Principles of naval engineering - Historic Naval Ships Association

Chapter 23. -GAS TURBINEnozzles at the pressure and volume required tomaintain the desired engine performance. At thesame time, the fuel control governor limits fuelflow to maintain operating conditions within safelimits.The fuel nozzles serve to introduce the fuelinto the combustion chamber. The fuel is sprayedinto the combustion chamber under pressure,through small orifices in the nozzles. Variouskinds of fuel nozzles are in use. Figure 23-16shows a simplex nozzle and a duplex nozzle.The simplex nozzle was used on some oldergas turbine engines. Most recent gas turbineengines use some kind of duplex nozzle. A duplexnozzle requires a dual manifold just ahead ofthe nozzles and a flow divider (before the manifold)to divide the fuel into primary and secondarystreams. The duplex type of nozzle providesa desirable spray pattern for combustionSECONDARYBFigure 23-16.— Fuel nozzles.A. Simplex. B. Duplex.147.150over twice the range of that provided by thesimplex nozzle.The fuel control assembly is the unit whichregulates the turbine rpm by adjusting fuel flowfrom the high pressure engine-driven pump tothe fuel nozzle. The major parts of the fuelcontrol assembly are shown in figure 23-17.Fuel enters the fuel control assembly and ispumped through a filter. High pressure fuel isrouted to the differential relief valve, then tothe fuel shutoff valve, and finally to the fuelnozzles.The speed setting lever on the outboard endof the governor is connected to a speed controldevice on the control console either by a cableor by an electric servomotor. At the fuel controlend, the lever is keyed to a pinion. Thispinion positions a rack which in turn controlsthe governor flyweight spring. The mechanismregulates gas producer speed according to theposition of the control lever. With the controllever in any particular position, variations fromthe preset speed are sensed by the governorflyweights and a compensating movement of thefuel control valve results. An externally adjustableneedle valve provides a constant minimumfuel flow during deceleration, when the governorvalve is closed, to prevent loss of combustion.An acceleration limiter, consisting of a needlevalve positioned by a shaft, arm, and bellows,is actuated by compressor discharge pressure.During acceleration, this mechanism controlsfuel flow to the point at which the governor flyweightmechanism and its fuel control valvetake over.LUBRICATING SYSTEM.-Because of thehigh operating rpm and the high operating temperaturesencountered in gas turbine engines,proper lubrication is of vital importance. Thelubricating system is designed to supply bearingsand gears with clean lubricating oil at thedesired pressures and temperatures. In someinstallations, the lubricating system also furnishesoil to various hydraulic systems. Heatabsorbed by the lubricating oil is transferredto the cooling medium in a lube oil cooler.The lubricating system shown in figure 23-18 has a combined hydraulic system— in thiscase, the hydraulic system is for the operationof a hydraulic clutch in a gas turbine propulsionServomechanisms are discussed in chapter 20 of thistext.607