Novel Lightweight Metal Foam Heat Exchangers

Abstract

An overview of open cell metal foam materials with application to advanced heat exchange devices is presented. The metal foam materials considered consist of interconnected cells in a random orientation. The manufacture of metal foam materials into complex heat exchange components is described. Experiments with flat foam panels brazed to copper sheets show increasing heat removal effectiveness with decreasing foam pore size at equivalent coolant flow rates. The high-pressure drop experienced with flow through small pore-size metal foam materials makes the use of larger pore size material more attractive. The paper will demonstrate that in certain configurations, particularly difficult geometries, confined spaces, high temperatures and demanding environments, metal foam is an excellent heat exchange medium.

Full text

1 of 7
Novel Lightweight Metal Foam Heat Exchangers
David P. Haack1, Kenneth R. Butcher1, T. Kim2 and T. J. Lu2
1Porvair Fuel Cell Technology, Inc., 700 Shepherd St., Hendersonville, NC 28792, USA
2Department of Engineering, University of Cambridge, Trumpington Street, Cambridge, UK
ABSTRACT
An overview of open cell metal foam materials with
application to advanced heat exchange devices is presented.
The metal foam materials considered consist of
interconnected cells in a random orientation. The
manufacture of metal foam materials into complex heat
exchange components is described. Experiments with flat
foam panels brazed to copper sheets show increasing heat
removal effectiveness with decreasing foam pore size at
equivalent coolant flow rates. The high-pressure drop
experienced with flow through small pore-size metal foam
materials makes the use of larger pore size material more
attractive. The paper will demonstrate that in certain
configurations, particularly difficult geometries, confined
spaces, high temperatures and demanding environments,
metal foam is an excellent heat exchange medium.
INTRODUCTION
New materials are needed in the development of
advanced, compact, and lightweight thermal systems to
satisfy new demands from emerging technologies. Three such
technologies are multi-functional fuel processors that demand
the capability for simultaneous heat exchange and chemical
reaction; highly efficient radiators for high power heat
rejection at low temperature differentials; and high-density
microelectronic circuits requiring high rates of heat removal.
Porvair Fuel Cell Technology is investigating metal foam as a
medium to use in the manufacture of highly effective, high
temperature-capable, geometry-flexible, and multi-functional
heat exchange devices. Two mechanisms have been found to
be important to the heat transfer enhancement associated with
the use of metal foam materials: specifically, interactions
between the solid foam material and a through-flowing fluid
[1-4]; and the importance of achieving a quality metal-to-
foam bond [3, 5]. Applications in electronics cooling and
compact heat exchangers have been investigated, revealing
promising advances in the rate of heat removal or transfer
under experimental conditions. Some applied research has
been performed, applying metal foam in unique designs to
radiators and advanced reactors [6]. While most of the test
results are proprietary, applied research is at Porvair Fuel Cell
Technology.
Metal foam heat transfer: A literature survey
Metal foam materials have been investigated for use in
heat exchange applications in the open literature. Studies
have attempted to describe thermal transport in ceramic and
metallic foams on a basic and applied basis. Younis and
Viscanta [7] have measured the volumetric heat transfer of
ceramic foam materials, and developed a Nusselt number
correlation fit to the experimental data. The volumetric heat
transfer rates measured were higher than those for packed
beds or sintered metals. Calmidi and Mahajan [3] studied the
solid-to-fluid thermal transport from a heated metal plate
brazed to aluminum metal foam. Results indicated a
significant contribution of the thermal transport resulted from
solid plate to foam contact, and subsequent fluid-to-foam
thermal transport. High quality joints are indicated as being
important to effective thermal transport. In a similar study,
Kim et al. [8] examined metal foam thermal transport
between two isothermal plates. Aluminum foam filled the
space between the plates, which were heated with flowing
water. The foam was put into mechanical contact by pressing
to minimize thermal contact resistance. Pressure drop friction
factors and modified Colburn J-factors were measured, and
comparisons were made to conventional louvered fins for
overall performance. Results indicated that the foam material
offered better heat transfer performance compared to a
louvered array, but at a greater pressure drop. Methods of
reducing pressure drop or improving the foam to solid
contact were not investigated. Lu et. al. [2] have developed a
model describing metal foam heat transfer, where the foam is
modeled by inter-connected cylinders. The analysis was
extended to electronics cooling, and to multi-layer heat
exchangers, with good performance predicted. Bastarows [5]
studied single side heating of a foam-filled channel for an
electronics cooling application. The experimental method
utilized both conductive epoxy bonding and brazing of the
metal foam to heated plates. Results indicated that brazed
foam materials are much more effective at heat removal than
epoxy-bonded samples. Measured heat exchange
performance indicated 3 times more heat removal capability
compared to a conventional fin-pin array.
