Overall Cooling Effectiveness Characteristic and Influence Mechanism on an Endwall With Film Cooling and Impingement

The cooling system is required to ensure gas turbine can work at high temperature, which has exceeded the material limitation. An endwall cooling test rig was built up to conduct the endwall cooling research. A detailed work was done for analyzing characteristics of endwall heat transfer and discussing the multi-parameter influence mechanism of overall cooling effectiveness. The main flow side heat transfer coefficient, adiabatic film cooling effectiveness and overall cooling effectiveness were measured in the experiments. The effects of coolant mass flowrate ratio (MFR) were considered through the measurement. In order to analyze how each of the parameters works on overall cooling effectiveness, a one-dimensional correlation was developed. The results showed that obvious enhancement could be found in cooling effectiveness by increasing coolant MFR, and the film jet can be easily attached to the surface after the acceleration of the main flow in the nozzle channel. Comparing with film cooling effectiveness, overall cooling effectiveness distribution is more uniform, which is due to the influence of internal cooling. The verified one-dimensional analysis method showed that the improvement in film cooling would be most efficient to heighten overall cooling effectiveness. The improvement in film cooling would be more efficient when film cooling effectiveness is in high level than in low level. However, the enhancement of internal heat transfer is more efficient when internal heat transfer coefficient is low.

Effect of Film Extraction on Impingement Heat Transfer Characteristics of Jet Arrays on Spherical-Dimpled Surfaces

Detailed heat transfer distributions are numerically investigated on a multiple jet impingement target surface with staggered arrays of spherical dimples where coolant can be extracted through film holes for external film cooling. The three dimensional Reynolds-averaged Navier-Stokes analysis with SST k-ω turbulence model is conducted at jet Reynolds number from 15,000 to 35,000. The separation distance between the jet plate and the target surface varies from 3 to 5 jet diameters and two jet-induced crossflow schemes are included to be referred as large and small crossflow at one and two opposite exit openings correspondingly. Flow and heat transfer results for the dimpled target plate with three suction ratios of 2.5%, 5.0% and 12.0% are compared with those on dimpled surfaces without film holes. The results indicate the presence of film holes could alter the local heat transfer distributions, especially near the channel outlets where the crossflow level is the highest. The heat transfer enhancements by applying film holes to the dimpled surfaces is improved to different degrees at various suction ratios, and the enhancements depend on the coupling effect of impingement and channel flow, which is relevant to jet Reynolds number, jet-to-plate spacing and crossflow scheme.

Conjugate Heat Transfer Optimization of the Nonuniform Impingement Cooling Structure of a HPT 2nd Stage Vane

This paper discusses the methodology of impingement cooling optimization of a gas turbine 2nd stage vane with 3D conjugate heat transfer (CHT) CFD analysis applied. The vane is installed with a novel impingement cooling structure in the leading cavity and a pin-fin array in the trailing edge. This study involves the optimization of the impingement cooling structure, including the location of the jet holes and the diameter of each hole. The generation of 3D model and CHT mesh was realized using an in-house code developed specifically for turbine cooling optimization. A constant pressure drop was assumed within the cooling system during optimization. To make the optimization computationally faster, a metamodel which can predict the detailed distribution of metal temperature on the vane surface was used in the second-level search together with a genetic algorithm. An optimal nonuniform impingement cooling structure in the leading cavity was automatically designed by the optimization process costing only dozens of CFD runs, which provided a more uniform temperature distribution on the vane surface and required no more coolant amount compared with the initial impingement cooling structure.

Conjugate Heat Transfer on Full-Coverage Film Cooling With Array Impingement Jets

