Importance of the vane exit Mach number on the axial clearance-related losses

Abstract

More efficient and physically smaller axial turbine designs are promoted to lower emissions and increase revenue. The physical size and the weight of the axial turbine can be minimised by adjusting the distance between successive stator and rotor rows. The influence of changing stator-rotor axial clearance can usually have either a positive or a negative influence on the turbine performance, and the reasons for this varying behaviour are not currently fully understood. A previous study revealed several design parameters that correlate with the efficiency curve shape. However, the effects of two parameters, namely the stator outlet Mach number and the reduced blade passing frequency, still remained unclear. Therefore, a novel approach is taken to analyse the correlations between the two design parameters and the axial clearance efficiency curve shape. Several different turbines are analysed using data available in the literature, and also new data are presented. The study suggests that the stator outlet Mach number correlates reasonably well with the efficiency curve shape, and it was further linked to five loss mechanisms and the rotational speed. Although the unsteady interaction plays an important role in the loss share, the level of unsteadiness did not correlate with the efficiency curve shape.

Stator-Rotor Interaction in Axial Turbine: Flow Physics and Design Perspective

“…According to the open literature, this trend is confirmed by some authors [42][43][44][45][46] but seems not general (as also reported and discussed by [41]), either for a lack of detailed data or for a case dependency in the stator wake-potential field coupling along the axial direction, in the axial gap region.…”

“…For low axial gaps, on the contrary, low-mixing takes places but a highly nonuniform flow enters in the rotor, leading to additional losses in the rotor itself. It is clear so far, how the axial gap is a parameter that has to undergo an optimisation process and this is the reason why it has been the focus of a number of research that gave different results, likely depending on the operating condition and stage loading [41].…”