A presentation on tip clearance flows in turbomachines, what causes it and some research into mitigating this effect

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
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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.