Chapter 5 Turbine Section PDF

Chapter 5: Turbine Section Introduction  Facts on Turbine Stage  Turbine assembly  Load on Turbine blade  Material Used on Turbine  Turbine blade cooling  Multi-pass Cooling  Film Cooling  Impingement Cooling  Ty...

Chapter 5: Turbine Section Introduction  Facts on Turbine Stage  Turbine assembly  Load on Turbine blade  Material Used on Turbine  Turbine blade cooling  Multi-pass Cooling  Film Cooling  Impingement Cooling  Types of turbine blades  Impulse Blade  Reaction Blade  Impulse-Reaction Blade  Nozzle Guide Vane  Creep  Turbine Case Cooling (TCC) / Active Clearance Control (ACC)  Blade attachment  Turbine technology 2 Facts on Turbine Stage 3 Facts on Turbine Stage The function of the turbine is to transform the energy of the gas (kinetic energy) into mechanical energy to drive the compressor and accessories A turbine stage includes a row of stationary vanes (stator or nozzle guide vanes) and a row of rotating blades (rotor or wheel). The gas coming from the combustion chamber flows first through the stationary vanes. The pressure is partly transformed into velocity because of the convergent passage between the vanes, and the flow is then directed towards the r o t o r blades. The kinetic energy of the gas then flows through the blades of the wheel on which mechanical forces are created resulting in the rotation of the wheel. 4 Turbine Assembly 5 Turbine Assembly https://www.summaryplanet.com/engineering/Jet-engine-design.html 6 Load on Turbine Blade  Turbine blades in a jet engine experience: ▪ Mechanical forces ▪ Creep ▪ Thermo-mechanical fatigue ▪ High temperature environment ▪ Oxidation ▪ Hot corrosion 7 Material Used on Turbine 8 Material Strength vs Temperature 9 Turbine Blade Cooling 10 Multi-pass Cooling 11 Film Cooling of Turbine Blades (Conduction)  Principle  Draw Cooling Air From Compressor  Creation of an Insulating Sublayer  Lower the Effective Gas Temperature in the Boundary Layer 12 Impingement Cooling (Forced Convection) With this cooling method, a high-velocity cold air jet is directed from a hole vertically onto the component surface to be cooled. As the cooling air jet hits the surface it is diverted in all directions parallel to the impingement surface. To ensure efficient impingement cooling of larger areas a great number of such holes must be arranged in one single row or in several rows 13 Turbine Blade Cooling 14 Chapter 7: Turbine Section Types of Turbine Blades 15 Types of Turbine Blades  Impulse blade  Reaction blade  Impulse-Reaction blade 16 Facts on Impulse Turbine Blade  The main characteristics of the impulse blade is to extract high impact energy from the kinetic energy of gas to do work such as turning the turbine, Wheel compressor and accessory gear box Gas  Gas will strike at the center of the blade (centre of gravity) to extract Nozzle energy for work. guide vane  The impulse blade cause no change in *Turbomeca Training notes pressure (due to straight duct) but velocity will decrease 17 Facts on Impulse Turbine Blade 18 Facts on Reaction Turbine Blade  The main characteristics of the reaction blade is to produce their turning force by an aerodynamic action (from the contour of the blade). Wheel  The reaction blade will result in reduction Gas of pressure and velocity.  High pressure gas goes through the Nozzle guide passage between the rotor blades vane produces in a shape of a convergent duct converting pressure energy to increase *Turbomeca Training notes relative velocity of gas. However, gas is used for turning the turbine causing the absolute velocity to drop. 19 Facts on Reaction Turbine Blade 20 Facts on Impulse-Reaction Turbine The benefit of Impulse blade is to generate high impact power to do work while the Reaction blade is to generate high speed gas (although drop in absolute velocity). The Impulse-Reaction blade combines the benefits of both types of design by incorporating the Impulse shape on the blade root and Reaction shape on the blade tip. To generate high speed gas out from the turbine section, the Impulse-Reaction blade is purposefully design to have the Reaction shape at the blade tip as it is understood that the velocity of air is always highest at the blade tip (V = rω). While the Impulse shape at the blade root will be useful for doing work. 21 Facts on Nozzle Guide Vane  The role of the Nozzle Guide Vane is to guide the Shroud gas from the Combustor to the 1st stage turbine rotor Blade  It converts pressure energy Root to increase kinetic (velocity) energy Disc Shrouded turbine Guide Vane (stator) 22 Creep / Turbine Case Cooling / Blade Attachment / Turbine Technology 23 Creep The resistance to creep is determined by measuring the elongation (A) of a metal sample subjected to both mechanical and thermal stresses at the same time. Three zones are visible on the curve, corresponding to primary, secondary and tertiary creep. When the elongation of the sample becomes permanent, it is called "primary creep". This happens normally very quickly, as soon as the test starts. The "secondary creep" is represented by the flat part of the graph. There is nearly no elongation and we may consider this part as the life of the blade. "Tertiary creep" is characterised by a very fast elongation until the failure occurs. 24 Creep Over A Period 25 Consequences of Creep Creep results first in a decrease of the clearance X, then turbine blades rubbing on the casing or even seizing. X Gas F X - Blade tip clearence T° F - Mechanical stress T° - Thermal stress *Turbomeca Training notes 26 Turbine Case Cooling (TCC)/ Active Clearance Control (ACC) The main purpose of TCC is to keep the blade tip clearance between the turbine blade and turbine casing in check. It works by releasing cold air from the atmosphere / compressor bleed air to shrink the turbine casing. 27 Fir Tree Attachment Blade Lock Fir tree root *Turbomeca Training notes 28 Turbine Technology 29 Recap  Purpose of turbine section  Stress faced by turbine blade  2 Cooling methods of turbine blade/NGV  Material used on turbine blades  3 Types of turbine blade and explain its performance  Blade attachment used for turbine blade  Define the different zones of creep and explain the consequence of creep  Name the type of latest turbine technology 30