Bladed (both axial and radial) type pumps and blowers as represented by the schematic Fig.G-10 possess a different type of output characteristics. Their output flow varies substantially with loading. On the other hand, expecially if loaded by high dissipance loads, their generated specific energy difference varies relatively very little and the performance may be modelled by the other basic source model - the ideal specific energy generator - Fig.G-11. In a larger range, an even better model of their characteristric could be obtained by a model composed of series connection of the ideal specific energy source and a restrictor - Fig.G-15. The restrictor serves to model the effect of internal losses, which decrease the available energetic drop and this decrease is roughly proportional to square of the mass flow rate through the machine.
An interesting fact is that the simplest connection of two nodes by a pipe which exhibits negligible loss (this is an acceptable simplification for many purposes, since friction loss is often - especially for short pipes - negligible in comparison with local losses in devices) according to Fig.G-12 may be represented by the degenerate case of ideal specific energy source with
= 0. On the other hand, severing a connection by blocking the pipe
as represented in Fig.G-13 may be modelled by the ideal flow source generating, of course, zero flow (so thgat its vertical characteristics corresponds to the vertical line
= 0. Note that the situation is reverse to that in electrical engineering, interconnection must be blocked here, simple broken pipe would represent nearly zero-dissipance outflow into atmosphere.
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Vaclav TESAR : "BASIC FLUID MECHANICS"
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