-High velocity gases pass through the turbine nozzle and impact the turbine blades, the turbine wheel rotates. -Each stageconsists of a turbine nozzle and wheel.
Turbine Disk Inspections-Typically inspected visually using a strong inspection light and magnifying glass. -Anycracks found on a turbine disk are cause for rejection and replacement.-If slight pitting exists, some manufacturers do allow continued use as long as the pitting can be blended by stoning and polishing.Single spool-In some engines, a single turbine wheel or stageabsorbs energy from the exhaust gas to drive the compressor and accessories. -Many engines use multiple turbine stagesbecause a single stage can't absorb enough of the energy. It has two or more stages of turbine turning a common shaftto drive the compressor. -One turbine blade can extract 250 HP. Multi-spool-Two or more sets of turbines driving different sections of the engine-Low, medium and / or high pressure compressor each section turns free and independent of each other.RADIAL INFLOW TURBINE-Primarily used on APUs -Low cost of manufacturing-High single stage efficiency, poor for multi stage-Gases flow inwards from stator vanes and out the center of the impeller.TURBINE BLADES-Typical temperature range turbine blade are subjected to is 700-1200 C-Blades are airfoil shaped designed to extract the maximum amount of energy from the flow of hot gases. -Blades are commonly precision cast and then ground down to a fine finish. -Today most turbine blades consist of cast nickel-based alloys. -Blades can fit loosely into a turbine disk when an engine is cold, but expand to fit tightly at normal operating temperatures. -The most commonly used method for attaching turbine blades is by fir treeslots cut into the turbine disk rim and matching bases cast or machined into the turbine blade base. -Blade may be retained in its groove by peening, welding, rivets, or lock tabs
Open ended blades -Used on high speed turbines with short blades-can be machine/ flat tip orprofile/ squealer tipShrouded blades -A shroud is attached to the tip of each blade. The shrouds of the blades contact each other, thereby providing support. -Reduces vibration substantially. -Prevent air from escaping over the blade tips, making the turbine more efficient. -More susceptible to blade growth, because of the added weight.Knife-edge seal-Is machined around the outside of the shroud -Reduces air losses at the blade tip. -Fits with a close tolerance into a shrouded ring mounted in the outer turbine case. Turbine blades types-are classified as impulse, reaction or impulse reaction typeimpulse blades-The blades merely change direction of the airflow coming from the turbine nozzle and cause relatively no change of gas pressure or velocity. The turbine wheel simply absorbs the force required to change the direction of airflow and converts it to rotary motion.
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