Aerodynamic unsteadiness generated upstream of the combustor basket via the complicated geometry of a modern gas turbine can lead to incomplete combustion, reduced efficiency, greater pressure drop, flashback, and reduced part life. The MidFrame section encompasses the main gas path from the compressor exit to the turbine inlet. Diffuser performance, support struts, transition pieces, and other flow obstructing geometries can lead to flow unsteadiness which can reduce performance. This study uses a combination of thermal anemometry, pressure microphone, and wall mounted accelerometer measurements to determine the primary unsteadiness frequencies and target their source. Diffuser performance is shown to have a significant impact on the downstream flow behavior. Inlet conditions are modified to provide a separated bottom wall and a fully attached compressor exit diffuser (CED) condition at an area average inlet Mach number of 0.26. Unsteadiness levels are seen to increase as a result of the separated inlet condition while the mean flow characteristics are slightly altered due to the varying exit trajectory of the main core from the CED, nevertheless the overall level of unsteadiness/turbulence is low for such a complex flow field (8 to 11 %). Results of this study can help diagnose and prevent the aforementioned issues for complicated geometries where simple flow experiments fall short.

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Experiments were performed to measure the heat transfer augmentation and film cooling effectiveness on a film-cooled annular surface subjected to unsteady passing wakes. The wakes can have a profound influence on the effectiveness of film cooling and heat transfer characteristics and it is the objective of an ongoing study to quantify that influence. As part of the study, three blowing ratios (M=0.25, 0.5, 0.75) were tested with discrete film injection (p/D=3) in this paper. The tests were performed for two wake Strouhal numbers (S=0.15, 0.3). The baseline cases involved a steady mainstream flow (S=0). Heat transfer augmentation was measured with passing wakes and with film cooling separately and then with combination of both. A numerical model replicating the annular geometry was used to predict film cooling effectiveness for the steady mainstream cases (S=0) and one transient case was attempted (M=0.5,S=0.3). The computations were performed with pressure-based Reynolds-Averaged Navier-Stokes solver and the realizable k-ε turbulence model. The results from the experiment and computations are compared with relevant published literature. The uncertainties in the experimental values are calculated to be ± 0.03 (absolute) for film cooling effectiveness, ± 3% for velocity measurements, and ± 6.5% for heat transfer coefficient ratio respectively. The passing wakes increased the heat transfer coefficients as high as 11% for the highest wake passing frequency for no film injection (M=0, S=0.3). The influence of the passing wakes was more significant with film injection with a heat transfer augmentation of 37% approximately for M=0.75,S=0.3. The displacement thickness to the film hole diameter ratio at the injection location was observed to be a pertinent parameter that dictates the heat transfer augmentation for the film injection experiments. The centerline film cooling effectiveness was greatly affected by the passing wakes with a maximum decrease of 15% observed for M=0.5,S=0.3. NomenclatureA = area (m 2 ) D = film cooling hole diameter (8mm) D = wake rod diameter (19mm) h = heat transfer coefficient (W/m 2 K) I = momentum flux ratio k = thermal conductivity (W/mK) M = blowing ratio (mass flux ratio) N = angular velocity of wake generator (RPM) n = number of wake generator rods p = pitch (mm) q′′ = heat flux (W/m 2 ) R = resistance of foil heater (ohm) Re = Reynolds number based on entry length; film hole diameter S = wake Strouhal number T = temperature (°C/K) Tu = turbulence intensity U = mean velocity (m/s) V = voltage (V) VR = velocity ratio w = width of heater strip (31 mm)Greek letters α = streamwise inclination angle of film hole (35°) β = compound angle of film hole (0°) δ = disturbance/boundary layer thickness (mm) ε = emissivity η = film cooling effectiveness θ = pitchwise distance θ * = momentum thickness (mm) ρ = density (kg/m 3 ); resistivity of heater material ߢ = ratio of specific heats (c p /c v )

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In the effort to increase turbine inlet temperature for greater efficiencies, more focus has been placed on the secondary and unsteady flow structures in gas turbine components. One such area that has seen great interest in past decades is the effect of unsteady wakes on film cooling. These wakes are primarily shed by upstream guide vanes or rotors. Relatively little data exists for annular endwall cooling in the presence of these wakes. Time resolved measurements of the film cooling-wake interaction were obtained using hot wire anemometry in a low speed, 30 degree annular sector open loop wind tunnel. In addition, time averaged measurements of the adiabatic film cooling effectiveness were determined for cylindrical holes. The film cooling effectiveness at three blowing ratios (0.25, 0.5, and 1.0) is reported at three wake Strouhal numbers (0, 0.1, and 0.3). Temperature Sensitive Paint was used to obtain spatially resolved temperature measurements. The experimental results are compared to numerical studies as well as experimental literature for several cases. The rotating wake is characterized by a velocity detriment and a local increase in turbulence. The effect of this wake is a reduction in film cooling effectiveness with increasing Strouhal number at weak injection rates (I < 0.3). For strong injection that would lead to liftoff, the effect of the wake is to promote reattachment and increase lateral spreading of the jet, resulting in increased effectiveness. Potential for active flow control exists for strong injection resulting in equal or better effectiveness at lower coolant flow rates.