502
13 Axial Compressors
tion anymore for positive mass flow rate and the operating point jumps to the point
B on the negative branch. Point B is a stable operating point (this may be verified by
a perturbation analysis) and a cycle ABCD is run through, as sketched in Fig. 13.26,
left. The cycle is sketched assuming that pressure does not change during the transition from the operating point at the maximum of the characteristic A to the operating
point at the negative branch B, and similarly for the transition from C to D. This can
only be attained with a very large reservoir. In reality, pressure decreases during the
transition from A to B. Similarly, pressure increases during the transition from C to
D. So, a real surge cycle rather looks as sketched in the right-hand part of Fig. 13.26.
The extension of the cycle depends on the buffer capacity of the reservoir. Negative flow rates may occur with very large reservoir capacity. When the flow rate
stays positive, the surge is called mild. When negative flow rates occur, the surge
is called deep. From the above description is clear that surge means that the flow as
a whole is pulsating and running through states with attached boundary layer flow
and states with massively separated boundary layer flow. So, we might also describe
the phenomenon as pulsating stall. The periodic forces on the blades typically are
so high that blades cannot withstand them and surge causes destruction. So, surge
is much more destructive than rotating stall and, therefore, surge is considered as
not allowable. Appearance of surge is only possible when there is a maximum in the
compressor characteristic. As discussed before, this maximum comes from rotating
stall. This means that both phenomena go together, which makes flow patterns quite
complex. In the right-hand part of Fig. 13.26 a sharp maximum is supposed. This
occurs with full-span rotating stall, which is produced at high rotational speed (see
next section). When the maximum is smooth, caused by part-span rotating stall at
low rotational speed, the precise location of the start of the surge cycle depends on
the shape of the load characteristic and the level of kinetic energy at the outflow of
the compressor. Often, with a very large reservoir, surge is assumed to start at the
maximum of the difference between total pressure at compressor inlet and static
pressure at compressor outlet [3]. For a smooth maximum in the characteristic for
total pressure ratio, this means a starting point left of the maximum.
13.5.5 Operating Range
Figure 13.27 sketches a field of characteristics for pressure ratio as a function of
mass flow rate and efficiency as a function of mass flow rate for an axial compressor. Choking is attained at higher rotational speeds. Characteristics are limited at
the low flow rate side by a line called the surge line (line with short dashes in
Fig. 13.27). But surge typically only occurs at higher rotational speed. The characteristic then has a sharp maximum and this maximum is the starting point of a possible surge cycle, independently from the capacity of the downstream reservoir and
the capacity of the compressor casing. At low rotational speed, the maximum typically is smooth and surge can only start at an operating point with lower flow rate
than at the maximum. The limiting phenomenon is then rotating stall if it becomes
13 Axial Compressors
tion anymore for positive mass flow rate and the operating point jumps to the point
B on the negative branch. Point B is a stable operating point (this may be verified by
a perturbation analysis) and a cycle ABCD is run through, as sketched in Fig. 13.26,
left. The cycle is sketched assuming that pressure does not change during the transition from the operating point at the maximum of the characteristic A to the operating
point at the negative branch B, and similarly for the transition from C to D. This can
only be attained with a very large reservoir. In reality, pressure decreases during the
transition from A to B. Similarly, pressure increases during the transition from C to
D. So, a real surge cycle rather looks as sketched in the right-hand part of Fig. 13.26.
The extension of the cycle depends on the buffer capacity of the reservoir. Negative flow rates may occur with very large reservoir capacity. When the flow rate
stays positive, the surge is called mild. When negative flow rates occur, the surge
is called deep. From the above description is clear that surge means that the flow as
a whole is pulsating and running through states with attached boundary layer flow
and states with massively separated boundary layer flow. So, we might also describe
the phenomenon as pulsating stall. The periodic forces on the blades typically are
so high that blades cannot withstand them and surge causes destruction. So, surge
is much more destructive than rotating stall and, therefore, surge is considered as
not allowable. Appearance of surge is only possible when there is a maximum in the
compressor characteristic. As discussed before, this maximum comes from rotating
stall. This means that both phenomena go together, which makes flow patterns quite
complex. In the right-hand part of Fig. 13.26 a sharp maximum is supposed. This
occurs with full-span rotating stall, which is produced at high rotational speed (see
next section). When the maximum is smooth, caused by part-span rotating stall at
low rotational speed, the precise location of the start of the surge cycle depends on
the shape of the load characteristic and the level of kinetic energy at the outflow of
the compressor. Often, with a very large reservoir, surge is assumed to start at the
maximum of the difference between total pressure at compressor inlet and static
pressure at compressor outlet [3]. For a smooth maximum in the characteristic for
total pressure ratio, this means a starting point left of the maximum.
13.5.5 Operating Range
Figure 13.27 sketches a field of characteristics for pressure ratio as a function of
mass flow rate and efficiency as a function of mass flow rate for an axial compressor. Choking is attained at higher rotational speeds. Characteristics are limited at
the low flow rate side by a line called the surge line (line with short dashes in
Fig. 13.27). But surge typically only occurs at higher rotational speed. The characteristic then has a sharp maximum and this maximum is the starting point of a possible surge cycle, independently from the capacity of the downstream reservoir and
the capacity of the compressor casing. At low rotational speed, the maximum typically is smooth and surge can only start at an operating point with lower flow rate
than at the maximum. The limiting phenomenon is then rotating stall if it becomes