Showerhead Film Cooling Performance of a Turbine Vane at High Freestream Turbulence in a Transonic Cascade

Abstract

One way to increase cycle efficiency of a gas turbine engine is to operate at higher turbine inlet temperature (TIT). In most engines, the turbine inlet temperatures have increased to be well above the metallurgical limit of engine components. Film cooling of gas turbine components (blades and vanes) is a widely used technique that allows higher turbine inlet temperatures by maintaining material temperatures within acceptable limits. In this cooling method, air is extracted from the compressor and forced through internal cooling passages within turbine blades and vanes before being ejected through discrete cooling holes on the surfaces of these airfoils. The air leaving these cooling holes forms a film of cool air on the component surface which protects the part from hot gas exiting the combustor.Design optimization of the airfoil film cooling system on an engine scale is a key as increasing the amount of coolant supplied yields a cooler airfoil that will last longer, but decreases engine core flow-diminishing overall cycle efficiency. Interestingly, when contemplating the physics of film cooling, optimization is also a key to developing an effective design. The film cooling process is shown to be a complex function of at least two important mechanisms: Increasing the amount of coolant injected reduces the driving temperature (adiabatic wall temperature) of convective heat transfer-reducing heat load to the airfoil, but coolant injection also disturbs boundary layer and augments convective heat transfer coefficient due to local increase in freestream turbulence.Accurate numerical modeling of airfoil film cooling performance is a challenge as it is complicated by several factors such as film cooling hole shape, coolant-to-freestream blowing ratio, coolant-to-freestream momentum ratio, surface curvature, approaching boundary layer state, Reynolds number, Mach number, combustor-generated high freestream turbulence, turbulence length scale, and secondary flows just to name a few. Until computational methods are able to accurately simulate these factors affecting film cooling performance, experimental studies are required to assist engineers in designing effective film cooling schemes. experimental data, uncertainty analysis, and computational work, all of which were used in the process of pursuing this research endeavor.iv Acknowledgments

Comparison of RANS and Detached Eddy Simulation Modeling Against Measurements of Leading Edge Film Cooling on a First-Stage Vane

“…The negative and decreasing slopes at s/C > 0.15 are similar for low and medium BR. The case at BR = 4.0 exhibits the most rapid decrease in η av with increasing s/C, consistently with the results reported in [10]. When evaluating the influence of BR on the measured effectiveness around the leading edge (-0.2 < s/C < 0.35), the blowing condition at the medium BR resulted in the most effective cooling performance.…”

“…For a quantitative assessment of the impact of BR on film cooling effectiveness, η values were laterally averaged over the span section shown in Fig. 12 ( showing rapid decay within -0.2 < s/C < -0.05 is consistent with [10], where measurements were performed on a showerhead film cooled vane, with the same hole spanwise angle as that assumed here, at high mainstream turbulence.…”

“…It emerged that increasing the BR up to the tested value of 2.9 resulted in increased lateral average adiabatic effectiveness level at low and high turbulence, due to a shallow injection angle of 25° [8,9]. Conversely, Nasir et al [10] reported that an increase in BR from 1.5 to 2.5 has an adverse effect on the adiabatic effectiveness on the pressure side and has negligible effect on the suction side. But they investigated a showerhead with hole spanwise angle of 45°, at an exit Mach number of Ma = 1.0.…”

On Film Cooling of Turbine Guide Vanes : From Experiments and CFD-Simulations to Correlation Development

“…Xue et al [24] demonstrated that compared to a showerhead only cooling case, the presence of an additional row of shaped holes can lead to a lower heat flux. The influence of exit Reynolds number/Mach number on the performance of showerhead cooling with respect to both film cooling effectiveness as well as Nusselt number has been investigated by Nasir et al [25]. In the presence of showerhead cooling, the AFE and overall cooling effectiveness of a cooling hole embedded in a trench on the pressure side is improved compared to the standard holes, as discussed by Albert and Bogard [26].…”

Evaluation of Predicted Heat Transfer on a Transonic Vane Using v2-f Turbulence Models

“…The paper discusses a new three-simulations technique to calculate vane surface recovery temperature, adiabatic wall temperature, and surface Nusselt number to completely characterize film cooling performance in a high speed flow. All of the 3D film cooling simulations are compared to the experimental data investigated by Nasir S. et al [11,12].…”