491
13.4 Compressor Blade Profiles
The same considerations as with stator vanes apply to rotor blades. Applying
positive lean and positive sweep at the hub is favourable. Sweep at the casing is
favourable to reduce the Mach number of the flow perpendicular to the blade. But
this can be obtained both with backward and forward sweep. Further, realisation
of a three-dimensional form is more difficult from a structural point of view. At
present, rotor blades have typically a slight general sweep (positive at the hub and
negative at the casing), as in Fig. 13.17, and a slight positive lean at the hub. The
sweep reduces corner stall at the hub. Note that corner stall is less intense with a
rotor than with a stator (Table 13.4). Back-sweep at the casing reduces the effective
Mach number. At present there are fan blades in turbofan engines with backward
sweep at the casing and with forward sweep at the casing. The sweep is applied to
reduce the Mach number of the flow attacking the leading edge. It is not fully clear
if sweep has to be positive or negative for optimal efficiency (and stall margin). The
same applies to the tip of transonic compressor rotors. Probably, forward sweep is
the most advantageous, since then the radial outward migration of boundary layer
fluid caused by centrifugal effect influences less the blade boundary layer at the tip
[8, 18].
13.4 Compressor Blade Profiles
13.4.1 Subsonic and Supercritical Cascades
The optimum Mach number distribution (boundary layer edge velocity) and the
corresponding profile shape for a stator application are shown in Fig. 13.18. Also
shown is the comparison between the optimum profile and a classical profile obtained by a NACA-65 thickness distribution on a circular camber line. The parameters of the stator cascade are:
• inlet Mach number: 0.773 (supercritical)
• flow turning: 39.6°
• pitch-cord ratio: 0.465
• axial velocity-density ratio (AVDR): 1.1.
The features of the optimal Mach number distribution are:
• The acceleration at the leading edge of the suction side runs over about 30 % of
the chord. The objective of the acceleration is to lower the pressure in order to
create pressure difference over the profile. The strength of the acceleration is
tuned such that the boundary layer stays laminar within this zone. The boundary
layer is kept in laminar state in order to minimise the friction and thus the contribution to the resulting drag.
• The deceleration at the suction side is optimised such that the boundary layer is
near to separation at the trailing edge. The boundary layer evolution is such that
the maximum possible tangential force is obtained, but in proportion to the drag
13.4 Compressor Blade Profiles
The same considerations as with stator vanes apply to rotor blades. Applying
positive lean and positive sweep at the hub is favourable. Sweep at the casing is
favourable to reduce the Mach number of the flow perpendicular to the blade. But
this can be obtained both with backward and forward sweep. Further, realisation
of a three-dimensional form is more difficult from a structural point of view. At
present, rotor blades have typically a slight general sweep (positive at the hub and
negative at the casing), as in Fig. 13.17, and a slight positive lean at the hub. The
sweep reduces corner stall at the hub. Note that corner stall is less intense with a
rotor than with a stator (Table 13.4). Back-sweep at the casing reduces the effective
Mach number. At present there are fan blades in turbofan engines with backward
sweep at the casing and with forward sweep at the casing. The sweep is applied to
reduce the Mach number of the flow attacking the leading edge. It is not fully clear
if sweep has to be positive or negative for optimal efficiency (and stall margin). The
same applies to the tip of transonic compressor rotors. Probably, forward sweep is
the most advantageous, since then the radial outward migration of boundary layer
fluid caused by centrifugal effect influences less the blade boundary layer at the tip
[8, 18].
13.4 Compressor Blade Profiles
13.4.1 Subsonic and Supercritical Cascades
The optimum Mach number distribution (boundary layer edge velocity) and the
corresponding profile shape for a stator application are shown in Fig. 13.18. Also
shown is the comparison between the optimum profile and a classical profile obtained by a NACA-65 thickness distribution on a circular camber line. The parameters of the stator cascade are:
• inlet Mach number: 0.773 (supercritical)
• flow turning: 39.6°
• pitch-cord ratio: 0.465
• axial velocity-density ratio (AVDR): 1.1.
The features of the optimal Mach number distribution are:
• The acceleration at the leading edge of the suction side runs over about 30 % of
the chord. The objective of the acceleration is to lower the pressure in order to
create pressure difference over the profile. The strength of the acceleration is
tuned such that the boundary layer stays laminar within this zone. The boundary
layer is kept in laminar state in order to minimise the friction and thus the contribution to the resulting drag.
• The deceleration at the suction side is optimised such that the boundary layer is
near to separation at the trailing edge. The boundary layer evolution is such that
the maximum possible tangential force is obtained, but in proportion to the drag