Unsteady Wake Over a Linear Turbine Blade Cascade With Air and CO₂ Film Injection: Part I—Effect on Heat Transfer Coefficients

The effect of unsteady wake flow and air (D.R. = 1.0) or CO₂ (D.R. = 1.52) film injection on blade heat transfer coefficients was experimentally determined. A spoked wheel-type wake generator produced the unsteady wake. Experiments were performed on a five-airfoil linear cascade in a low-speed wind tunnel at the chord Reynolds number of 3 × 10⁵ for the no-wake case and at the wake Strouhal numbers of 0.1 and 0.3. Results from a blade with three rows of film holes in the leading edge region and two rows each on the pressure and suction surfaces show that the Nusselt numbers are much higher than those for the blade without film holes. On a large portion of the blade, the Nusselt numbers “without wake but with film injection” are much higher than for “with wake but no film holes.” An increase in wake Strouhal number causes an increase in pressure surface Nusselt numbers; but the increases are reduced at higher blowing ratios. As blowing ratio increases, the Nusselt numbers for both density ratio injectants (air and CO₂) increase over the entire blade except for the transition region where the effect is reversed. Higher density injectant (CO₂) produces lower Nusselt numbers on the pressure surface, but the numbers for air and CO₂ injections are very close on the suction surface except for the transition region where the numbers for CO₂ injection are higher. From this study, one may conclude that the additional increases in Nusselt numbers due to unsteady wake, blowing ratio, and density ratio are only secondary when compared to the dramatic increases in Nusselt numbers only due to film injection over the no film holes case.