Pressure energy
In Fig.A-23 there is, quite analogically to
Fig.A-21, an element of pipeline system
in which work is done
at the price of pressure energy. While at the
input
there is
pressure,
in the element output
there is a lower pressure
. The work required to
Fig.A-23
get the marked fluid column of length
into the element
is again calculated as a product of force and distance - this time
the force in question
(it is expressed in Fig. A-14)
and the distance is equal to the lenth of fluid column
.
If the decrease of pressure energy of fluid is evaluated as the
difference between the input work and output work, the result
... where
is the pressure drop across the element. As a result
we obtain the expression for the pressure energy
.
For specific value
- again, the transition to specific values does not apply
to the intensity factor, which in this
case is pressure. An important fact is the formal
similarity of expressions for the both kinds of energy, pressure and
position one. Similarly, it is possible to find
at least certain similarity also in other kinds of energy.
Here in both cases we find invariance (which need not be the case for the other cases)
of the specific value of extensity factor:
we work here with the gravity acceleration
as if it were a constant (which, strictly speaking, it is not - but
its variations, e.g. with increasing height above sea level, are
negligible for technical calculations) and since we limit here our attention
to the model of incompressible fluid, there is also
Compressibility effect: are worth at least a brief note,
even though we shall not handle them further in this textbook :
in compressible case,
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This is page Nr. A11 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS" Any comments and suggestions concerning this text may be mailed to the author
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