503
13.5 Performance Characteristics and Operating Range
so intense that there is risk for damage. Conventionally, start of rotating stall is
considered as a limiting instability phenomenon. Also conventionally, the limiting
instability line in the performance chart is called the surge line, although in practice
surge only occurs for the higher rotational speeds.
A possible operating line, imposed by the load, is drawn with short and long
dashes in Fig. 13.27. An example of a unique operating line is with steady state
operation of the compressor of an aero-engine. The compressor load is generated by
the downstream turbine and the downstream nozzle. For a given rotational speed,
there is an operating margin against instability and choking. Often, operating range
is expressed by the difference between the choking and surge mass flow rates divided by the choking mass flow rate at the design pressure ratio (horizontal line
in the chart). But it is rather difficult to determine where exactly choking begins.
Therefore, mostly the notion of surge margin is used. Also, avoidance of surge is
much more critical than avoidance of choking. Surge margin may mean the difference in pressure ratio between the instability and optimum efficiency values for the
design mass flow rate as a fraction of the optimum efficiency value (vertical line in
the chart). Expressed this way, the surge margin is typically around 25 % for a compressor in an aero-engine [3]. A similar expression is based on the mass flow rates
at instability and optimum efficiency at the design pressure ratio (horizontal line in
the chart). The operating line is not unique because of load transients. For instance,
when an aero-engine is accelerated from lower speed to higher speed by fuel flow
increase, the transient operating line comes closer to the surge line than the line of
steady operating points. Actually, the maximum acceleration is limited by the margin between the line of steady operating points and the surge line.
Fig. 13.27 Chart of characteristics of an axial compressor
13.5 Performance Characteristics and Operating Range
so intense that there is risk for damage. Conventionally, start of rotating stall is
considered as a limiting instability phenomenon. Also conventionally, the limiting
instability line in the performance chart is called the surge line, although in practice
surge only occurs for the higher rotational speeds.
A possible operating line, imposed by the load, is drawn with short and long
dashes in Fig. 13.27. An example of a unique operating line is with steady state
operation of the compressor of an aero-engine. The compressor load is generated by
the downstream turbine and the downstream nozzle. For a given rotational speed,
there is an operating margin against instability and choking. Often, operating range
is expressed by the difference between the choking and surge mass flow rates divided by the choking mass flow rate at the design pressure ratio (horizontal line
in the chart). But it is rather difficult to determine where exactly choking begins.
Therefore, mostly the notion of surge margin is used. Also, avoidance of surge is
much more critical than avoidance of choking. Surge margin may mean the difference in pressure ratio between the instability and optimum efficiency values for the
design mass flow rate as a fraction of the optimum efficiency value (vertical line in
the chart). Expressed this way, the surge margin is typically around 25 % for a compressor in an aero-engine [3]. A similar expression is based on the mass flow rates
at instability and optimum efficiency at the design pressure ratio (horizontal line in
the chart). The operating line is not unique because of load transients. For instance,
when an aero-engine is accelerated from lower speed to higher speed by fuel flow
increase, the transient operating line comes closer to the surge line than the line of
steady operating points. Actually, the maximum acceleration is limited by the margin between the line of steady operating points and the surge line.
Fig. 13.27 Chart of characteristics of an axial compressor