Gustavo A. Ledezma

Turbulence Model Assessment for Conjugate Heat Transfer in a High Pressure Turbine Vane Model

An assessment of steady state Reynolds Averaged Navier-Stokes (RANS) models has been undertaken for conjugate heat transfer of an internally cooled high-pressure turbine vane with and without film cooling. The assessment includes near wall treatment and different 2-equation Eddy Viscosity Models (EVM) and 6-equation Reynolds Stress Models (RSM) models. The present study was conducted using CFX v11.0 with unstructured tetrahedral meshes with near wall prism layers. The validation cases are the 1983 NASA C3X internally cooled vane and the 1988 NASA C3X internally and film cooled vane. Internal cooling for both cases is achieved with ten radial cooling channels of constant cross-sectional area. Film cooling is achieved for the same airfoil geometry but with three separately fed upstream plenums feeding various rows of film cooling holes. Predictions obtained with the different modeling strategies are compared to documented metal surface pressures and temperatures and the differences are discussed. A conjugate heat transfer assessment is made using the vane Biot number. In general good agreement with experimental data is obtained for wall integration meshes with the k-ω and SST turbulence models.Copyright

Overall and Adiabatic Effectiveness Values on a Scaled Up, Simulated Gas Turbine Vane: Part II—Numerical Simulations

Conjugate heat transfer (CHT) calculation techniques for the heat transfer analysis of high-pressure turbines (HPT) have been developed during the past few years. Thus, it has become possible to take into account the coupling of the film, internal cooling, external gas flow and the metal diffusive heat transfer. The coupling problem may become extremely important in regions such as the airfoil leading edge and the vicinity of film hole breakout region where heat fluxes and thermal gradients are high. This article presents the results obtained using fully coupled 3D CHT simulations of a simplified film-cooled leading edge model and a NASA C3X vane with suction side film cooling. The results for the two cases are compared against experimental data obtained at University of Texas at Austin. The numerical simulations were conducted using the k-ω turbulence model. The leading edge model overall effectiveness predictions are in good agreement with the experiments, especially in the low blowing ratio range (1≤M≤2). For the C3X vane, the CHT results tend to underpredict the midspan and laterally averaged effectiveness due to film liftoff. However, the quantitative agreement is still reasonably good. The different levels of overall effectiveness agreement found between all cases are discussed.Copyright

The Effects of Conjugate Heat Transfer on the Thermal Field Above a Film Cooled Wall

Common gas turbine heat transfer analysis methods rely on the assumption that the driving temperature for heat transfer to a film cooled wall can be approximated by the adiabatic wall temperature. This assumption implies that the gas temperature above a film cooled adiabatic wall is representative of the overlying gas temperature on a film cooled conducting wall. This assumption has never been evaluated experimentally. In order for the adiabatic wall temperature as driving temperature for heat transfer assumption to be valid, the developing thermal boundary layer that exists above a conducting wall must not significantly affect the overriding gas temperature. In this paper, thermal fields above conducting and adiabatic walls of identical geometry and at the same experimental conditions were measured. These measurements allow for a direct comparison of the thermal fields above each wall in order to determine the validity of the adiabatic wall temperature as driving temperature for heat transfer assumption. In cases where the film cooling jet was detached, a very clear effect of the developing thermal boundary layer on the gas temperature above the wall was measured. In this case, the temperatures above the wall were clearly not well represented by the adiabatic wall temperature. For cases where the film cooling jet remained attached, differences in the thermal fields above the adiabatic and conducting wall were small, indicating a very thin thermal boundary layer existed beneath the coolant jet.Copyright

Experimental Measurements and Computational Predictions for an Internally Cooled Simulated Turbine Vane

In this study the conjugate heat transfer effects for an internally cooled vane were studied experimentally and computationally. Experimentally, a large scale model vane was used with an internal cooling configuration characteristic of real gas turbine airfoils. The cooling configuration employed consisted of a U-bend channel for cooling the leading edge region of the airfoil and a radial channel for cooling the middle third of the vane. The thermal conductivity of the solid was specially selected so that the Biot number for the model matched typical engine conditions. This ensured that scaled nondimensional surface temperatures for the model were representative of those in the first stage of a high pressure turbine. The performance of the internal cooling circuit was quantified experimentally for internal flow Reynolds numbers ranging from 10,000 to 40,000. The external surface temperature distribution was mapped over the entire vane surface. Additional measurements, including internal surface temperature measurements as well as coolant inlet and exit temperatures, were conducted. Comparisons between the experimental measurements and computational predictions of external heat transfer coefficient are presented.

