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

The effect of unsteady wake flow and air (D.R.=0.97) or CO₂ (D.R.=1.48) film injection on blade film effectiveness and heat transfer distributions 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. A model turbine blade with several rows of film holes on its leading edge, and pressure and suction surfaces (−0.2<X/C<0.4) was used. Results show that the blowing ratios of 1.2 and 0.8 provide the best film effectiveness over most of the blade surface for CO₂ and air injections, respectively. An increase in the wake Strouhal number causes a decrease in film effectiveness over most of the blade surface for both density ratio injectants and at all blowing ratios. On the pressure surface, CO₂ injection provides higher film effectiveness than air injection at the blowing ratio of 1.2; however, this trend is reversed at the blowing ratio of 0.8. On the suction surface, CO₂ injection provides higher film effectiveness than air injection at the blowing ratio of 1.2; however, this trend is reversed at the blowing ratio of 0.4. CO₂ injection provides lower heat loads than air injection at the blowing ratio of 1.2; however, this trend is reversed at the blowing ratio of 0.4. Heat load ratios under unsteady wake conditions are lower than the no wake case. For an actual gas turbine blade, since the blowing ratios can be greater than 1.2 and the density ratios can be up to 2.0, hence, a higher density ratio coolant may provide lower heat load ratios under unsteady wake conditions.