Effect of nozzle exit area on the performance of a turbojet engine

The thrust produced by a gas turbine engine can be optimized by suitably changing the nozzle geometry in conjunction with other control parameters. However, varying the nozzle geometry influences the performance of the upstream rotating machinery and shifts the equilibrium running line on the compressor map. In the present study, to investigate the influence of the nozzle area on engine performance, experiments are conducted on a small gas turbine engine. A total of three tests are conducted. The first being a baseline test, without any nozzle area reduction. For the subsequent tests, the jet pipe exit area is reduced using a suitably designed throttle cone, simulating nozzle area reduction. The engine component performances are monitored using suitably placed pressure sensors, thermocouples and microphones. The results show that when the throttle setting was unaltered, with a 23% decrease in the nozzle exit area, engine rotor speed reduced to ∼55,000 RPM from the initial setting of ∼70,000 RPM, EGT increased by ∼250 K, thrust reduced by ∼30% and OASPL remained relatively constant. All of the above is caused by a reduction in air mass flow rate ingested by the engine. However, when the same nozzle exit area reduction was imposed at constant rotor speed, engine operation moved along a constant speed line on the compressor map, moving closer to the surge line. The EGT increased by ∼200 K, thrust increased by ∼18% and in order to maintain the same rotor speed, fuel-air-ratio increased by ∼41%. The dominant effect when operating at constant rotor speed is the increase in fuel flow rate, which contributes to an improved engine performance as compared to the constant throttle setting case.