Loops are not only convenient concepts for calculations based upon Kirchhoff's second law, but they are actually basic units of fluid systems. The task of a fluid system is usually spatial transport of fluid - the aim may be to transport mass of fluid, but quite of then the real reason is fluid transport of power and in some cases transport of information. There is usually some fluid flow source, necessary to impart energy to fluid, and some load in which the energy is dissipated. Fig. G-6 presents schematic diagram of such transfer of power by fluid flow between the two elements of a single loop.
Fig.G-6
There are cases where the return branch of the loop is really missing: the source may be arranged for just a one-time action (pressurised vessel) or the fluid may be somehow consumed in the load (e.g. by burning - in the case of fluid fuel supplied to an engine). There are, however, many instances of gas system in which the return branch is only seemingly missing and in fact is formed by atmosphere (if fluid is a liquid, the role of atmosphere may be played by some large storage vessel) - Fig.G-7.
Fig.G-7




SOURCES - flow generators: pumps, blowers or even simple storage vessels - are, of course, essential components of fluid systems. Not only there would be without a source no flow in a system, but source properties influence in a decisive manner the processes taking place in a system (for example, different transitional phenomena in a CQ circuit take place if an identical circuit consisting of simple two elements is supplied
Fig.G-8
from constant specific energy source or from constant flow source). Since properties of real fluid flow generators as expressed by their characteristics may be sometimes very complicated, it is a useful idea to start the initial discussion of system behaviour using some simplified models. Unfortunately, behaviour of the two most important groups of fluid flow source we have already learned about - positive displacement machines discussed in chap. [E] and centrifugal pumps and blowers discussed in chap. [F] - differs so much that it cannot be represented by a single common simple model and it is necessary to introduce two basic models.
Note in Fig.G-8 and all subsequent illustrations that absolute values are used for expressing the generated energetic drop. This to observe the convention that represents a drop (i.e. decrease in flirection of flow) across an element due to loss, as introduced in chap. [D]
Positive displacement type pumps and blowers generate flow which is given by drive speed and the volume displaced during one operating cycle.
Fig.G-9
At a constant speed operation (very common in practice because of general industrial use of asynchronous electric motors) the characteristics in the co-ordinates shown e.g. in Fig.G-9 is very nearly vertical - being very little influenced by loading. A suitable basic model for this behaviour is the ideal flow generator. Also a source represented by very large vessel (pressurised gas storage vessel or fishpond as source of water avalable under its dam) behaves according to this pattern. Deviations from this model encountered in real displacement machines machones are due to the fact that aspecially at high some fluid tends to escape through inevitable leaks.
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This is page Nr. G03 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS"
Any comments and suggestions concerning this text may be mailed to the author to his address tesar@fsid.cvut.cz

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