We report an investigation of the total cooling effectiveness of a film cooled surface with staggered array impingement jet cooling using infra-red thermography. Heat transfer experiments were carried out using three film cooled test plates of different thermal conductivities: stainless steel (with a thermal conductivity, k = 13.4 W/mK), Corian® (k = 1 W/mK), and polycarbonate (k = 0.2 W/mK). The effects of conduction through the test plates and convective heat transfer due to the arrayed impingement jets were analyzed. The inclination angle of the film cooling holes was 35° and that of the impingement jet holes was 90°. The film and impingement jet holes on each plate were arranged in a staggered pattern, and the film cooling holes and impingement jet holes were also positioned in a staggered pattern. The jet Reynolds number based on the hole diameter was Rejet = 3,000 and the equivalent blowing rate was M = 0.3. The ratio of the target surface height to the hole diameter was varied in the range 1 < H/d < 5. The diameter of both the film cooling holes and impingement jet holes was 5 mm. The total cooling effectiveness was investigated with and without the impingement jets. When the impingement jets were added to the internal cooling, the averaged total cooling effectiveness was enhanced about 8.4%. The stainless steel plate was found to exhibit better cooling performance with more uniform temperature distribution. The total cooling effectiveness was increased up to 0.87 in the stainless steel plate, and the maximum deviation of total cooling effectiveness in the stainless steel was reduced to 85% from that in polycarbonate plate along the lateral direction. The total cooling effectiveness was related to the Biot number of the film cooled plate, however, the effect of the H/d ratio was not significant.

Heat Transfer Implications of Acoustic Resonances in HP Turbine Blade Internal Cooling Channels

In an attempt to investigate the acoustic resonance effect of serpentine passages on internal convection heat transfer, the present work examines a typical high pressure turbine blade internal cooling system, based on the geometry of the NASA E3 engine. In order to identify the associated dominant acoustic characteristics, a numerical FEM simulation (two-step frequency domain analysis) is conducted to solve the Helmholtz equation with and without source terms. Mode shapes of the relevant identified eigenfrequencies (in the 0–20kHz range) are studied with respect to induced standing sound wave patterns and the local node/antinode distributions. It is observed that despite the complexity of engine geometries, as a first order approximation, the predominant resonance behavior can be modeled by a same-ended straight duct. Therefore, capturing the physics observed in a generic geometry, the heat transfer ramifications are experimentally investigated in a scaled wind tunnel facility at a representative resonance condition. Focusing on the straight cooling channel’s longitudinal eigenmode in the presence of an isolated rib element, the impact of standing sound waves on convective heat transfer and aerodynamic losses are demonstrated by liquid crystal thermometry, local static pressure and sound level measurements. The findings indicate a pronounced heat transfer influence in the rib wake separation region, without a higher pressure drop penalty. This highlights the potential of modulating the aero-thermal performance of the system via acoustic resonance mode excitations.

Numerical Investigation of Secondary Flow Vortex Core Structure in the Two-Pass Rectangular Channel With 45° Ribs

The secondary flow which is generated by the angled rib is one of the key factors of heat transfer enhancement in gas turbine blade cooling channels. However, the current studies are all based on the velocity vector and streamline, which limit the research on the detailed micro-structure of secondary flow. In order to make further targeted optimization on the flow and heat transfer in the cooling channels of gas turbine blade, it is necessary to firstly investigate the generation, interaction, dissipation and the influence on heat transfer of secondary flow with the help of new topological method. This paper reports the numerical study of the secondary flow and the effect of secondary flow on heat transfer enhancement in rectangular two-pass channel with 45° ribs. Based on the vortex core technology, the structure of secondary flow can be clearly shown and studied. The results showed that the main flow secondary flow is thrown to the outer side wall after the corner due to the centrifugal force. Then it is weakened in the second pass and a new main flow secondary flow is generated at the same time near the opposite side wall in the second pass. The Nusselt number distribution has also been compared with the secondary flow vortex core distribution. The results shows that the heat transfer strength is weakened in the second pass due to the interaction between the old main flow secondary flow and the new one. These two secondary flows are in opposite rotation direction, which reduces the disturbance and mass transfer strength in the channel.

Build Direction Effects on Additively Manufactured Channels

With the advances of Direct Metal Laser Sintering (DMLS), also generically referred to as additive manufacturing, novel geometric features of internal channels for gas turbine cooling can be achieved beyond those features using traditional manufacturing techniques. There are many variables, however, in the DMLS process that affect the final quality of the part. Of most interest to gas turbine heat transfer designers are the roughness levels and tolerance levels that can be held for the internal channels. This study investigates the effect of DMLS build direction and channel shape on the pressure loss and heat transfer measurements of small scale channels. Results indicate that differences in pressure loss occur between the test cases with differing channel shapes and build directions, while little change is measured in heat transfer performance.

