456
12 Thrust Gas Turbines
12.7.3 Turbofan with Separate Flows
Consider the engine from Exercise 12.7.1 transformed into a turbofan engine. This
is achieved by mounting upstream of the hot nozzle an additional turbine, driving
the fan. Take the bypass ratio equal to 5 and the pressure ratio of the fan equal to
1.7. As infinitesimal fan efficiency choose 0.90. First, determine the power required
to drive the cold flow part of the fan. Then, determine the pressure ratio of the additional turbine, assuming that the mechanical efficiency of the transmission between
the turbine and the fan equals 1. Determine the flow velocities of the cold and the
hot flow and the resulting specific thrust. Determine thermal efficiency, propulsive efficiency and overall efficiency. Compare with the results of Exercise 12.7.1.
Compare also with the results in Table 12.2. Repeat the calculation for a pressure
ratio of the fan 2.5 (attainable in two stages). Evaluate the improvement compared
to pressure ratio 1.7.
12.7.4 Turbofan with Mixed Flows
Consider the turbofan engine with pressure ratio of the fan 1.7 from the foregoing
exercise and transform it into an engine with mixed flows. The available flows are
not perfectly suitable for this, as they do not have the same pressure and the same
velocity. Therefore assume a 3 % loss when mixing, compared to the overall pressure attained from the mass-averaged value. As total enthalpy after mixing assume
the mass-averaged value. Evaluate if flow mixing generates any advantage.
12.7.5 Optimisation of Turbine Inlet Temperature with a
Turbofan Engine
Repeat the calculations of Exercises 12.7.1 and 12.7.3 with turbine inlet temperature T 03 = 1500 K and fan pressure ratio 1.7. Observe that the thermodynamic efficiency lowers somewhat, but the propulsive efficiency increases considerably, so
that the overall efficiency improves. This observation shows that turbine inlet temperature may be optimised.
12.7.6 Helicopter Rotor
The BK117 helicopter (Eurocopter) has a maximum take-off weight of 3100 kg.
The rotor diameter is11 m. Calculate the rotor shaft power required to take off with
maximum weight, ignoring ground effect, in other words, assuming that the helicopter is free in the air, and ignoring the downward force exerted to the fuselage by
the rotor flow. Take into account that the incoming velocity of the air is zero with
12 Thrust Gas Turbines
12.7.3 Turbofan with Separate Flows
Consider the engine from Exercise 12.7.1 transformed into a turbofan engine. This
is achieved by mounting upstream of the hot nozzle an additional turbine, driving
the fan. Take the bypass ratio equal to 5 and the pressure ratio of the fan equal to
1.7. As infinitesimal fan efficiency choose 0.90. First, determine the power required
to drive the cold flow part of the fan. Then, determine the pressure ratio of the additional turbine, assuming that the mechanical efficiency of the transmission between
the turbine and the fan equals 1. Determine the flow velocities of the cold and the
hot flow and the resulting specific thrust. Determine thermal efficiency, propulsive efficiency and overall efficiency. Compare with the results of Exercise 12.7.1.
Compare also with the results in Table 12.2. Repeat the calculation for a pressure
ratio of the fan 2.5 (attainable in two stages). Evaluate the improvement compared
to pressure ratio 1.7.
12.7.4 Turbofan with Mixed Flows
Consider the turbofan engine with pressure ratio of the fan 1.7 from the foregoing
exercise and transform it into an engine with mixed flows. The available flows are
not perfectly suitable for this, as they do not have the same pressure and the same
velocity. Therefore assume a 3 % loss when mixing, compared to the overall pressure attained from the mass-averaged value. As total enthalpy after mixing assume
the mass-averaged value. Evaluate if flow mixing generates any advantage.
12.7.5 Optimisation of Turbine Inlet Temperature with a
Turbofan Engine
Repeat the calculations of Exercises 12.7.1 and 12.7.3 with turbine inlet temperature T 03 = 1500 K and fan pressure ratio 1.7. Observe that the thermodynamic efficiency lowers somewhat, but the propulsive efficiency increases considerably, so
that the overall efficiency improves. This observation shows that turbine inlet temperature may be optimised.
12.7.6 Helicopter Rotor
The BK117 helicopter (Eurocopter) has a maximum take-off weight of 3100 kg.
The rotor diameter is11 m. Calculate the rotor shaft power required to take off with
maximum weight, ignoring ground effect, in other words, assuming that the helicopter is free in the air, and ignoring the downward force exerted to the fuselage by
the rotor flow. Take into account that the incoming velocity of the air is zero with