Investigation on interactions of unsteady wakes and film cooling on an annular endwall

In recent decades, greater interest in the effect of rotational wakes on gas turbine film cooling applications has produced increasing numbers of studies on these unsteady phenomena. Wakes are primarily shed from upstream components such as transition duct walls, stator vanes, and rotors. Studies have shown that in areas of unsteady flow, the best performing parameters in conventional steady investigations may not be the best for unsteady applications. One common method of modeling the unsteady wake interaction in subsonic flows is the use of spoke wheel type wake generators using cylindrical rods to produce the velocity detriment and local increase in turbulence intensity. Though the impact of wakes have been studied for decades on airfoil losses and boundary layer transition, only recently has the film cooling and wake interaction been investigated. The existing work is primarily on leading edge models and airfoil cascades. The primary parameter characterizing the unsteady wakes is the dimensionless or reduced frequency known as the Strouhal number. The film cooling jet itself has dominant frequencies resulting from the shear and the jet trailing wake shedding, depending on the injectant flow rate. There exist great deficiencies in the fundamental understanding of the interaction of these two frequencies. Heat transfer considerations are also relatively recent being studied only since the early 1990\u27s. Heat transfer coefficients and film cooling effectiveness have been reported for leading edge and linear airfoil cascades. In the case of the linear cascade, no data can be taken near the endwall region due to the varying tangential velocity of wake generating rod. The current work expands on this initiative incorporating a sector annular duct as the test setting for the rotating wakes focusing on this endwall region.; Studies in to the effect of the rods in this alternate orientation include film cooling effectiveness using temperature sensitive paint, impact of wake rod to film cooling hole diameter ratio, and time accurate numerical predictions and comparisons with experimental work. Data are shown for a range of momentum flux ratios and Strouhal numbers. The result of this work sets the stage for the complete understanding of the unsteady wake and inclined jet interaction

Investigation On The Effects Of Wake Rod To Film Cooling Hole Diameter Ratio In Unsteady Wake Studies

In recent decades, greater interest in the effect of rotational wakes on gas turbine film cooling applications has produced increasing numbers of studies on these unsteady phenomena. Wakes are primarily shed from upstream components such as transition duct walls, stator vanes, and rotors. Studies have shown that in areas of unsteady flow, the best performing parameters in conventional steady investigations may not be the best for unsteady applications. One common method of modeling the rotor-stator interaction in subsonic flows is the use of spoke wheel type wake generators using cylindrical rods to produce the velocity detriment and local increase in turbulence intensity. Among the results published to date, no mention on the potential effect of the wake rod to film cooling hole diameter ratio has been addressed. Disagreement among investigators concerning the trailing edge thickness and the effect of boundary layer growth has led to diameter ratios from 0.5 to 5.6 in open literature. This investigation measures the effect of the diameter ratio on the adiabatic film cooling effectiveness over three diameter ratios of 2.375, 4.75, and 9.5 in order to determine any dependence on this typically unidentified parameter. Blowing ratios of 0.25, 0.5, and 0.75 will be tested at wake Strouhal numbers of 0 and 0.3 in order to determine the effects for weak and strong injection rates. Measurements are taken in a low speed induced flow annular duct with jet Reynolds numbers of 2000 to 8000. Among the cases studied no discernable trend based on the diameter ratio is found, indicating that this effect is negligible in wake impact comparisons. The effect of wakes is consistent for both weak and strong injection with similar magnitude of wake impact for each diameter ratio tested. The existence or lack of dependence on diameter ratio allows for more purposeful comparisons among present and future investigations which use this method of producing unsteady wakes. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved

Heat Transfer And Film Effectiveness Study On A Pitchwise Curved Surface With Unsteady Wake Interaction

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. © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved

Experimental Investigation On Aerodynamic Unsteadiness In A Full Scale Gas Turbine Midframe Sector

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. Copyright © 2013 by ASME

Interaction Of Rotational Wakes And Coolant Film In A Sector-Annular Duct

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 \u3c 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. Copyright © 2011 by ASME

Experimental Study Of Unsteady Wake Effect On A Film-Cooled Pitchwise-Curved Surface

Unsteady wake interactions with the near wall flow field occur when a surface is exposed to fluid flow past upstream rotating bodies. In the case of gas turbines, understanding such interactions are essential to better design cooling schemes on endwalls. In view of this, an experimental study has been conducted to determine the heat transfer coefficient and film cooling effectiveness on a pitchwise-curved surface which is subjected to unsteady passing wakes generated using a wake rod in a wall-normal orientation. The mainstream Mach number was maintained approximately constant at 0.03. A single row of cylindrical film holes with pitch to diameter ratio of 3 and inclined at 35° to the test surface are used for discrete film injection. The coolant to mainstream mass flux ratio (M) is varied between 0.25 and 1. A spoke-wheel type wake generator is used to produce unsteady wakes at two wake Strouhal numbers (S = 0.15, 0.3). Measurements are made for (i) steady mainstream flow (S = 0) which serves as a baseline case, (ii) mainstream flow with unsteady wakes, (iii) steady mainstream flow with film injection (iv) both (ii) and (iii) combined. The unsteady passing wakes mitigated jet lift-off at high coolant to mainstream momentum flux ratios. The maximum increase in film effectiveness was measured to be ≅16.35% at the jet centerline for M = 0.75, S = 0.3 at x/D = 2. At high coolant to mainstream mass flux ratios, a combination of increasing film jet turbulence and strong interaction with the mainstream, deteriorates film cooling effectiveness but increases the heat transfer coefficient. Heat transfer augmentation increased by ≅7.6% for the highest wake passing frequency (S = 0.3) without film injection. A combination of unsteady passing wakes and film injection resulted in a maximum pitch-averaged and centerline heat transfer augmentation of ≅28% and 31.7% respectively