Cascades Tunnel

P Karthikeyan

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 of blade rows in actual machines and optimization of blade row design- In sonic velocity condition, cascades can gives information on

compressibility effects- The angle at which the air is turning for minimum loss can be

determined- Profile drag coefficient from which the cascade efficiency can

be estimated

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 radial

direction (both inward and outward)

Behavior of flow though cascades

• Models of flow in actual machines can be constructed in stationary row of blades (cascades) by maintaining the geometric, dynamic and kinematic similarities

• Measurement usually consists of pressures, velocities and flow angles on the upstream and downstream of the cascades

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

• 2-d flow conditions obtained in a tunnel of rectangular section simplifies the interpretation of the test results

Cascade Tunnel

• Main parts of Cascade tunnel- Blower- Diffuser- Settling chamber- Honeycomb chamber- Contraction zone- Test section

Turntable- Instrumentation

Pitot tubes Yaw meters

• Cylindrical• Claw

Boundary layer suction is applied to prevent the contraction of the flow and to overcome the 3d effects

Cascade tunnel

Cascade Tunnel

Cascade tunnel

• Tunnel consists of an arrangement whereby a stream of air can be sucked or blown through a no. of blades set out in the form of a st. cascade- Blower can be either axial or centrifugal type

• Air from the blower is supplied to the settling chamber through short diffuser

• Due to large cross sectional area of the settling chamber, the flow velocity is reduced to a small value

• Settling chamber contains wire gauges and honeycomb will straighten the flow and remove turbulence before it is expanded into contraction zone

• Contraction can be achieved simultaneously or achieved in two stages- Profile of the converging walls must be designed carefully in

order to avoid separation and thickening of boundary layers

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

• The height and the length of the cascade are made as large 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 do to this the passage of the test section is further contracted and flow is accelerated- This will result in stalling of the flow at top blade

Cascade tunnel

• Boundary layer suction on the walls is applied to prevent the contraction of the air flow as it flows the down stream of the tunnel

• Turntable- The cascades are mounted on the turn table- 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 distortions at the ends A and B is minimized by employing minimum no of blades- Cascade of seven blades of aspect ratio (h/l) is found to be

good compromise

Cascade tunnel

• Means are provided for traversing pressuring and flow direction measurement over two planes- Usually at a distance of one chord upstream and

downstream of the cascade- Various quantities such as velocity, direction of flow are

measured

Blade nomenclature

Blade nomenclature

Performance parameters

• Stagnation pressure loss

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

• The loss is more at the trailing edge

Performance parameters

• 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 down from the blade (stall) and so the losses are higher

St.

Pr.

loss

+ _

Performance parameters

• Blade loading- Blade loading can be assessed in terms of static pressure

coefficient- Cp distribution gives an idea about the chord wise load

distribution

Cp

x/C

Air deflection

• Air deflection is the difference b/w air inlet and outlet angle

Objective of the cascade is to turn the air at maximum angle with minimum loss

Air deflection

• Practice is to select a deflection which corresponds to definite proportion of the stalling deflection

» s = Stalling deflection

* mainly depends on the pitch chord ratio and air outlet angle 2