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.
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, a higher density ratio coolant may provide lower heat load ratios under unsteady wake conditions.
In film cooling situations, there is a need to determine both local adiabatic wall temperature and heat transfer coefficient to fully assess the local heat flux into the surface. Typical film cooling situations are termed three temperature problems where the complex interaction between the jets and mainstream dictates the surface temperature. The coolant temperature is much cooler than the mainstream resulting in a mixed temperature in the film region downstream of injection. An infrared thermography technique using a transient surface temperature acquisition is described which determines both the heat transfer coefficient and film effectiveness (non-dimensional adiabatic wall temperature) from a single test. Hot mainstream and cooler air injected through discrete holes are imposed suddenly on an ambient temperature surface and the wall temperature response is captured using infrared thermography. The wall temperature and the known mainstream and coolant temperatures are used to determine the two unknowns (heat transfer coefficient and film effectiveness) at every point on the test surface. The advantage of this technique over existing techniques is the ability to obtain the information using a single transient test. Transient liquid crystal techniques have been one of the standard techniques for determining h and η for turbine film cooling for several years. Liquid crystal techniques do not account for non uniform initial model temperatures while the transient IR technique measures the entire initial model distribution. The transient liquid crystal technique is very sensitive to the angle of illumination and view while the IR technique is not. The IR technique is more robust in being able to take measurements over a wider temperature range which improves the accuracy of h and η. The IR requires less intensive calibration than liquid crystal techniques. Results are presented for film cooling downstream of a single hole on a turbine blade leading edge model.
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