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Course description

Course Objectives

The course aims to present an in depth coverage of all key aspects of gas turbine engine and component design and performance for both design point and off-design conditions. On completion of this course, the delegate should be able to:

• make a selection of the major performance parameters according to engine application.
• select an appropriate layout for the engine.
• understand the limitations imposed on design point selection by the need for adequate off design

Who Should Attend?

It is expected that delegates attending this course will already have a good understanding of the fundamentals of gas turbine design and performance. Course will also be of value to more experienced engineers in the gas turbine sector who have a need for an overview of the design and performance of the main engine component technologies beyond their recent experience. Alternatively, previous attendance on either of Cranfield’s Gas Turbine Appreciation or Gas Turbine Performance courses would qualify the delegate ideally for this course.

Course Topics

Gas Turbine Fundamentals
A brief review of the fundamental fluid mechanics applied to the gas turbine engine; adiabatic and isentropic flow, static and stagnation conditions. Mass flow functions and choking.

Gas Turbine Performance
Ideal cycles. Component efficiencies. Cycle efficiencies and their relationship with specific consumption and air miles per gallon. Design-point analysis of turbojet, turboprop and turbofan (bypass) cycles. Influence of pressure ratio, peak temperature, bypass ratio and flight conditions on specific thrust and fuel consumption. Use of non-dimensional groupings. Simplified off-design analysis with constant component efficiencies and specific heats. Use of digital computer; iterative techniques. Presentation of results. Comparison of behaviour of different engine types; choice of engine parameters for given duty. Thrust augmentation methods, variable geometry and other modifications to basic cycle. Engine-intake matching. Transient behaviour of engines.

Axial Compressor Design and Performance
Overall problems of diffusing air flow. The axial compressor stage, stage loading, diffusion parameters, flow coefficient, reaction. The overall stage characteristic, real and ideal, stall and choke. Radial Equilibrium considerations, design for free vortex, prescribed vortex, graded work, etc. The overall compressor characteristic, surge line, running line, effect of changes in inlet pressure and temperature. Off-Design performance, use of variable IGV's, air bleed, multi-spooling. Stage matching. Choice of annulus geometry, tip speed, etc.

Axial Turbine Design and Performance
Problems of expanding air flow and the importance of passage shape. Choice of blade profile shape, prescribed velocity distribution. The axial turbine stage, velocity triangles, reaction, stage loading and flow coefficients; limiting values. The real and ideal characteristic. Design for maximum power, effect of Mach number, effect of choking and changes of inlet temperature and pressure. Factors affecting efficiency and correlations. Choice of design values according to application. Choice of overall annulus geometry and layout; rising line, constant mean diameter and falling line. A complete single-stage high pressure turbine aerodynamic design is carried out by the delegates in class. Subsequently, a computer programme is used interactively to undertake design optimisation of a single stage low pressure turbine.

Blade Cooling
NGV and blade cooling, definition of terms, methods of cooling; impingement film and transpiration. The design compromise between aerodynamics, cooling and mechanical integrity.

Combustion Systems
Burning velocity; effects of pressure, temperature and turbulence. Methods of measuring burning velocity. Chemical reaction rates. Reaction rate parameters applicable to practical combustion systems.

Performance criteria of combustion chambers; combustion efficiency, stability and ignition performance, temperature traverse quality. Design criteria; determination of chamber dimensions and pressure loss to meet stipulated performance requirements. Relative merits of tubular, annular and tubo-annular chambers.

Fuel injection methods; spray injection, vaporising tubes, airblast atomisers. Combustion chamber aerodynamics; diffuser characteristics, influence of inlet velocity profile on pressure loss and temperature traverse quality.

Problems of ground starting and altitude re lighting. Relative merits of weak, stoichiometric and rich primary zones.

