Design and off-design performance of a supersonic axial flow turbine with different stator–rotor axial gaps

The effects of changing the stator–rotor axial gap on the performance and flow field of a low-reaction supersonic axial turbine are studied at design and off-design conditions. The objectives are gaining a better understanding of the effects and giving information and recommendations that a designer can use in his work. Three different axial gaps are modelled numerically with computational fluid dynamics at design and low off-design conditions. The geometry with the smallest axial gap is also measured at intermediate off-design conditions. The efficiency of the turbine decreases when the axial gap increases. The efficiency decrement is steeper at the off-design than the design conditions. It is concluded that the efficiency drop is mainly caused by the increased total pressure losses at the axial gap. It is recommended that the axial gap should be as small as possible. The smaller axial gap makes the rotor inlet flow angle distribution less curved, which is recommended to be taken in account when designing the turbine rotor for the studied turbine type. At the rotor outlet, the changing axial gap changes the absolute flow angle.