Determination of the thermal properties of ceramic sponges B. Dietrich a, * , G. Schell b , E.C. Bucharsky b , R. Oberacker b , M.J. Hoffmann b , W. Schabel a , M. Kind a , H. Martin a a Institute of Thermal Process Engineering, Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, 76128 Karlsruhe, Germany b Institute for Ceramics in Mechanical Engineering, Karlsruhe Institute of Technology (KIT), Haid-und-Neu-Strasse 7, 76131 Karlsruhe, Germany article info Article history: Received 30 April 2009 Received in revised form 13 August 2009 Accepted 25 August 2009 Available online 23 October 2009 Keywords: Ceramic sponge Two-phase thermal conductivity Open-celled foam Krischer model Thermal properties abstract The knowledge of thermal properties of technical components or internals in chemical reactors is often a key characteristic for planning and designing chemical engineering processes. As an alternative to packed beds or packings, sponges turned out to be used in new application fields in chemical and process engi- neering. Therefore an experimental study was performed to investigate the two-phase thermal conduc- tivity of solid ceramic sponges made of alumina, mullite and oxidic-bonded silicon carbide (OBSiC) at moderate temperatures. A two-dimensional model is used for analysing the measured temperature pro- files and for calculating the thermal conductivity. It can be observed, that the thermal conductivity increases with decreasing porosity and is nearly constant when the pore size (ppi number) is varied. The thermal conductivity data are modelled by an approach similar to the well known Krischer model. Compared to a packed bed of spherical particles, the values of the thermal conductivity of sponges turn out to be about five times higher. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Solid sponges (i.e. open-celled foams) belong to the cellular materials. The term ‘‘foam” typically stands for cellular materials produced by foaming of liquids with closed cells (gas bubbles). Be- cause of the open-celled solid network structure, the word ‘‘sponge” seems to be more appropriate for the material investi- gated. The key characteristics of solid sponges are a high and con- tinuously accessible porosity of typically about 75–95%, which leads to a low pressure drop for fluids flowing through them [1], combined with advantageous heat transfer properties. In addition to the fluid phase, sponges have a second, solid, continuous phase. This fact leads to a higher radial heat transfer in comparison to packed beds. In general, the properties of cellular materials depend on their geometric structures and on the material properties of the solid phase. The properties of sponges can be modified over a wide range. This encourages the use of solid ceramic sponges in various applications in chemical and process engineering, including single- and multiphase flow. Some examples are solar receivers, gas burn- ers or alternatives for column packings. Today, the manufacturing process is considered to be advanced enough to open a route to new industrial areas ranging from catalyst supports, hot-gas or molten-metal filters, membranes, gas burners, lightweight con- struction, sound and heat insulation to energy absorption applica- tions [2–6]. For all these mentioned applications, the knowledge of thermal properties is of particular interest. 2. Thermal conductivity 2.1. Literature study Heat transfer in porous materials is described by the ‘‘two- phase thermal conductivity” depending on solid body properties, like the thermal conductivity k s , porosity w, pore density (ppi num- ber) and cell geometry, as well as the fluid media characteristics, such as thermal conductivity k f , viscosity l f and the fluid velocity u. The two-phase thermal conductivity k 2ph of sponges can be understood as the superposition of the conduction of the solid, conduction of the fluid, free and forced convection as well as radi- ation within the cells. In this contribution, only heat conduction without forced con- vection and radiation is considered. Experimental data are ob- tained under steady state heat transfer conditions in closed systems at moderate temperatures (negligible radiation) and at atmospheric pressure. Furthermore, the effect of free convection inside the sponge structure on heat transfer is negligible because of the horizontal placement of the experimental equipment, which is described in Section 3.5. In the future, experiments with a verti- cal placement of the measuring cell will be performed in order to investigate the effect of free convection. The results will be pub- lished in a further paper. Table 1 gives a short overview of existing models in the literature for calculating the two-phase thermal conductivity without flow. 0017-9310/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2009.09.041 * Corresponding author. Tel.: +49 721 608 6830; fax: +49 721 608 3490. E-mail address: dietrich@kit.edu (B. Dietrich). International Journal of Heat and Mass Transfer 53 (2010) 198–205 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt