In order to continue increasing the efficiency of gas turbines, an important effort is made on the thermal management of the turbine stage. In particular, understanding and accurately estimating the thermal loads in a vane passage is of primary interest to engine designers looking to optimize the cooling requirements and ensure the integrity of the components. This paper focuses on the measurement of endwall heat transfer in a vane passage with a three-dimensional (3D) airfoil shape and cylindrical endwalls. It also presents a comparison with predictions performed using an in-house developed Reynolds-Averaged Navier–Stokes (RANS) solver featuring a specific treatment of the numerical smoothing using a flow adaptive scheme. The measurements have been performed in a steady state axial turbine facility on a novel platform developed for heat transfer measurements and integrated to the nozzle guide vane (NGV) row of the turbine. A quasi-isothermal boundary condition is used to obtain both the heat transfer coefficient and the adiabatic wall temperature within a single measurement day. The surface temperature is measured using infrared thermography through small view ports. The infrared camera is mounted on a robot arm with six degrees of freedom to provide high resolution surface temperature and a full coverage of the vane passage. The paper presents results from experiments with two different flow conditions obtained by varying the mass flow through the turbine: measurements at the design point (ReCax=7.2×105) and at a reduced mass flow rate (ReCax=5.2×105). The heat transfer quantities, namely the heat transfer coefficient and the adiabatic wall temperature, are derived from measurements at 14 different isothermal temperatures. The experimental data are supplemented with numerical predictions that are deduced from a set of adiabatic and diabatic simulations. In addition, the predicted flow field in the passage is used to highlight the link between the heat transfer patterns measured and the vortical structures present in the passage.
Shaped holes are used on modern turbine blades for their higher performance and greater lateral coolant spreading compared to classic streamwise angled holes. This study incorporates measurements and observations from a shaped hole geometry undertaken at ETH Zurich in which a row of laterally expanded diffusely shaped holes is compared to the classic row of streamwise round holes. Infrared measurements provide high-resolution data of the adiabatic effectiveness and three dimensional velocity measurements are carried out through stereoscopic Particle Image Velocimetry. Both experiments are run for similar operating conditions allowing a comparison to be made between the flow structure and the thermal performance. The adiabatic effectiveness is seen to be higher for shaped holes compared to cylindrical holes: in particular the laterally averaged values are higher due to a larger lateral spreading of the coolant. The work presented here shows the first results on the limited influence of the density ratio on the thermal performance. The performance is also influenced by the vortical structure. The typical counter-rotating vortex pair which is completed by another pair of anti-kidney vortices is observed with their strength being clearly reduced compared to the example with cylindrical holes. The doubled structure and the reduced strength change the behavior of the jet, explaining the higher performance of a jet with shaped holes. The vertical motion leading to lift-off is reduced, so the jet remains close to the surface even at high blowing rates. The goal of this article is to present data for the thermal performance and flow field of shaped holes and then explain the relationship between the two.
A numerical investigation of the effect of stochastic surface roughness on vane endwall heat transfer was conducted. The effect of equivalent sand grain roughness height was explored and compared with available experimental data. Steady-state computations using ANSYS CFX 14.0 in conjunction with the shear stress transport turbulence model were performed. Computations were conducted for fully turbulent flow conditions, since this best reproduces the conditions for the corresponding measurements. Roughness measurements were conducted at different locations along the vane passage. Exploration of these measurements indicated roughness Reynolds number values from the transitional and fully rough regime. The roughness model supplied in CFX was applied to explore the impact of surface roughness on heat transfer. Numerical heat transfer results in the vane passage were determined from a set of computations at the same operating point consisting of an adiabatic and a heat flux calculation. Calculations were conducted with a systematic variation of equivalent sand grain roughness heights and compared with experimental data. Results are presented for smooth and rough wall calculations at two different flow conditions.
In order to continue increasing the efficiency of gas turbines, a significant effort is being made to reduce losses induced by secondary flows in turbine stages. In addition to their impact on aerodynamic losses, these vortical structures are also the source of large heat transfer variations across the passage. A substantial reduction of the secondary flow losses can be achieved with a contoured endwall. However, a change in the vortical pattern can dramatically impact the thermal loads on the endwalls and lead to higher cooling requirements in those areas. This paper focuses on heat transfer measurements made in a passage with either flat or contoured endwalls. The experimental data are supplemented with numerical predictions of the heat transfer data. The measurements are carried out on an isothermal endwall equipped with symmetric airfoils. The paper presents measurements at M = 0.3, corresponding to a Reynolds number ReCax=4.6×105. An infrared camera is used to provide high-resolution surface temperature data on the endwall. The surface is equipped with an insulating layer (Kapton), allowing the calculation of heat flux through the endwall. The heat transfer quantities, namely the heat transfer coefficient and the adiabatic wall temperature, are then derived from a set of measurements at different isothermal plate temperatures. The numerical predictions clarify the link between the change in the heat transfer quantities and the changes in the flow field due to endwall contouring. Finally, numerically predicted heat transfer data are deduced from a set of adiabatic and diabatic simulations that are compared to the experimental data. The comparison focuses on the differences in the regions with endwall contouring, where a significant difference in the heat transfer coefficient between flat and contoured endwalls is measured but underpredicted numerically.
The introduction of endwall contouring in the design of modern gas turbines has helped to improve the aerodynamic performance. In fact, the management of secondary flows and the control of purge air flow are limiting the generation of losses and enhancing the use of coolant air. The impact of such geometrical features on the endwall thermal loads is then of primary interest for designers in charge of optimizing the cooling of the components and ensuring their mechanical integrity. This paper focuses on heat transfer measurement on a contoured vane endwall installed in an axial turbine. The measurements were performed on a dedicated platform installed in the axial turbine rig of ETH Zurich, using a quasi-isothermal boundary condition and an infrared camera traversed by a multi-axis robot-arm. Due to the complex geometry, a mis-attachment of the insulating Kapton layer was observed in several regions of the passage and corrupted the measurements of about 20% of the endwall. An experimental correction method based on the surface response to laser step heating was developed. A specific setup was constructed and used to map the surface response of a calibration plate with flat bottom holes and the heat transfer platform. A model linking the response to the bubble thickness was obtained and used to successfully correct the results. The heat transfer data were obtained for two turbine operating conditions at ReCax = 720000 and 520000. The correction technique, commonly used for defects detection, has been applied in a quantitative manner to provide successful correction of the measurements for different operating conditions.
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