Outline of paper
This paper will discuss open-cell metal foam fabrication
techniques, the use of metal foam in complex heat exchanger
designs, and the potential to mass manufacture metal foam

2 of 7
materials. In addition, the heat exchange properties of a high
temperature-capability metal foam alloy (Porvair Fuel Cell
Technology’s FeCrAlY material) will be presented.
OPEN-CELL POROUS METAL FOAMS: FABRICATION
AND ASSEMBLY
Cellular morphology of metal foams
The metal foam structure, shown in Fig. 1, consists of
ligaments forming a network of inter-connected
dodecahedral-like cells. The cells are randomly oriented and
mostly homogeneous in size and shape (Fig. 1a, a result of
the manufacturing method used to create the metal foam
precursor material). The triangular-shaped edges of each cell
are hollow (Fig. 1b), a result of the manufacturing technique.
Pore size may be varied from approximately 0.4 mm to 3
mm, and the net density from 3% to 15% of a solid of the
same material. Metal foam from Porvair Fuel Cell
Technology is available in alloys and single-element
materials. Common materials include copper, stainless steel,
and high temperature iron-based alloys (e.g., FeCrAlY).
Metal foam thermal properties
Heat transfer enhancement using porous metal foams
depends on both the cellular structure of the foam material,
and the thermal properties of the metal foam. Metal foam
thermal conductivity is dependent upon the overall density of
the piece and the metal from which the foam is made.
Conductive pathways through the porous material are limited
to the ligaments of the material. Experimental measurements
have determined a functional relationship between the foam
thermal conductivity and density as λsρr
1.8<λf<λsρr
1.65 ,
where
ρ
r
is the foam relative density,
λ
f is the foam
conductivity, and
λ
s
is the solid conductivity [6]. Higher
material conductivity is associated with higher density
materials, and significant increase in thermal conductivity
results from an increase in material density. On the other
hand, heat transfer by metal foams due to thermal dispersion
effects is proportional to cell size [4].
Metal foam fabrication and capabilities
Metal foams have been manufactured for many years
using a variety of novel techniques. Metallic sintering, metal
deposition through evaporation, electrodeposition or
chemical vapor decomposition (CVD), and investment
casting (among numerous other methods) have created open
cell foams. In foam creation through metal sintering, metallic
particles are suspended in slurry and coated over a polymeric
foam substrate. The foam skeleton vaporizes during heat
treatment and the metallic particles sinter together to create
the product. This method is thought to be the most cost-
effective and the most amenable to mass production. The
CVD method utilizes chemical decomposition of a reactive
gas species in a vacuum chamber to deposit material onto a
heated substrate (polymer or carbon/graphite, depending
upon the temperature of the deposition process). Production
rates are limited in this method by the rate at which material
is deposited on the substrate. Highly refractory metals and
ceramics may be created with this method with high quality.
Molten metal infiltration is utilized to make aluminum and
copper foam materials [9]. With this method, a foam
precursor is coated with a ceramic casing and packed into
casting sand. The casting assembly is heated to decompose
the precursor and harden the casting matrix. Molten metal is
then pressure infiltrated into the casting, filling the voids of
the original matrix. After solidification, the material is broken
free from the mold. The method has the advantage of being
capable of producing a pieces in widely used metals and
alloys with solid struts. However, the process requires several
processing steps and specialized equipment, and does not
lend itself to rapid production processes.
