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First Measurements of Thermal Accommodation Coefficients on Ceramic Surfaces



During the last decade the interest has risen in micro or nano scaled technological products which have many advantages due to their size. The size plays also a significant role in the area of micro and nanoscale heat transfer. Applications were developed using the effect of an increasingly incomplete heat transfer with decreasing dimensions for better thermal insulation such as nano structured porous media, e.g. Aerogels or ceramics. Despite the technical utilization research is still in progress concerning a better understanding of the gas-surface interactions on a molecular scale or for example in micro- and nanoflow using simulation techniques such as Molecular Dynamics or Direct Simulation Monte Carlo. This study is, however, focused on the experimental determination of thermal accommodation coefficients αth on ceramic surfaces as no measurement data are available despite the significant influence of αth on the effective thermal conductivity of porous ceramic in the high Knudsen number range. For the determination of αth a parallelplate instrument is used, although most measurements in the past have been arranged with the hot-wire method. The main advantage is due to simpler machining of specimens and the possible analysis of the effect of roughness. The facility is built following the well-known guarded-hot-plate-arrangement using a unidirectional heat flux through the gas layer due to a small temperature difference, which is provided by two different heating foils. The measurements take place between 30 and 100 °C as well as in the transition regime. We present the measurement device and results of the calibration and a first result on a porous ceramic specimen as using a parallel-plate instrument is challenging due to the small heat fluxes and therefore maintaining a homogeneous temperature field.


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