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Description of the Flow in a Two-Stage Low-Pressure Turbine With Hub Cavities

In gas turbine, multi-stage row blading and technological effects can exhibit significant differences for the flow compared with isolated smooth blade rows. Upstream stages promote 
a non-uniform flow field at the inlet of the downstream rows that may have large effects on 
mixing or boundary layer transition processes. The rows of current turbines (and compressors) 
are already very closely spaced. Axial gaps between adjacent rows of approximately 
1/4 to 1/2 of the axial blade chord are common practice. Future designs with higher loading 
and lower aspect ratios, i.e., fewer and bigger blades, and the ever present aim at minimizing 
engine length or compactness, will aggravate this condition even further. Interaction 
between cascade rows will therefore keep increasing and need to be taken into account 
in loss generation estimation. Also the cavities at hub platform induce purge flow 
blowing into main annulus and additional losses for the turbine. A robust method to 
account for the loss generated due to these different phenomena needs to be used. The 
notion of exergy (energy in the purpose to generate work) provides a general framework 
to deal with the different transfers of energy between the flow and the gas turbine. This 
study investigates the flow in a two-stage configuration representative of a low-pressure 
turbine including hub cavities based on large eddy simulation (LES). A description of the 
flow in the cavities, the main annulus, and at rim seal interface is proposed. The assessment 
of loss generated in the configuration is proposed based on an exergy analysis. The study of 
losses restricted to boundary layer contributions and secondary flows show the interaction 
processes of secondary vortices and wake generated in upstream rows on the flow in downstream 
rows