Clocking Effects of Inlet Non-Uniformities in a Fully Cooled High-Pressure Vane: A Conjugate Heat Transfer Analysis

A high-pressure vane equipped with a realistic film-cooling configuration has been studied. The vane is characterized by the presence of multiple rows of fan-shaped holes along pressure and suction side while the leading edge is protected by a showerhead system of cylindrical holes. Steady three-dimensional Reynolds-Averaged Navier-Stokes (RANS) simulations have been performed. A preliminary grid sensitivity analysis with uniform inlet flow has been used to quantify the effect of spatial discretization. Turbulence model has been assessed in comparison with available experimental data. The effects of the relative alignment between combustion chamber and high-pressure vanes are then investigated considering realistic inflow conditions in terms of hot spot and swirl. The inlet profiles used are derived from the EU-funded project TATEF2. Two different clocking positions are considered: the first one where hot spot and swirl core are aligned with passage and the second one where they are aligned with the leading edge. Comparisons between metal temperature distributions obtained from conjugate heat transfer simulations are performed evidencing the role of swirl in determining both the hot streak trajectory within the passage and the coolant redistribution. The leading edge aligned configuration is resulted to be the most problematic in terms of thermal load, leading to increased average and local vane temperature peaks on both suction side and pressure side with respect to the passage aligned case. A strong sensitivity of both injected coolant mass flow and heat removed by heat sink effect has also been highlighted for the showerhead cooling system.

Novel Gas Turbine Blade Leading Edge Cooling Configuration Using Advanced Double Swirl Chambers

The Double Swirl Chambers (DSC) cooling technology, which has been introduced and developed by the authors, has the potential to be a promising cooling technology for further increase of gas turbine inlet temperature and thus improvement of the thermal efficiency. The DSC cooling technology establishes a significant enhancement of the local internal heat transfer due to the generation of two anti-rotating swirls. The reattachment of the swirl flows with the maximum velocity at the center of the chamber leads to a linear impingement effect on the internal surface of the blade leading edge nearby the stagnation line of gas turbine blade. Due to the existence of two swirls both the suction side and the pressure side of the blade near the leading edge can be very well cooled. In this work, several advanced DSC cooling configurations with a row of cooling air inlet holes have been investigated. Compared with the standard DSC cooling configuration the advanced ones have more suitable cross section profiles, which enables better accordance with the real blade leading edge profile. At the same time these configurations are also easier to be manufactured in a real blade. These new cooling configurations have been numerically compared with the state of the art leading edge impingement cooling configuration. With the same configuration of cooling air supply and boundary conditions the advanced DSC cooling presents 22–26% improvement of overall heat transfer and 3–4% lower total pressure drop. Along the stagnation line the new cooling configuration can generate twice the heat flux than the standard impingement cooling channel. The influence of spent flow in the impinging position and impingement heat transfer value is in the new cooling configurations much smaller, which leads to a much more uniform heat transfer distribution along the chamber axial direction.

Adjoint Optimization of an Internal Cooling Channel U-Bend

This paper addresses the optimization of a two-dimensional U-bend passage of an internal serpentine cooling channel for reduced total pressure loss by means of a steepest-descent method. A steady-state incompressible flow is considered at a Reynolds number of 40,000 based on the bulk velocity at the domain inlet. The two-equation k-ε model is used for primal turbulence modeling. After only 30 design iterations, the gradient-based optimization results in a reduction of total pressure loss by 46% compared to the baseline geometry. To obtain the required objective gradients efficiently, a continuous adjoint approach is implemented in the OpenFOAM environment. Adjoint governing equations and boundary conditions are derived from state equations for steady-state, incompressible, turbulent flows under the assumption of frozen turbulence. Two different methods are proposed for modifying the shape of internal and external curves defining the duct geometry. The first method makes use of direct displacement of boundary grid points, allowing for a wide design space. The second, novel parameterization utilizes a projection of the surface sensitivities to an underlying Bézier curve. In this case, the Bézier control points are used as design variables. A comparison of both methods demonstrates a slightly lower performance improvement by the Bézier-based approach due to the reduced design freedom. This approach has, however, several practical advantages. Previous studies already addressed this optimization problem using gradient-free methods, but were limited in the degrees of freedom given to the shape variation. The present gradient-based optimization allows for a much larger design space and hence is used to compare the different methodologies. It shows that both optimizations result in similar shapes, although the gradient-based method allows for a slightly larger reduction in pressure loss due to the wider design space, while converging faster towards the optimum.