PPL Lesson 2

• The compressor section's primary function is to supply air in sufficient quantity to satisfy the requirements of the combustion burners.
• A secondary function of the compressor is to supply bleed air for various purposes in the engine and aircraft.
• Centrifugal Flow Compressor has a large frontal area for a given airflow and losses in turns between stages.
• Axial Flow Compressor has good efficiencies over only a narrow rotational speed range, is difficult to manufacture and expensive, is relatively heavy, and has high starting power requirements which are partially overcome by split compressors.
• The centrifugal compressor, sometimes called a radial outflow compressor, is one of the earliest compressor designs and is still used today in some smaller engines and auxiliary power units (APU's).
• Centrifugal flow compressors offer several advantages including simplicity of manufacture, relatively low cost, low weight, low starting power requirements, and operating efficiency over a wide range of rotational speeds.
• Centrifugal compressors have a high pressure rise per stage that can be around 8:1.
• Generally centrifugal compressors are limited to two stages due to efficiency concerns.
• Centrifugal compressors consist of: Impeller, Diffuser, and Manifold.
• The rotor blades are usually made of stainless steel with the latter stages being made of titanium.
• Stator vanes are the stationary blades located between each row of rotating blades in an axial flow compressor, acting as diffusers for the air coming off the rotor, decreasing its velocity and raising its pressure.
• Axial flow compressor stator blades act as diffusers at each stage, partially converting high velocity to pressure.
• Centrifugal flow compressors have high pressure rise per stage, efficiency over wide rotational speed range, simplicity of manufacture and low cost, low weight, and low starting power requirements.
• Stator vanes may be secured directly to the compressor casing or to a stator vane retaining ring, which is secured to the compressor case.
• As the compressor rotor rotates, centrifugal force keeps the blades in their correct position, and the airstream over each blade provides a shock absorbing or cushioning effect.
• Most stator vanes are attached in rows with a dovetail arrangement and project radially toward the rotor axis.
• Stator vanes help prevent swirling and direct the flow of air coming off each stage to the next stage at the appropriate angle.
• Like rotor blades, stator vanes have an airfoil shape.
• The angle of attack of stator vanes can be fixed or variable.
• The base, or root of a rotor blade often fits loosely into the rotor disk, allowing for easy assembly and vibration damping.
• Stator vanes are often shrouded at their tips to minimize vibration tendencies.
• The second compressor section of a twin-spool compressor is called the high pressure, high speed, or N2 compressor and is typically driven by a single stage high-pressure turbine at the front of the turbine section.
• Axial flow compressor rotor blades increase the compression of the air at each stage and accelerate it rearward through several stages.
• A low pressure compressor is typically driven by a two-stage turbine at the rear of the turbine section.
• Axial flow compressors have high peak efficiencies, small frontal area for given airflow, straight-through flow, allowing high ram efficiency, and increased pressure rise by increasing number of stages, with negligible losses.
• The rotor blades used in an axial flow compressor have an airfoil cross-section with a varying angle of incidence, or twist, which compensates for the blade velocity variation caused by its radius.
• The impeller's function is to take air in and accelerate it outward by centrifugal force.
• Compressors having only one impeller are referred to as single-stage compressors while compressors having two impellers are referred to as double-stage compressors.
• When two impellers are mounted back-to-back a double-sided or double-entry impeller is created.
• The diffuser acts as a divergent duct where the air spreads out, slows down, and increases in static pressure.
• The diffuser is an annular chamber provided with a number of vanes forming a series of divergent passages into the manifold.
• The diffuser vanes direct the flow of air from the impeller to the manifold at an angle designed to retain the maximum amount of energy imparted by the impeller.
• The compressor manifold distributes the air in a smooth flow to the combustion section.
• The mani­fold has one outlet port for each combustion cham­ber so that the air is evenly divided.
• The axial-flow compressor has two main elements: a rotor and a stator.
• Each consecutive row of rotor blades and stator vanes constitutes a pressure stage.
• Unlike a centrifugal compressor, which is capable of compressor pressure ratios of 15:1, a single stage in an axial flow compressor is capable of producing a compressor pressure ratio of only 1.25:1.
• High compressor pressure ratios are obtained by adding more compressor stages.
• Single spool compressor - the compressor and turbine are connected by a single shaft and rotate as a single unit.
• Dual-spool or Twin-spool compressors - The front section of a dual-spool compressor is called the low pressure, low speed, or N1 compressor.