Experimental Measurements of Ingestion Through Turbine Rim Seals: Part 1—Externally-Induced Ingress

This paper describes a new research facility which experimentally models hot gas ingestion into the wheel-space of an axial turbine stage. Measurements of CO2 gas concentration in the rim-seal region and inside the cavity are used to assess the performance of two generic (though engine-representative) rim-seal geometries in terms of the variation of concentration effectiveness with sealing flow rate. The variation of pressure in the turbine annulus, which governs this externally-induced (EI) ingestion, was obtained from steady pressure measurements downstream of the vanes and near the rim seal upstream of the rotating blades. Although the ingestion through the rim seal is a consequence of an unsteady, three-dimensional flow field and the cause-effect relationship between pressure and the sealing effectiveness is complex, the experimental data is shown to be successfully calculated by simple effectiveness equations developed from a previously published orifice model. The data illustrate that, for similar turbine-stage velocity triangles, the effectiveness can be correlated using a non-dimensional sealing parameter, Φo. In principle, and within the limits of dimensional similitude, these correlations should apply to a geometrically-similar engine at the same operating conditions. Part 2 of this paper describes an experimental investigation of rotationally-induced (RI) ingress, where there is no mainsteam flow and consequently no circumferential variation of external pressure.

Application of a CFD-Based Film Cooling Model to a Gas Turbine Vane Cascade With Cylindrical and Shaped Hole Endwall Film Cooling

This paper presents the application of a CFD-based film cooling model to a gas turbine vane cascade test rig. The experimental investigations feature aerodynamic and endwall film cooling measurements on a first stage gas turbine vane in a linear cascade. An extended version of a previously developed cylindrical hole film cooling model has been employed, which now includes modeling of shaped hole cooling flows. The computational domain extends approximately one axial chord length upstream of the leading edge and downstream of the trailing edge of the vane. Adjacent solid parts are included by means of a conjugate heat transfer analysis to account for conduction effects. A hybrid mesh with resolved boundary layers and high spatial mesh resolution in the near-wall region is being used. This meshing approach ensures that the near-wall mesh resolution requirements of the film cooling model are satisfied, while maintaining a manageable total node count. Results obtained using the film cooling model are compared to surface distributions of film cooling effectiveness from the experimental cascade. Due to the moderate node count (≈ 3.5 × 106), CFD calculations including film cooling flows can be performed at comparatively low computational cost. The film cooling model, which previously had been validated against flat plate measurement data and applied to single cooling hole configurations only, is therefore shown to be a viable tool for the thermal design of gas turbine components with film cooling.

Flow Visualization and Conjugate Heat Transfer Study From Shower Head Impinging Jets

Computational and experimental investigations on a flat circular disk are reported with a constant heat flux imposed on its bottom surface and a shower head of air jets impinging on the top surface. The shower head consists of a central jet surrounded by four neighboring perimeter jets. Lamp black flow visualization technique and computations using shear stress transport (SST) κ-ω turbulence model are employed to describe the complex interaction of the wall jets and the associated flow structure. Thermochromic liquid crystal measurement technique is used for surface temperature measurement. The formations of saddle point, nodal point of attachment, nodal point of separation, flow separation line and the up-wash flow are identified. It is observed that the flow topology is practically independent of Reynolds number within the investigated range but is significantly altered with the spacing between the jet orifice and the target surface. A strong correlation between the Nusselt number and the pressure distribution is noticed. Local variation of heat transfer rate with varying plate spacing to jet diameter ratio is significant but its effect on the area weighted average heat transfer rate is small. When compared with a single jet of equal mass flow rate and Reynolds number, the shower head jets provide higher heat transfer rate but require more power for pumping.

Influence of Film Cooling Hole Angles and Geometries on Aerodynamic Loss and Net Heat Flux Reduction

