Investigations of heat transfer at high temperatures (T > 600 °C) in ceramic sponges

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Packed beds of pellets or honeycombs are typical means for enhancing the efficiency of chemical engineering appliences (e.g. reactors). Advantages of packed beds include a high cross mixing and a high specific surface area whereas honeycombs yield lower pressure drops. Disadvantages of packed beds are the high pressure drop and high thermal resistances due to point contact between two pellets while honeycombs do not allow cross mixing of the fluid. In a new and innovative approach, these commonly used structures are to be replaced by sponges (also called open-celled foams). Sponges are network structures of high porosities. Further key characteristics include a comparatively low pressure drop, a high specific surface area and a high cross mixing of the fluid. Due to the continuous solid phase hot spots can nearly be avoided and a homogeneous temperature distribution can be achieved. While advantages of pellet structures and honeycombs are combined in sponges, disadvantages of these two structures are reduced at once.To dimension chemical engineering equipment, reliable correlations for the two-phase (= effective) heat transfer parameters are required. In the past, the applicants successfully developed heat transfer correlations and models at moderate temperatures neglecting radiation. For high temperature applications, the radiation must be included in the models as an additional heat transfer mechanism beside heat conduction. However, only few publications presenting only few experimental data exist in literature dealing with this topic. Consequently, the aim of the project is to establish a wide experimental data basis investigating different sponge types (variation of material, cell diameter and porosity) as well as to extend the own correlations in order to provide the possibility of calculating heat transfer in sponges at moderate and high temperature reliably. Typically, heat transfer models are based on a homogeneous or a heterogeneous approach. The first considers the sponge as quasi-homogeneous system with superposed properties. Here, the two-phase thermal conductivity with and without flow must be known. The latter type of model considers the sponge as a two-phase system. Here, for describing heat transfer two energy balances are coupled by a term containing the heat transfer coefficient. Both approaches will be pursued in this project leading to the experimental determination of both the thermal conductivity and the heat transfer coefficient at temperatures up to about 1000 °C. For modeling heat transfer based on radiation, optical parameters (transmittance, emissivity and reflectance) of the sponges will be determined by Fourier Transform Infrared Spectroscopy.