Mini/micro channels and metal foams
for enhanced heat exchangers
Gian Luca Morini
Dipartimento di Ingegneria
Industriale, Alma Mater Studiorum Universitą di
Bologna
Viale Risorgimento 2, 40136
Bologna gianluca.morini3@unibo.it
ABSTRACT
In the last twenty years, many research works were
focused on the optimization of heat transfer in heat exchangers. Tubes are scaled
down from macro-metric sizes to micrometric dimensions in order to improve
their performances. Finned surfaces are replaced with porous metallic media to
maximize the heat transfer surface per unit of volume.
However, the reduction of the inner dimensions
of the channels implies a series of negative effects which cannot be ignored
during the design of a new micro heat exchanger. In many heat exchangers the
adopted number of parallel microchannels is very large and this aspect
introduces a new problem related to the distribution of the working fluid among
the channels. During the talk it will be shown, by means of numerical and
experimental results, in which way the non-uniform distribution among the
channels can be controlled by optimizing the shape of the inlet and outlet
manifolds, or by introducing an additional pressure loss at the entrance of
each channel. In addition, when the thickness of the solid region among the
channels can become of the same order of magnitude of the hydraulic diameters
of the channels the conjugate heat transfer between the solid walls and the
fluids cannot be ignored both along the axial and transverse direction. The
presence of a non-negligible axial and transverse heat conduction changes the
behavior of the heat exchanger in terms of overall performances and the impact
is different if the adopted flow configuration changes from counter-current flow
to co-current flow or to cross flow configuration.
On the other hand, the replacement of
conventional finned surfaces with metal foam surfaces is not always convenient.
In this talk, it is shown how the adoption of metal foams with high porosity
might guarantee similar pressure drops with respect to the conventional finned
heat exchangers but, in terms of overall heat transfer coefficients, high values of porosity are responsible for a
lower surface-to-volume ratio of the foam-based extended surfaces, yielding a
strong penalization on the heat transfer rate. Moreover, the small contact area
between metal fibers and tubes proved to strongly increase the contact thermal
resistance between metal foams and tubes and, consequently, the overall thermal
performance of the heat exchanger are reduced. The
total thermal resistance is also influenced by the bonding technique adopted to
build the foam-based heat exchangers. The experimental results underline that
the replacement of the fins conventionally used in water-to-air heat exchangers
with metal foam surfaces can be suitable only in presence of low specific air
flow rates and a reduced contact thermal resistance between foam and tubes.