Numéro : 1994 - Year : 1985
Stability and coupling of crossflow fans
G. HEID, Ingénieur, Institut de Mécanique des Fluides de Lille
This study is the continuation of a research program concerning the realization, experimentation and optimisation of a crossflow fan used to provide the lift or the lift and propulsion forces in surface effect vessels.
The aeraulic characteristic of this type of fan is at first sight particularly well suited in reducing ramming, and this has led into equipping a NES 4000 L2 model at the "Bassin d'Essais des Carènes" with a rack of two motofans working in parallel. The tests carried out with this model showed the existence on one hand of an effect of coupling and on the other hand of an effect of surging in certain areas of operation. The results presented are of two series of tests dealing with each of the two complementary subjects.
Surging
The existence of an effect of stationary waves is shown. Recordings of
dynamic transitions on the fan characteristics are given and it is shown how the operating point of the fan evolves once the surging begins to appear or disappear. The cycles of surging and the respective influences of circuit parameters on the characteristics of surging are presented. Finally, it is shown that it is possible to deduce a statistical model of the phenomena and to apply to it the laws of similarity dealing to the occurrence of the phenomenon on fans of larger size.
Coupling
The analogy of this phenomenon with surging and also the way one can theoretically deduce an overall characteristic from the characteristics of two crossflow fans mounted in parallel are shown. The areas of stability or instability were found experimentally and compared with experimental recordings. In particular, the large categories of dynamic behaviour are given and compared with theoretical predictions. Finally, the advantage of a solution based on the voluntary dissymmetry of the characteristics of the two fans is shown.
In conclusion, future developments of studies dealing with these problems and in particular the adaptation of the theoretical conclusions to more complex circuits is passed in review.
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