Outlet triangle

The outlet triangle is determined by geometrical dimensions of flow channel and selected blade angle β2B. The blade angle β2B strongly affects the transmission of energy in the impeller therefore is has to be chosen very carefully.

beta2

Similar to the inlet the velocity triangles in cross sections 2 and 3 are different due to blockage of the flow channel by blades τ2 in section 2.

For determination of β2B it is important to be aware about the deviation between flow angle and blade angle. The direction of the relative flow w2 at impeller outlet does not follow exactly with the blade contour at angle β2B. The flow angle β2 is always smaller than blade angle β2B due to the slip velocity. This difference is called deviation angle δ:

The deviation angle should not exceed 10°…14°, in order to limit increased turbulence losses by asymmetric flow distribution.

A reduced flow angle β2 results in smaller circumferential component of absolute speed cu2, which is - according to Euler's equation - dominant for the transmission of energy. Blade angle β2B is estimated by cu2,∞ for blade congruent flow (see figure). Therefore an estimation of slip is necessary.

Slip can be estimated by empirical models. Two different possibilities are available in CFturbo (not for Turbines):

¢ (1) Decreased output by PFLEIDERER

¢ (2) Outflow coefficient by WIESNER

Blade angle β2B must be determined to reach the desired energy transmission - respectively the required head/ pressure difference - under consideration of slip velocity. Common blade angles β2B are within the range of 15°…45°. For pumps angles of 20°…27° are commonly used, for ventilators 50° should not be exceeded at all.

The following recommendations for common blade angles β2B exist due to optimal efficiency:

Pumps:	15°...45°, commonly used 20°...27°
Ventilators:	not higher than 50°
Compressors:	35°...50°, unshrouded impellers up to 70°...90°
Turbines:	radius dependent, see sine rule

Radial machines - except for turbines - with low specific speed nq usually have similar values for β2B. The blades for this type of impellers are often designed with a straight trailing edge (β2B=const.).

Possible warnings

Problem	Possible solutions
Leading edge blade angle ßB1 < 10°
Unusual low inlet blade angles.	Too small inlet angles indicate too high inlet cross section. Decrease suction diameter dS (Main dimensions)
Leading edge blade angle ßB1 > 40° (pumps, ventilators only)
Unusual high inlet blade angles for pumps and ventilators.	Too high inlet angles indicate too low inlet cross section. Increase suction diameter dS (Main dimensions) and/or reduce blade blockage
Trailing edge blade angle ßB2 < 10°
Unusual low outlet blade angles.	Too small outlet angles indicate too high outlet cross section. Decrease impeller diameter d2 or outlet width b2 (Main dimensions)
Trailing edge blade angle ßB2 > 90°
Unusual high outlet blade angles. As a result the blades are forward curved.	Increase impeller diameter d2 or outlet width b2 (Main dimensions); Check deviation (slip) δ
The deviation (slip) between blade and flow δ > 20° (pumps, ventilators, compressors only)
Unusual high deviation (slip) between blade and flow direction at outlet. This indicates too high blade loading.	Increase the impeller diameter and/or the number of blades (Main dimensions)
The blade angles are not within the valid range.
Unusual high blade angles. Usage of CFturbo is limited to outlet angles between 0° and 180°.	Increase impeller diameter d2 or outlet width b2 (Main dimensions)

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