High volume manufacturing
Of the methods suitable to produce metal foam materials,
the metal sintering method offers the most promise for mass
production. Necessary production equipment is easily
automated and yields high-quality, low-cost metal foam
materials for use in a variety of applications.
Capability of manufacturing complex assemblies
To effectively use metal foam materials in heat exchange
devices it is necessary to combine the material with tubes and
sheets for flow control and heat transfer. Development
efforts have taken place at Porvair Fuel Cell Technology to
successfully combine a variety of metal foam materials with
solid structures. Several proprietary components have been
constructed combining tubes and other solid materials to
construct advanced, multifunctional heat exchange devices
for a variety of customers. An important consideration in the
formation of the advanced heat exchangers is the quality of
the bond joint between foam and solid material through
which heat is transferred [5]. Figure 2 is a photograph of a
developmental component consisting of tubes imbedded in a
metal foam matrix, generating an advanced high-temperature
radiator. Metallurgical bonding between the tube wall and
the foam matrix was achieved by sintering during material
heat treatment. Figure 3 shows an SEM micrograph of the
joint region, which shows good bonding between the foam
and tube. Assemblies have also been manufactured through a
proprietary co-sintering technique. Figure 4 shows an
example of a foam-filled tube manufactured with this
method. Complex assemblies combining metal foam with
metal packaging are in the design stage to create an advanced
two-phase heat exchange component for use in fuel cell fuel
processing systems at Porvair Fuel Cell Technology.
HEAT TRANSFER MEASUREMENTS
An experimental program was performed at Cambridge
University for the purpose of measuring the heat transfer
effectiveness of metal foams under varying flow conditions.
Experimental procedure
The experimental apparatus mainly consists of four
sections: coolant supplier, test section, test model, and data
acquisition system. A photograph of the test rig with a sample
inserted in its test section is shown in Figure 5.
Air is used as a coolant and is forced through the channel

3 of 7
inlet by a suction type air blower. A total of four static
pressure taps are placed along the flow direction on the upper
copper skin. An asymmetrical isoflux (constant wall heat
flux) boundary condition is imposed on the lower copper skin
by a heating element (silicone-rubber etched foil from
Watlow
TM Inc.). Five thin foil (each 0.05 mm thick) T-type
copper-constantan thermocouples (from Rhopoint Inc.) are
inserted on the lower skin along the flow direction. There are
two additional T-type thermocouples, positioned separately at
the inlet and outlet of the test section to measure the coolant
temperature at each location. A temperature scanner with
reading resolution of
±
0.1K
is used to record and analyze
temperature readings from all thermocouples simultaneously.
The experiments were run for several minutes until the
flow inside the channel became hydraulically and thermally
stabilized. All measurements were performed under steady
state conditions. A Pitot tube was positioned before the test
section to measure stagnation and static pressures at the inlet.
Because the blockage ratio of the pitot tube (tube diameter
(0.51 mm) to channel height (12 mm)) is small, wall
interference from the Pitot tube is expected to be negligible.
Measurement uncertainties
During experimentation, measurements were repeated
until significant data repetition was ensured (i.e., 5%
uncertainty interval). An uncertainty analysis was performed
following the method suggested by Kline and McClintock
[10]. The maximum heat loss through the insulation materials
was estimated to be less than 2 percent of input heat flux. The
heat loss through the perspex side-walls was estimated to be
negligible through a conduction heat loss analysis. The
thermal conductivity kf of air varies slightly in the operating
temperature range of 300.0 K to 350.0 K. An arithmetic mean
value is used for kf, with uncertainty estimated to be within
6.6 %. From these, the uncertainty in the measured heat
transfer coefficient and Nusselt number was estimated to be
less than 7.0% and 9.6%, respectively, whilst the uncertainty
in the pressure drop and friction factor measurements was
estimated to be less than 5.0% and 7.8%, respectively, using a
root-sum-square method.
