Axial Turbine Design Considerations - Turbomachinery Aerodynamics - Lecture Slides, Slides of Turbomachinery

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Axial Flow Turbine

Blade design - Profiles

Selection of

1) Design requirements , T*g-max , α * exit , M * 2

2. Design constraints : for both HPT & LPT –

i) n 1 , n 2 , U m1 , D m1 , U m2 ,D m

ii) Blade and Disk Stress levels iii) Materials Technology iv) Blade cooling Technology

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ηT

Airfoils for Gas Turbine

Airfoil Geometry Selection

Blade loading vs Blade Spacing

If the spacing between the blades is small, the blades provide maximum guidance of the fluid , but the surface area goes up and the surface friction loss (primary loss) goes up.

Zweifel criterion specifies: Zw =ΔH0T/ΔH0T-ideal≈ 0.

Which in terms of blade tangential loads , Zw = 2. s/c .cos^2 α 2 (tanα 1 + tanα 2 ) The above criterion allows the designed to arrive at a minimum loss blade spacing. However modern design exploration has proven that the specification is valid for 60 o^ < α 2 <70 0

• Airfoils designs are generally done in cascade format
• A starting airfoil shape (T6 or T106) with appropriate starting camber matching the flow turning angle (Δα) is selected
• The airfoil is suitably modified by interactive or Direct method of numerical analysis
• At the final stage when a acceptable blade loading has been reached, Indirect method may be used by specifying a Cp distribution over the blade to arrive at a final airfoil shape in cascade

Supersonic Turbine airfoils