Rotor-only Preswirler-Rotor- Preswirler-Rotor-Straightener
2.7 loading factor equation
As reported previously, blading design for low-speed axial fans is based on the Blade Element Momentum Theory. The principal design formula de-rives from momentum considerations on an airfoil cascade and is referred as Loading Factor equation:
CL= 2 σ
cam
cos βm
∆cu
w2m − CDtan βm (40)
where
• cam is the local mean axial velocity;
• ∆cu is the difference in terms of tangential velocity through the blade;
• wmis the local mean velocity relative to the blade;
• CL: is the blade element lift coefficient (with respect to the mean veloc-ity wm);
• σbl = nr2πrbl·ch: is the local blade solidity;
• CD: is the blade element drag coefficient (with respect to the mean velocity wm);
Several formulations of Eq. 40 exist; this general form is presented as de-rived in [36, p. 19].
The Loading Factor equation is obtained neglecting the stress terms due to the blade element wake [12, p. 185]. However, it is extremely useful for blading design purpose, containing the basic ingredients of the blade design inverse method: the velocity angles β1, β2, βm obtained from the required velocity distribution (and thus, from the required pressure rise), the solidity distribution σbl(i.e., the chord length for a given number of blades) and the aerodynamic characteristics of the blade section CL and CD.
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3 M E T H O D S
This Chapter presents the numerical and experimental verification meth-ods that have been used during the research to evaluate fan blading behav-ior, performance and efficiency. The numerical CFD model has been used to evaluate the local (i.e., the velocity field) and global (i.e., performance and efficiency) behavior of the fans that have been designed, before the manu-facturing of the prototypes. The Standard facilities have been used for the experimental validation of the design indications as well as to obtain data that have been used in the construction of the fan databases that are pre-sented in Chapter4.
All the fan global performance (i.e., pfand psf) and related efficiencies are defined and evaluated according to the ISO 5801 Standard. The numerical CFD model is presented first, followed by a description of the experimental apparatus.