Abstract
The accuracy of model testing rests on the applicability of the affinity laws interrelating the behaviour of geometrically similar machines running at various speeds. To be confident of this applicability it is necessary to ensure, with sufficient accuracy, the similarity of the flow patterns within the prototype and model.
With a perfect liquid considerable guidance as to the flow regime in a pump or turbine can be gained from partial streamline maps based purely on hydrodynamics. Approximate maps are given for the relative flow within an idealized centrifugal impeller, and the respects in which these fall short of representing the regime throughout a complete pump, even with perfect liquid, are discussed.
The actual flow patterns, both in model and prototype, will differ from the flow regime ruling in the case of a perfect liquid, and in general from one another, mainly by reason of viscosity. If cavitation occurs, further differences in behaviour will be caused, notably by reason of gas content and surface tension and by the downward limit of absolute pressure.
The nature, location, and extent of these relative departures, and their effect on the applicability of the affinity laws, are examined in the light on the flow pattern afforded by the streamline maps.
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