Turbofan:Bearings: Difference between revisions

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=Bearings and cooling=
=Bearings and cooling=


Rotational speed achieved by the engine will probably be around 40000rpm. At this speed, regular ball bearings may overheat or suffer from a too fast wear. In real engines, bearings are constantly lubricated by an oil bath, which poses problems with regards to oil pressurization and leaks in other parts of the engine. Seals are consequently placed close to bearings to prevent leaks, generally carbon leaks. Accessories like oil pumps, pipes, fixations, filters and so on, are also required.
Rotational speed achieved by the engine will probably be above 40000rpm. At these speeds, regular ball bearings may overheat or suffer from a too fast wear. In real turbine engines, bearings are constantly lubricated by oil jets, which poses problems with regards to oil pressurization and leaks in other parts of the engine. Seals are consequently placed close to bearings to prevent leaks, generally carbon seals. To enforce the seal, oil is contained in a casing with an internal pressure lower than the external pressure built from compressed air. That way, air can enter the oil casing, but oil cannot leak outside, in other parts of the engine.
 
Accessories like oil pumps, pipes, fixations, filters, tanks, heat exchangers and so on, are also required.


==Bearings==
==Bearings==


===Ball bearings===
===Ball bearings===
Ball or roller bearings are the obvious way to guide rotating parts. They provide good mechanical constraints on the axis orthogonal to the rotation, they are inexpensive and their integration is reasonably simple.
Ball or roller bearings are the obvious way to guide rotating parts. They can handle high mechanical constraints radially or even axially, they are inexpensive and their integration is reasonably simple.


[http://en.wikipedia.org/wiki/Silicon_nitride#Bearings Silicon nitride bearings] have lots of improvements over regular metal ball bearings. Balls are more than 60% less heavy, thus having a lower inertia at high speeds, implying a more softer contact with the tracks, allowing longer lifetime or higher reachable speeds. They also require less lubrication. Fortunately, silicon nitride bearings have reached marked with a large production, and are not over-expensive.
[https://en.wikipedia.org/wiki/Silicon_nitride#Bearings Silicon nitride bearings] have lots of improvements over regular metal ball bearings. Balls are more than 60% less heavy, thus having a lower inertia at high speeds, implying a more softer contact with the tracks, allowing longer lifetime or higher reachable speeds. They also require less lubrication. Fortunately, silicon nitride bearings have reached market with a large production, and are not over-expensive.


===Alternate bearings===
===Alternate bearings===
Fluid or magnetic bearings should be considered. They allow much higher rotation speeds and lower friction, but have two main drawbacks. At standby state, they release the hard constrain on moving parts orthogonally to the rotating axis. In reduced-size turbomachinery, where rotor and stator have to be adjusted to tens of microns, it's quite complicated to use those bearings. The second drawback is that they require more external hardware, to pressurize the fluid or to provide magnetic energy.
Fluid or magnetic bearings should be considered. They allow much higher rotation speeds and lower friction, but have two main drawbacks. At standby state, they release the radial constrain on moving parts. In reduced-size turbomachinery, where rotor and stator have to be adjusted to tens of microns, it seems quite complicated to use those bearings. The second drawback is that they require more external hardware, to pressurize the fluid or to provide magnetic energy.


However, magnetic bearings have been demonstrated in [http://books.google.com/books?id=AXtqMugS3TQC&lpg=PP1&pg=PA263#v=onepage&q&f=false this paper] <ref>S. Jana, V. Arun Kumar and M. Ananda. '''5-axes levitation of a rotor towards indigenization of the magnetic bearing technology'''. In ''Air breathing engines and aerospace propulsion: proceedings of NCABE 2004'', november 2004.</ref>, in which axial position accuracy is measured below 150µm for a 4kg rotor at around 2000rpm. The rotor position sensor has a resolution of 2µm per mV. However, no indication is given about the resting position of the rotor and how that impacts the clearance between rotor and stator.
However, magnetic bearings have been demonstrated in [http://books.google.com/books?id=AXtqMugS3TQC&lpg=PP1&pg=PA263#v=onepage&q&f=false this paper] <ref>S. Jana, V. Arun Kumar and M. Ananda. '''5-axes levitation of a rotor towards indigenization of the magnetic bearing technology'''. In ''Air breathing engines and aerospace propulsion: proceedings of NCABE 2004'', november 2004.</ref>, in which axial position accuracy is measured below 150µm for a 4kg rotor at around 2000rpm. The rotor position sensor has a resolution of 2µm per mV. Unfortunately, no indication is given about the resting position of the rotor and how it impacts the clearance between rotor and stator.


