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Consider a small turbojet installed into a model radio-controlled aircraft which has the following operating conditions; Freestream static temperature and pressure Flight speed (u) Engine air intake frontal area (4) Turbine Inlet Temperature (704) No afterburner installed Exhaust Jet Velocity (u) Exhaust static temperature (T) 288 K and 100000 Pa. 14 m/s 0.031 m2 600 K 324 m/s 374 K Take the following values to be constant through the engine; y =1.4 R=287J/kg.K Cp = 1005J/kg.K Qs = 45x10ºJ/kg You may assume the following; • the exhaust nozzle flow is fully expanded to atmospheric pressure; no stagnation pressure loss in the combustion chamber; a combustion efficiency equal to 1.0; and, all component isentropic efficiencies are equal to 1.0. Using the data, and assumptions, above, calculate the following; the compressor pressure ratio (Pos/Poz); • the fuel-air ratio (f); • the thrust (J) in Newtons; and, the exhaust nozzle exit area to maintain the engine intake velocity at the flight speed. .
Consider a low-bypass turbojet which has a low-pressure multi-stage compressor at the engine front rather than a large single-stage fan. The engine is equipped with an afterburner and a single variable outlet-area exhaust nozzle. Using the data tabulated below, calculate the specific thrust when the afterburner is off and then switched on. Conditions for Afterburner Off and on Freestream static temperature and pressure 220 K and 19 kPa. Flight speed (u) 251 m/s Bypass ratio (B) 2 y = 1.36 R=287J/kg.K Cp = 1084J/kg.K - Single Mixed Nozzle - Afterburner Off Bypass duct outlet conditions (Toy) and (P) 343 K and 90 kPa Turbine outlet conditions (Ts) and (Pos) 947 K and 72 kPa Fuel-air ratio (8) 0.023 Single nozzle is choked at exit Single Mixed Nozzle - Afterburner On Bypass duct outlet pressure (Pog) 90 kPa Turbine outlet pressure (Pos) 72 kPa Afterburner outlet temperature (76) 1600 K Single nozzle isentropic efficiency 0.9 Fuel-air ratio (S3 + far) 0.04 Single nozzle is fully expanded at exit 2(b). (5 marks) Calculate the approximate percentage increase in exhaust nozzle exit area required to ensure that the nozzle exit flow is fully expanded to the atmospheric pressure when the afterburner is on.