Blading
The blading is called the set of blades of a compressor and a turbine . A distinction is made between rotor blades and guide blades .A ring of rotor blades with the associated ring of guide blades is called a step . The blading of the turbine or compressor can be multi-stage.
The guide vanes are permanently installed in the housing of the compressor or the turbine and guide the working fluid at the optimum angle onto the rotor blades, which are located on rotating shafts . The mechanically usable power between the machine and the fluid is coupled via the rotor blades . (In general, turbines drive an attached work machine , often a generator , propeller, or fan; a compressor is generally driven by a motor .)
When specifying the number of stages of a compressor or a turbine, the number of rotor blade rings is decisive: A five-stage compressor has five rotor blade rings. In the case of compressors , the guide vane rings are usually assigned to the preceding rotor blade ring, and in the case of turbines they are usually assigned to the following rotor blade ring.
Fluid mechanics
In the guide vanes, the enthalpy converted per stage is either completely or partially converted into flow energy. In the impellers, the flow energy is converted into a circumferential force through the deflection . In principle, fewer stages are required to reduce an enthalpy difference in a turbine than to build up the same difference in a compressor. This is related to the fact that the accelerated flow of a turbine is much less at risk of stall than the decelerated flow in a compressor.
The ratio of the enthalpy converted into flow energy in the rotor blades of a turbine stage to the total enthalpy gradient of a turbine stage is called the degree of reaction . Usually a degree of reaction of 0.5 is achieved in positive pressure turbines. In constant pressure turbines , the degree of reaction is 0; the entire enthalpy gradient of a stage is converted into flow energy in the guide vanes, the pressure in the rotor blades of the stage remains constant.
charges
Particularly in the case of turbines, the rotor blades are exposed to particular loads. A high operating temperature in combination with the tensile stresses in the radial direction is particularly critical. Over time, these loads cause the blades to creep . In the course of their life, the blades become longer and longer, which in the worst case can lead to the blade touching the outer casing of the step, thereby blocking the step. For aerodynamic reasons and for a high degree of efficiency, however, the smallest possible gap between the blade and the housing is usually desired. For example, a few millimeters of soft metal can be attached to the upper edge of a shovel, which "grinds off" when it comes into contact with the housing during the first few runs and thus leads to an optimal fit; but are possible z. B. also a brush seal or a labyrinth seal .
Vibration loads are also critical : the blade begins to "flutter". This can lead to material fatigue.
High operating temperatures have a positive effect on the Carnot efficiency .
In gas turbines , the high loads make highly resilient materials necessary. The materials used for the rotor blades limit the efficiency of the turbine, as they only allow a limited operating temperature. Often, however, turbine blades have cooling air ducts which, for example, can generate a thin layer of cooling air over the blade surface. This reduces the effective surface temperature and decouples it somewhat from the actual hot gas temperature after the combustion chamber of the gas turbine.
Materials
Turbine blades for gas turbines are made of nickel superalloys , tungsten-molybdenum alloys or titanium alloys . The blades are protected by coatings for higher resistance to temperatures and erosion such as pitting , also known as "pitting corrosion". The coating for heat shielding is called thermal barrier coating or TBC for short. Further measures to make the blades more heat-resistant consist of sophisticated cooling duct systems . This technique is used in both the guide vanes and the rotor blades.