Test samples
The samples fabricated by Porvair Fuel Cell Technology
consisted of FeCrAlY metal foam bonded on top and bottom
to a thin copper sheet (1 mm thick). Bonding was achieved
by brazing. Metal foam pore size included 10, 30 and 60 PPI
(pores per inch). Foam relative density was set to 5%, 7.5%
and 10 %. Table 1 shows the specifications for each heat
exchange test sample. The thermal conductivity of solid
FeCrAlY alloy is taken to be 16
W
/
mK
. The sandwiched
foam specimens were trimmed to fit into the test section of a
heat sink channel of size 0.127 m (W)
×
0.127 m (L)
×
0.012
m (H).
RESULTS AND DISCUSSION
Permeability and inertial coefficient
The measured pressure drop across the FeCrAlY foam
samples is presented in Figure 6 as a function of mean flow
velocity U
m
(at the test section inlet). The modified Darcy
equation:
−dP
dx 1
µU
m
=1
K+ρIC
K
Um
µ(1)
is subsequently used to determine the permeability, K, and
inertial coefficient, I
C
, for each sample, with
µ
and
ρ
denoting separately the dynamic viscosity and density of air.
The results are listed in Table 1, and are found to be similar
to those reported for aluminum foams [3-4]. Pressure drop is
found to be highly dependent upon material pore size, and
less dependent upon material density.
Thermal resistance and heat removal performance
The surface temperature T
w
(x)of the copper plates was
measured from thermocouples, where x denotes the
longitudinal axis. Linear variation of T
w
with x was observed
under the isoflux boundary condition. The thermal
performance of FeCrAlY foams as a heat sink medium was
characterized by the local heat transfer coefficient h and local
Nusselt number Nu defined as:
h(x)=q
T
w
(x)−T
in
(2)
Nu(x)=h(x)
kf/Dh(3)
where q, T
in
, kf and D
h
are heat flux, coolant temperature
at inlet, coolant thermal conductivity, and hydraulic diameter
of the heat sink channel. These are averaged over the sample
length to obtain the mean heat transfer coefficient and mean
Nusselt number as:
h=1
L
h(x)dx
0
L
∫ (4)
Nu =1
L
Nu(x)dx
0
L
∫ (5)
Reynolds number is based on the measured permeability,
Re
K
=ρU
m
K/µ.
Fig. 7a plots the averaged Nusselt number as a fucntion
of Re
K
for the FeCrAlY samples with a fixed pore density
(30 PPI) but different relative densities, whilst Fig. 7b plots
Nu
as a function of Re
K
for samples at 10% and 15%
relative densities and different pore densities. The results for
all FeCrAlY samples are summarized in Figure 8, and
compared with those taken from [3] for aluminium foams.
The results of Fig. 7a indicate that for a foam with a fixed
pore density, a higher relative density is favored for improved
heat transfer rate, although the corresponding flow resistance
is higher. On the other hand, at a fixed relative density and a
fixed value of Re
K
, the 60 PPI foam sample removes more
heat than the 10 PPI sample (Fig. 7b). The importance of the
material density can also be seen in this figure through a

4 of 7
comparison of 30 ppi material performance at a slightly
lower relative density (10% compared to 15%). The higher
density large pore material was found to outperform the
slightly less dense smaller pore material. Figure 7b also
shows that, at a given pumping power (implied from
permeability), the maxium value of Re
K
, (Re
K
)
max
, is
limited by the flow resistance of each sample, with
(Re
K
)
max
increasing as the flow resistance is decreased (or,
equivalently, as the peamibility is increased). Consequently,
the highest Nusselt numbers obtained from the 10 PPI foam
with 15% relative density nearly double that of the 60 PPI
foam having the same relative density, indicating that the
former has the best heat transfer efficiency at a specified
pumping power.
