Ewald Lutum

Advanced evaluation of transient heat transfer experiments using thermochromic liquid crystals

Abstract An advanced evaluation method for transient heat transfer experiments using thermochromic liquid crystals (TLCs) combining the advantages of standard hue and maximum intensity methods is presented. In order to obtain a global evaluation of locally correct heat transfer coefficients by using the one-dimensional solution of Fouriers equation, assuming heat conduction in a semi-infinite medium with a convective boundary condition, local input values have to be identified from measurements of the fluid and surface temperatures. For that reason, two different approaches have emerged. First, a two-dimensional numerical method has been adapted to evaluate the transient fluid temperature distributions in multi-pass systems from a few local measurements. Additionally, on the basis of latest calibration and indication experience of TLCs, especially in complex passages, an innovative temporal indication analysis method using a neural network has been implemented in the process of heat transfer evaluation.

Performing Heat Transfer Experiments in Blade Cooling Circuits Using a Transient Technique With Thermochromic Liquid Crystals

Blades and vanes in the first stages of modern gas turbines are exposed to high thermal loads. As the resulting temperatures exceed the temperature stability of the material, it is necessary to validate the internal cooling capability of these parts experimentally, before proving their reliability in service. In the present state of the art a commonly used experimental method for evaluating heat transfer coefficients is given by the transient technique using thermochromic liquid crystals (TLCs). Though it is easily applicable for short single-pass and simple-shaped cooling channels, additional aspects must be considered for small engine-representative 3D cooling circuits that have a complex multi-pass system. Mass flow splits and space-time-dependent fluid temperatures may be noted as examples. To point out and suggest solutions for these characteristics, we therefore present a procedure for conducting transient heat transfer experiments using TLCs with respect to engine-representative, complex 3D cooling circuits in particular. General design and dimensioning guidelines are given by means of exemplary geometries. Furthermore, an approach for the experimental setup, preparation of models, and test procedure is discussed. The experiments were conducted with an engine-representative Reynolds number and Mach number and the same heat flux direction as in a real blade. We evaluated the present method and performed an uncertainty analysis where the technique demonstrated robustness for a variety of investigated geometries.© 2008 ASME

A computational investigation of the effect of surface roughness on heat transfer on the stator endwall of an axial turbine

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.

High Resolution Heat Transfer Measurement Technique on Contoured Endwall With Non-Uniform Thermal Resistance

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.Copyright

Effect of purge air on rotor endwall heat transfer of an axial turbine

In order to gain in cycle efficiency, turbine inlet temperatures tend to rise, posing the challenge for designers to cool components more effectively. Purge flow injection through the rim seal is regularly used in gas turbines to limit the ingestion of hot air in the cavities and prevent overheating of the disks and shaft bearings. The interaction of the purge air with the main flow and the static pressure field of the blade rows results in a nonhomogenous distribution of coolant on the passage endwall which poses questions on its effect on endwall heat transfer. A novel measurement technique based on infrared thermography has been applied in the rotating axial turbine research facility LISA of the Laboratory for Energy Conversion (LEC) of ETH Zürich. A 1.5 stage configuration with fully three-dimensional airfoils and endwall contouring is integrated in the facility. The effect of different purge air mass flow rates on the distribution of the heat transfer quantities has been observed for the rated operating condition of the turbine. Two-dimensional distributions of Nusselt number and adiabatic wall temperature show that the purge flow affects local heat loads. It does so by acting on the adiabatic wall temperature on the suction side of the passage until 30% of the axial extent of the rotor endwall. This suggests the possibility of effectively employing purge air also as rotor platform coolant in this specific region. The strengthening of the secondary flows due to purge air injection is observed, but plays a negligible role in varying local heat fluxes. For one test case, experimental data is compared to highfidelity, unsteady Reynolds-Averaged Navier–Stokes simulations performed on a model of the full 1.5 stage configuration. J. Glob. Power Propuls. Soc. | 2017, 1: 211–223 | https://doi.org/10.22261/F29ZWY 211

High Resolution Heat Transfer Measurements on the Stator Endwall of an Axial Turbine

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 3D airfoil shape and cylindrical endwalls. It also presents a comparison with predictions performed using an in-house developed 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 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.Copyright

Aero-Thermodynamic Aspects of Film Cooling in Regions of Separated Flow on the Pressure Side of a High-Lift HPT Blade

Due to the ever increasing demand for cost-optimised designs, modern engine design concepts lead to more and more highly loaded HP turbine blades. In order to achieve the high lift required, turbine airfoils will have to cope with main flow diffusion up to separation both on suction and pressure side. Thus, for film cooled HP turbine blades and vanes, the possible aerodynamic and aero-thermal interaction of highly loaded blade rows and film cooling needs to be addressed. The first results to be presented from this ongoing work within the European 5th Frame-Work-Project AITEB jointly comprises experimental high-speed cascade wind-tunnel as well as numerical investigations with state-of-the-art 3D-RANS CFD. Steady and unsteady experimental results detailing the row characteristic of the highly-loaded T120 HP-turbine cascade set the stage for detailed numerical investigations with and without coolant injection from rows of holes on the pressure side surface as well as comparative numerical calculations with different codes and turbulence models. Despite the current focus of the experimental work on aerodynamic topics, the numerical results to be presented comprise thermodynamic investigations and detailed studies on optimised coolant injection geometries as well.Copyright