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8/12/2019 Turbine Blades Titanium

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Cascade

Cascade definition

- A row of blades representing blade ring of the actual turbo

machine

Importance of cascade tests

- Cascade tests are useful in predicting the performance ofblade rows in actual machines and optimization of blade row

design

- In sonic velocity condition, cascades can gives information oncompressibility effects

- The angle at which the air is turning for minimum loss can bedetermined

- Profile drag coefficient from which the cascade efficiency canbe estimated


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Cascade types

Cascade Types

- Rectilinear cascade, blades are arranged in a st. line- Annular cascade, blades are arranged in annulus (closer to real

life situation)

- Radial cascade,flow through the ring of blades in radialdirection (both inward and outward)


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Behavior of flow though cascades

Models of flow in actual machines can be constructed

in stationary row of blades (cascades) by maintainingthe geometric, dynamic and kinematic similarities

Measurement usually consists of pressures, velocitiesand flow angles on the upstream and downstream of thecascades

By using st. cascade, the mechanical complication ofthe test rig is reduced

2-d flow conditions obtained in a tunnel of rectangularsection simplifies the interpretation of the test results


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Cascade Tunnel

Main parts of Cascade tunnel

- Blower- Diffuser- Settling chamber- Honeycomb chamber- Contraction zone

- Test section Turntable

- InstrumentationPitot tubes

Yaw meters

Cylindrical

Claw

Bou ndary layer suct ion is appl ied to prevent the contract ion of the f low

and to overcom e the 3d effects


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Cascade tunnel


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Cascade Tunnel


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Cascade tunnel

Working or test section receives uniform flow from

contraction The exit section is oblique to receive the cascade of

blades in an inclined position

To obtain a 2-d flow, a cascade of large no of bladesare required

- This in turn requires a large test section and high flow rates

The height and the length of the cascade are made aslarge as the available air supply will allow

- This is to eliminate the inference effects due to tunnel walls

The contraction always induce the boundary layer doto this the passage of the test section is furthercontracted and flow is accelerated

- This will result in stalling of the flow at top blade


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Cascade tunnel

Boundary layer suction on the walls is applied to

prevent the contraction of the air flow as it flows thedown stream of the tunnel

Turntable- The cascades are mounted on the turntable

- The table can be rotated so that the angle of incidence of the

air can be varied The disturbance in the flow in the central region of

the cascade communicated due to flow distortionsat the ends A and B is minimized by employingminimum no of blades

- Cascade of seven blades of aspect ratio (h/l) is found to begood compromise


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Cascade tunnel

Means are provided for traversing pressuring and

flow direction measurement over two planes- Usually at a distance of one chord upstream anddownstream of the cascade

- Various quantities such as velocity, direction of flow aremeasured


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Blade nomenclature


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Blade nomenclature


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Performance parameters

Stagnation pressure loss

- Loss depends on the magnitude of inlet velocity. So it isexpressed in terms of dynamic head at inlet (dimensionless)

The loss is more at the trailing edge


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Loss is minimum (fairly constant for wide range of

incidence Loss rises more rapidly when the incidence angle is

positive or negative on the larger side

At extreme incidence, the flow of air breaks downfrom the blade (stall) and so the losses are higher

St.Pr.loss

+_


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Blade loading

- Blade loading can be assessed in terms of static pressurecoefficient- Cp distribution gives an idea about the chord wise load

distribution

Cp

x/C


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Air deflection

Air deflection is the difference b/w air inlet and

outlet angle

Object ive of the cascade is to turn the air at maximum angle with

min imum loss


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Air deflection

Practice is to select a deflection which corresponds

to definite proportion of the stalling deflection

s= Stalling deflection

* main ly depends on the pi tch c hord ratio and air out le t angle 2

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