450
12 Thrust Gas Turbines
better result, compared to separate flows, even with optimised velocity ratio, may
easily be explained. The efficiency of the mechanical energy transfer (turbine,
transmission, fan) is the bypass efficiency, which is significantly lower than unity
(about 0.88 in Table 12.3). The thermal energy transfer by the mixing is, in principle, lossless. So, it is clearly advantageous to mix, even if there is some loss during mixing. Not too high bypass ratios, as in military engines ( b = 0.5–1), enable a
complete mixing of the hot and the cold flow. A lobed mixer surface is sometimes
applied to enhance mixing (see further: Fig. 12.17). Application of mixed flows is
then always favourable. Complete mixing cannot be realised at higher bypass ratios,
because of the limited length of the mixing zone in practise. Some engines with
moderate bypass ratio have two versions. An example is the V2500 (IEA). In this
engine the bypass ratio is about five. It exists with separate nozzles, but there is also
a version with the cold flow brought to the rear of the engine through a long duct.
This is favourable for noise reduction, as the engine is then encapsulated. The cold
flow is partially mixed with the hot flow without mixer surface.
12.5.3 Intercooling and Recuperation
Figure 12.13 is a sketch of a turbofan engine with intercooling between the IP
compressor (booster) and the HP compressor. The cooling air comes from part
of the outlet flow of the fan. There is recuperation of outlet heat by a series of
heat exchangers preheating the compressor air before entry to the combustion
chamber. The thermodynamic benefits of the system are similar as with power
gas turbines, but it is, of course, crucial that pressure losses in the heat exchangers be low. Such engines are studied nowadays (2014), but these are not used
in practise yet.
Table 12.3 Performance of a double-jet engine (turbofan) for T 03 = 1675 K and b = 10
r cold
p 06 /p 05
s hot
2
s cold
2
T 2 /T 1
b
h
d p
h h
1.50
1.424
2.910
1.443
1.791
0.891
0.646
1.55
1.199
2.721
1.474
1.825
0.888
0.659
1.60
1.010
2.521
1.504
1.853
0.885
0.669
1.65
0.850
2.309
1.533
1.874
0.882
0.676
1.70
0.715
2.079
1.560
1.887
0.880
0.681
1.75
0.601
1.825
1.587
1.890
0.878
0.682
1.80
0.504
1.534
1.612
1.881
0.876
0.679
12 Thrust Gas Turbines
better result, compared to separate flows, even with optimised velocity ratio, may
easily be explained. The efficiency of the mechanical energy transfer (turbine,
transmission, fan) is the bypass efficiency, which is significantly lower than unity
(about 0.88 in Table 12.3). The thermal energy transfer by the mixing is, in principle, lossless. So, it is clearly advantageous to mix, even if there is some loss during mixing. Not too high bypass ratios, as in military engines ( b = 0.5–1), enable a
complete mixing of the hot and the cold flow. A lobed mixer surface is sometimes
applied to enhance mixing (see further: Fig. 12.17). Application of mixed flows is
then always favourable. Complete mixing cannot be realised at higher bypass ratios,
because of the limited length of the mixing zone in practise. Some engines with
moderate bypass ratio have two versions. An example is the V2500 (IEA). In this
engine the bypass ratio is about five. It exists with separate nozzles, but there is also
a version with the cold flow brought to the rear of the engine through a long duct.
This is favourable for noise reduction, as the engine is then encapsulated. The cold
flow is partially mixed with the hot flow without mixer surface.
12.5.3 Intercooling and Recuperation
Figure 12.13 is a sketch of a turbofan engine with intercooling between the IP
compressor (booster) and the HP compressor. The cooling air comes from part
of the outlet flow of the fan. There is recuperation of outlet heat by a series of
heat exchangers preheating the compressor air before entry to the combustion
chamber. The thermodynamic benefits of the system are similar as with power
gas turbines, but it is, of course, crucial that pressure losses in the heat exchangers be low. Such engines are studied nowadays (2014), but these are not used
in practise yet.
Table 12.3 Performance of a double-jet engine (turbofan) for T 03 = 1675 K and b = 10
r cold
p 06 /p 05
s hot
2
s cold
2
T 2 /T 1
b
h
d p
h h
1.50
1.424
2.910
1.443
1.791
0.891
0.646
1.55
1.199
2.721
1.474
1.825
0.888
0.659
1.60
1.010
2.521
1.504
1.853
0.885
0.669
1.65
0.850
2.309
1.533
1.874
0.882
0.676
1.70
0.715
2.079
1.560
1.887
0.880
0.681
1.75
0.601
1.825
1.587
1.890
0.878
0.682
1.80
0.504
1.534
1.612
1.881
0.876
0.679