This document discusses a study that tested the performance of heat exchangers constructed from fins with anisotropic micro-grooved surfaces. Two prototype heat exchangers were constructed with fins that had microchannels etched into them to create anisotropic wettability. Testing was conducted to measure contact angles, critical sliding angles, water retention during dip testing, and changes in air-side pressure drop between dry and wet conditions. The results showed that the micro-grooved fin surfaces, in combination with an alkyl silane coating, could reduce water retention by over 27% and decrease the wet air-side pressure drop by 30-36% compared to a baseline heat exchanger. The document provides context on related research and describes the experimental methods

1. Condensate drainage performance of a plain fin-and-tube
heat exchanger constructed from anisotropic
micro-grooved fins
A.D. Sommers a,
*, R. Yu b
, N.C. Okamoto c
, K. Upadhyayula c
a
Department of Mechanical and Manufacturing Engineering, Miami University, 650 East High Street, Oxford, OH 45056, USA
b
Department of Chemical and Paper Engineering, Miami University, 650 East High Street, Oxford, OH 45056, USA
c
Department of Mechanical and Aerospace Engineering, San Jose´ State University, One Washington Square, San Jose´, CA 95192, USA
a r t i c l e i n f o
Article history:
Received 27 September 2011
Received in revised form
10 April 2012
Accepted 10 May 2012
Available online 15 May 2012
Keywords:
Air conditioning
Condensate
Anisotropy
Dehumidification
Micro-channel
Pressure drop
a b s t r a c t
In this research, the feasibility of using non-homogeneous, chemically-modified aluminum
surfaces to more effectively manage condensate formation on a round-tube, plain-fin heat
exchanger is explored. Surfaces having an anisotropic, patterned wettability may be
preferred in HVAC&R applications since droplet motion can be restricted to one direction
(i.e. downward with gravity) thereby mitigating condensate carryover into the occupied
space. In this work, micro-grooved aluminum surfaces were created and then used to
construct two prototype heat exchangers for testing. The results have shown that the
micro-grooved structure of the fin surface in combination with an alkyl silane coating can
reduce water retention by more than 27 percent. The new fin surface design was also
shown to decrease the air-side core pressure drop of the prototype heat exchangers during
wet operation by an average of 30e36 percent over the baseline heat exchanger, with the
actual percentage strongly dependent on the flow rate.
ª 2012 Elsevier Ltd and IIR. All rights reserved.
Performance en termes d’e´vacuation du condensat d’un
e´changeur de chaleur a` tubes ailete´s compose´ d’ailettes
anisotropiques a` microrainures
Mots cle´s : Conditionnement d’air ; Condensat ; Anisotropie ; De´shumidification ; Micro-canal ; Chute de pression
* Corresponding author. Tel.: þ1 513 529 0718; fax: þ1 513 529 0717.
E-mail address: sommerad@muohio.edu (A.D. Sommers).
www.iifiir.org
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ijrefrig
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 7 6 6 e1 7 7 8
0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved.
doi:10.1016/j.ijrefrig.2012.05.006

2. 1. Introduction
Plain fin-and-tube heat exchangers are widely used in
a variety of applications in the air-conditioning, refrigeration,
and process industries. In these systems, when the cold
surface of the heat exchanger is exposed to warm humid air,
condensation forms on the heat transfer surface. While
condensation can be desirable to dehumidify the air, water
retention is problematic. Water retention increases the air-
side core pressure drop, has the potential to reduce the air-
side heat transfer coefficient, leads to corrosion, and
provides a moist environment for biological activity.
Furthermore, controlling the direction of condensate move-
ment on the surface is as important as the physical removal of
those droplets. Homogeneously modified surfaces permit
(even facilitate) condensate blow-off into the occupied space
which is undesirable. Thus, new and innovative approaches
are needed to better manage condensate formation on heat
exchangers and further reduce the core volume, material cost,
and flow noise. Because the air-side thermal resistance is
dominant in these applications, even modest enhancements
in the air-side thermal-hydraulic performance could lead to
smaller, lighter, quieter, and more energy-efficient systems.
The purpose of this research was to provide a critical assess-
ment of the use of anisotropic, micro-grooved fins to improve
the thermal-hydraulic performance of this particular heat
exchanger geometry and to mitigate the problems associated
with water retention on the coil. Specific objectives of this
study included measuring the dynamic contact angles and
critical inclination angles of representative fins and evalu-
ating the dip testing performance and air-side pressure drop
of full-scale prototype heat exchangers under both dry and
wet operating conditions.
Although extensive research has been reported on using
topography and chemistry to alter surface wettability in other
applications, there has not yet been a comprehensive study on
the use of these techniques in conventional HVAC&R systems
where the energy impact could potentially be quite large. In
fact, very few papers have been published on the
hydrophobizing of metal substrates, and studies concerned
with the manufacture of hydrophobic aluminum surfaces are
especially rare in the literature. Shibuichi et al. (1998) and Qian
and Shen (2005) describe wet chemistry techniques for
producing hydrophobic aluminum surfaces. However, these
methods produce many cracks and small rectangular pits in
the surface which could be problematic in applications were
thermal cycling is necessary. Bayiati et al. (2004), Ji et al. (2002)
and Elkin et al. (1999) focused on the selective deposition of
fluorocarbon (FC) films on metal surfaces, in an effort to
control surface wettability. Hydrophobic surface properties
were achieved; however, the durability of the coating on the
surfaces was a problem. In a paper with deicing applications,
Somlo and Gupta (2001) prepared a weakly hydrophobic 6061
aluminum alloy surface through a dipping process involving
dimethyl-n-octadecilcholoro-silane (DMOCS) and studied the
tensile strength of the ice/DMOCS interface. No contact angle
information was provided, and the adherence and longevity of
the DMOCS coating to the aluminum surface was not quan-
tified. Guo et al. (2006) obtained super-hydrophobic aluminum
alloy 2024 surfaces using wet chemical etching followed by
modification of the surface with a cross-linked silicone elas-
tomer, perfluorononane (C9F20), or perfluoropolyether (PFPE).
