Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

70 Fluid Mechanics, Thermodynamics of Turbomachinery
(a)
Total pressure loss coefficient, x
(b)
0.15
0.10
0.05
Incidence, i = a1 – a¢ 1 deg
–30 –20 –10
Incidence, i deg
0 10
FIG. 3.12. (a) Compressor cascade characteristics (Howell 1942). (By courtesy of the
Controller of H.M.S.O., Crown copyright reserved.) (b) True variation of total pressure
loss versus incidence angle—a replot of the data in (a).
There is a pronounced increase in total pressure loss as the incidence rises beyond a
certain value and the cascade is stalled in this region. The precise incidence at which
stalling occurs is difficult to define and a stall point is arbitrarily specified as the incidence
at which the total pressure loss is twice the minimum loss in total pressure.
Physically, stall is characterised (at positive incidence) by the flow separating from the
suction side of the blade surfaces. With decreasing incidence, total pressure losses again
rise and a “negative incidence” stall point can also be defined as above. The working
range is conventionally defined as the incidence range between these two limits at
which the losses are twice the minimum loss. Accurate knowledge of the extent of the
working range, obtained from two-dimensional cascade tests, is of great importance
when attempting to assess the suitability of blading for changing conditions of
operation. A reference incidence angle can be most conveniently defined either at the