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Carmen Ruan

on 17 October 2014

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Modules of gas turbine
Maintenance activities
Propeller aerodynamics
Propeller control
intakes design
combustion chambers
design of blades
blades materials
Maintenance activities
A propeller is an airfoil and like a wing it will generate an aerodynamic force much the same way. It has a leading and trailing edge, camber and a chord line. The propeller is rotated by the engine and this creates thrust and moves the aircraft forward.
Propeller aerodynamics
Propeller control
Intakes design: inlet section
provide clean and unrestricted airflow to engine.
ensurre minimum airflow losses
the air inlet duct must :
atmospheric conditions (dust, salt, pollution, FOD (birds, nuts, bolts)...
temperature (icing conditions)
Designed and produced as a separate system of the engine
provides the turbine with all the air needs is an efficient manner.
straighten the air to provide the combustor with the proper airflow direction
raises the temperature of the air as the air is compressed and moved rearward
Effort is made to keep the air flowing smoothly though the compressor to minimize airflow losses due to friction and turbulence
all components used are shaped in the form of airfoils to maintain the smoothes airflow possible
types of compressor
There are three basic categories : Axial compressor, centrifugal compressor and mixed flow compressor
combines axial and radial components to produce a diagonal airflow compressor stage
achieves a pressure rise by adding kinetic energy/velocity to a continuous flow of fluid through the rotor or impeller. This kinetic energy is then converted to an increase in potential energy/static pressure by slowing the flow through a diffuser.
Compresses its working fluid by first accelerating the fluid and then diffusing it to obtain a pressure increase.
Axial compressor
Centrifugal compressor
mixed flow compressor
mixed flow compressor
Combustion Chamber
control the burning of large amounts of fuel and air.
release the heat in a manner that the air is expanded and accelerated to give a smooth and stable stream of uniformly-heated gas all starting and operating conditions.
accomplish the task with minimum pressure loss and maximum heat release.
The cooling air is critical, the flame temperature is higher than the metal can endure.
types of combustion chamber
Annular type: Annular combustors do away with the separate combustion zones and simply have a continuous liner and casing in a ring (the annulus).
Can type: Can combustors are self-contained cylindrical combustion chambers. Each "can" has its own fuel injector, liner,interconnectors,casing. Each "can" get an air source from individual opening.
Cannular type: Like the can type combustor, can annular combustors have discrete combustion zones contained in separate liners with their own fuel injectors. Unlike the can combustor, all the combustion zones share a common air casing.
the turbine converts the gaseous energy of the air/burned fuel mixture out of the combustor into mechanical energy to drive the compressor, throufgh a reduction gear, the propeller.
converts e energy by expanding the hot, high pressure gases to a lower temperature and pressure.
the efficiency of the turbine is determined by how well extracts mechanical energy from the hot, high-velocity gasses.
Effects of turbine temperature
Compromises are made in turbine design to achieve the optimum balance of power, efficiency, cost, engine life...
The last investigations iallows a higher inlet temperature due to improved materials and design
design of turbine blades
A turbine blade is the individual component which makes up the turbine section of a gas turbine. The blades are responsible for extracting energy from the high temperature, high pressure gas produced by the combustor. The turbine blades are often the limiting component of gas turbines.
A key limiting factor is the performance of the materials available for the hot section. Two main problems creep* and fatigue*, which two ways of resolution made by :

Specific Materials like superalloys and ceramic coating: Modern turbine blades often use nickel-based superalloys that incorporate chromium, cobalt, and rhenium : Rene 77,Rene N5, CMSX4

Methods of cooling(by air or liquid cooling):
Internal cooling: Convection cooling(air), impingement cooling
External cooling: Film cooling(air), Cooling effusion(air), Pin fin cooling, Transpiration cooling
detection problems and corrective actions
Boroscope inspection

PHYSICAL PROBLEMS : Dirt, build-up, fouling, worn bearings, worn seals, excessive blade tip clearences, burned or warped turbine, fuel nozzels, erosion, corrosion, foreign object damage…

Four ways for a detection of problems :
1. the thermodynamic gas path
2.vibration of bearing, rotors, etc..
4. controls and inspections(boroscope)
Indications action
s: turbine blade failures account or 25.5% of gas turbine failures. Turbine blade
oxidation, corrosion and erosion
is normally a longtime process with material losses occurring slowly over a period of time. However, damage resulting from impact by a
foreign object
is usually sudden. Both of them will result in parameter changes similar. Parameter changes that can readily be seen by the operator: an increase fuel flow, exhaust gas temperature, and compressor discharge pressure. It result in a decrease in turbine efficiency.

Corrective actions
: Geometry changes resulting from oxidation, corrosiom, erosion, or impact damage can be corrected only during overhaul by removal and replacement of the damaged parts. Problems initially discovered by gas path analysis can be verified by a boroscope inspection.

Compressor fouling normally occurs due to foreign deposits on the airfoils. The compressor fouling is indicated by a drop in compressor efficiency(decrease of rotor speed and airflow , low pressure compressor discharge pressure) and a reduction in load capacity.
For maintenance the equipment manufacturer normally specifies cleaning agents (tipically carbo-blast and water-wash are specified)

Foreign object damage (FOD) is defined as material (nuts, bolts, ice…) ingested into the engine from outside the envelope. FOD is almost always accompanied by an increase in vibration.

Domestic object damage (DOD) is defined as objects from any other part of the engine itself.

Compressor FOD symptoms are similar to compressor fouling.
Turbine FOD will cause a decrease in turbine efficiency similar to blade and vane erosion.

DOD are the result of small loose parts within the engine, or break loose after operations is initiated.

When used counjuction with gas path, vibration, and trending analysis techniques, boroscope inspections often provide the final step in the process of identifying an internal problem.

Boroscope inspections are useful providing a general view of the condition of critical components.
Variable pitch:The purpose of varying pitch angle with a variable pitch propeller is to maintain an optimal angle of attack (maximum lift to drag ratio) on the propeller blades as aircraft speed varies
Feathering: the blades can be rotated parallel to the airflow to reduce drag in case of an engine failure, increase the gliding distance and maintain altitude(multi-engine aircraft) with the reduced power from the remaining engines
Reverse pitch: This is used to help slow the plane down after landing in order to save wear on the brakes and tires, but in some cases also allows the aircraft to back up on its own
Pilot’s Handbook of Aeronautical Knowledge. Oklahoma City: U.S. Federal Aviation Administration. 2008. FAA-8083-25A.
Brooks, David S. (1997). Vikings at Waterloo: Wartime Work on the Whittle Jet Engine by the Rover Company. Rolls-Royce Heritage Trust. ISBN 1-872922-08-2.
Golley, John (1997). Genesis of the Jet: Frank Whittle and the Invention of the Jet Engine. Crowood Press. ISBN 1-85310-860-X.
Hill, Philip; Peterson, Carl (1992), Mechanics and Thermodynamics of Propulsion (2nd ed.), New York: Addison-Wesley, ISBN 0-201-14659-2
Kerrebrock, Jack L. (1992). Aircraft Engines and Gas Turbines (2nd ed.). Cambridge, MA: The MIT Press. ISBN 978-0-262-11162-1.Brooks, David S. (1997).
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