Question 4

Question 4a
In an ideal unmixed–exhaust turbofan engine where the exhaust exit pressures of both the core engine and bypass duct are equal to ambient pressure, the thrust produced by the core engine 𝐹𝐶, and the fan 𝐹𝐹 are:

Where
𝑚̇ 𝐶 = mass flowrate of air through core engine
𝑚̇ 𝐹 = mass flowrate of air through bypass duct
𝑚̇𝑓 = fuel mass flowrate
𝑉0 = forward velocity
𝑉9 = core engine exhaust nozzle exit velocity
𝑉19 = bypass duct nozzle exit velocity

Question 4a(i)
For a turbofan engine, explain the term specific thrust. If its bypass ratio 𝛼 = 𝑚̇ 𝐹/𝑚̇ 𝐶, show that the specific thrust can be expressed as:

Question 4a(ii)
Define the term thrust specific fuel consumption of a turbofan engine, and hence show that it can be expressed in terms of its specific thrust as in the following:

Question 4b
At forward speed of 750 km/h, a turbofan engine with bypass ratio of 5 has a fuel-air ratio of 0.03. At the exhaust nozzles of both the bypass duct and core engine, the gases are fully expanded. The bypass exhaust exits at 450 m/s while the core engine exhaust exits at 500 m/s. Analyse the engine specific thrust.