Leakage Flows in Turbine Cascades

P M V SubbaraoProfessor

Mechanical Engineering Department

In a Large Group, a Fraction of Parcels Will Try to Look for Short Cuts ….

Gaps in Turbines

• In any kind of turbomachinery a certain gap is inevitably required between the rotating and stationary components.

• The minimization of the gap is more than desirable but mechanical considerations and mainly thermal expansion of the materials set certain limits.

• The presence of the tip gap over the blade usually eliminates the stagnation at the endwall.

• The stangation zone typically occurs in the corner between the endwall and blade leading edge in the no-tip-gap configuration.

• Therefore, a horse-shoe vortex does not feature at the tip endwall unless the tip gap is very small.

Leakage losses - Impulse wheel and diaphragm construction

Leakage losses - Reaction drum rotor construction

Classification of Tip Clearance Flow

• The flow at the tip endwall approaching the tip region above the leading edge of the blade is divided into two streams.

• These two streams are aiming towards low pressure regions at the suction surfaces of the neighbouring blades.

• – a main stream of the tip leakage flow going through the tip gap over the blade and

• – a stream of cross-flow going across the blade-to-blade passage.

3D Blade Cascade with a Tip Clearance

The tip leakage flow leaving the tip gap separates from the endwall under conditions of adverse pressure gradient and forms a tip leakage vortex.

• The cross-flow blocked by the development of the tip leakage vortex also separates from the endwall and rolls up into a passage vortex.

• The stream dividing line between the tip leakage and cross-flow lies at the pressure side of the blade tip.

• The tip leakage and passage vortices are characterised by the opposite sense of rotation.

• Usually, a dominant structure is the tip leakage vortex. • The relations between the circulation and size of the two

structures depend on many factors such as • the tip gap size and • flow turning angle.

Influence of Leakage Losses on Exit Velocity Triangle

Effect of leakage flow on Velocity triangle : Hub Section

Effect of leakage flow on Velocity triangle : Tip Section

Modifications of the rotor leading edge

positive metal angle, +12o

negative metal angle, -30o.

Baseline metal angle is at -19o .

Labyrinth Seals

• The labyrinth design provides a high resistance path against steam leakage while allowing sufficient clearance between the rotor and the stationary components

• Labyrinth seals reduce the amount of leakage flow that by-pass the main flow path through the rotor and increasing efficiency thus avoiding rotor rubs during operation..

• Numerous labyrinth geometries are used by the turbine designers depending on the application.

• All of them consist of an inlet and exit cavity and a number of closed cavities in between.

• The size of the cavities needs to be relatively large compared to the blade height to allow for the axial displacement of the rotor due to thermal growth or axial thrust variations.

Labyrinth Seals

New Geometrical Parameters

Effect of leakage flow on incidence angle

• Leakage Losses account for a substantial part of the total Group-1 losses in shrouded turbines.

• The leakage flow that is driven by the pressure difference across the rotor through the labyrinth, gets circulated inside the rotor inlet cavity by the toroidal vortex.

• A part of this flow re-enters the main flow causing extensive mixing in the interaction zone.

• Prior mixing of the cavity and main flows changes the incidence angle at the tip section of the blade.

• The difference in flow angle between the re-injected cavity flow and the main flow is what facilitates the off-design incidence on the downstream blade row.

Challenging Efforts to Contain the Leakage Losses with Seals

The Cavities in Seals• Cavity design plays a key role in the cavity vortex formation

and positioning. • The lower side of the cavity vortex should be close to the

interaction zone. • A Cavity design should push the vortex to be even lower, but

not extending beyond the cavity limit.

Test Rigs for Measurements

Shroud leading edge design

Inlet cavity configurations

Casing contour at the Outlet

Efficiency Gains due to New Ideas

Recovery of Pressure Coefficient

Effect of Cavity Volume on Turbine Performance

Cavity Size Vs Losses

Losses by stage and section for a 700 MW turbine.Source: Toshiba