517
14.2 Kinematic Parameters
Figure 14.8 sketches the h-s diagram of the compression process for R = 0.55.
The kinetic energy at the rotor outlet amounts to about half the rotor work. So
the diffuser has a very crucial role. Blade backward sweep increases the degree of
reaction and decreases the kinetic energy to be transformed by the diffuser. Losses
within the rotor are quite small, due to the lossless contribution to the static enthalpy
rise by the centrifugal force. The enthalpy rise is
With the velocity triangles in Fig. 14.3, the centrifugal force contributes to the enthalpy rise for about 90 % (taking an average w 1 at the rotor inlet). As usual, station 2 is the outlet of the rotor. Losses downstream of this station are accounted
to the subsequent stator part, encompassing a mixing space, a vaned or vaneless
diffuser and a volute. The biggest loss is the mixing loss immediately downstream
of the rotor (see discussion in the later Sect. 14.5.2). With a low degree of reaction
( R = 0.55), mainly the stator loss determines the overall efficiency. Compressor efficiency is generally defined between the total state at the machine inlet, which is
the rotor eye inlet (state 00), and the total state at the machine outlet, which is the
volute outlet (state 03). The total state at the machine inlet is generally taken as the
reference state for determination of dimensionless quantities. We further adhere to
this convention. This way, the classic definition of isentropic internal efficiency is
(see Fig. 14.8):
The third kinematic parameter, the flow coefficient, follows from efficiency optimisation. It is commonly assumed, as a first approximation, that maximum efficiency
is obtained by minimising the Mach number of the relative velocity at the inducer
inlet, at the casing position (tip): w 1t [3, 4]. The main objective of this minimum is
.
2
2
2
2
2
1
2
1
2
2
2
1
1
1
(
)
h h
u u
w
(
w )
− =
−
+
−
.
D
D
h
D
D
−
=
=
=
−
03s
00
0s
0s
tt
03
00
0
h
h
h
h
h
h
h
W
Fig. 14.8 h-s diagram of a
centrifugal compressor with
radial end blades (R = 0.55)
14.2 Kinematic Parameters
Figure 14.8 sketches the h-s diagram of the compression process for R = 0.55.
The kinetic energy at the rotor outlet amounts to about half the rotor work. So
the diffuser has a very crucial role. Blade backward sweep increases the degree of
reaction and decreases the kinetic energy to be transformed by the diffuser. Losses
within the rotor are quite small, due to the lossless contribution to the static enthalpy
rise by the centrifugal force. The enthalpy rise is
With the velocity triangles in Fig. 14.3, the centrifugal force contributes to the enthalpy rise for about 90 % (taking an average w 1 at the rotor inlet). As usual, station 2 is the outlet of the rotor. Losses downstream of this station are accounted
to the subsequent stator part, encompassing a mixing space, a vaned or vaneless
diffuser and a volute. The biggest loss is the mixing loss immediately downstream
of the rotor (see discussion in the later Sect. 14.5.2). With a low degree of reaction
( R = 0.55), mainly the stator loss determines the overall efficiency. Compressor efficiency is generally defined between the total state at the machine inlet, which is
the rotor eye inlet (state 00), and the total state at the machine outlet, which is the
volute outlet (state 03). The total state at the machine inlet is generally taken as the
reference state for determination of dimensionless quantities. We further adhere to
this convention. This way, the classic definition of isentropic internal efficiency is
(see Fig. 14.8):
The third kinematic parameter, the flow coefficient, follows from efficiency optimisation. It is commonly assumed, as a first approximation, that maximum efficiency
is obtained by minimising the Mach number of the relative velocity at the inducer
inlet, at the casing position (tip): w 1t [3, 4]. The main objective of this minimum is
.
2
2
2
2
2
1
2
1
2
2
2
1
1
1
(
)
h h
u u
w
(
w )
− =
−
+
−
.
D
D
h
D
D
−
=
=
=
−
03s
00
0s
0s
tt
03
00
0
h
h
h
h
h
h
h
W
Fig. 14.8 h-s diagram of a
centrifugal compressor with
radial end blades (R = 0.55)