Comparison was made with selected test data for
aluminum foams having solid struts (these foams were
processed via the expensive investment casting technique). In
general, at a fixed value of Reynolds number Re
K
under
forced air convection, the FeCrAlY samples remove 30~50 %
of heat that is removed by aluminum foams of a similar pore
size and density, although the thermal conductivity of
FeCrAlY (~16
W
/
mK
) is an order of magnitude smaller
than that of pure aluminum (~200
W
/
mK
). However, if air is
replaced by water as the coolant, it is expected that FeCrAlY
foams and aluminum foams will have similar heat transfer
characteristics, because the thermal conductivity of high
porosity foams, whether ceramic or metal, is roughly the
same as the thermal conductivity of the coolant [4].
CONCLUSION
New applications for highly effective, multi-functional
heat exchange devices are driving the development of metal
foam components. Metal foam materials have the potential
to increase heat transfer rates from solid surfaces by
conducting heat to the material struts and inducing a high
interaction between the struts and a through-flowing fluid.
New manufacturing techniques developed at Porvair Fuel
Cell Technology allow effective, low-cost, high-volume
manufacturing, and new assembly techniques are being
developed to manufacture complex assemblies of foam and
solid metals to form heat exchange devices. Several
prototype devices have been constructed for industry to
increase performance and reduce size, cost and weight.
Heat transfer and pressure drop measurements reveal that
high rates of heat removal are possible with FeCrAlY foam, a
high-temperature metal. While small pore-size material is
advantageous for achieving high rates of heat removal,
pressure drop will be higher. Larger pore size materials can
achieve higher Nussalt numbers at high rates of flow with
relatively low fan power required. Increasing material
density was found to increase Nussalt numbers at a given rate
of coolant flow. Bare metal conductivity of aluminum is
approximately ten times that of FeCrAlY, however, the heat
transfer performance of aluminum foam is only 2-3 times
greater than FeCrAlY. This demonstrates that the foam
structure, and therefore the turbulence induced in the process
fluid substantially improves heat transfer performance.
Future work will examine the performance of copper
materials in a similar arrangement. A theoretical model will
be developed to enable flexible design of advanced heat
exchange concepts using metal foam materials.
ACKNOWLEDGEMENTS
This work was supported by Porvair Fuel Cell
Technology, Inc., the US Office of Naval Research
(ONRIFO/ONR Contract No. N00014-01-1-0271), and by
UK Engineering and Physical Scientific Research Council
(EPSRC grant number EJA/U83). The authors would like to
thank Mr. Alberic du Chene of Cambridge University for
providing Figure 1.
REFERENCES
1. A.-F. Bastawros, A.G. Evans, and H.A. Stone, Evaluation
of Cellular Metal Heat Dissipation Media, Technical
Report MECH-325, DEAS, Harvard University, March
1998.
2. T.J. Lu, H.A. Stone and M.F. Ashby, Heat transfer in
open-cell metal foams, Acta Mater 46 (1998) 3619-3635.
3. V.C. Calmidi and R.L. Mahajan, Forced Convection in
High Porosity Metal Foams, Trans. of ASME, J of Heat
Transfer 122 (2000) 557-565.
4. M.L. Hunt and C.L. Tien, Effects of Thermal Dispersion
on Forced Convection in Fibrous Media, Int. J. Heat
Mass Transfer 31 (1988) 301-309.
5. A.-F. Bastawros, Effectiveness of Open-cell Metallic
Foams for High Power Electronic Cooling, IMECE
Paper, Thermal Management of Electronics, ASME
Proc. HTD-361-3/PID-3, 211-217.
6. M.F. Ashby, A. Evans, N.A. Fleck, L.J. Gibson, J.W.
Hutchinson, H.N.G. Wadley, Metal Foams: A Design
Guide, Butterworth-Heinemann, Boston, ISBN 0-7506-
7219-6.
7. L.B. Younis and R. Viskanta, Experimental
determination of the volumetric heat transfer coefficient
between stream of air and ceramic foam, Int. J. of Heat
Mass Transfer, 36 (1993) 1425-1434.