Overall and Adiabatic Effectiveness Values on a Scaled Up, Simulated Gas Turbine Vane: Part I—Experimental Measurements

Recent advances in computational power have made conjugate heat transfer simulations of fully conducting, film cooled turbine components feasible. However, experimental data available with which to validate conjugate heat transfer simulations is limited. This paper presents experimental measurements of external surface temperature on the suction side of a scaled up, fully conducting, cooled gas turbine vane. The experimental model utilizes the matched Bi method, which produces non-dimensional surface temperature measurements that are representative of engine conditions. Adiabatic effectiveness values were measured on an identical near adiabatic vane with an identical geometry and cooling configuration. In addition to providing a valuable data set for computational code validation, the data shows the effect of film cooling on the surface temperature of a film cooled part. As expected, in nearly all instances the addition of film cooling was seen to decrease the overall surface temperature. However, due to the effect of film injection causing early boundary layer transition, film cooling at a high momentum flux ratio was shown to actually increase surface temperature over 0.35 < s/C < 0.45.Copyright

Experimental and Numerical Investigations of the Heat Transfer and Flow Field in a Trailing Edge Cooling Geometry: Part 2 — LES and Hybrid RANS/LES Study

This paper presents the results of a numerical study on the predictive capabilities of Large Eddy Simulation (LES) and hybrid RANS/LES methods for heat transfer in the trailing edge (TE) geometry experimentally investigated in Part 1. The experimental validation data includes 2D wall contours and laterally-averaged values of adiabatic cooling effectiveness. The simulations were conducted at three different blowing ratio values. The comparison with the experimental data shows a general advantage of LES and hybrid RANS/LES methods against steady-state RANS. The results obtained by means of the WALE LES model and the Improved Delayed Detached Eddy Simulation (IDDES) hybrid RANS/LES method were comparable. The presented grid dependence study shows the importance of adequate grid resolution for the predictive capabilities of trailing edge cooling LES. Furthermore, the importance of considering TE slot lands simulation quality in the numerical method assessment is discussed. Potential directions of future research needed to improve simulation reliability are outlined.Copyright

Heat Transfer in Multiple Parallel High Aspect Ratio Ducts With Triangular Trench Enhancement Features

Detailed heat transfer coefficient distributions and pressure drop have been obtained for high aspect ratio (AR = Width/Height = 12.5) ducts with triangular trench enhancement features oriented normal to the coolant flow direction. Numerical and experimental approaches analyze the performance of triangular trenches for six geometrically identical ducts branching from a common plenum. The numerical approach is based on a Reynolds Averaged Navier Stokes (RANS) turbulence model with an unstructured mesh. A transient liquid crystal (TLC) technique is used to experimentally calculate Nu on the ducts surfaces. Reynolds number (Re = 7080, 14800, and 22400, with respect to the duct hydraulic diameter are explored. As Computational Fluid Dynamics (CFD) and TLC results are both detailed, qualitative and quantitative comparisons are made. Experimental results show the closest and furthest ducts from the entrance of the plenum are considerably affected, as recirculation zones develop which partially choke the inlet the respective ducts. Results from the experiments are compared to CFD predictions from Duct 4. In addition, the experimental data are recalculated with the maximum bias in TLC temperature to indicate an improved matching between CFD and experimental methods to demonstrate that CFD captures the wall heat transfer coefficient trends similar to experimental results. The triangular trenches enhance heat transfer in the ducts two-fold compared to smooth wall Dittus-Boelter Nusselt number correlation for flow in tubes.Copyright

The Optimal Distribution of Pin Fins for Blade Tip Cap Underside Cooling

This article presents a geometric optimization study to maximize the total heat transfer rate between an array of discrete pin fins and the surrounding serpentine cooling flow. The fins are installed on the tip cap underside of a High Pressure Turbine blade model. The study has three parts. In the first, the numerical model is validated against experimental data obtained with liquid crystal thermography. In the second part, the heat and fluid flow performance of the pin fin assembly is simulated numerically, using RANS turbulence models in the range 25,000 < Re < 100,000 and Pr ∼ 0.7. The effect of varying the spacing and the tip cap boundary condition is investigated. In the last part of the study it is shown that the optimal spacing between the pin fins can be correlated following the same theoretical arguments derived in previous investigations that used simpler geometries.Copyright