Figure 15. Heat transfer coefficient distribution on 2nd stage blade meanline section
It’s clear from the charts that the gas heat transfer coefficient in a counter-rotating turbine is lower
than in the traditional axial flow turbine with the same temperature conditions, it’s especially
noticeable at the leading edge. If we consider the heat balance the equation for a cooled turbine
blade in its simplest form can be written:
Where Tb, Tg and Tc is blade,
gas and coolant temperature, hg and hc are the gas-side and coolantside
heat transfer coefficients, and Sg and Sc are the wetted perimeters of the blade profile and
combined coolant passages respectively.
Discussion of obtained results. As a result of the heat transfer coefficient comparison sources a
few potential benefits from these differences, which could be useful in counter-rotating turbines in
case of equality of other thermal conditions in flow and materials. If it is necessary to apply cooling
to this blade, we can consider that lower gas heat transfer coefficient enables to use less cooling air
or higher cooling air temperatures to obtain the same resulting blade temperature as in the initial
case , .
Also, the second benefit is coming from the counter-rotating turbine part arrangement specialties.
As counter-rotating blades are joined to the second shaft in the tip section, they suffer not tensile,
but compression stresses and that’s why it could be possible to use ceramic materials for the second
shaft blades, due to the nature of ceramic materials to resist to compression stresses.