[http://en.wikipedia.org/wiki/Foil_bearing Foil bearings] are a particular type of fluid bearing, that "Unlike aero or hydrostatic bearings, foil bearings require no external pressurisation system for the working fluid, so the hydrodynamic bearing is self-starting". In [http://b-dig.iie.org.mx/BibDig/P06-0351/pdfs/track-16/GT2006-90791.pdf this other paper] <ref>Hooshang Heshmat, Michael J. Tomaszewski, James F. Walton II. '''Small gas turbine engine operating with high temperature foil bearing'''. In ''proceedeings of GT2006 ASME Turbo Expo 2006: Power for land, sea and air'', may 2006.</ref>, a small centrifugal turbojet is built to evaluate the ability of [http://www.miti.cc/products-services.html MiTi]'s product, a foil bearing, to sustain very high rotation speeds (120'000rpm) and high temperature (800°C). The bearing has a low spacing between the rotor's journal and the stator fixation, but it is secured, in this paper, using a ball bearing on the compressor side, where the temperature is low. They planned to make a dual-foil bearing, we'll need to check on that. MiTi also demonstrated a [http://www.miti.cc/newsletters/20_150mm_foil_journal_bearing%20_hybrid_foil_magnetic_bearing.pdf hybrid foil magnetic bearing], that has the advantages of magnetic bearings at low speeds and those of foil bearings at high speeds.
[https://en.wikipedia.org/wiki/Foil_bearing Foil bearings] are a particular type of fluid bearing, that "Unlike aero or hydrostatic bearings, foil bearings require no external pressurisation system for the working fluid, so the hydrodynamic bearing is self-starting". In [http://b-dig.iie.org.mx/BibDig/P06-0351/pdfs/track-16/GT2006-90791.pdf this other paper] <ref>Hooshang Heshmat, Michael J. Tomaszewski, James F. Walton II. '''Small gas turbine engine operating with high temperature foil bearing'''. In ''proceedings of GT2006 ASME Turbo Expo 2006: Power for land, sea and air'', may 2006.</ref>, a small centrifugal turbojet is built to evaluate the ability of [http://www.miti.cc/products-services.html MiTi]'s product, a foil bearing, to sustain very high rotation speeds (120'000rpm) and high temperature (800°C). The bearing has a low spacing between the rotor's journal and the stator fixation, but it is secured, in this paper, using a ball bearing on the compressor side, where the temperature is low. They planned to make a dual-foil bearing, we'll need to check on that. MiTi also demonstrated a [http://www.miti.cc/newsletters/20_150mm_foil_journal_bearing%20_hybrid_foil_magnetic_bearing.pdf hybrid foil magnetic bearing], that has the advantages of magnetic bearings at low speeds and those of foil bearings at high speeds.


==Use of lubricating oil for cooling==
==Use of lubricating oil for cooling==
In real-world jet engines, cooling is the primary function of oil when conventional bearings are used, even more important than lubrication. That's well explained in [https://www.youtube.com/watch?v=WAia8PwMvQM this AgentJayz video]. A high flow rate of oil is then required, with a heat exchanger somewhere along the oil path. The other fluid for the heat exchanger can be air from the bypass duct or fuel, but in our highly size-constrained engine environment, we'll probably have to move some of the engine's equipment to the wings. But that will have to be studied after the bearing type has been chosen obviously.


==Oil displacement without external pumping==
==Oil displacement without external pumping==


[http://en.wikipedia.org/wiki/Screw_conveyor Screw pumping] will be used as a way to move the oil through the engine, i.e. parts that have to be lubricated and cooled. the work of the turbine will thus directly drive the oil pumping without requiring external accessories. However, cooling the oil may require external hardware, and sealing is absolutely required.
[https://en.wikipedia.org/wiki/Screw_conveyor Screw pumping] may be used as a way to move the oil through the engine, i.e. parts that have to be lubricated and cooled. The idea is to use the work of the turbine to directly drive the oil pumping without requiring external accessories. That does not solve the fact that cooling the oil may require external hardware, and that sealing is mandatory.


==External hardware required for lubrication==
==Other hardware required for lubrication and bearing cooling==


A cooling device will be required if the oil gets too hot, which is likely. A basic oil-to-air heat exchanger should be sufficient.
Sensors will be required too, at least for oil temperature and displacement confirmation. Oil temperature informs about the status of the engine's bearings. Oil displacement sensor is required to ensure that there is no problem with the oil/cooling flow in the engine and that the measured temperature is not bogus.