Water contact angles exceeding 150
were achieved; however,
this study also did not involve detailed surface patterning.
Other germane studies (Wu et al., 2006; Sun et al., 2008;
Kannan and Sivakumar, 2008; Zhu et al., 2006; Baldacchini
et al., 2006) have shown that increased surface roughness
can lead to improved hydrophobicity, but these studies did not
specifically examine the effect of micro-grooved surfaces on
condensate drainage performance. Other studies involving
heat exchanger testing and the effect of surface wettability on
thermal-hydraulic performance should also be noted (Hong
and Webb, 1999; Kim and Kang, 2003; Kim et al., 2002, 2003;
Wang and Chang, 1998; Wang et al., 2002; Mimaki, 1987; Shin
and Ha, 2002). These papers discuss the effect of various
surface treatments on the thermal-hydraulic performance of
fin-and-tube heat exchangers. For example, Wang et al. (2002)
reported that the air-side heat transfer performance of
a hydrophilic coating, plain-fin surface was degraded by up to
Nomenclature
A area
D diameter
Dh hydraulic diameter, 4AminL/Atot
f Fanning friction factor, ((2$DPcore$rair)/G2
)
(Amin/Atot)
G mass velocity, rVmax
L fin length (flow depth)
N number of tubes, fins, etc.
DP pressure drop
ReDh
air-side Reynolds number, VmaxDh/n
t time
T temperature
V velocity
w micro-channel width
Greek symbols:
a critical sliding angle
d micro-channel depth
L micro-channel aspect ratio
n kinematic viscosity
q contact angle
r density
Subscripts:
A advancing
HX heat exchanger
max maximum
min minimum
R receding
tot total
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3. 20% as compared to the uncoated surface for small fin pitches
(i.e. Fp ¼ 1.23 mm). This degradation was attributed to the loss
of condensate droplets on the fin surface which act as
a “roughening mechanism” to help mix the airflow. The air-
side pressure drop was also observed to be lower for the
hydrophilic coating surface by as much as 40% for the plain fin
geometry.
Although the technical literature is replete with papers
describing surface modification techniques, few studies have
focused on the application of these techniques to metallic
surfaces or the testing of full-scale heat exchangers possess-
ing a modified surface wettability. Thus, this study fills an
important gap in the literature by demonstrating the benefits
and feasibility of these surfaces for application in HVACR
systems. The specific objective of this research study was to
develop a low-cost method of fabricating anisotropic
aluminum fins and then to evaluate the performance of a few
prototype heat exchangers constructed from this fin material
for possible application in HVACR systems. The heat
exchangers studied in this project were constructed from
aluminum fin samples having four different surface wetta-
bilities. Various methods were used to assess the behavior of
water on these novel surfaces including contact angle
measurements, critical inclination angle measurements,
dynamic dip testing, and air-side pressure drop measure-
ments. The measurement of dynamic contact angles and
critical inclination angles were used to assess changes in the
wettability of the fin surface, while the water retention data
collected during dip testing and changes in air-side pressure
drop between dry and wet conditions were used to evaluate
the overall drainage performance of the heat exchangers.
Based on these findings, recommendations were then made
for an improved fin surface design that minimizes water
retention. (Note: Although it was not specifically studied in
this initial phase of the project, the impact of the new fin
surface design on the coil capacity and the longevity of the
silane coating are both currently being examined.)
2. Experimental method
2.1. Fabrication of the anisotropic fin surfaces
A photolithographic method was developed to create the
anisotropic surface pattern on aluminum fin samples for
chemical etching. Prior to the photolithographic process, the
fins were cleaned in an ultrasonic bath of acetone for 15 min
and then dried by nitrogen gas. Afterward, the aluminum
fins were spin-coated with a positive photoresist (S1813) at
2000 rpm for 30 s and baked for 1 min at 110
C to prevent
mask sticking. A standard 1:1 mask aligner was then used to
align a photomask over the aluminum substrate and expose
the photoresist to UV light for 10.1 s using a lamp attached
to a 350 W power supply. The aluminum fins were then
developed for 40 s using a 1:4 solution of CD-26 developer to
water. (Note: CD-26 developer is an alkaline liquid contain-
ing tetra-methyl ammonium hydroxide (TMAH) in concen-
trations exceeding 2.0%.) Finally, the fins were etched using
Transene Aluminum Etchant Type A and then rinsed using
deionized water and measured. The aluminum used for the
fins was Al alloy 1100 due to its high aluminum content
(99.9% pure Al) and thus its ability to be easily etched. It
should be pointed out that wet etching is inherently
isotropic, and as a result the channels created using this
method are rounded and thus do not possess vertical side-
walls. Vertical sidewalls tend to be more effective at
imparting anisotropic wettability to the surface. Thus, it was
expected that these surfaces with rounded sidewalls would
possess reduced anisotropy and reduced hydrophobicity as
compared to surfaces having vertical sidewalls
Fig. 1 e Photo of a representative fin sample prepared for Coil #2 with SEM image showing underlying micro-structure after
wet etching.
