I am wondering why turbines are not implemented with controllable pitch blades (on the rotor system)? I assume the reason is that the coupling between the stator and rotor systems posed by typical controllable pitch mechanisms poses an unacceptable frictional loss for such a high speed/ low torque application.
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$\begingroup$ Turbines are a giant category of machines. On the extreme end, a 100MW gas turbine for power generation has no need to be because its already 90+% efficient with good off-design and transient performance, while the materials are already stretched to their mechanical limits. Can you be more specific about what size/type or turbine you think would benefit from being variable pitch? $\endgroup$– ericksonlaCommented Aug 20, 2018 at 16:07
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2 Answers
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There are turbos with variable geometry, in fact my car is fitted with one.
This is obvious with the actuator visible which, when the ignition is switched on, the actuator is cycled from min to max and back to min.
There are two classic issues that put the engine error light on - one is the actuator fails (usually soldered joints apparently) and two is that the vanes seize due to lots of low speed use clogging the vanes (a good regular "italian service" on the motorway is one solution.
Here is a link to some further info.
To add the info based on the comments, turbos for cars tend to be around 2" to 2.5" max and the turbine blades very thin (rotating around 180000rpm), trying to put a control system to control the pitch of those blades would be a challenge due to size and the effects of play in the mechanisms.
The images below are from the Jaguar x type manual pages 1129 and 1130, showing low and high engine speed operation and the effects of the vane position:
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$\begingroup$ Variable geometry turbos arent the same thing as controllable pitch turbos although they share a common objective. $\endgroup$ Commented Aug 20, 2018 at 14:41
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$\begingroup$ Well, the vanes are moving on the ones I am talking about... If you are talking about the turbine blades themselves then the simple answer is that there is an issue of size (with required strength) and also the variance die to the tolerances. $\endgroup$ Commented Aug 20, 2018 at 14:48
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$\begingroup$ My question was vague and your answer was correct given my original wording. With respect to your last comment, what is the size issue and what is variance die and its relation to controllable pitch blades? $\endgroup$ Commented Aug 20, 2018 at 15:04
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$\begingroup$ die is due - typo, but turbos for cars tend to be around 2" to 2.5" max and the turbine blades very thin (rotating around 180000rpm), trying to put a control system to control the pitch of those blades would be a challenge due to size and the effects of play in the mechanisms. $\endgroup$ Commented Aug 20, 2018 at 15:15
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$\begingroup$ Maybe update your answer so i can reword you? $\endgroup$ Commented Aug 20, 2018 at 15:18
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Turbine Fuel Efficiency: Fitting a Pitch
Here is an excellent answer to my question.
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$\begingroup$ @MahendraGunawardena as this is link only, should it be converted to a comment? $\endgroup$ Commented Jun 10, 2019 at 5:22
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$\begingroup$ @SolarMike, In this situation I would leave it as an answer. $\endgroup$ Commented Jun 10, 2019 at 10:16
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$\begingroup$ Can you please add an explanation in the body of the answer as to why the link a good answer? $\endgroup$ Commented Jun 10, 2019 at 10:17