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92. The Simplex spray nozzle shown in fig. 10-16
was first used on early jet engines. It consists of a
chamber, which induces a swirl into the fuel, and a
fixed-area atomizing orifice. This fuel spray nozzle
gave good atomization at the higher fuel flows, that
Fuel system
114
Fig. 10-15 Various stages of fuel atomization.
is, at the higher fuel pressures, but was very unsatisfactory
at the low pressures required at low engine
speeds and especially at high altitudes. The reason
for this is that the Simplex was, by the nature of its
design, a ’square law’ spray nozzle; that is, the flow
through the nozzle is proportional to the square root
of the pressure drop across it. This meant that if the
minimum pressure for effective atomization was 30
lb. per square inch, the pressure needed to give
maximum flow would be about 3,000 lb. per square
inch. The fuel pumps available at that time were
unable to cope with such high pressures so the
variable port spray nozzle was developed in an effort
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 relationship
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eventually superseded by the Duplex and the Duple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
the fuel to the manifolds (fig. 10-18). As the fuel flow
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
away. With this arrangement it is possible to supply
fuel to the swirl chamber at a high pressure all the
time, As the fuel demand decreases with altitude or
reduction in engine speed, more fuel is spilled away
from the swirl Chamber, leaving less to pass through
the atomizing orifice. The spill spray nozzles’
constant use of a relatively high pressure means that
even at the extremely low fuel flows that occur at
high altitude there is adequate swirl to provide
constant and efficient atomization of the fuel.
96. The spill spray nozzle system, however,
involves a somewhat modified type of fuel supply
and control system from that used with the previous
types. A means has to be provided for removing the
Fuel system
115
Fig. 10-16 A Simplex fuel spray nozzle.
Fig. 10-17 A variable port or Lubbock fuel
spray nozzle.
spill and also for controlling the amount of spill flow
at various engine operating conditions. A
disadvatage of this system is that excess heat may
be generated when a large volume of fuel is being
recirculated to inlet. Such heat may eventually lead
to a deterioration of the fuel.
97. The airspray nozzle (fig. 10-19), carries a
proportion of the primary combustion air (Part 4) with
the injected fuel. By aerating the spray, the local fuelrich
concentrations produced by other types of spray
nozzle are avoided, thus giving a reduction in both
carbon formation and exhaust smoke. An additional
advantage of the airspray nozzle is that the low
pressures required for atomization of the fuel permits
the use of the comparatively lighter gear-type pump.
98. A flow distributor (fig. 10-20) is often required to
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