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4. manufactured using more costly techniques (i.e. reactive ion
etching). Examining the potential significance of this
difference was one of the objectives of this work. Two
different micro-grooved fin surfaces were producedd(i)
those etched for 25 min with micro-channels
14.88 Æ 0.36 mm wide, 9.56 Æ 1.41 mm deep, and (ii) those
etched for 50 min with micro-channels 49.84 Æ 1.03 mm wide,
15.95 Æ 1.96 mm deep (see Fig. 1). More than 40 measure-
ments were performed in arriving at these values. The fin
surfaces with the 50 mm wide channels had a greater depth
than those with the 15 mm wide channels because the
substrates were etched twice as long (i.e. 50 min vs. 25 min).
This longer etching time may have also resulted in the larger
observed standard deviation in the measured micro-channel
dimensions for this surface.
A round tube, plain fin heat exchanger geometry was
selected for the prototype heat exchanger construction. The
layout for these coils is shown in Fig. 2. The fin spacing was
3.175 mm (or, 8 FPI), and the flow depth was 3.49 cm (or, 1.375
inches). Two different anisotropic heat exchangers were
constructed for testing (Coils #2 and #3) as well as a baseline
coil (Coil #1) and a coil with homogeneously modified fins (Coil
#4) as shown in Table 1. For Coils #2 and #3, the alkyl silane
coating (i.e. dimethyldichlorosilane) was applied by immer-
sion for approximately 1 h at 30 deg C. For Coil #4, the fins were
exposed to a 300 W oxygen plasma for approximately 10 min
in a planar plasma etching system prior to coil assembly. The
oxygen plasma was not intended to etch the surface but rather
make the fin surface more hydrophilic. Note: An oxygen
plasma is used in the semiconductor industry to remove
residual photoresist from a surface during the ashing process.
Thus, it was expected that this surface might be more
hydrophilic than the baseline surface (Yamamoto et al., 1992).
2.2. Contact angle and critical sliding angle
measurements
Dynamic contact angles were measured on representative fin
samples using a Rame´-Hart contact angle goniometer. For
these tests, water droplets were injected onto the surface of
the fin using a 2.0 mL, high-precision micro-syringe. The
advancing and receding angles were then measured by
increasing or decreasing the volume of water droplets injected
on the surface until the maximum or minimum volume was
achieved without a change in the droplet contact area (see
Fig. 3). After performing these measurements, the contact
angle hysteresis which is the difference between the
advancing and receding angles was calculated. Surfaces with
low contact hysteresis generally retain less water than those
with high contact angle hysteresis. The resolution of the
goniometer was less than 1
. The uncertainties (a ¼ 0.05)
associated with these measurements are shown in Table 3.
The critical sliding angle which characterizes when
a droplet of a given volume first begins sliding on an inclined
surface was also measured and analyzed using recorded
images and compared among all fabricated surfaces (see
Fig. 3). A high-resolution CCD camera was used to record
profile images of the droplets from a location parallel to the
base during the process. For these tests, droplet volumes
between 20 mL and 70 mL were considered. Because these
surfaces exhibit an anisotropic wettability, the critical sliding
angle was measured with the micro-channels aligned parallel
to gravity since this orientation is preferred for drainage.
It should be noted that the critical sliding angle and contact
angle hysteresis are important parameters that are frequently
used to assess drainage behavior (i.e. Cannon and King, 2009;
Lu et al., 2007; O¨ ner and McCarthy, 2000; Chen et al., 1999; Jopp
et al., 2004; Fu¨ rstner et al., 2005). For example, Chen et al.
(1999) argued that contact angle hysteresis is more impor-
tant than the maximum recorded contact angle when char-
acterizing surface hydrophobicity and suggested that the term
“ultrahydrophobic” be reserved for surfaces possessing very
low critical sliding angles. The idea was again later supported
by O¨ ner and McCarthy (2000). The critical sliding angle (like
contact angle hysteresis) also represents a dynamic
measurement of surface hydrophobicity. In Fu¨ rstner et al.
(2005), critical sliding angles for three different plant repli-
cate surfaces were presented where the mean value was
calculated from 5 to 12 individual measurements, and in
Cannon and King (2009), the critical sliding angle was reduced
by more than 40
for a flat silicone surface by embossing
micro-structural features into the surface using a microcast
metal master. In all cases, lower critical sliding angles were
equated with increased hydrophobicity.
Fig. 2 e Schematic of the relevant coil geometry.
Table 1 e Description of the fin surfaces used for heat exchanger prototype construction.
Coil #1 Coil #2 Coil #3 Coil #4
Fin Surface
Conditions
Baseline
Surface
Channels: 15 mm width,
9e10 mm depth þ silane coating
Channels: 50 mm width,
15e16 mm depth þ silane coating
Fin surface
exposed to a 300 W
O2 plasma for 10 min
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5. 2.3. Dynamic dip testing measurements
Dynamic dip testing was performed by fully immersing the
heat exchanger prototypes in a tank of room temperature
water and then quickly dropping the water level below the
heat exchanger and recording the mass of the heat exchanger
as shown in Fig. 4. The retention of water on the heat
exchanger was then recorded as a function of time using
a high-precision balance and standard RS232 data recording
software. The tank was positioned using a mechanical jack to
allow for a smooth, consistent, and rapid lowering of the
water level. The tank was 24.7 cm by 34.9 cm by 27.9 cm in size
and was constructed of transparent Plexiglas which permitted
visual inspection of the specimen inside the tank. With this
setup, experiments could be performed with the test spec-
imen tilted from the vertical orientation.