Emissions
Gaseous pollutants, mechanism of production of CO, NOx, UHC and aldehydes. Advanced combustor designs - variable geometry, staged fuel, LPP concepts. Carbon formation and exhaust smoke, use of alternative and residual fuels in gas turbines and the problems of combustion, pollution and corrosion that are encountered in using them.

Reheat Systems
Relationship between stability, burning length and combustion efficiency. Flame stabilisation and propagation, Design criteria. Problems of low and high frequency instability.

Diffusers
The application of diffusers to GT. engines and their effect on neighbouring components. Performance parameters, use of design charts, the effect of non-uniform flow distribution. Techniques for reducing diffuser length and raising pressure recovery.

Combustor Cooling
Reasons for cooling. Flame temperatures versus maximum metal temperatures. Typical heat release rates - heat transfer rates to the combustor wall. Need for regenerative cooling. Fuel cooling, in relation to more advanced fuels. High-pressure air cooling - external convection, internal convection - film cooling. Pressure drop limitations. Effect of cooling air on TTQ. Trend of cooling requirements versus pressure ratio. Cooling techniques - wigglestrips, machined rings etc. Possibilities of transpiration cooling. Heat balance across chamber walls. Radiation and convection contributions. Estimation of film temperature - correlation equation. Radiant heat transfer. Overall assessment of wall temperatures, and influence of input parameters on final wall temperature.

Engine Noise
Principles of acoustic noise measurement. Sources and mechanisms.

Electronic Control
Types of system and their relationship with the engine.
Fuel Systems: Variation of fuel requirements with operating condition. Fundamental types of control and methods of metering. Effect of burner limitations on the fuel system. Pumps. Scheduling systems (e.g. simple flow control, proportional flow control). Over-ride controls on speed, temperature, acceleration etc. Governor systems (e.g. combined acceleration and speed control, electric), advantages and disadvantages.

Fuels Technology
Hydrocarbon fuel molecular structure and behaviour. Conventional petroleum fuel types and preparation. Laboratory test methods and results. Significance of test results in fuel handling and combustion performance. Aviation fuel specifications, and reasons for recent amendments. Current problems: thermal stability, linear temperature, smoke formation, etc. Expected changes in fuel quality. Alternative fuels for use in the short and long term.

Intakes and Exhausts
Intakes: General performance requirements of intakes. Pressure recovery, external drag. Subsonic and supersonic intake characteristics. Variable geometry systems. Flow distortion. Exhaust Systems: general performance requirements of nozzles. Efficiency and off-design problems. Influence of area ratio on exhaust thrust and external drag. Variable geometry systems. Thrust reversers. Silencers. Noise suppressers.

Materials Technology
Relationship of structure, design and operational properties of metals. Specific materials; nickel, cobalt and chromium alloys, the refractory metals, cermets, ceramics, titanium and composites, their availability, properties, potentialities and limitations for engine applications.

Mechanical Integrity
Origin of loads on gas-turbine components. Factors and strength criteria for proof, ultimate, creep, fracture and fatigue cases. Integrity of specific components such as discs, blades, shafts, combustion chambers, casings, flanges, etc. Cumulative creep, effect of mean stress, surface conditions and stress concentration on fatigue. Cumulative fatigue, double-Goodman diagram and rainflow methods. Examples of finite-element analysis.

Fatigue and Fracture Mechanics - two parts of the same process. Griffith Criterion for fracture. The stress analysis approach. Fracture Toughness in plane strain and plane stress. Geometric and flaw shape effects. Effects of cyclic loading. Paris Law, mean stress effects. Threshold Stress Intensity, Environmental effects. Practical Examples - discs, rolling contact bearings. Types of vibration encountered in the gas turbine. Blade modes of vibration, including centrifugal and thermal effects. Analytical and experimental methods of determining natural frequencies, holography, laser-doppler, etc. Production of frequency diagram and methods of overcoming vibration problems. Disc vibration with transfer matrix solution. Critical speeds of shafts and alleviation by means of squeeze-film damper bearings.