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It is well known that in porous materials moisture condensation occurs under certain physical conditions as a result of water vapor diffusion. In the temperature range below 0°C, ice crystals grow in the pores of the material generating a frost structure.
In pores of variable cross-sectional areas, frost growth is nonuniform. Therefore, a graphical procedure is developed with which the mass and location of frost growth in pores with any cross-sectional function can be determined. By this graphical procedure it is also possible to develop some general statements about the frost density distribution in pores at greater frost contents. It is shown, assuming a time-constant temperature function, that at the beginning of frost growth the diffusion flux in any cross section of the pore has a maximum value, and that it decreases continuously with increasing frost growth.
The results developed by means of the graphical procedure are confirmed by measurements made on capillaries with alternating cross-sectional areas. Afterwards, the diffusion and frost growth in pore-models of ordered sphere-packings are then investigated theoretically by the use of the same graphical procedure. It is shown, above all, that normally the frost growth does not begin at the smallest cross-sectional areas of the pores.
Finally, employing the graphical results, a method is developed through which the diffusion resistance factor of a porous material can be approximately calculated as a function of frost density.
heat transmission, thermal insulation, frost, water vapor, porous materials
Professor, Institut fur Technische Thermodynamik und Thermische Verfahrenstechnik, Universitat Stuttgart,