0625 cm2.
Wei and Honda [55] studied effects of different sizes of fins
similar to the Yao et al. [53] work. They created these fins
by the dry etching process (Figure 28). The square fins with
different heights and a pitch of twice the fin thickness were
fabricated. In their experiments, they observed that the fins with
higher length and height performed the best under atmospheric
conditions with FC-72 as working fluid. The highest heat flux
they achieved was 84.5 W/cm2 at a wall superheat of 20 K.
Chu et al. [56] fabricated micro pillar arrays on undoped silicon using deep reactive ion etching. A 300-nm-thick thermal
Figure 23 Pool boiling curve for surfaces developed by Forrest et al., adapted
from reference [49].
Figure 24 SEM of TiO2 coated surface. © [Elsevier]. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.
heat transfer engineering
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Figure 25 Silicon microchannel substrate with nanowires. © [IOP]. Reproduced by permission of IOP. Permission to reuse must be obtained from the
rightsholder.
oxide layer was developed to improve the wettability of the surface. When tested with water, they obtained a highest heat flux
of 207.9 ± 9.9 W/cm2 at a wall superheat of 39.3 ± 1.8 K. They
observed that the fins with lower diameter, lower spacing, and
larger heights performed the best when tested for pool boiling
heat transfer performance.
Jung and Kwak [57] prepared a surface using anodizing
technique. The silicon surface was prepared by anodizing in
dimethylforamide (DMF)-based electrolyte (DMF:HF:H2 O =
92:4:4) and hydrofluoro acid-based solution (HF:H2 O:C2 H6 O =
20:20:40) for 25 min under a direct current density of
20 mA/cm2. The AFM images are shown in Figure 29. When
tested with FC-72, it reached critical heat flux at 23.32 W/cm2.
Jo et al. [58] created hydrophobic and hydrophilic coatings
for silicon substrates without using microstructures capable of
trapping water. They prepared hydrophilic surfaces using oxidation procedure in a furnace, leaving behind a 5000 Å thick
SiO2 layer. The authors did not mention specific parameters
for preparation of this surface. For hydrophobic surface, they
used a special Teflon material. The oxidized substrate was spin
coated with photoresist, heated at 90◦ C for 90 sec, and then patterned by exposing to ultraviolet (UV) radiation. A mixture of
FC-40 and AF1600 was spin coated on the developed samples,
subjected later to an evaporation procedure, and finally cleaned
with acetone and methanol. In their work they observed that the
hydrophobic surface provided better heat transfer performance
than a hydrophilic surface, but had a very low critical heat flux.
Figure 26 Pool boiling curves of surfaces developed by Yao et al. with water
as working fluid. Adapted from reference [53].
heat transfer engineering
Carbon Nanotube Coated Surfaces
Recent studies have shown that carbon nanotubes (CNT) have
displayed high thermal conductivities of 3000–5000 W/mK
[59, 60]. Thus, CNT are good candidates to create nanoporous
surfaces to enhance heat transfer owing to its thermal
properties.
Ujereh et al. [61] used different arrays of carbon nanotubes to
create nanoporous surfaces on copper and silicon substrates for
enhancement of heat transfer. A dendrimer, which is a branched
polymer with small voids, containing iron particles in the voids
was developed as catalyst, enabling low-temperature synthesis.
A small amount of 0.42 g of polyamidoamine with amine peripheral groups was dissolved in 20 mL water. Another solution
of 0.5 g FeCl3 ·6H2 O was made with 20 mL water, and the solutions were stirred vigorously together for 2 h. A 30- to 100-nm
layer of titanium was deposited on the top surface of the clean
substrate using an electron beam evaporator. The substrate was
dipped in a catalyst for different durations, and the dendrimer
was removed by mild calcination at 550◦ C for 30 min, leaving
behind nonagglomerated catalyst. The carbon nanotubes were
grown using plasma enhanced chemical vapor deposition technique at a temperature of 900◦ C under a methane gas flow rate of
5–10 sccm and a hydrogen gas flow rate of 50 sccm for 20 min.
Flat, grid-structured, and island-patterned surfaces were coated
with nanotubes on silicon substrates, and flat and microstud
surfaces on copper substrates. Figures 30a, 30b, and 30c show
different configurations used for carbon nanotube depositions.
