Page 125 - The Jet Engine

Fuel system

and pressure increases, the pressurizing valve
moves to progressively admit fuel to the main
manifold and the main orifices. This gives a
combined flow down both manifolds. In this way, the
Duplex and Duple nozzles are able to give effective
atomization over a wider flow range than the Simplex
spray nozzle for the same maximum fuel pressure.
Also, efficient atomization is obtained at the low flows
that may be required at high altitude. In the combined
acceleration and speed control system (para. 51),
the fuel flow to the spray nozzles is apportioned in
the F.F.R.

95. The spill type fuel spray nozzle can be
described as being a Simplex spray nozzle with a
passage from the swirl chamber for spilling fuel
Fig. 10-16 A Simplex fuel spray nozzle. away. With this arrangement it is possible to supply
fuel to the swirl chamber at a high pressure all the
is, at the higher fuel pressures, but was very unsatis- time, As the fuel demand decreases with altitude or
factory at the low pressures required at low engine reduction in engine speed, more fuel is spilled away
speeds and especially at high altitudes. The reason from the swirl Chamber, leaving less to pass through
for this is that the Simplex was, by the nature of its the atomizing orifice. The spill spray nozzles'
design, a 'square law' spray nozzle; that is, the flow constant use of a relatively high pressure means that
through the nozzle is proportional to the square root even at the extremely low fuel flows that occur at
of the pressure drop across it. This meant that if the high altitude there is adequate swirl to provide
minimum pressure for effective atomization was 30 constant and efficient atomization of the fuel.
lb. per square inch, the pressure needed to give
maximum flow would be about 3,000 lb. per square 96. The spill spray nozzle system, however,
inch. The fuel pumps available at that time were involves a somewhat modified type of fuel supply
unable to cope with such high pressures so the and control system from that used with the previous
variable port spray nozzle was developed in an effort types. A means has to be provided for removing the
to overcome the square law effect.
93. Although now only of historical value, the
variable port or Lubbock fuel spray nozzle (fig. 10-
17) made use of a spring-loaded piston to control the
area of the inlet ports to the swirl chamber. At low fuel
flows, the ports were partly uncovered by the
movement of the piston; at high flows, they were fully
open. By this method, the square law pressure rela-
tionship was mainly overcome and good atomization
was maintained over a wide range of fuel flows. The
matching of sets of spray nozzles and the sticking of
the sliding piston due to dirt particles were, however,
difficulties inherent in the design, and this type was
eventually superseded by the Duplex and the Duple
fuel spray nozzles.
94. The Duplex and the Duple spray nozzles
require a primary and a main fuel manifold and have
two independent orifices, one much smaller than the
other. The smaller orifice handles the lower flows and
the larger orifice deals with the higher flows as the
fuel pressure increases. A pressurizing valve may be
employed with this type of spray nozzle to apportion Fig. 10-17 A variable port or Lubbock fuel
the fuel to the manifolds (fig. 10-18). As the fuel flow spray nozzle.

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