Experimental determination of the local heat transfer coefficient for MEGAPIE target window using infrared thermography

In the scope of the Megawatt Pilot Experiment (MEGAPIE), i.e. a liquid metal target for a spallation neutron source, an experimental investigation (KILOPIE) of the target window cooling has to be performed. A reason to perform the KILOPIE experiment is that, in the area of the proton beam entry window, the values and distribution of the local convection heat transfer coefficient 'a' for MEGAPIE conditions are unknown. The liquid metal, in this case lead-bismuth eutectic (LBE), is simultaneously used as target material and coolant. A hemispherical flow geometry made of T91 steel is used for the mockup of the proton beam entry window in an experimental set-up for a determination of the local convection heat transfer coefficient 'a'. In KILOPIE two complementary methods are used for a determination of the local convection heat transfer coefficient 'a', the first one is the two dimensional Heat Emitting Temperature Sensitive Surface (2D-HETSS) method develop at PSI (Platnieks et al.) and the second is an improved two-dimensional and dynamic infrared thermography (2DD-IRT) method, also developed at PSI (Patorski et al.). In this paper only the methodology of improved 2DD-IRT will be presented. The experimental activities will be performed at the beginning of 2004 at Forschungszentrum Karlsruhe (FZK) using the THEADES loop of the KALLA laboratory and will be continued at Paul Scherrer Institute (PSI) using the new, consisting of two independent pumped circuits, PSI-LBE-Double-Loop. A specially tailored 0.053 mm thick Aluchrom YHf heating foil is used, which allows to apply a uniform and constant heat flux deposition on the outer surface of the hemispherical mockup-specimen of the target window. The optical non-contact IRT equipment measures the outer surface temperature of the mockup-specimen dish with a high space and time resolution, e.g. 1.25 mrad and 20 Hz. The 100 mm diameter of the mockup-specimen dish with approximately 5000 pixels, i.e. temperature measurement points, area of 1.6 mm2 is covering the area of interest, corresponding to the approx. 60 mm diameter of the proton beam footprint and results in temperature contour plots with good resolution. The dynamic capability of 2DD-IRT is essential for the investigation of the change of the local convection heat transfer coefficient 'a' especially during transient load cases, i.e. during changes of the flow rate of the coolant. The accuracy of the temperature measurement is ±1%. The knowledge of constant heat flux and temperature differences between inner surface and coolant allow a two-dimensional dynamic determination and visualization of the local convection heat transfer coefficient 'a'. In other words, the determination of the local convection heat transfer coefficient 'a' is a result of ratio of the known local heat flux from the Aluchrom YHf heating foil to the difference between the local inner surface temperature and the bulk temperature of the LBE coolant. (author)