Analytical and experimental researches on the characteristics of a three-shaft gas turbine are presented in the paper. The concerned gas turbine engine includes a twin-shaft gas generator and a third shaft — the free turbine for power generation. The free turbine uses a variable-angle nozzle (VAN) for fuel saving and load-adjusting flexible.

The engine performance is calculated by using the multi-dimensional Newton-Raphson method, which is based on a set of matching constraints described by the error matrix and the performance maps of the related components. It has a better accuracy and stability than those of some other methods (e.g. parameter cycle method) and is well suitable for solving the off-design problem. The suggested prediction method, presented as a computer program written in Fortran 95, is extremely flexible and can be employed to various engine configurations.

Experiments are carried out on the concerned engine by controlling the stagger angle of the VAN, then adjusting the fuel consumption for different outputs. The accuracy of the prediction method is validated by the experiment.

Detailed performance characteristic of the engine is investigated under five different stagger angles of the VAN when the T3 value was controlled from 85% to 105% of its design value, revealing how the T3 and the stagger angle of the VAN affect the gas turbine performance. The results are useful for the engine operation and components optimization.

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