Some useful data on hydraulic losses
The traditional loss coefficient represents relative magnitude of dissipated energy in relation to the kinetic energy in some specified cross section: this is usually the smallest
Fig.D-25 Some useful
values on loss coefficients
in bends and possibilities
of decreasing the resistance
by limiting the extent
of the separation region.

cross section of the device. It is an advantage that the value is the same for a whole family of geometrically similar devices of different size,
Fig.D-26
so that experimental data obtained on a single member of the family (perhaps even a reduced-size model) are applicable for all other members. Obviously, also the presentation of such experimental data in tabelar form - such as the table Fig.D-25 - is simpler and easier.
Fig.D-27








Among various cases of local losses, the Borda loss Fig.D-26, due to sudden enlargement of cross-sectional area, has particular importance. It is the only case which it is possible to calculate (though not exactly), as is demonstrated in chap. [H]. Also in many other cases of local losses a similar character of processes may be traced down: if the loss is due to flow separation, Fig.D-18, there is always some enlargement of actual cross-sectional area downstream from the separation. Were the area ratio known, it would be possible to calculate (at least approximately) the loss effect by using the Borda formula. The Borda loss is, of course, loss of available pressure - but it should be stressed
Fig.D-28 Dissipance coefficient data for different geometry of entrances into a pipe.
that unless the area ratio is infinite, the pressure actually increases in the downstream direction as the flow passes through the enlargement. The Borda loss, as shown in Fig.D-27, represents the fact that pressure increase is lower than the one which would correspond to the ideal pressure coefficient.
Fig.D-29
Fig.D-30 Fig.D-31















On the other hand, a catalogue of devices, intended for their selection for use in a pipeline system, such as shown for valves in the small sample
Fig.D-32
in Figs.D-29 to Fig.D-32, should preferrably contain values of dissipance - since these include not only the information about influence of the shape, but also an information about inluence of size and of properties (specific volume) of the used fluid. Values in the examples Fig.D-29 to D-32 are given for the fully open state. To evaluate dissipance of partly open valves, it is necessary to multiply the fully-open dissipance value by a coefficient dependent upon the relative magnitude of plug lift above its seat. There are actually two sets of such coefficients, one valid for valve with sufficiently long constant-diameter pipe downstream, the other for valve located at an entrance to a vessel. In the former case, there is some pressure increase downstream from the smallest cross section, as predicted by the Borda loss theory.


Going to another page: click
This is page Nr. D11 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

WWW server administrators: Jiri Kvarda, Zdenek Maruna ...... Contact: webmaster@vc.cvut.cz
Last change : 9.02.1998