Prior to each dynamic dip test, the heat exchanger was first
suspended over the tank to make sure that the heat exchanger
was level and to check the alignment. After the balance was
zeroed, the tank of water was raised to immerse the specimen,
and the water was agitated to remove the air bubbles from the
air-side heat transfer surfaces. Once this was accomplished,
the reservoir was lowered, and mass readings were recorded
every 0.2 s beginning at the instant when the water level
reached the bottom of the heat exchanger. These dip testing
experiments were conducted five times per test condition
which is recommended to ensure the same dropping velocity
(Liu and Jacobi, 2008; Zhong et al., 2005), and the data was then
averaged to determine the final steady water retention. Dip
testing data for the heat exchangers in both vertical and
horizontal orientations, as well as with and without the silane
coating, were measured.
2.4. Air-side pressure drop measurements
These experiments were performed in an open-circuit wind
tunnel comprised of five major sections: a flow-conditioning
chamber, contraction, test section, flow measurement
system, and diffuser as shown in Fig. 5. The air flow was
drawn from the inlet plenum, through honeycomb flow
straighteners, screens, and a 9:1 area contraction into the test
section for air flow rates between 0.0236 m3
sÀ1
and
0.135 m3
sÀ1
(or, 50 CFM and 285 CFM). These volumetric flow
rates correspond to face velocities of 0.57 m sÀ1
to 3.3 m sÀ1
.
The test section (17.93 cm  23.01 cm  60.96 cm) was con-
structed from Plexiglas and permitted flow measurements
both upstream and downstream of the heat exchanger and
visual access to the specimen. The test section had the same
cross-sectional area as the heat exchangers, resulting in no
bypass flow around the coil. Pressure drop data were collected
for both dry and wet conditions.
For all tests, the inlet air temperatures remained within the
range of 24.9e25.8
C. For the dry tests, relative humidity was
30% Æ 3%. For the wet tests, the heat exchanger was con-
nected to an isothermal bath and chilled by circulating
a water/glycol mixture cooled below the dew point through
the tubes. Steam was generated at the inlet of the wind tunnel
to ensure a consistent humidity level of 100% for all wet tests.
Relative humidity was also measured downstream of the test
section to verify that it remained at 100%. The relative
humidity meter had an uncertainty of Æ2% up to 95%. (A
somewhat increased uncertainty is expected above 95% but
should have no effect on the results.) The difference in relative
humidity between wet and dry tests resulted in a maximum
deviation in the air density of 2.2% among tests, which is
Table 3 e Fin surface contact angle measurement error
data.
Surface Advancing
angle error
Receding
angle error
Coil #1 Baseline
surface
Uncoated Æ2.3
; 2.6% Æ2.0
; 8.1%
Alkyl silane Æ3.9
; 3.7% Æ7.9
; 11.8%
Coil #2 15 mm
Channels
Uncoated Æ2.7
; 4.3% Æ2.2
; 12.0%
Alkyl silane Æ5.0
; 5.0% Æ9.2
; 15.1%
Coil #3 50 mm
Channels
Uncoated Æ1.2
; 1.5% Æ1.2
; 5.4%
Alkyl silane Æ2.2
; 2.1% Æ8.1
; 11.2%
Coil #4 Oxygen
plasma
Oxygen plasma Æ3.1
; 3.3% Æ2.3
; 7.2%
Fig. 3 e Schematic showing how (a) advancing/receding contact angles and (b) critical sliding angles were measured.
Table 2 e Fin surface contact angle data.
Surface Advancing
angle
Receding
angle
Contact
angle
hysteresis
Coil #1 Baseline
surface
Uncoated 87.8
25.2
62.6
Alkyl silane 105.8
67.0
38.8
Coil #2 15 mm
Channels
Uncoated 62.6
18.7
43.9
Alkyl silane 100.6
60.9
39.7
Coil #3 50 mm
Channels
Uncoated 73.7
22.2
51.5
Alkyl silane 104.5
72.6
31.9
Coil #4 Oxygen
plasma
Oxygen plasma 91.9
31.5
60.4
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6. accounted for in the friction factor calculations discussed
later. During each test, the fin surface temperature was also
checked at several downstream locations (near the tube outlet
where temperature is highest) to ensure that the local fin
surface temperature was below the dew point temperature
everywhere and thus ensure that the coil was completely wet.
For these tests, the fin surface temperatures varied by
approximately 2
C from the coldest to warmest location, and
the fin surface temperatures near the outlet ranged from 9.6 to
10.7
C. During the initial setup of the apparatus, fin temper-
atures were measured at several locations to verify that these
fins near the tube exit were the highest temperature fins. Once
steady state was achieved and a uniform rate of condensate
shedding was visually observed, multiple data points were
collected for each flow rate and then averaged. These data
were then used to calculate and compare changes in air-side
pressure drop associated with wet operation.
The core pressure drop across the heat exchanger was
measured using four pressure tapsdtwo upstream and two
downstream located in the center of the top and side panels of
the test section. A high-precision electronic manometer with
an uncertainty of Æ0.0003 inches of water including both the
random and bias error was used to measure the pressure drop
across the heat exchanger. Random error was calculated using
two standard deviations of 30 pressure drop readings recorded
at an average flow rate. The final calculated uncertainty in DP
ranged from a maximum of 2.3% for the dry test at the lowest
air flow rate to a minimum of 0.2% for the wet test at the
highest air flow rate (95% confidence level). The repeatability
of the tests if a heat exchanger was removed and reinserted
was approximately 7%. This was due to small variations in the
placement of the heat exchangers. In this case, however, the
pressure drop for both wet and dry tests went up or down by
the same percentage. In no case was a heat exchanger repo-
sitioned between wet and dry tests.
