$ 24 (b)] THE STEAM-JET. 130
After the steam attains its full velocity in the tube D, and in
passing any cross-plane, as GH, it must continually push ahead
of it steam subjected to the lower pressure p₁, doing the work
=
U-144p 144p(x,₂+w₂).
(113)
As the pressure drops along the nozzle, the steam expands;
and if we imagine two cross-planes close together and enclosing a
small body of steam between them, then the static pressure in
the substance will be less at the right plane than at the left one.
This small difference of pressure produces a corresponding accel-
eration, and the pressure-work of the expansion is thus gradually
changed into kinetic energy of the jet.
The expansion, unless very special conditions are imposed,
is essentially adiabatic: for after any particular action has become
established, and each portion of the inner surface of the nozzle
has assumed the temperature of the part of the jet which touches
it, then only as heat is conducted along the nozzle or radiated
from it will there be any chance for a transfer of heat between jet
and guiding-surface.
The adiabatic expansion-work is-see Eq. (74), § 13 (k)-
U₂=778(q,+x4—qz−x+y).
(114)
And the net work expended in accelerating the current, per pound
of steam, is
U=U₁+UÂ−U₂
(115)
This is the same as the effective work of Cycle B, Fig. 26 and
$16 (b), corresponding to U in Tables 16 A and 16 B; and if we
reduce it to heat-units, using the symbol E for the energy AU
given to the jet, we have
E=AP₁(zu₁+w₁) - AP₂(z+w₂)+4₂+×4 −qa− xyl
=9+9+A(P₁₁-P₂w₂).
(116)
This equation embodies the simpler method of calculating the net work of Cycle B, referred to under $16 (b); except that in that case the last term, A(Pw₁-Pw), which represents the water-volume effect, was omitted.
