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                                 C. Hutter, Ph. Rudolf von Rohr1
        ETH Zurich, Institute of Process Engineering, Sonneggstrasse 3 - 8092 Zurich; CH


Highly exothermic reactions in a continuous reaction system shall be conducted in a
controllable environment also for continuous operation. The investigations in the micro
reaction area have shown that an approach to handle such kind of systems is to scale down the
reactor size. Nevertheless the volumetric performance of these devices is low. An approach to
combine the advantages of microsystems with the performance of macro devices is the
application of metal foam. Foams are widely used in the heat exchanger technology [1] but
they can also be used as static mixing elements [2]. Therefore such a device enables better
reaction control and improved selectivity due to its excellent heat and mass transfer behavior.
We present measurements of the heat transfer within a circular tube reactor (inner diameter
7mm) filled with foam of different pore sizes (20 and 30 ppi). The experimental facility
consists of a plug flow reactor of 1m length with a heating coil, which is insulated to the
outside. The temperature of the heating fluid (water) was measured on the in- and the outlet of
the coil and the temperatures of the main stream (water) were recorded at five equally
distributed positions in the center of the tube. By applying the heat balance the heat transfer
coefficients could be obtained and compared to an empty tube reactor. The investigations took
place in a range of Reynolds numbers between 600 and 7200 defined with the empty reactor
dimension and the Darcian velocity. The observed heat transfer with inserted foam elements
was up to two times higher than without foam. It could be shown that the foam with bigger
pores (20ppi) offers a better heat transfer performance. Results of simultaneous PIV (Particle
image velocimetry) / LIF (Laser induced fluorescence) measurements delivered an
explanation for this behavior. The flow behind a 20ppi element featured an increased
turbulence intensity compared to 30 ppi foam. This leads to a bigger convection heat transfer
coefficient. Consistent with the increased heat transfer of the 20ppi foam the mass transfer
was measured increased also. Further results indicated the importance of the contact between
the foam and the wall, which was observed as the limiting step in the heat transfer
mechanism. The comparison of the pressure drop measurements of the two foam types shows
only small differences.
The presented results show a strongly increased heat transfer due to the bigger surface area
implemented by the metal foam. A new technology will be presented to guarantee a good
contact between the foam and the wall of the tube which was detected as a limiting resistance
in heat transfer.

[1] K. Boomsa et al., Mechanics of Materials 35 (2003)
[2] S. Ferouillat et al., Chemical Engineering and Processing 45 (2006)

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