Transient Heat Conduction in Capillary Porous Bodies

Abstract

Capillarity is a well known phenomenon in physics and engineering. Porous materials such as soil, sand, rocks, mineral building elements (cement stone or concrete, gypsum stone or plasterboards, bricks, mortar, etc.), biological products (wood, grains, fruit, etc.) have microscopic capillaries and pores which cause a mixture of transfer mechanisms to occur simultaneously when subjected to heating or cooling. In the most general case each capillary porous material is a peculiar system characterized by the extremely close contact of three intermixed phases: gas (air), liquid (water) and solid. Water may appear in them as physically bounded water and capillary water (Chudinov, 1968, Twardowski, Richinski & Traple, 2006). Both the bounded water and the capillary water can be found in liquid or hard aggregate condition. Physically bounded water co-operates with the surface of a solid phase of the materials and has different properties than the free water. The maximum amount of bounded water in porous materials corresponds to the maximal hygroscopicity, i.e. moisture absorbed by the material at the 100% relative vapour pressure. The maximum hygroscopicity of the biological capillary porous bodies is known as fibre saturation point. Capillary water fills the capillary tube vessels, small pores or sharp, narrow indentions of bigger pores. It is not bound physically and is called free water. Free water is not in the same thermodynamic state as liquid water: energy is required to overcome the capillary forces, which arise between the free water and the solid phase of the materials. For the optimization of the heating and/or cooling processes in the capillary porous bodies, it is required that the distribution of the temperature and moisture fields in the bodies and the consumed energy for their heating at every moment of the process are known. The intensity of heating or cooling and the consumption of energy depend on the dimensions and the initial temperature and moisture content of the bodies, on the texture and microstructural features of the porous materials, on their anisotropy and on the content and aggregate condition of the water in them, on the law of change and the values of the temperature and humidity of the heating or cooling medium, etc. (Deliiski, 2004, 2009). The correct and effective control of the heating and cooling processes is possible only when its physics and the weight of the influence of each of the mentioned above as well as of many other specific factors for the concrete capillary porous body are well understood. The summary of the influence of a few dozen factors on the heating or cooling processes of the