The flow rate was determined by measuring the pressure
drop across ASME standard orifice plates. The uncertainty in
the calculated flow rate ranged from 3.6% at the lowest CFM to
2.1% at the highest. These uncertainties were found using the
standard method of Kline and McClintock (1953). The velocity
profiles in the wind tunnel test section were also checked
using a Pitot static tube and shown to be flat to within 2% for
the highest test velocity.
3. Results and discussion
3.1. Dynamic contact angle data
Advancing and receding contact angles on four different kinds
of aluminum surfaces with and without the alkyl silane
coating were measured. For the uncoated case, these
measured data show that the grooved surfaces (Coils #2 and
#3) have lower contact angle hysteresis than the baseline
surface (see Table 2). That is, the presence of the micro-
grooves on the fin surface tends to reduce contact angle
hysteresis. Moreover, surfaces having the alkyl silane coating
exhibit more hydrophobicity and lower contact angle hyster-
esis than those without the coating. Contact angle hysteresis
is the absolute difference between the advancing and receding
Fig. 4 e Apparatus used for performing the dip testing experiments.
Fig. 5 e Wind tunnel used to perform air-side pressure drop experiments.
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7. contact angles, and its magnitude is often used as a gauge of
hydrophobicity (O¨ ner and McCarthy, 2000; Chen et al., 1999;
Jopp et al., 2004; Fu¨ rstner et al., 2005). In these papers, small
contact angle hysteresis is linked to a reduced affinity for
water retention. Thus, these data suggest that the new micro-
channel fin surface design should exhibit an improved ability
to shed water droplets from the surface. For the surface
treated with the oxygen plasma, very little difference in the
contact angle hysteresis was observed with respect to the
baseline surface (i.e. 60.5
vs. 62.6
). It should also be noted
that the surface with the highest contact angle hysteresis was
the baseline surface. The calculated errors associated with
these measurements are shown below in Table 3. The abso-
lute error of the receding contact angle measurements was
comparable to those calculated for the advancing contact
angle. However, the relative error associated with the receding
contact angles was larger due to the small angles being
measured.
3.2. Critical sliding angle data
By themselves, large contact angles do not ensure that
a surface easily sheds water. Therefore, the critical sliding
angle is a useful criterion when evaluating the water drainage
behavior of surfaces. In Fig. 6, the measured sliding angles are
shown for the four prepared surfaces. As expected, the critical
sliding angle decreases as the water droplet volume increases.
For a given water droplet volume, smaller sliding angles imply
better surface drainage characteristics. Fig. 6 shows that for
water droplets smaller than 60 mL, the new fin surface designs
have smaller critical sliding angles than the baseline surface.
This means that the new fin surface designs could lead to
improved drainage behavior of the surface. Overall, the fin
material used to make Coil #2 (15 mm wide channels) and Coil
#4 (oxygen plasma) performed the best. For water droplet
volumes larger than 60 mL, the differences between surfaces
were insignificant. Here, the gravitational force was compa-
rable to the surface tension retaining force so the benefit of the
channels was not as fully realized. Thus, perhaps of greatest
importance, these data suggest that the most significant
enhancement can be realized for small droplets such as those
formed during condensation conditions. There are generally
two kinds of condensation that manifest in HVACR sys-
temsddropwise condensation and filmwise condensation. In
many systems, dropwise condensation is more common than
filmwise condensation (Hampson, 1951; Helmer, 1974;
McQuiston, 1976). In this mode, the condensate forms on the
surface as small droplets. As the droplets grow, they coalesce
with neighboring droplets and eventually gain enough mass
to drain from the surface. Of particular relevance then are fin
surface designs that target small droplets. The data shown in
Fig. 6 suggest that the new surfaces perform well even at fairly
small water droplet volumes.
3.3. Dynamic dip testing data
Data for the four full-scale heat exchangers are presented in
Figs. 7 and 8. As expected, the overall water retention
decreased sharply in the first 10 s after which time water
drainage was observed to be more gradual. At 1500 s, changes
in the overall water retention on the coil were very small
(i.e. 0.15 g per min). Thus, steady state water retention was
declared after 25 min (or, 1500 s) of drainage.
Tests were performed in the vertical orientation (i.e. stan-
dard drainage path) with and without the alkyl silane coating.
In addition, dip testing experiments were performed for the
heat exchangers in a horizontal orientation with surface
coating. These tests were designed to evaluate the ability of
these fins to inhibit water droplet migration across the fin in
the usual direction of air flow. Thus, these tests were
comparative in nature (with respect to the baseline coil) and
served to assess the potential of the surface at minimizing
condensate “blow-off” in application. Therefore, for the
vertical orientation tests, low water retention is regarded as
Fig. 6 e Critical sliding angle measurements for the
prototype surfacesdsmaller angles were observed for the
enhanced surfaces.
Table 4 e Dynamic dip testing measurement error data.
Untreated vertical Treated vertical Treated horizontal Treated horizontal 10
Coil #1 37.7 Æ 2.6 g, 6.8% e e e
Coil #2 39.8 Æ 1.2 g, 3.0% 23.2 Æ 2.2 g, 9.6% 43.6 Æ 1.5 g, 3.5% 30.2 Æ 1.8 g, 6.0%
Coil #3 39.0 Æ 2.2 g, 5.6% 27.2 Æ 1.1 g, 4.0% 46.8 Æ 2.6 g, 5.5% 38.3 Æ 3.6 g, 9.4%
Coil #4 e 43.7 Æ 1.6 g, 3.3% 53.2 Æ 1.2 g, 2.3% 40.4 Æ 3.5 g, 8.6%
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8. good performance; whereas for the horizontal tests, water
retention comparable to the baseline coil is regarded as a good
indicator of performance.
