Of course, energy cannot be
lost in the literal sense. That is why in the chapter heading above
the term is written in the quotation marks. We use this expression to accentuate that conversion into thermal energy has
one-directional, irreversible character. The energy converted into heat
cannot be used any more for other changes - at least in the present
context and especially in liquids. This irreversibility is the consequence
of general tendency of processes in nature towards higher
disorder (- change into heat is associated with increase in entropy, which is
basically a measure of disorder): thermal energy
is energy of chaotic motion of fluid molecules.
A mechanical model might be quite instructive: let us imagine
a rotating wheel supported by bearings. Its rotation slows
down due to bearing friction and air drag - even up to a halt
without any outside action. Where did the disappeared kinetic energy of rotating wheel
come to ? In is not lost
but converted into heat (heating of bearings due to friction is a measurable effect). It would be
a futile attempt to try to reverse the proces - and to put the wheel into rotation by heating the bearings
with a torch. Why ? For such an attempt to be successful, it would be necessary
for the chaotic thermal motions to order themselves - and this would be associated with
decrease of entropy.
Irreversibility of the change
is also caused by
heat being lost by transfer into surrounding material. The new component
may be written - as in the components treated in previous chapters - as
a product of an intensity factor, which in this case is temperature
,
and an extensity factor, which is here the thermal capacity
. The expression for specific values
is
. Specific capacity
may be for the present purposes considered to be a material constant,

Its numerical
values for several most important fluids is found in the table Fig.D-7.
The increase in thermal energy therefore manifests itself as an increase in temperature
of fluid -



- this leads to heat flux into the surroundings in the direction of the temperature gradient.
The converted part of the energy is thus no more accessible
and therefore lost for our purposes (Fig.D-1).
The notion of "loss" as escape of a part of available energy
was also supported by the fact that generated temperature differences are very small
and almost immeasurable. Let as assume an example of dissipation of kinetic energy of
a submerged water jet
according to
Fig.C-25 for a rather large difference in surface heights,
equal to
5 metres.
In this case the discharge velocity (cf. formula in Fig.C-25) is
. The "lost" energy is
.
For water, there is
(Fig.D-7).
In spite of this being an exceptionally
high loss (usual magnitudes for flow in pipeline
elements are by several decimal odrers smaller),
the resultant temperature increase
is as low as
- this is practically not measurable
and, moreover, such an increase would almost immidiately disappear by conduction into the
surrounding liquid or pipeline system components.
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This is page Nr. D01 from textbook
Vaclav TESAR : "BASIC FLUID MECHANICS"
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