Blade solidity

Blade solidity is an important design parameter for the axial flow impeller and is defined as the ratio of blade chord length to spacing.

• Blade Solidity = c/s

Where

• ${\displaystyle s=2\pi r_{m}/n_{b}}$ is the spacing
• ${\displaystyle r_{m}}$ is the mean radius
• ${\displaystyle n_{b}}$ is blade number
• Chord length c is the length of the chord line

In case of an axial flow impeller, the mean radius is defined in terms of hub (${\displaystyle r_{h}}$,inner radius) and tip radius (${\displaystyle r_{t}}$,outer radius) as :

• ${\displaystyle r_{m}=[(r_{t}^{2}+r_{h}^{2})/2]^{0.5}}$

Blade solidity affects various turbomachinery parameters, so to vary those parameters, one needs to vary blade solidity. However, there are some limitations imposed by aspect ratio (span/chord) and pitch. If an impeller has only a few blades (i.e a large pitch), it will result in less lift force and in a similar manner for more blades (i.e. very low pitch), there will be high drag force.

Blade solidity should not be confused with rotor solidity, which is the ratio of the total area of the rotor blades to the swept area of the rotor.