Characteristics of flow and thrust variations in a jet engine with a variable area nozzle

A numerical analysis of the internal flow field in an exhaust converging-diverging nozzle of a jet engine has been carried out. Variable area nozzle is required in a jet engine for optimal expansion and adjusting back pressure in the engine when the afterburner is operated. Steady-state and transient analyses are carried out for each condition with and without afterburner operation and as a function of the location of the nozzle flap. For a baseline power condition, mixture composition of combustion products is calculated by the chemical equilibrium analysis and the cold flow analysis is carried out based on the mixture composition. For the afterburner operating condition, additional fuel is injected from the inlet boundary and numerical analyses of reactive flow fields are carried out. With variable area nozzle adopted, combustion fields are variable in time, leading to periodically variable thrust. When nozzle flap is in a closed state, nozzle internal pressure and temperature are increased upon the afterburner operation and moment of a force at the pivot point is increased as well. These undesirable phenomena can be solved by control of a variable area nozzle. And, thrust level is also maximized by nozzle variation. During nozzle variation, unsteadiness effect is observed because of response lag of internal flow to nozzle variation. Both the design pressure and the optimum expansion of the nozzle can be attained by adopting the independent operation of a variable area nozzle.

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