Turbine

Jet Engine > Turbine

After air has passed through the combustor, the jet engine has achieved the change in momentum of air required to create thrust and make the plane fly.  So what does a turbine do?

If we look at the engine so far, the fan section sucks in air, the compressor squashes it down, and the combustor ignites it.  To get enough air into the engine to make a plane move fast enough, the fan section and compressor have to work hard requiring a lot of power.  The problem is that power requires fuel, and fuel is heavy and expensive.  So to get around this problem Brayton in 1870 realised the hot air coming out of the back could be used to drive the front sections of the engine.  The very fast hot air coming out of the combustor passes over the turbines which causes them to turn the same as a windmill.  By connecting the back of the engine to the front of the engine via a shaft, hot air turns the turbine which turns the shaft, which turns the fan section and compressor, which then sucks in new air.  Basically the turbines reclaim energy from the air and recycles it back into the engine meaning a far more efficient engine that requires less fuel and therefore a substantially lighter plane.

Like the compressor, turbines have both rotating and static blades, or rotors and stators (aka nozzle guide vanes).  An ‘impulse turbine’ means that as the air flows across the stators the pressure drops.  A ‘reaction turbine’ is where the stators just change the direction of the air and the pressure drop occurs over the whole turbine section.  Modern engines use a combination of both these designs.  In some cases the turbine section has no stators and so to deal with the complex air flow, counter rotating blades can be used.  This means the HP turbines may rotate one way and the IP turbines another.  The turbine is designed to make sure the right amount of air passes through the blades, the right amount of power is achieved to drive the early parts of the engine, and, lastly that the engine is efficient.

When we think about the air passing through the turbines, it is hot and travelling very fast.  This means the turbine blades have air passing over them which is about 1700°C, once again too high for metals to cope with.  To deal with this, an interpass cooling system and thermal barrier coatings are used.  Cool air is taken from the compressor, bypassing the combustor, and is then fed into the turbine blades.  These blades are have a number of channels for the cool air to travel through.  This cool air is in fact about 700°C and therefore only provides critical but a small amount of cooling.  This means the material the blades are made out of needs to be strong enough to handle the stresses and temperature, and do so whilst being hollow and full of holes for air to flow through.

Due to stress and temperature, turbine blades are one of the most demanding components as unlike the combustor, turbines are a moving part.  Thinking about what materials are available, ceramics look like the answer in this case because they are strong, corrosion resistant and perform well at high temperatures beyond what metals can achieve.  The problem with using ceramics is that they are brittle which means there is little or no time between something being damaged and catastrophic failure.  This introduces a number of issues in operating engines long term as you want to be able to diagnose damage before failure and ceramics give you a very short window to do this.

Another way of harnessing the properties of ceramics is to use them as coatings.  Thermal barrier coatings (TBCs) are where a ceramic layer is applied a metal component in order to protect it from the harsh conditions.  TBCs are mostly used in the turbine section.  Ceramics have low thermal conductivity meaning that they insulate the turbine blade resulting in the metal  underneath remaining cooler.  Another advantage is that ceramics are inert and therefore protect the blade from oxidation and hot corrosion.

Being a moving part in the hottest parts of the engine, modern turbines are made out of nickel superalloy single crystals.  By using a single crystal, grain boundaries are eliminated resulting in a stronger material that is more creep resistant.  CMSX-4 is an ultra-high strength single crystal nickel superalloy with added rhenium able to handle turbine stresses up to 1160°C.  In some cases below 1130°C CMSX-3 is used instead due to costs.  Currently RR3000 and RR3010 (CMSX-10 & CMSX-10+) are the newest nickel single crystals with LCF/HCF lives approximately 2-3 times that of CMSX-4, and the ability to handle high enough temperatures that reduce the need for cooling in IP turbine blades.