Sensors will be required too, at least for oil temperature and displacement. Oil temperature may inform about the status of the engine, and with sufficient experiments and modeling can be used to infer turbine temperature. Oil displacement sensor is required to ensure that there is no problem with the oil/cooling flow in the engine and that the measured temperature is not bogus.
Simple oil filters should also be put somewhere on the oil lines to prevent the more obvious failures.


==References==
==References==

Revision as of 03:05, 15 October 2012

Bearings and cooling

Rotational speed achieved by the engine will probably be above 40000rpm. At these speeds, regular ball bearings may overheat or suffer from a too fast wear. In real turbine engines, bearings are constantly lubricated by oil jets, which poses problems with regards to oil pressurization and leaks in other parts of the engine. Seals are consequently placed close to bearings to prevent leaks, generally carbon seals. To enforce the seal, oil is contained in a casing with an internal pressure lower than the external pressure built from compressed air. That way, air can enter the oil casing, but oil cannot leak outside, in other parts of the engine.

Accessories like oil pumps, pipes, fixations, filters, tanks, heat exchangers and so on, are also required.

Bearings

Ball bearings

Ball or roller bearings are the obvious way to guide rotating parts. They can handle high mechanical constraints radially or even axially, they are inexpensive and their integration is reasonably simple.

Silicon nitride bearings have lots of improvements over regular metal ball bearings. Balls are more than 60% less heavy, thus having a lower inertia at high speeds, implying a more softer contact with the tracks, allowing longer lifetime or higher reachable speeds. They also require less lubrication. Fortunately, silicon nitride bearings have reached market with a large production, and are not over-expensive.

Alternate bearings

Fluid or magnetic bearings should be considered. They allow much higher rotation speeds and lower friction, but have two main drawbacks. At standby state, they release the radial constrain on moving parts. In reduced-size turbomachinery, where rotor and stator have to be adjusted to tens of microns, it seems quite complicated to use those bearings. The second drawback is that they require more external hardware, to pressurize the fluid or to provide magnetic energy.

However, magnetic bearings have been demonstrated in this paper [1], in which axial position accuracy is measured below 150µm for a 4kg rotor at around 2000rpm. The rotor position sensor has a resolution of 2µm per mV. Unfortunately, no indication is given about the resting position of the rotor and how it impacts the clearance between rotor and stator.

Foil bearings are a particular type of fluid bearing, that "Unlike aero or hydrostatic bearings, foil bearings require no external pressurisation system for the working fluid, so the hydrodynamic bearing is self-starting". In this other paper [2], a small centrifugal turbojet is built to evaluate the ability of MiTi's product, a foil bearing, to sustain very high rotation speeds (120'000rpm) and high temperature (800°C). The bearing has a low spacing between the rotor's journal and the stator fixation, but it is secured, in this paper, using a ball bearing on the compressor side, where the temperature is low. They planned to make a dual-foil bearing, we'll need to check on that. MiTi also demonstrated a hybrid foil magnetic bearing, that has the advantages of magnetic bearings at low speeds and those of foil bearings at high speeds.

Use of lubricating oil for cooling

In real-world jet engines, cooling is the primary function of oil when conventional bearings are used, even more important than lubrication. That's well explained in this AgentJayz video. A high flow rate of oil is then required, with a heat exchanger somewhere along the oil path. The other fluid for the heat exchanger can be air from the bypass duct or fuel, but in our highly size-constrained engine environment, we'll probably have to move some of the engine's equipment to the wings. But that will have to be studied after the bearing type has been chosen obviously.

Oil displacement without external pumping

Screw pumping may be used as a way to move the oil through the engine, i.e. parts that have to be lubricated and cooled. The idea is to use the work of the turbine to directly drive the oil pumping without requiring external accessories. That does not solve the fact that cooling the oil may require external hardware, and that sealing is mandatory.

Other hardware required for lubrication and bearing cooling

Sensors will be required too, at least for oil temperature and displacement confirmation. Oil temperature informs about the status of the engine's bearings. Oil displacement sensor is required to ensure that there is no problem with the oil/cooling flow in the engine and that the measured temperature is not bogus.

Simple oil filters should also be put somewhere on the oil lines to prevent the more obvious failures.

References

  1. S. Jana, V. Arun Kumar and M. Ananda. 5-axes levitation of a rotor towards indigenization of the magnetic bearing technology. In Air breathing engines and aerospace propulsion: proceedings of NCABE 2004, november 2004.
  2. Hooshang Heshmat, Michael J. Tomaszewski, James F. Walton II. Small gas turbine engine operating with high temperature foil bearing. In proceedings of GT2006 ASME Turbo Expo 2006: Power for land, sea and air, may 2006.