The atomizing spray nozzle, as distinct from the
vaporizing burner, has been developed in five fairly distinct
types, the Simplex, the variable port (Lubbock), the Duplex
or Duple, the spill type, and the airspray nozzle.
The Simplex spray nozzle shown in Figure 7e16 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 atom-
ization at the higher fuel flows, that is, at the higher fuel
pressures, but was very unsatisfactory at the low pressures
required at low engine speeds and especially at high alti-
tudes. 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 3000 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.