The limited number of experimental investigations on similarity operation in the literature is not sufficient to state the validity of similarity laws for cross-flow fans. This is also due to the operating conditions occurring in practical applications, where the values of impeller size and rotational speed generally result in low Reynolds numbers and, then, in a non-negligible influence of viscous forces on performance. The analysis is further complicated by the high influence of the casing shape on performance, which can lead to induced effects on similarity operation. An experimental test program is presented in this paper, aimed at understanding the influence of Reynolds number on performance for different casing shapes. Five impellers having similar shape and different dimensions were tested, each one matched with five casings at different rotational speed. The results show that similarity laws can be applied with good approximation above critical blade Reynolds numbers of around 4000 to 15,000, depending on the geometrical characteristics of the casing. The strong scale effect, shown by some authors in the literature, is not observed in these tests.

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