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End + Pon Ctrl Home Gr ME4528 Propulsion Systems Ender Spring Senester Exam Paper 2011/12 QUESTION 2 (25 marks) Para 3 ore 2/a). (15 marks) Consider a turbojet fitted with an exhaust nozzle capable of varying the nozzle exit cross- sectional area (A). Use the various key engine operating parameters given in the table below to calculate the following: 1. The specific thrust (J/m,) produced when the nozzle is choked at exit: il. The choked exhaust exit area ( A, - A,) which maintains the air velocity into the inlet (4) at the flight speed; ili. The specific thrust (J/,) when the exhaust is fully expanded at exit; and, iv. The exhaust exit area ( A,) which would allow the exhaust to be fully expanded at exit assuming the choked exit area calculated in part-li is now located at the nozzle inlet ( 4 = 4g). Freestream static temperature and pressure (T, and p.) Flight speed (u) Compressor inlet area (A) Burner fuel-air ratio (S) Exhaust nozzle inlet conditions (Tos and Pos) Exhaust nozzle isentropic efficiency (n.) 220 K and 20000 Pa 249 m/s 0.785 m2 0.030 1439 K and 338276 Pa 1.0 Take the following values to be constant through the engine; y = 1.36 R=287J/kg.K c, = 1084J /kg.K 2(b). (10 marks) Describe, with the aid of diagrams, the various exhaust nozzle cross-sectional area variations associated with the phrases 'choked at nozzle exit' and 'fully expanded at nozzle exit'. Note, each diagram should indicate the locations of the important values of local Mach number and local static pressure.