Mini/micro channels and metal foams for enhanced heat exchangers

Gian Luca Morini

Dipartimento di Ingegneria Industriale, Alma Mater Studiorum Università 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.