The hydraulic structures generated in FracTherm can be exported as DXF files. DXF is a common format for CAD (Computer Aided Design) and CAM (Computer Aided Manufac­turing) applications. Thus it was possible to produce an absorber model using a computer — controlled milling machine (plexiglass, scale 1:8, 325.5 mm x 250 mm, Fig. 7).

In order to investigate the flow through a fractal hydraulic structure, some experiments with ink were carried out. At first, pure water flowed through the absorber. Afterwards, ink was injected into the inlet tubing. The results can be seen in Fig. 8. The pictures a) and b) are part of a movie (3 and 4 seconds after injecting, respectively). It can be observed how the ink spreads within the fractal structure. Picture c) shows the ink distribution within the whole absorber model. The pictures a) and b) reveal a laminar profile (parabolic shape). It can also be stated that the profiles are not exactly symmetric, but that the maximum flow speed has moved from the centre of the channels to one of the channel walls (direction in­dicated by arrows). This is probably an effect of secondary flows which occur in curved channels (so called Dean vortices): the fluid moves to the outer wall of a curved channel due to centrifugal forces. Since an asymmetric flow profile can lead to a non-uniform volume flow distribution in the following bifurcation, further investigations on this effect are necessary.

Concerning the thermal behaviour of the absorber, the aim is to obtain a uniform heat transfer from the absorber surface into the fluid. In the experiment shown in Fig. 9 the pro­cess was inverted: after cold water had flowed through the absorber, hot water was injec­ted and the temperature distribution on the surface was observed using thermography. The picture in Fig. 9 was taken 114 seconds after injection. The temperature profile on the right hand side is taken along the white line in the thermography picture on the left hand side (with increasing time from front to back). Apart from the wavy structure — resulting from the periodic changes between fluid channels and solid material — it can be stated that the temperature distribution is rather uniform all over the profile, which also indicates a uniform heat transfer.