FUEL SPRAY NOZZLES
89. The final components of the fuel system are the fuel spray nozzles, which have as their essential function the task of atomizing or vaporizing the fuel to ensure its rapid burning. The difficulties involved in this process can be readily appreciated when one considers the velocity of the air stream from the compressor and the short length of combustion system (Part 4) in which the burning must be completed.
90. An early method of atomizing the fuel is to pass it through a swirl chamber where tangentially disposed holes or slots imparted swirl to the fuel by converting its pressure energy to kinetic energy. In this state, the fuel is passed through the discharge orifice which removes the swirl motion as the fuel is atomized to form a cone-shaped spray. This is called 'pressure jet atomization'. The rate of swirl and pressure of the fuel at the fuel spray nozzle are important factors in good atomization. The shape of the spray is an indication of the degree of atomization as shown in fig. 10-15. Later fuel spray nozzles utilize the airspray principle which employs
high velocity air instead of high velocity fuel to cause atomization. This method allows atomization at low fuel flow rates (provided sufficient air velocity exists) thus providing an advantage over the pressure jet atomizer by allowing fuel pumps of a lighter con- struction to be used. 91. The atomizing spray nozzle, as distinct from the vaporizing burner (Part 4), has been developed in five fairly distinct types; the Simplex, the variable port (Lubbock), the Duplex or Duple, the spill type and the air spray nozzle.
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
is, at the higher fuel pressures, but was very unsatis- factory 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 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 beemployed 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