This numerical study focuses on the determination of macroscopic (effective) properties from pore scale calculation. These results will be applied to heat exchangers design. The computational domain -representative of heat exchanger section- is a parallelepiped filled with metallic foam, heated on one face and crossed by a forced fluid flow. Conjugate heat transfer and fluid flow are computed using finite volume approach on the actual solid matrix and pore space topology obtained from X-ray tomograms. Calculated heat transfer coefficient and flow law parameters are in good agreement with literature data. An active foam length is defined and measured in order to provide optimal design characteristic for foamed heat exchanger.