The document discusses tip clearance flows in turbomachines, focusing on their causes, effects, and optimization techniques. Tip clearance is defined as the space between the rotor and casing, which can significantly impact performance by generating vortices and causing losses in efficiency. Recommendations for optimization include minimizing blade angles, maintaining narrow clearance heights, operating in the transonic regime, and employing specific blade contouring techniques.

1. Tip Clearance Flow
in
Turbomachines
1
Appalla Aditya Shiva
1005 – 17 – 745302
M.E. Turbomachinery

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Contents:
Basis of tip – clearance flows in turbomachines.
Effects of tip – clearance flow.
Determinants of tip – clearance effect.
Estimation using determinants of tip – clearance flows.
Optimisation of tip – clearance flows.

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Basis of tip – clearance
flows in turbomachines:
• Tip clearance – radial
distance between rotor and
casing of a turbomachine.
• Tip clearance flow is the flow
across the vane from
pressure to suction side,
deviating from vane –
congruent flow.
• Accounts for nearly 1/3rd of
total accrued losses in axial
turbomachines

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Basis of tip – clearance
flows in turbomachines:
• Vortices are created as this
flow mixes with passage flow
(normal flow) and disturbs
overall aero -
thermodynamic behaviour.
• Scrapping flow by blade
opposes this flow.
• Contributes to loss in specific
work.

5. ■ Effects of tip – clearance
flow:
• Flow separation across the blade.
• Heat generation and abrasive
thermal loading
• Loss in fluid work.
• Blockage of fluid – surge in
compressors.
• Noise production.
• Cavitation in hydro – turbomachines.
■ Determinants of tip – clearance
flow:
• Blade – tip clearances.
• Blade loading.
• Relative motion effect.
• Thermal effects.
• End wall and blade boundary layer
5

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Analysis of Determinants:
1. Blade curvature
• Vortices lie in the upper half of
most blade profiles, generating
great energy losses.
• Positive or forward curved
vanes experience less vortex
losses.
• Negative or backward curving –
increases wall vortex flows,
sometimes.

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Analysis of Determinants:
2. Blade – tip clearances
• Pressure loss increases with tip
clearance height, in almost all
cases of subsonic and
transonic flow.
• However, the best results have
been seen at 1% - 3% of blade
height in subsonic flows.
• Such effect is negligible in
turbines generally in transonic
regimes.

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Analysis of Determinants:
3. Blade contouring:
(a) Squealer Blade:
• Guided grooves on the
suction side streamlines the
flow and decreases tip
leakage flow.
• However, recessed pressure
side faces blade loading.

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Analysis of Determinants:
3. Blade contouring:
(b) Axisymmetric Blade and casing:
• Provides smoother tip flow, thus
lower chances of adverse
pressure gradients.
• Reduces tip leakage flow, thus
increasing efficiency and
maintaining nearly 100% mass
flow rate.

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Analysis of Determinants:
3. Blade contouring:
(c) Blade tip winglet:
• Eliminates flow separation
completely, gives 100% mass
flow across the stage.
• Pressure side winglet increases
stall performance by nearly 34%,
without any loss in efficiency.

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Conclusion:
1. Lower blade angles, preferably close to 16° near the outlet.
2. Clearance height as narrow as possible – 0.5% to 1% of blade height.
3. Operate within transonic regime.
4. Use better blade contours – axisymmetric is optimum.
5. Use winglets to fully streamline the flow.