NOZZLE GUIDE VANES AND SHROUDED BLADES

The nozzle guide vanes are aerofoil shaped vanes arranged in a static ring ahead of a turbine disc.

They control the velocity, pressure, and direction of the gas flow prior to its entry to the turbine blades.

Gas passing through the convergent vane spaces accelerates whilst dropping its pressure and temperature.

The direction of the gas is altered to let it enter the turbine blades at the most effective angle.

You will have already read that they have a lower outlet area than that of their inlet area due to convergent vane spacing.

You will also have read that the nozzle guide vane spacing for impulse reaction blades does vary from convergent at the inner radius to virtually parallel at the outer radius.

The overall effect, however, is described as being convergent.

As the hot gas leaves the combustion chamber it directly impinges on the high-pressure nozzle guide vanes.

The converging ducts between these vanes raise the gas velocity to Mach one and reduce the gas pressure and temperature.

This is the highest gas velocity to be found in the engine.

The high-pressure nozzle guide vanes normally reach the choked, condition during maximum power operation.

The high-pressure nozzle guide vanes have the highest heat to metal contact in the gas turbine engine and have to be continually cooled in operation.

The design of the vane section is critical and must be carefully matched to the turbine it serves.

Restricted gas flow through the vanes could create backpressure, which would push the compressors towards the stall and surge.

Excessively high gas flow through the vanes would affect the air velocity through the compressor causing its rear stages to choke.

The high-pressure nozzle guide vanes experience the highest direct gas temperature contact in the engine. This ranges from about 900°C to 1100°C.

Although the flame temperature in the combustion chamber is much higher at 2000°C, there is no direct contact with the chamber due to its cooling system.

The vanes do not experience any rotational stress and primarily designed to resist buckling under thermal stress.

It is emphasized that it is the high-pressure nozzle guide vanes that are at most risk from heat damage. Excessive temperatures could melt them.

Due to the temperature drop through each turbine stage. The low-pressure turbine nozzle guide vanes face a much-reduced risk in comparison to the HP nozzle guide vanes.

MATERIALS

The NGVs manufactured in segments by investment casting from high content nickel alloys such as the Nimonic range.

These have discussed under turbine blade manufacture. The HPNGVs are of hollow construction to permit the flow of HP compressor delivery air through their interiors to act as cooling air.

The vanes perforated with cooling holes that allow this air to exit the interior and form a film of cooling air over the vane surface.

NGVs frequently coated with a ceramic material to increase their heat resistance.

SHROUDED BLADES

If gas were to leak across the tips of the rotor blade it would create turbulence and friction, which would create heat and drop the gas pressure.

This would be pressure not available for conversion to work or thrust. This is described as tip loss.

The tip loss is inversely proportional to the square of the turbine blade tip speed.

The answer would seem to indicate spinning the turbine faster but that is a problem in itself.

Firstly, the tip could reach the sonic speed that would produce shock waves.

Secondly, the compressor at the other end of the turbine shaft will have a limiting rotational speed.

Thirdly, the centrifugal force on the blade is proportional to the square of its velocity.

SHROUDED-BLADES

The most common method used to prevent blade tip gas leakage and, therefore, increase blade efficiency, is to manufacture a blade with a T sectioned shroud at the tip.

When the blades are installed their shrouds touch each other to form a peripheral ring at the blade tips.

The shrouds prevent gas leakage across the tips of the blades making them more efficient and also reduce blade vibration.

The shroud does increase the centrifugal force though and this limits the rotational speed and the working temperature of the blade.

Shrouded blades are also manufactured with knife-edge projections on the top of their shrouds.

These are aligned with static rub strip seal rings in the turbine casing, which are used to reduce the tip working clearance to a minimum.

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