On the silicon substrates, a fully coated low-density surface
performed better compared to other configurations with FC-72,
having a critical heat flux of 18.1 W/cm2 at a wall superheat
of 7.9◦ C. For copper, the carbon nanotube-coated microstuds
performed better at low wall superheats, resulting in a critical
heat flux of 26 W/cm2 at a wall superheat of 10◦ C, but the critical heat flux of the uncoated studs was 32.3 W/cm2 at a wall
superheat of 39◦ C.
Figure 27 SEM images of the pin fins periodic microchannels. © [ASME].
Reproduced by permission of ASME. Permission to reuse must be obtained
from the rightsholder.
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Figure 28 SEM of chips with square fin dimensions (a) 50μ m × 60 μm, (b) 50 μm × 200 μm, and (c) 50 μm × 270 μm. © [Elsevier]. Reproduced by
permission of Elsevier. Permission to reuse must be obtained from the rightsholder.
Figure 29 AFM images of surface created by Jung and Kwak. © [Elsevier].
Reproduced by permission of Elsevier. Permission to reuse must be obtained
from the rightsholder.
Weibel et al. [62] used a similar technique to deposit carbon nanotubes over sintered copper surfaces. Three layers of
60-nm-thick Ti, 10-nm-thick Al, and 3-nm-thick Fe were deposited directly on the sintered copper samples using an electron beam deposition system. Fe provides sites for growth of
carbon nanowires. The temperature of the system was ramped
up to 900◦ C, and nitrogen gas at 10 torr was passed through
the system. Flow rates were set to 50 sccm for H2 and 10 sccm
for CH4 . They found that carbon nanotube-coated surfaces reduced the wall superheat by almost 72% when tested with water.
For a patterned surface with the carbon nanotube coating, there
was a 30% reduction in supearheat temperatures at high heat
fluxes.
McHale et al. [63] deposited carbon nanotubes on porous
sintered copper surfaces and tested the performance for pool
boiling. Copper surfaces were first degreased in acetone and
methanol baths and cleaned in a standard piranha solution for
5 min to remove inorganic materials. Graded 140–170 mesh
copper spherical copper particles were used to prepare a 90to 106-μm-thick film. These particles were poured in a ceramic mold over the copper substrate and sintered at 950◦ C
in an environment of nitrogen and hydrogen gas mixtures for
60 min. The furnace cycle consisted of 30 min room temperature
purge, ramping to 950◦ C in 30 min, and then cooling to 50◦ C in
150 min. The porosity of the sintered layer was 65%. Based on
the work by Powell [64], McHale et al. [63] grew multiwalled
carbon nanotubes by the microwave–plasma chemical vapor deposition process under partial vacuum at 900◦ C for 10 min on a
catalyst consisting of titanium, iron, and aluminum. To achieve
coating with carbon nanotubes, the surfaces were coated with a
100-nm-thin Ti layer, followed by application of catalyst (ferric chloric hexahydrate [FeCl3 ·6H2 O]) using airbrushing technique. The substrates were then heated at 150◦ C to remove
volatile components and water, followed by microwave–plasma
chemical vapor deposition and metallization resulting in growth
of carbon nanotubes. When tested with HFE-7300 and deionized water, they achieved heat fluxes of 27 W/cm2 and 55 W/cm2
(not CHF) due to limitations of cooling systems in their test
setup.
Figure 30 (a) Copper substrate with microstuds with carbon nanotube deposits. (b) Silicon substrate with “island pattern.” (c) Silicon substrate with “grid”
pattern. Adapted from reference [61]. © [Elsevier]. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.
heat transfer engineering
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CONCLUSIONS AND FUTURE DIRECTIONS
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A number of different techniques to create porous surfaces
for enhanced pool boiling heat transfer with different materials
have been reviewed in this paper. A brief outline of the review
is presented here.
Figure 31 Enhanced surface used by Launay et al. © [Elsevier]. Reproduced
by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.
Launay et al. [65] deposited carbon nanotubes on enhanced
microstructures prepared by micromachining, as shown in Figure 31. Carbon nanotubes were grown using a technique developed by Wei et al. [66]. They first covered the silicon with a
100-nm-thick silica layer by a thermal oxidation process. The
photoresist was then spin coated and was photo cured.
