Fig.F-18 Basic two-dimensional considerations: decomposition of velocity vectors
into two mutually orthogonal components. The component in radial direction
is denoted by index "1", while the other component, in tangential direction
is written with index "2".
Note the general tendency of shapes of characteristics: generated output energetic difference decreases with increasing flow rate . Practical usage of characteristics is the subject of the next chapter [G].

In practice, characteristics are sometimes still plotted using old systems of quantities (which, after all, may have some practical advantages). For example, characteristics of pumps are often plotted with an equivalent height difference in place of the specific energy on the vertical axis (- since pumps are most often used for transporting liquid between two levels, direct reading of the level difference is quite useful). The mutual conversion between the two systems is, of course, quite simple: = . In a similar manner, characteristics of blowers



and ventilators may be encountered with pressure difference plotted on the vertical axis (conversion: = ). One of the advantages of using co-ordinates and is the fact that the product . = is power in proper units (watts) without any need for conversions.
The basic parabolic shape, as derived in Fig.F-9 and shown in the practical examples of Fig.F-16 and Fig.F-19, may be for some pumps and blowers with complex three-dimensional processes taking place in their blade systems, replaced by rather complex shapes. Especially at small flow rates the behaviour is often complicated by effect of flow separation from blade surfaces. Nevertheless, the equation in Fig.F-9 is usually a good approximation at least in the vicinity of optimum operating point (cf. Fig.F-16).
Usually, the optimum operating point coincides with the point in which the value of the power is maximum.

Since the last century, there have been attempts at finding some general features in the seemingly unsurmountable variety of pump and blower behaviour as presented in their characteristics.
Fig.F-19
First successful move in this direction was the discovery of the possibility to convert characteristics obtained at different speeds onto just a single curve in nondimensional co-ordinates - Fig.19. The practical advantage of this finding is universality of presentation: the behaviour is characterised irrespective of the speed of the motor or drive. Another advantage is the validity of the non-dimensional characteristic irrespective of the size of the machine: experimental investigations mea be performed on a scaled-down model (cheaper and easier to handle) - and yet the model and the real machine posses the same characteristic in the dimensionless co-ordinates.


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This is page Nr. F07 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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