A numerical 2D study of the unsteady flow in the first stage of an industrial axial flow gas turbine is presented. The study focuses on the influence of the axial spacing between stator and rotor on the unsteady flow field and relevant design parameters. For six different axial spacings between 18.5 % and 60 % of rotor chord length, time-averaged and time-resolved results are presented, focusing mainly on the pressure distribution on the rotor, stage efficiency and flow angle variation.

The instantaneous pressure and entropy contours at midspan are presented for different rotor positions. The underlying physical principles are described making a clear distinction between the impact of the potential flow interaction and the wake-blade interaction. The pressure fluctuations over the stator and rotor blades and the seal air injection gap as well as the effects of axial spacing are investigated. The major focus of this paper is to examine the variation of stage efficiency and rotor incidence angle with a changing axial gap.

A simple design rule is derived in order to optimize the vane-blade spacing and it is shown that the gap chosen for the real life machine offers a good compromise between the opposing design requirements.

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