Turbine design engineers have to ensure that film cooling can provide sufficient protection to turbine blades from the hot mainstream gas, while keeping the losses low. Film cooling hole design parameters include inclination angle (α), compound angle (β), hole inlet geometry and hole exit geometry. The influence of these parameters on aerodynamic loss and net heat flux reduction is investigated, with loss being the primary focus. Low-speed flat plate experiments have been conducted at momentum flux ratios of IR = 0.16, 0.64 and 1.44. The film cooling aerodynamic mixing loss, generated by the mixing of mainstream and coolant, can be quantified using a three-dimensional analytical model that has been previously reported by the authors. The model suggests that for the same flow conditions, the aerodynamic mixing loss is the same for holes with different α and β but with the same angle between the mainstream and coolant flow directions (angle κ). This relationship is assessed through experiments by testing two sets of cylindrical holes with different α and β: one set with κ = 35°, another set with κ = 60°. The data confirm the stated relationship between α, β, κ and the aerodynamic mixing loss. The results show that the designer should minimise κ to obtain the lowest loss, but maximise β to achieve the best heat transfer performance. A suggestion on improving the loss model is also given. Five different hole geometries (α = 35.0°, β = 0°) were also tested: cylindrical hole, trenched hole, fan-shaped hole, D-Fan and SD-Fan. The D-Fan and the SD-Fan have similar hole exits to the fan-shaped hole but their hole inlets are laterally expanded. The external mixing loss and the loss generated inside the hole are compared. It was found that the D-Fan and the SD-Fan have the lowest loss. This is attributed to their laterally expanded hole inlets, which lead to significant reduction in the loss generated inside the holes. As a result, the loss of these geometries is ≈ 50% of the loss of the fan-shaped hole at IR = 0.64 and 1.44.

Application of Artificial Neural Network for the Heat Transfer Investigation Around a High-Pressure Gas Turbine Rotor Blade

This paper presents the preliminary results of using artificial neural networks in the prediction of gas side convective heat transfer coefficients on a high pressure turbine blade. The artificial neural network approach which has three hidden layers was developed and trained by nine inputs and it generates one output. Input and output data were taken from an experimental research program performed at the von Karman Institute for Fluid Dynamics by Camci and Arts [5,6] and Camci [7]. Inlet total pressure, inlet total temperature, inlet turbulence intensity, inlet and exit Mach numbers, blade wall temperature, incidence angle, specific location of measurement and suction/pressure side specification of the blade were used as input parameters and calculated heat transfer coefficient around a rotor blade used as output. After the network is trained with experimental data, heat transfer coefficients are interpolated for similar experimental conditions and compared with both experimental measurements and CFD solutions. CFD analysis was carried out to validate the algorithm and to determine heat transfer coefficients for a closely related test case. Good agreement was obtained between CFD results and neural network predictions.

Effects of Guide Vanes on the Tip Heat Transfer Enhancement of a Turbine Blade

Gas turbine blade tips encounter large heat load as they are exposed to the high temperature gas. A common way to cool the blade and its tip is to design serpentine passages with 180-deg turns under the blade tip-cap inside the turbine blade. Improved internal convective cooling is therefore required to increase the blade tip life time. This paper presents numerical predictions of turbulent fluid flow and heat transfer through two-pass channels with and without guide vanes placed in the turn regions using RANS turbulence modeling. The effects of adding guide vanes on the tip-wall heat transfer enhancement and the channel pressure loss were analyzed. The guide vanes have a height identical to that of the channel. The inlet Reynolds numbers are ranging from 100,000 to 600,000. The detailed three-dimensional fluid flow and heat transfer over the tip-walls are presented. The overall performances of several two-pass channels are also evaluated and compared. It is found that the tip heat transfer coefficients of the channels with guide vanes are 10∼60% higher than that of a channel without guide vanes, while the pressure loss might be reduced when the guide vanes are properly designed and located, otherwise the pressure loss is expected to be increased severely. It is suggested that the usage of proper guide vanes is a suitable way to augment the blade tip heat transfer and improve the flow structure, but is not the most effective way compared to the augmentation by surface modifications imposed on the tip-wall directly.

Optimized Impingement Configurations for Double Wall Cooling Applications

Double wall cooling is a very effective technique for increasing heat transfer in hot gas path components utilizing a narrow channel near the surface of the component. Multiple techniques exist to increase the heat transfer within the narrow channel, including the use of impingement jets, turbulators and microchannels. A preliminary study has been performed using computational fluid dynamics (CFD) to determine the heat transfer benefits of double wall cooling technology when compared to a smooth wall square channel and a ribbed wall square channel. Conjugate CFD simulations of flow through an aluminum channel were performed to include the effects of conduction through the solid and convection within the main channel. The design for the preliminary study consists of a square main channel and a narrow impingement channel connected by a series of holes creating impingement jets on the outer surface of the impingement channel. The study examines multiple parameters to increase heat transfer without increasing the pumping power required. The parameters studied include diameter of impingement jets, jet-to-jet spacing, number of impingement jets, and jet-to-wall spacing. Results show that the impingement channel height-to-diameter ratio has a strong impact on heat transfer effectiveness. This study also provides a new optimization methodology for improving cooling designs with specific targets.

