 |
Fig.G-15
|
The constant specific energy generator (this is, for incompressible fluid, practically equivalent to a constant pressure source) is practically realisable only as a rather complex device comprising a pressure controller (regulator). Nevertheless, it is a useful concept - it may serve as a building block for models of elements. Such models are used to simulate more complex behaviour (graphically
 |
Fig.G-16
|
represented by a more complex shape of the characteristic curve). By way of an example, behaviour of turbomachines (pumps, ventilators, blowers,...) as we have learned it in chap.[F],
may be reasonably well modelled by a model (Fig.G-15) consisting of the constant specific energy source and a quadratic restrictor connected in series to this source. Fig.G-16 shows a practical example in which such representation was found nearly perfect. Even if the complete characteristic of turbomachines are sometimes more complex, this model is usually applicable at least in the most important region near the optimum operating point.


In steady states, the only relevant characterisation quantity of an element is its dissipance. Although the relations discussed are valid for elements of any sort, we shall discuss them on the particular example of restrictors (since these are the element the task of which is just to posses some dissipance). The other two characterisation quantities,
capacitance and
inertance make themselves felt only in unsteady flow situations.


- two two-terminal elements may be connected either in series or in parallel, Fig.G-17. Let us consider two different restrictors:
the first one is characterised by dissipance
, while the other one has dissipance value
. The equation of characteristic for the first restrictor is
=
and the similar equation for the second restrictor is
=
In the case of connection in series
in Fig.G-18, the first Kirchhoff law applied to the connection node between the two restrictors leads to the result of both flows being equal: 
= 
=
. Any practical operation of this simple system will be possible only if it is connected with some fluid-supplying source. Let us assume, for simplicity, use of the source generating constant specific work
. Connecting this source will change the system into a loop. The second Kirhhoff law applied to this loop leads to the conclusion of there being

+

=
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Vaclav TESAR : "BASIC FLUID MECHANICS"
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