The Organic Rankine Cycle (ORC) offers a great potential for recovering waste heat and using low-temperature sources for power generation. However, the ORC thermal efficiency is limited by the relatively low temperature level, and, therefore, designing ORC components with high efficiencies and minimized losses is of major importance. Such an approach requires the use of a specially designed closed cascade wind tunnel. This contribution presents the design of the contraction zone shape. The ideal shape can be defined by a sixth order polynomial yielding a smooth curve for the nozzle profile. Due to pressure vessel costs, it is not possible to realize the whole contraction zone as one piece for this wind tunnel. Instead, a piece-wise conical design approach is chosen. Classical nozzle design guidelines do not offer an analytical solution to this flow problem. Therefore, computational fluid dynamics (CFD) in combination with Stratford’s separation criterion is used for an optimization study of a piece-wise conical contraction zone. Different combination of numbers of components, length, and inflection points are investigated. The optimization minimizes the flow deviation of the chosen profile to the optimal shape in two steps: a geometrical approach to the optimal shape and an optimization of the flow field within the contraction zone. The geometrical optimization yields a profile with minor deviation to the ideal shape. For the flow field optimization, a CFD analysis is used to minimize flow separations at the break points between the single conical pieces, especially those at the far end of the contraction zone. All shapes are investigated by Stratford’s separation criterion, which is adopted to conical pieces. The presented analysis indicates that the flow field optimization yields a much better approach for the fluid dynamics of the wind tunnel than the geometrical approach.

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