Kinematics

<< Click to Display Table of Contents >>

Navigation:  Impeller > Airfoil/ Hydrofoil design > Blade properties >

Kinematics

Previous pageReturn to chapter overviewNext page

► Impeller | Blade properties

[ Axial machines only ]

The following parameters together with the chosen profile describe the blade at each span:

γ

stagger angle

l

chord length

φ

camber angle (Lieblein method)

Two methods are available for the determination of the scaling (solidity) and staggering of the profiles:

Blade element momentum method   [only fans]
for low pressure applications (high specific speed nq)

Lieblein method   [pumps, fans]
for high pressure applications (low specific speed nq)

 

On the tabsheet Profile properties the stagger angles and solidity are calculated.

Sweep correction μ

Sweep correction is used when the profile properties are calculated. A sweep correction < 1 means that stagger angle and chord length are overestimated to compensate for losses introduced by blade sweeping.

Efficiency values that are relevant for the profile properties calculation are displayed for information as hint.

Limitations

The design methods are valid only within certain scopes:

The deceleration should no be smaller than the DeHaller criterion:

.

 

In a pipe flow having a swirl a dead water zone is built at small radii. Strscheletzky and Marcinowski stated that the diameter of such a dead water zone should be smaller than the hub diameter of an impeller. From this they derived the following criteria for single stage machines:

,

and for multi stage machines:

.

 

From boundary layer analysis the diffusion number applied for profiles with a maximum thickness of 10% was derived:

.

Special NACA-measurements yield a scope to be fulfilled of DF0.1 ≤ 0.6.

Possible warnings

Problem

Possible solutions

Automated blade angles/ stagger angles/ camber angles/ chord lengths are active.
Values may adapt to changing input parameters.

The values are updated automatically when input parameters are modified.

To fix the values uncheck "Automatic" calculation. Then one must manually start the calculation if required.

Automated blade angles/ stagger angles/ camber angles/ chord lengths are NOT active.
Values are fixed, but may not reflect input parameters.

The values are not updated automatically when input parameters are modified.

To be sure that all parameter modifications are considered one shall switch to an automatic calculation by checking the "Automatic" option.

The blade sweep yields a sweep correction factor of x and is different from the value currently set in blade properties (y).

See warning.

Set μ according to value given in blade sweep.

Radial equilibrium calculation failed.

There is not for all constellations a solution of the radial equilibrium.

Try to change Bezier-points of cu-curve in case of variable load or switch to free vortex.

Automatic mode not possible in case of failed radial equilibrium calculation.

In case of failed radial equilibrium calculation the velocity triangles are not correctly determined and cannot be used for setting the blade angles or stagger angles + chord length.

Uncheck "Automatic" and set blade angles or stagger angles + chord length manually.