In the case of micro-channels without the coating, Fig. 7b
shows that Coil #2 and Coil #3 actually retained more water at
steady state than the baseline coil (i.e. Coil #1). That is
because, in the absence of the alkyl silane surface coating, the
grooved micro-structure increases the hydrophilicity of the
surface. It should be pointed out that in some HVACR
applications, a super-hydrophilic surface is desired in which
case these surfaces might be preferred since there is no reli-
ance on a chemical coating. However, for the fin surfaces with
micro-channels and the alkyl silane coating, Fig. 7d shows
that Coil #2 and Coil #3 hold less water than the baseline
surface (with no coating). In this case, the grooved micro-
structure increases the hydrophobicity of the surface thus
facilitating water droplet shedding. The initial dynamic
drainage characteristics were also a little different among the
four heat exchangers. Coils #2 and #3 were observed to drain
the water more rapidly than both the baseline heat exchanger
and Coil #4 as shown in Fig. 7c. These data also support the
assertion that condensate retention per unit area is generally
inversely proportional to the receding contact angle (Min and
Webb, 2001). In general, the surfaces with the lowest receding
contact angles also exhibited the smallest water retention. In
all cases, water retention on Coil #4 was larger than on the
other heat exchangers due to the exposure of the fin material
to the oxygen plasma which made the surface more hydro-
philic. Initially, it was speculated that the water on this coil
would drain from the surface in sheets and thus might retain
less water than the other coils. These results disprove this
initial hypothesis. Thus, as compared to the baseline coil, Coil
#2 decreased water retention by 38.5% and Coil #3 by 27.9%,
while Coil #4 increased water retention by 16.1%. These data
20
30
40
50
60
70
80
90
-5 0 5 10 15 20
baseline untreated, vertical
coil#2 untreated, vertical
coil#3 untreated, vertical
coil#4 plasma, vertical
WaterRetention(g)
Time (sec)
20
30
40
50
60
70
80
90
-5 0 5 10 15 20
baseline untreated, vertical
coil#2 treated, vertical
coil#3 treated, vertical
coil#4 plasma, vertical
WaterRetention(g)
Time (sec)
20
25
30
35
40
45
50
55
60
800 900 1000 1100 1200 1300 1400 1500 1600
baseline untreated, vertical
coil#2 untreated, vertical
coil#3 untreated, vertical
coil#4 plasma, vertical
WaterRetention(g)
Time (sec)
20
25
30
35
40
45
50
55
60
800 900 1000 1100 1200 1300 1400 1500 1600
baseline untreated, vertical
coil#2 treated, vertical
coil#3 treated, vertical
coil#4 plasma, vertical
WaterRetention(g)
Time (sec)
Vertical, With Coating
(“Steady State” Data)
Vertical, No Coating
(Transient Data)
Vertical, No Coating
(“Steady State” Data)
Vertical, With Coating
(Transient Data)
Baseline Coil
Coil #2
Coil #3
Coil #4
Baseline Coil
Coil #4
Coils
#2 3
a
dc
b
Fig. 7 e Dynamic dip testing data for the prototype heat exchangers in the vertical orientationd(a) untreated coils
(transient); (b) untreated coils (steady state); (c) coils treated with the alkyl silane (transient); and (d) coils treated with the
alkyl silane (steady state). These data show that coils #2 and #3 (with the treated surface) drained 38.5% and 27.9% more
water, respectively, than the baseline coil.
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9. show that the anisotropic micro-channel surface structure in
combination with the alkyl silane treatment enhances water
drainage and can significantly decrease water retention for
this heat exchanger.
For the treated surfaces in the horizontal orientation,
increased water retention was observed on all coils as
compared to the vertical orientation (see Fig. 8). This addi-
tional water retention is believed to be the result of the U-
shaped header plate that was placed on the coils during
construction. It is thought that this header plate may have
served to trap a small quantity of water when placed in this
orientation. Thus, tests were performed with the heat
exchangers oriented at both a 0
and 10
angle of inclination
with respect to the horizon. This inference is supported by
Fig. 8c and d which show a decrease in the water retention for
Coils #2, 3, and 4 when the heat exchanger was oriented at
a 10
angle of inclination with respect to the horizon which
would have allowed water trapped in this U-shaped header to
drain. Thus, although the overall trends were not affected,
these data may be more appropriate when evaluating the
effect of the micro-channels on the water retention in this
orientation. When considered collectively, these data
(Fig. 8aed) further suggest that the grooved micro-structure
impedes water drainage across the channels as was shown
by earlier data. Thus, the anisotropic wettability associated
with the grooved micro-structure appears effective at
promoting water drainage in only one direction while simul-
taneously preventing water drainage in the other. This is best
observed for Coil #3 which exhibited more than a 25% reduc-
tion in water retention when oriented vertically but retained
approximately the same amount of water as Coil #4 when
oriented horizontally at 10
inclination. Although dip testing
data were not collected for Coil #1 in the horizontal orienta-
tion, this finding suggests that the micro-channels may be
used to help prevent condensate carryover into the occupied
spacedsomething recently supported by Sommers et al.
(2012). The measurement error associated with these
dynamic dip testing data can be found in Table 4.
