Figure 4. Counter-rotating turbine design
Selection of second shaft rotation speed is dictated by overall engine layout, i.e. joined compressor
part rotation speed, gearbox presence or absence etc. Its natural that higher rotation speed allows to
obtain more specific work from every stage of turbine.
Search region for the second shaft rotation speed was chosen in ranges 10-15% less than primary
shaft rotation speed. To receive exact resulting speed the design of experiment study was performed.
In the figure below response surface between total-to-total efficiency vs first and second shaft
rotation speeds (X and Y axis correspondingly) is shown. Basing on the highest total-to-total
efficiency criterion rotation the speed of 4500 RPM was chosen for the second shaft.
Figure 5. Efficiency chart vs rotation speeds