Two-Shaft Jet Engine
Simplified analysis. The maximum practical pressure, or temperature,
ratio for the compressor of a single-shaft jet engine is limited. The volume
flow rate at the inlet determines the diameter and the rotating speed is
limited by the highest allowable Much number at the inlet stage. As the
pressure increases within a compressor, the volume flow rate reduces and
the air temperature rises. The hydraulic diameters of the flow passages
continuously decrease from inlet to outlet and the corresponding pressure
ratio produced by each stage becomes smaller and smaller. Eventually, the
unit becomes heavy, inefficient, and impractical.
The increase in air temperature through a compressor lowers the Much
numbers of the flow about the blades. It is thus reasonable to use two
compressors; then, after a moderate pressure ratio is developed, the flow is
passed to another compressor having a smaller diameter and a greater
rotating speed. In the same way, the use of two turbines also offers
advantages. Designs with two separate shafts, as shown in Fig. 1.22, are
thus recommended. The reduced size of the inner spool offers the additional
benefit of a reduction in weight. Although the pressure ratio of each
compressor is smaller than that used in Fig. 1.12a, the overall pressure ratio
and efficiency can be increased markedly. Figure 1.23 presents typical maps
of the compressors. The trends of the turbine maps of Fig. 1.13 are still
appropriate.
The hierarchy of pressure ratios across the turbine is similar to that of
the gas generators previously described. An example is shown in Fig. 1.24
for several overall pressure ratios up to 8 across the turbines and engine
nozzle. A logarithmic scale is used for the ordinate so that the constantpressure
ratios across components appear as parallel lines. All elements
were assumed to be choked at the arbitrary overall pressure ratio of 8. The
low-pressure turbine begins to unchoke when the pressure ratio is 6, while
the high-pressure turbine is practically choked down to an overall pressure
ratio of 3.4.
Simplified analysis. The maximum practical pressure, or temperature,
ratio for the compressor of a single-shaft jet engine is limited. The volume
flow rate at the inlet determines the diameter and the rotating speed is
limited by the highest allowable Much number at the inlet stage. As the
pressure increases within a compressor, the volume flow rate reduces and
the air temperature rises. The hydraulic diameters of the flow passages
continuously decrease from inlet to outlet and the corresponding pressure
ratio produced by each stage becomes smaller and smaller. Eventually, the
unit becomes heavy, inefficient, and impractical.
The increase in air temperature through a compressor lowers the Much
numbers of the flow about the blades. It is thus reasonable to use two
compressors; then, after a moderate pressure ratio is developed, the flow is
passed to another compressor having a smaller diameter and a greater
rotating speed. In the same way, the use of two turbines also offers
advantages. Designs with two separate shafts, as shown in Fig. 1.22, are
thus recommended. The reduced size of the inner spool offers the additional
benefit of a reduction in weight. Although the pressure ratio of each
compressor is smaller than that used in Fig. 1.12a, the overall pressure ratio
and efficiency can be increased markedly. Figure 1.23 presents typical maps
of the compressors. The trends of the turbine maps of Fig. 1.13 are still
appropriate.
The hierarchy of pressure ratios across the turbine is similar to that of
the gas generators previously described. An example is shown in Fig. 1.24
for several overall pressure ratios up to 8 across the turbines and engine
nozzle. A logarithmic scale is used for the ordinate so that the constantpressure
ratios across components appear as parallel lines. All elements
were assumed to be choked at the arbitrary overall pressure ratio of 8. The
low-pressure turbine begins to unchoke when the pressure ratio is 6, while
the high-pressure turbine is practically choked down to an overall pressure
ratio of 3.4.
No comments:
Post a Comment