serial, meandering structure is relatively high due to the channel length, it is lower in paral­lel structures. However, parallel structures can lead to a non-uniform flow distribution (de­pending on the ratio of the diameters and lengths of the risers and the header channels, [4, 5]).

In contrast to the mentioned designs, many natural hydraulic structures such as blood vessels or leaf venation show a multiple branched, fractal geometry. Since natural designs are optimised with respect to energy efficiency, the idea of transferring their construction principles to technical applications seems to be a promising approach (Fig. 1 c), [6]). If channel designs with both lower pressure drop and a uniform flow distribution can be ob­tained, it will be possible to rise the energy efficiency of heat exchangers.