3.4. Air-side pressure drop data
The data of the air-side pressure drop for both the dry and
wet tests are presented in Figs. 9 and 10 and show that all
three prototype coils had a lower pressure drop penalty
20
30
40
50
60
70
80
90
-5 0 5 10 15 20
coil #2 treated, horizontal
coil #3 treated, horizontal
coil #4 plasma, horizontal
WaterRetention(g)
Time (sec)
20
40
60
80
100
120
-5 0 5 10 15 20
coil #2 treated, horizontal 10 deg
coil #3 treated, horizontal 10 deg
coil #4 plasma, horizontal 10 deg
WaterRetention(g)
Time (sec)
20
25
30
35
40
45
50
55
60
800 900 1000 1100 1200 1300 1400 1500 1600
coil #2 treated, horizontal
coil #3 treated, horizontal
coil #4 plasma, horizontal
WaterRetention(g)
Time (sec)
20
25
30
35
40
45
50
55
60
800 900 1000 1100 1200 1300 1400 1500 1600
coil #2 treated, horizontal 10 deg
coil #3 treated, horizontal 10 deg
coil #4 plasma, horizontal 10 deg
WaterRetention(g)
Time (sec)
Horizontal, With Coating
(“Steady State” Data)
Horizontal, With Coating
(Transient Data)
Horizontal, 10º Inclination,
With Coating (“Steady State” Data)
Horizontal, 10º Inclination,
With Coating (Transient Data)
ba
dc
Fig. 8 e Dynamic dip testing data for the prototype heat exchangers in the horizontal orientationd(a) treated coils at
0
inclination (transient); (b) treated coils at 0
inclination (steady state); (c) treated coils at 10
inclination (transient); and (d)
treated coils at 10
inclination (steady state).
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10. during wet operation than the baseline coil (albeit to varying
degrees). Fig. 11 shows the increase in the wet coil pressure
drop as compared with the dry coil pressure drop. The delta
P (i.e. wet DP À dry DP) for all cases increased monotonically
with the air flow rate up to 0.097 m3
sÀ1
(or, 205 CFM);
however, at 0.135 m3
sÀ1
(or, 285 CFM), the delta P dropped.
(Note: These flow rates correspond to face velocities of
1.14 m sÀ1
and 1.58 m sÀ1
, respectively.) At these higher air
flow rates, the droplets appeared smaller (partly due to
evaporation) and some of the droplets appeared to be
entrained in the air flow thereby mitigating the pressure
drop penalty associated with wet operation as shown in
Fig. 11.
Coil #3 had the lowest pressure drop increase of all the
prototype coils, while Coil #2 had a lower pressure drop
increase than the two remaining heat exchangers with
Fig. 9 e Air-side pressure drop data during dry tests.
Fig. 10 e Air-side pressure drop data during wet tests.
Fig. 11 e The change in pressure drop between dry and wet
conditions. An average reduction of more than 40% in the
pressure drop penalty was exhibited by Coil #3 as
compared to the baseline coil for air flow rates less than
0.10 m3
sL1
.
Fig. 12 e The percentage increase in friction factor for wet
versus dry conditions.
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11. isotropic fins. Because of slight variations between the coils,
direct comparison of pressure drop values is not as helpful
as a comparison of the percentage change in pressure drop.
Fig. 12 show the percentage increase in friction factor for
the wet case versus the dry case e wet f minus dry f divided
by dry f. The densities used in these calculations are aver-
ages of the air inlet and exit densities. In this figure, we
clearly see at the lowest flow rates, Coils #2 and #3 result in
a much smaller increase in friction factor under wet oper-
ation as compared to dry operation, and hence fan power
will also be reduced. These improvements continue for Coil
#3 to the highest flow rates. We also see that Coil #4 results
in the greatest increase in friction factor as compared to dry
operation at the lowest flow rates. This is to be expected
since Coil #4 was slightly more hydrophilic than the base-
line coil as shown by the dip testing results in Tables 4 and
5. At higher flow rates, the performance of the three coils
drew close together. It was initially thought that the fin
surface of Coil #4 would promote filmwise condensation
(rather than dropwise condensation) thus allowing water to
drain from the surface in “sheets” rather than as discrete
droplets. For all coils, however, the droplets were large and,
in some cases, spanned the fins at low air flow rates. At high
air flow rates, droplets did not always fall via gravity but
were sometimes entrained by the air going through the heat
exchanger, and they were also smaller due to evaporation.
These factors combined to result in a smaller difference in
the pressure drop between the wet and dry coils.
In summary, the increase in the air-side pressure drop
associated with wet operation was the smallest for Coil #2 and
Coil #3. For both Coils #2 and #3, the most significant
enhancement over the baseline coil was observed at
0.0236 m3
sÀ1
(or, 50 CFM) where the air flow was least able to
assist in water drainage. Coil #4 showed the largest percent
increase in delta P for wet operation as compared to dry
operation while Coil #3 showed the smallest percent increase
in delta P. More specifically, the increase in pressure drop
associated with wet operation for the baseline coil was 68%,
48%, and 29% larger than Coil #3 for 0.0236 m3
sÀ1
,
0.0480 m3
sÀ1
, and 0.0724 m3
sÀ1
, respectively, as calculated
below:
% increase ¼
ðwet DP À dry DPÞHX1 À ðwet DP À dry DPÞHX3
ðwet DP À dry DPÞHX1
(1)
All of the wet tests were performed using an inlet relative
humidity of 100% to maintain consistency from test to test. In
application, however, the relative humidity is typically lower
than 100%. While similar behavior is expected with a lower
inlet relative humidity, it is expected that less condensate
would form on the fins, and thus the percentage increase in
pressure drop during wet operation would be somewhat less
than what was found here.
3.5. Comparison of assessment methods
After completing these measurements, a comparison of the
various assessment methods and their conclusions was per-
formed. The results from this comparison are summarized
below in Table 5. For the untreated surfaces, Coil #2 and Coil
#3 with the grooved micro-structure were shown to be more
hydrophilic than Coil #1. This caused the two heat exchangers
to retain more water and possess smaller apparent contact
angles than the baseline coil while still exhibiting small
contact angle hysteresis. In the end, all three independent
tests arrived at the same conclusiondmicro-grooves in the
absence of a chemical coating increase the hydrophilicity of
an aluminum fin.
