Page 421 - Advanced thermodynamics for engineers

17.3 AIRCRAFT GAS TURBINES 411

Using these data the performance parameters can be calculated based on the following criteria:
• the pressure and temperature of the gas leaving the fan and entering the bypass duct;
• the mass flow along the bypass duct can be evaluated from the total flow and the bypass ratio;
• the thrust from the cold stream (bypass) can be evaluated;
• in a two-spool arrangement, the high-pressure (HP) turbine drives the HP compressor and a
power balance must be achieved on that shaft; i.e. there is no net output;
• then the low-pressure (LP) turbine drives the fan according to following conditions

m
_ m t c p a DT 0 12 ¼ h _ m c c p g DT 0 56 (17.78)
thus

_ m t c p a c p a
¼ ¼ðb þ 1Þ (17.79)
DT 0 56 DT 0 12 DT 0 12
m
m
_ m c h c p g h c p g
The value of b can vary over a wide range (0.3 < b < 10) and hence it has a major effect on the
temperature drop in the turbine driving the fan.
has been found it is possible to evaluate the other conditions in the turbines.
Once DT 0 56
• If the hot and cold streams are mixed the resulting gas conditions should be evaluated from mass,
enthalpy and momentum balances.
Example 2 (based on Saravanamuttoo et al. (2001))
The following data apply to a twin-spool turbofan engine, similar to Fig. 17.23, with the fan driven
by the LP turbine and the compressor by the HP turbine. Separate cold and hot nozzles are used.

Overall pressure ratio 19.0
1.65
Fan pressure ratio, r f
3.0
Bypass ratio m b /m c
Turbine inlet temperature 1300 K
Fan, compressor and turbine 0.90
polytropic efficiencies, h N
Isentropic efficiency of each 0.95
propelling nozzle
Mechanical efficiency of 0.99
each spool
Combustion pressure loss 1.25 bar
Total air mass flow, m t 115 kg/s

Evaluate the thrust under sea level conditions where the ambient pressure and temperature are
1.0 bar and 288 K.