Rico Poser

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

Transient Heat Transfer Experiments in Complex Passages

Transient heat transfer experiments were performed in a model of a multi-pass gas turbine blade cooling circuit. The inner surface of the Plexiglas model was coated with thermochromic liquid crystals in order to determine the internal heat transfer coefficients. A change in inlet temperature is applied using a pre-cooled heat exchanger. As for simple geometries the analytical solution of Fourier’s equation can often be directly used for data evaluation, one ought to pay attention to complex passages. The reason has to be seen that the flow in complex passages has to be characterized by local and time dependent fluid temperatures. As a direct consequence data evaluation might be limited to small evaluation areas especially far downstream. Otherwise the uncertainties in the heat transfer results will increase substantially. In the present study the sensitivity of the transient method for complex passages has been analyzed theoretically and applied experimentally.Copyright