At the Institute for Micro Process Engineering of the Forschungszentrum Karlsruhe, micro heat exchangers are manufactured out of single foils of base metal alloys. The characterisation of the thermohydraulic properties of microchannel heat exchangers is done using water as heat transfer medium on both passages at temperatures of 10°C and 95°C. The present publication will give an overview of the numerical simulation as well as experimental results for crossflow and counterflow microchannel heat exchangers. A comparison of three crossflow heat exchangers with different microchannel structures, and two different types of counterflow microchannel heat exchangers is shown. For comparison, the heat transfer rate, the overall heat transfer coefficients and efficiencies as well as pressure drop obtained from experiment and theory is shown. For numerical simulation, two models have been used. An easily accessibly method is to use classical engineering codes based on the Nusselt theory (VDI Wa¨rmeatlas, 1994). A more detailed model is to use computational fluid dynamics (CFD) with the commercially available tool FLUENT ®, where best estimation codes have been applied for numerical simulation. Both numerical calculations are a helpful complement to predict thermal and hydrodynamic behaviour of the microchannel heat exchangers.

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