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Inlet triangle |
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Inlet triangle |
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The inlet triangle is defined by inflow parameters and geometrical dimensions on leading edge.
Between inlet area and leading edge the swirl is constant because transmission of energy from rotating impeller to fluid occurs in blade area only.
Cross sections 0 and 1 (see Main dimensions) are different only due to blockage of the flow channel by blades (τ1) in section 1. This results in an increased meridional velocity cm.
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Selected blade angle β1B does only indirectly influence the velocity triangle due to blade blockage. Differences between selected blade angle β1B and flow angle β1 is referred as the incidence angle: i = β1B-β1
In general an inflow without any incidence is intended (i=0). If i≠0 the flow around the leading edge shows high local velocities and low static pressure:
i > 0: β1 < β1B → stagnation point on pressure side
i < 0: β1 > β1B → stagnation point on suction side
A small incidence angle i can be profitable for best efficiency point. Calculation of β1B inside CFturbo gives inflow without incidence.
For pumps and ventilators β1B should be lower than 40° due to best efficiency.
For pumps, because of cavitation β1B should be as small as possible; with regard to efficiency not smaller then 15…18°.
For compressors the optimal blade angle β1B is about 30°.
If the radius of leading edge varies from hub to shroud the blade angle β1B does not remain constant. A higher radius on shroud results in a lower value for β1B- the blade is curved on leading edge.
Problem |
Possible solutions |
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Leading edge blade angle β1 > 40° (pumps, ventilators only) |
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Unusual high inlet blade angles. Small inlet angles are typical for pumps and ventilators. |
Too high values indicate too small inlet cross section. |
Leading edge blade angle ß1 < 10° |
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Unusual low inlet blade angles. |
Too small inlet angles indicate too high inlet cross section. Decrease suction diameter dS (Main dimensions) |
The blade angles are not within the valid range. |
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Unusual high blade angles. Usage of CFturbo is limited to inlet angles between 0° and 90° (turbines 180°). |
Increase suction diameter dS (Main dimensions) |
[ Turbine rotors only ]
In case of turbines the following calculation of incidence by Aungier can be used. This is based on the empirical equation of the outflow coefficient by Wiesner.
According to decreased energy transmission the slip coefficient γ is defined:
The cu-difference is called slip velocity.
The smaller the slip coefficient, the higher is the deviation of flow compared to the direction given by blade (γ=1: no incidence).
The empirical equation by Wiesner adapted to the incidence is:
with correction factor
Circumferential component of blade flow at zero incidence can be calculated as follows:
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(const. inflow swirl)