8. S.Y. Kim, J.W. Paek, B.H. Kang, Flow and Heat
Transfer Correlations for Porous Fin in a Plate-Fin Heat
Exchanger, Trans. Of the ASME, J. of Heat Transfer,
122 (2000) 572-578.
9. G.J. Davies, S. Zhen, Review: Metallic foams: their
production, properties and applications, J. Material Sci.,
18 (1983) 1899-1911.
10. S.J. Kline and F.A. McClintock, Describing
Uncertainties in Single-Sample Experiments, Mechanical
Engineering (1953) 3-8.

5 of 7
Table 1. Specifications of FeCrAlY foams
Sample No. S-1 S-2 S-3 S-4 S-5 S-6 S-7
Pore size (PPI) 10 10 30 30 60 60 30
Relative density (%) 5 15 5 10 5 15 7.5
Permeability, K (×107m2)1.67 0.5 1.0 0.5 0.33 0.11 1.0
Inertial Coefficient, I
C
0.093 0.13 0.15 0.164 0.24 0.49 0.2
(a) (b)
Figure 1. SEM images of reticulated metal foam structure (FeCrAlY). Interconnected tortuous
pathways create turbulence in through-flowing fluids.
Figure 2. Metal foam compact heat exchanger for high
temperature service. Foam material is PFCT’s FeCrAlY.

6 of 7
Figure 3. SEM micrograph of a foam strut sintered
to a solid tube. Bonding region shows
metallurgical sintering between foam and solid.
Figure 4. Example assemblies manufactured in a
proprietary co-sintering technique (patent applied
for).
Figure 5. Test apparatus showing parallel plates for flow, and a foam sample with
insulation.
Um
dP/L[kPa/m]
0 2 4 6 8 10 12 14 16
0
20
40
60
80
100
120 S-1
S-2
S-3
S-4
S-5
S-6
S-760 ppi
10%relativedensity
10 ppi
5%relativedensity
60 ppi
5%relativedensity
30 p pi
5%relativedensity
30 ppi
10%relativedensity
30 ppi
7.5%relativedensity
10 ppi
10%relativedensity
Figure 6. Static pressure drop per unit length results from the
Porvair foam samples.
60 ppi
15% relative density
10 ppi
15% relative density

7 of 7
+
++
+
++
+++
ReK
50 10 0 15 0 200 25 0 300
10 0
20 0
30 0
40 0
50 0
60 0
70 0
80 0
90 0
10 00 S-1,(10 p pi,5%)
S-2,(10 p pi,1 5% )
S-3,(30 p pi,5% )
S-4,(30 p pi,1 0% )
S-5,(60 p pi,5% )
S-6,(60 p pi,1 5% )
S-7,(30 p pi,7.5% )
Sample1 ofERG
Sample3 ofERG
Sample4 ofERG
+
Figure 8. Comparison of FeCrAlY foams (Porvair) with aluminum
foams (ERG, data from [3]).
Nu
ReK
50 100 150 200 250 300
50
100
150
200
250
300
350
400
S-3
S-4
S-7
30ppi
7.5%Relativedensity
30ppi
5%Relativedensity
30ppi
10%Relativedensity
(a)
(b)
Figure 7. Average Nusselt number of Porvair foams (FeCrAlY) as a function of Reynolds number:
(a) fixed pore size (30 PPI ) (b) 10% and 15% relative densities.
R
e
K
50 100 150 200
50
100
150
200
250
300
3
5
0
400
S
-
2
S
-
4
S
-
6
6
0
p
p
i
1
5
%
r
e
l
a
t
i
v
e
d
e
n
s
i
t
y
1
0
p
p
i
1
5
%
r
e
l
a
t
i
v
e
d
e
n
s
i
t
y
3
0
p
p
i
1
0
%
r
e
l
a
t
i
v
e
d
e
n
s
i
t
y
Nu
Nu