Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

172 Fluid Mechanics, Thermodynamics of Turbomachinery
(5.38)
Now since the change in swirl velocity across the rotor is equal and opposite to the
swirl produced by the guide vanes, the work done per unit mass flow, C pDT 0 is equal
to Ucy1. Thus the second term in eqn. (5.37) is
Combining eqns. (5.35), (5.36) and (5.39) in eqn. (5.37) then,
(5.39)
(5.40a)
The foregoing exercise can be repeated for the second type of fan having outlet straightening
vanes, and assuming frictionless flow through the “straighteners”, the rotor blade
element efficiency becomes
(5.40b)
Some justification for ignoring the losses occurring in the guide vanes is found by
observing that the ratio of guide vane pressure change to rotor pressure rise is normally
small in ventilating fans. For example, in the first type of fan
the tangential velocity cy1 being rather small compared with the blade speed U.
FIG. 5.18. Method suggested by Wislicenus (1947) for obtaining the zero lift line of
cambered aerofoils.