As mentioned before, more attention has to be paid to flow phenomena at bifurcations. Therefore simulations were carried out using CFD (Computational Fluid Dynamics). As can be seen from Fig. 10, only the first bifurcations at the absorber inlet were taken into consideration. A constant outlet pressure was assumed. A mass flow of 0.01 kg/s (7.2 l/ (m2h); laminar flow) and 0.1 kg/s (72.1 l/(m2h); turbulent flow), respectively, was chosen as an inlet condition. Fig. 10, which shows the results of a high-flow simulation, confirms the effects observed in the flow experiments: the fluid tends to prefer the inner channels of the fractal structure. A cross section of the first left branch also reveals the Dean vortices pre­sumed to be the cause for the described effect (indicated by vectors in the detail picture).

72.1 l/(m2h) (turbulent)

3.24e 01 I 2 92e 01 2.59Є-01 2.27Є-01 194e01 1.62Є-01

і 1.30e01 9.72e02 6.4ве 02 I 3 24e 02 H O. OOe+OO

Conclusions

An algorithm which is capable of generating fractal hydraulic structures on a given area with fluid in — and outlet was developed and served as a basis for the programme FracTherm. With this programme it is also possible to carry out hydraulic as well as thermal simulations and visualise the results. A total collector efficiency factor F can be determined. The simulated F of a fractal absorber was high compared to measured val­ues of conventional fin absorbers. DXF files can be exported from FracTherm, which al­lows computer aided manufacturing. Flow experiments with ink indicated secondary flows at bifurcations. Thermography pictures showed that a uniform heat transfer can be ob­tained. CFD simulations confirmed the secondary flow effects observed in the flow experi­ments.

In order to be able to compare fractal structures with conventional ones, it is intended to use FracTherm also for simulations with serial and parallel hydraulic structures. Moreover, further CFD simulations as well as experiments are planned to investigate flow phenom­ena and determine the pressure loss coefficients at bifurcations. Variations of the FracTherm parameters will show their influence on the total energy efficiency. In a further step these variations can be carried out automatically and thus lead to an optimisation us­ing "evolution strategy" [7]. The simulation environment ColSim can be used to carry out dynamic system simulations which will also take thermal capacity effects into considera-

tion. Finally, it is intended to produce and measure prototype absorbers. Thus a comparis­on of fractal absorbers and other heat exchangers with conventional ones will be possible.

Acknowledgement

The author likes to thank the German Federal Environmental Foundation (DBU) for fund­ing this research work in its scholarship programme.

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