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

WWW.SURVIVALEBOOKS.COMFM 1-506manifold, the entire assembly (diffuser and manifold) isoften referred to as the diffuser.The impeller’s function is to pick up and accelerateair outward to the diffuser. Impellers may be either oftwo types – single entry or double entry. Both are similarin construction to the reciprocating engine superchargerimpeller. The double-entry type is similar to two back-toback impellers. However, because of much greater combustionair requirements in turbine engines, theseimpellers are larger than supercharger impellers.The principal differences between the two types ofimpellers are size and ducting arrangement. The doubleentrytype has a smaller diameter but is usually operatedat a higher rotational speed to ensure enough airflow.The single-entry impeller permits convenient ductingdirectly to the impeller eye (inducer vanes) as opposedto the more complicated ducting necessary to reach therear side of the double-entry type. Although slightlymore efficient in receiving air, the single-entry impellermust be large in diameter to deliver the same quantity ofair as the double-entry type. This of course, increases theoverall diameter of the engine.Included in the ducting for double-entry compressorengines is the plenum chamber. This chamber is necessaryfor a double-entry compressor because air mustenter the engine at almost right angles to the engine axis.To give a positive flow, air must surround the enginecompressor at a positive pressure before entering thecompressor.Multistage centrifugal compressors consist of two ormore single compressors mounted in tandem on the sameshaft. The air compressed in the first stage passes to thesecond stage at its point of entry near the hub. This stagewill further compress the air and pass it to the next stageif there is one. The problem with this type of compressionis in turning the air as it is passed from one stage to thenext.The diffuser is an annular chamber provide with anumber of vanes forming a series of divergent passagesinto the manifold. The diffuser vanes direct the flow ofair from the impeller to the manifold at an angle designedto retain the maximum amount of energy imparted by theimpeller. They also deliver the air to the manifold at avelocity and pressure satisfactory for combustion chambers.The compressor manifold diverts the flow of air fromthe which, which is an integral part of the manifold, intothe combustion chambers. The manifold will have oneoutlet port for each chamber so that the air is evenlydivided. A compressor outlet elbow is bolted to each ofthe outlet ports. These air outlets are constructed in theform of ducts and are known by a variety of namesincluding "air outlet ducts", "outlet elbows," and "combustionchamber inlet ducts.” These outlel ducts performavery important part of the diffusion process. Theychange the airflow direction from radial to axial. Thediffusion process is completed after the turn. To help theelbows perform this function efficiently, turning vanes(cascade vanes) are sometimes fitted inside the elbows.The vanes reduce air pressure losses by presenting asmooth, turning surface.The centrifugal compressor is used best on smallerengines where simplicity, flexibility, and ruggedness areprimary requirements. These have a small frontal areaand can handle high airflows and pressures with low lossof efficiency.Centrifugal-flow compressors have the following advantages:High pressure rise per stage.Efficiency over wide rotational speed range.Simplicity of manufacture with resulting lowcost.Low weight.Low starting power requirements.They have the following disadvantages:Large frontal area for given airflow.Impracticality if more than two stages becauseof losses in turns between stages.Axial-Flow CompressorAxial-flow compressors have have main elements arotor (drum or disc type) and a stator. These compressorsare constructed of several different materialsdepending on the load and operating temperature. Thedrum-type rotor consists of rings that are flanged to fitone against the other so that the entire assembly can beheld together by through bolts. This type of constructionis satisfactory for low-speed compressors wherecentrifugal stresses are low (Figure-3-7). The rotor (disctype)assembly consists of—Stub shafts.Discs.Blades.Ducts.Air vortex spoilers.Spacers.Tie bolts.Torque cones.3-5