Flexible Non-Intrusive Heat Flux Instrumentation for the AFRL Research Turbine

This paper describes a large scale heat flux instrumentation effort for the AFRL HIT Research Turbine. The work provides a unique amount of high frequency instrumentation to acquire fast response unsteady heat flux in a fully rotational, cooled turbine rig along with unsteady pressure data to investigate thermal loading and unsteady aerodynamic airfoil interactions. Over 1200 dynamic sensors are installed on the 1 & 1/2 stage turbine rig. Airfoils include 658 double-sided thin film gauges for heat flux, 289 fast-response Kulite pressure sensors for unsteady aerodynamic measurements, and over 40 thermocouples. An overview of the instrumentation is given with in-depth focus on the non-commercial thin film heat transfer sensors designed and produced in the Heat Flux Instrumentation Laboratory at WPAFB. The paper further describes the necessary upgrade of data acquisition systems and signal conditioning electronics to handle the increased channel requirements of the HIT Research Turbine. More modern, reliable, and efficient data processing and analysis code provides better handling of large data sets and allows easy integration with the turbine design and analysis system under development at AFRL. Example data from cooled transient blowdown tests in the TRF are included along with measurement uncertainty.

Thermal Analysis of Cooling System in a Gas Turbine Transition Piece

This paper presents thermal analyses of the cooling system of a transition piece, which is one of the primary hot components in a gas turbine engine. The thermal analyses include heat transfer distributions induced by heat and fluid flow, temperature, and thermal stresses. The purpose of this study is to provide basic thermal and structural information on transition piece, to facilitate their maintenance and repair. The study is carried out primarily by numerical methods, using the commercial software, Fluent and ANSYS. First, the combustion field in a combustion liner with nine fuel nozzles is analyzed to determine the inlet conditions of a transition piece. Using the results of this analysis, pressure distributions inside a transition piece are calculated. The outside of the transition piece in a dump diffuser system is also analyzed. Information on the pressure differences is then used to obtain data on cooling channel flow (one of the methods for cooling a transition piece). The cooling channels have exit holes that function as film-cooling holes. Thermal and flow analyses are carried out on the inside of a film-cooled transition piece. The results are used to investigate the adjacent temperatures and wall heat transfer coefficients inside the transition piece. Overall temperature and thermal stress distributions of the transition piece are obtained. These results will provide a direction to improve thermal design of transition piece.

LES and RANS Assessment of Rib Cooled Channel Related to SGT-800 Combustor Liner

The main parts of the annular combustor liner walls of the Siemens gas turbine SGT-800 are convectively cooled using rib turbulated cooling. Due to the serial system of cooling and combustion air there is a potential of further reduction of total combustor pressure drop by improvements of the cooling system. Apart from the rib cooling, also the cooling channel bypass entrance is related to a significant part of the total cooling system pressure drop. In this study, an investigation is performed for a rib cooled channel which is related to the considered combustor liner and where empirical correlations are available in order to evaluate the methodology used. The study includes an assessment of the Reynolds Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) models available within commercial Computational Fluid Dynamics (CFD) codes and includes also an investigation of model size when using periodic boundaries for LES simulations. It is well known that a small geometrical distance in the direction of the periodic boundaries may have a strong effect on the flow field but is often neglected in practice in order to speed up LES calculations. Here the effect is assessed in order to show what size is required for accurate results, both for time averaged and transient results. In addition too small domains may be affected by spurious low frequencies originating from the periodic boundaries requiring additional simulation time for time converged statistics, but also the averages may be significantly affected. In addition the simulation period for time converged statistics is evaluated in order to show that larger model size in the periodic direction does not necessarily require longer practical simulation time, due to the fact that larger volumes may be used for the combined time and space averaging. The aim is to obtain practical guidelines for LES calculations for internal cooling flows. Then the study is extended step by step to investigate the importance due to high Reynolds number, variable fluid properties and large temperature gradients in order to cover the ranges and specifics required for SGT-800 engine conditions.