The exposed silica was etched out to create the required pattern for deposition of carbon nanotubes. The carbon nanotubes
were grown in a furnace using the chemical vapor deposition
(CVD) technique. Initially, pressure of the system was reduced
to 10–3 torr vacuum, backfilled with flowing argon gas to about
100 mtorr, and gradually heated to 800◦ C. A mixture of xylene
[C8 H10 ] and ferrocene [Fe(C5 H5 )2 ] was preheated at 150◦ C and
sublimed in the CVD furnace. This lead to deposition of multiwalled carbon nanotubes on the SiO2 surfaces, whereas no
nanotube growth was seen on the silicon surface. They developed a pin fin array on the boiling surface as shown in Figure 32.
When tested with PF-5060, the maximum heat flux obtained was
27 W/cm2 at wall superheat of about 40◦ C, while with deionized
water, a maximum heat flux of 130 W/cm2 at wall temperature
of 75◦ C was recorded.
Figure 32 SEM of pin fin array nanotubes. © [Elsevier]. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.
heat transfer engineering
1. Porous surfaces play a significant role in enhancing pool
boiling heat transfer.
2. Sintering is one of the easiest techniques to manufacture a
porous surface when using metals, with the only limitation
being the expensive equipment required. An enhanced surface can be sintered in a variety of ways, including use of
powder particles, metal mesh, and so on. Different sizes of
copper particles sintered together yielded superior results.
3. Electrodeposition is another alternative to create porous surfaces, using hydrogen bubbles evolved as dynamic templates.
Multistage electrodeposition was employed to gain a better control over porosity of the deposit and improve bonding with the base substrate. This technique produced better results than the conventional electroplating techniques.
Nanowires were also created using this technique. Variety of
surfaces were created using an anodizing technique.
4. Various nonconventional techniques like soldering of particles, binding the particles with binder, spray painting the a
coating, alloy–dealloying, and unidirectional solidification
under inert gas were used. Thin films were created using
oxidation, layer-by-layer deposition, and a TiO2 film using
TiO2 –ethanol solution.
5. Similar enhanced surfaces were studied on silicon substrate
with semiconductor cooling as a potential application. Various surfaces were created using different forms of etching
techniques and tested for pool boiling heat transfer with different fluids (non-dielectric).
6. Carbon nanotubes have excellent heat transfer properties,
and were deposited on different flat and microscale enhanced
surfaces. Using different forms of chemical vapor deposition
is the most common technique used to deposit carbon nanotubes and its metal composites on the base surface.
7. Critical heat flux for all surfaces were not clearly stated.
Also, most of the authors did not give data on repeatibility
and longevity of the surfaces.
There is a need to develop a process that will give precise
control over size of pores, their uniformity and distribution,
and porosity. Also, the size of particles plays a critical role and
needs to be optimized. Thickness of the enhancement surface
needs to be optimal in order to reduce the thermal resistance,
elevate the maximum heat flux, and reduce wall superheat. Also,
the surface should offer more nucleation sites and release the
vapors as rapidly as possible. The surfaces should be tested for
repeatability and reliability in the long run.
vol. 35 no. 10 2014
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C. M. PATIL AND S. G. KANDLIKAR
ACKNOWLEDGMENTS
This work was done at the Thermal Analysis, Microfluidics
and Fuel Cell Laboratory at Rochester Institute of Technology.
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Chinmay M. Patil is currently pursuing his master’s
degree in mechanical engineering in the Kate Gleason College of Engineering at Rochester Institute of
Technology (RIT). He comes from Pune, India, and
pursued his bachelor’s degree in mechanical engineering at University of Pune. He is currently engaged
in creating micro-/nanostructures for enhancement in
pool boiling heat transfer.
Satish G. Kandlikar is the Gleason Professor of Mechanical Engineering at RIT. He received his Ph.D.
degree from the Indian Institute of Technology in
Bombay in 1975 and was a faculty member there
before coming to RIT in 1980. He has worked extensively in the area of flow boiling heat transfer and
CHF phenomena at microscale, single-phase flow in
microchannels, high heat flux chip cooling, and water management in PEM fuel cells. He has published
more than 200 journal and conference papers. He is
a fellow of the ASME and a former associate editor of ASME Journal of Heat
Transfer. He received RIT’s Eisenhart Outstanding Teaching Award in 1997
and Trustees Outstanding Scholarship Award in 2006. He received the 2008
Rochester Engineer of the Year award from the Rochester Engineering Society.
He is the recipient of the 2012 ASME Heat Transfer Memorial Award. Currently he is working on Department of Energy- and GM-sponsored projects on
fuel cell water management under freezing conditions, and a National Science
Foundation-sponsored project on developing nanostructures for enhanced pool
and flow boiling.
vol. 35 no. 10 2014