For the treated surfaces, Coil #3 had the lowest contact
angle hysteresis and the largest receding contact angle which
suggests that Coil #3 was the most hydrophobic of the
surfaces examined. Coil #3 also had the lowest wet pressure
drop which makes it the most attractive to the HVACR
industry. On the other hand, Coil #2 had the lower critical
sliding angle and water retention in dip testing so Coil #2
performed better in the absence of air flow. This observed
difference might be explained by “droplet pinning” against the
micro-channels in the case of air flow. Because the micro-
channels on Coil #2 are narrower than on Coil #3 (i.e. 15 mm
vs. 50 mm), a water droplet experiences the effect of more
channels on Coil #2 than Coil #3. Because these channels
impede droplet movement in the streamwise direction, it is
possible that droplets may become “pinned” on the fin surface
when the air flow is sufficiently large. It is important to note,
however, that for the tests performed on the full-scale heat
exchangers (i.e. dynamic dip testing and wet pressure drop),
Coil #3 and Coil #2 consistently outperformed the baseline
coil; thus the effect is small.
Finally, it is important to note and should be emphasized
that the micro-channels in this study were produced by wet
chemical etching which causes the sidewalls to be rounded
and not truly vertical. Thus, the overall anisotropy and
hydrophobicity of these surfaces is likely reduced as
compared to surfaces manufactured using more costly
techniques (i.e. reactive ion etching, micro-milling, laser
machining, etc.). Furthermore, the fin spacing adopted in
this study was fairly large (i.e. 3.175 mm). Although larger
savings could potentially be realized in geometries with
significant condensate bridging, this would be offset some-
what by the smaller savings that might be expected when
dealing with a lower inlet relative humidity than what was
tested here. In terms of the potential impact, the savings
Table 5 e Comparison of the various assessment
methods.
Untreated surfaces and heat exchangers
(Objective: To increase hydrophilicity)
Apparent contact angle Coil #2 Coil #3 Coil #1
Contact angle hysteresis Coil #2 Coil #3 Coil #1
Water retention in
dip testing
Coil #2 Coil #3 Coil #1
Treated surfaces and heat exchangers
(Objective: To increase hydrophobicity)
Apparent contact angle Coil #3 Coil #1 Coil #2 Coil #4
Contact angle hysteresis Coil #3 Coil #1 Coil #2 Coil #4
Critical sliding angle Coil #4 Coil #2 Coil #3 Coil #1
Water retention in
dip testing
Coil #2 Coil #3 Coil #1 Coil #4
Wet pressure drop Coil #3 Coil #2 Coil #4 Coil #1
Note: Arranged from “Best” to “Worst” with the best performer
shown in bold.
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12. that would result from adopting this anisotropic fin surface
design in plain fin, round tube heat exchangers would likely
originate in two forms: (1) a reduction in the air-side core
pressure drop under wet operating conditions, and (2)
a reduction in the apparent latent load on the coil. First, if
the heat exchanger stays dry, then the fan does not have to
work as hard to deliver the same volumetric flow rate of
conditioned air. The second and perhaps more significant
potential savings would be the reduced apparent latent load
on the heat exchanger. In air-conditioning systems, the
water retained on the evaporator coil after the air-
conditioner stops operating is re-evaporated into the
space. This means that this water must be recondensed
again effectively increasing the apparent latent load on the
air-conditioner as compared to the real latent load in the
space.
While the results of this study seem promising, a few
ancillary issues should be more completely investigated to
help advance this technology, namelyd(a) the durability of
the fin surface and fin surface coating to fouling and
thermal cycling, (b) the heat transfer performance of the
full-scale prototypes, (c) the effect of inlet relative humidity,
and (d) the appropriateness of these surfaces for use in
refrigeration systems (not investigated in this work). Follow-
up testing that addresses some of these issues is currently
underway.
4. Conclusions
This study reports an assessment of the condensate
drainage behavior of four full-scale heat exchanger proto-
typesdtwo constructed using smooth, isotropic fins (i.e.
Coils #1 and #4) and two constructed using micro-grooved,
anisotropic fins (i.e. Coils #2 and #3) where the fin surface
contained parallel micro-channels tens of microns in width
and depth. Experiments were performed on these heat
exchangers with and without an alkyl silane coating
applied to the surface. Contact angle hysteresis and critical
sliding angles on the enhanced surfaces were generally
found to be smaller than the baseline surface, and during
dynamic dip testing experiments, the steady-state water
retention was reduced by more than 27% for the prototype
coils with the coating. Additionally, the air-side pressure
drop of these enhanced coils was found to be less severe
during wet operation as compared with the baseline coil
(i.e. Coil #1). For the coils constructed using the micro-
grooved surfaces, the increase in air-side pressure drop
during fully wet conditions was generally lower by an
average of 30% for Coil #2 and 36% for Coil #3 as compared
to the baseline coil, although this percentage varied
significantly with the flow rate. The wider, deeper micro-
channels of Coil #3 performed better in wet air-side
pressure drop testing, while the narrower, shallower
micro-channels of Coil #2 performed better in dynamic dip
testing experiments. While the individual return on any
HVACR application is expected to be small, the breadth of
applications is naturally quite large.
Acknowledgments
The authors gratefully acknowledge financial support from
a California Energy Commission Energy Innovation Small
Grant (EISG Grant #55699A/07-23).
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