FM 1-506 Fundamentals of Aircraft Power Plants ... - Survival Books

WWW.SURVIVALEBOOKS.COMFM 1-506a low-pressure compressor in front and a high-pressurecompressor in the rear. Both compressors(lowand high)are driven by two different shafts that connect to differentturbines. The starter is usually connected to the high­-pressure compressor because it reduces the torque requiredto start the engine. With the rear (high-pressure)compressor turning at governed Speed, the front (lowpressure)compressor (not governed) is automaticallyrotated by its turbine. Rotation speed is whatever speedwill ensure an optimum flow of air through the compressor.With the front and rear compressor rotors workingin harmony instead of interfering with each other, compressionrates can be increased without decreasing efficiency.Due to the added length of the engine this typeof compressor is found on turbojet aircraft.Most gas turbines in Army aircraft have a combinationof an axial compressor (front) and a centrifugalcompressor (rear). The usual combination is a five- orseven-stage axial- flow compressor and a centrifugal-flowcompressor. The axial compressor and centrifugal compressorcombination is mounted on the same shaft; thecompressors turn in the same direction and at the samespeed. By combining the the manufacturer makes themost of the advantages of both compressors small frontaIarea, increased compression ratios, and shortenedoverall engine length. Using the centrifugal-flow compressorboosts compression and increases efficiency ofthe turbine engine. The centrifugal compressor alsoshortens the length of the engine. If the centrifugal compressorwere not added, the manufacturer would have toadd more stages of axial compression to equal that of thecentrifugal compressor.COMBUSTION SECTIONThe combustion section contains the combustionchambers, igniter plugs, and fuel nozzle or fuel injectors.It is designed to burn a fuel-air mixture and to delivercombusted gases to the turbine at a temperature notexceeding the allowable limit at the turbine inlet.Theoretically, the compressor delivers 100 percent of itsair by volume to the combustion chamber. However, thefuel-air mixture has a ratio of l5 parts air to l part fuel byweight. Approximately 25 percent of this air is used toattain the desired fuel-air ratio. The remaining 75 percentis used to form an air blanket around the burninggases and to dilute the temperature, which may reach ashigh as 3500ºF, by approximately one-half. This ensuresthat the turbine section will not be destroyed by excessiveheat.The air used for burning is known as primary air; thatused for cording is secondary air. Secondary air is controlledand directed by holes and louvers in the combustionchamber liner. Igniter plugs function during startingonly; they are shut off manually or automatically. Combustionis continuous and self-supporting. After engineshutdown or failure to start, a pressure-actuated valveautomatically drains any remaining unburned fuel fromthe combustion chamber. The most common type usedin Army gas turbine engines is the external annularreverse-flow type.The primary function of the combustion section is,of course, to bum the fuel-air mixture, thereby addingheat energy to the air. To do this efficiently, the combustionchamber must–Provide the means for mixing the fuel and air toensure good combustion.Bum this mixture efficiently.Cool the hot combustion products to a temperaturewhich the turbine blades can withstandunder operating conditions.Deliver the hot gases to the turbine section.The location of the combustion section is directlybetween the compressor and turbine sections. The combustionchambers are always arranged coaxially with thecompressor and turbine, regardless of type, since thechambers must be in a through-flow position to functionefficiently.All combustion chambers contain the same basicelements:A casingA perforated inner liner.A fuel injection system.Some means for initial ignition.A fuel drainage system to drain off unburnedfuel after engine shutdown.There are currently three basic types of combustionchambers, varying in detail only:The multiple-chamber or can type.The annular or basket type.The can-annular type.Can-Type Combustion ChamberThe can-type combustion chamber is typical of thetype used on both centrifugal and axial-flow engines(Figure 3-11). It is particularly well suited for thecentrifugal compressor engine since the air leaving thecompressor is already divided into equal portions as it3-9