Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

270 Fluid Mechanics, Thermodynamics of Turbomachinery
Incidence losses
At off-design conditions of operation with the fluid entering the rotor at a relative
flow angle, b 2, different from the optimum relative flow angle, b 2,opt, an additional loss
due to an effective angle of incidence, i2 = b 2 - b 2,opt, will be incurred. Operationally,
off-design conditions can arise from changes in
(i) the rotational speed of the rotor,
(ii) the rate of mass flow,
(iii) the setting angle of the stator vanes.
Because of its inertia the speed of the rotor can change only relatively slowly, whereas
the flow rate can change very rapidly, as it does in the pulsating flow of turbomachine
turbines. The time required to alter the stator vane setting angle will also be relatively
long.
Futral and Wasserbauer (1965) defined the incidence loss as equal to the kinetic
energy corresponding to the component of velocity normal to the rotor vane at inlet.
This may be made clearer by referring to the Mollier diagram and velocity diagrams
of Figure 8.10. Immediately before entering the rotor the relative velocity is w2¢.
Immediately after entering the rotor the relative velocity is changed, hypothetically, to
w2. Clearly, in reality this change cannot take place so abruptly and will require some
finite distance for it to occur. Nevertheless, it is convenient to consider that the change
in velocity occurs suddenly, at one radius and is the basis of the so-called shock-loss
model used at one time to estimate the incidence loss.
The method used by NASA to evaluate the incidence loss was described by
Meitner and Glassman (1983) and was based upon a re-evaluation of the experimental
data of Kofskey and Nusbaum (1972). They adopted the following equation
devised originally by Roelke (1973) to evaluate the incidence losses in axial flow
turbines:
(8.46)
Based upon data relating to six stators and one rotor, they found values for the exponent
n which depended upon whether the incidence was positive or negative. With the
present angle convention,