Experimental Study And Mechanism Analysis Of Thermal Conductivity Measurement Accuracy Of Wet Porous Media

Thermal conductivity of porous media with different moisture content had been measured, and the accuracy and the influential factors had been studied. Besides, a series of experimental observations were conducted for fully understanding the pore scale water morphology, distribution and evolution in porous media and results revealed that there is some certain inherent relationship between the thermal conductivity measurement relative error and pore water morphologies. A model has been formulated to describe the influence of porous media matrix structure and pore wall thermal properties on pore scale water porphologies. This model can determine the critical transformation moisture content of the water morphology and distribution in pores. By experimental study and theoretical analysis, the influential mechanism of matrix physical properties and moisture content on wet porous media thermal conductivity measurement accuracy has been revealed.Thermal conductivity of piled stainless steel beads, glass beads and sands with different particle size had been measured by a Hot Disk constant analyzer. The repeatability and relative error of the measured values and also their variation versue the moisture content had been analysed. The results showed that the thermal conductivities of samples with relatively high water content are easy to be measured correctively but very difficult for the stainless steel beads, glass beads and sands with the moisture content less than 50%, 35% and 25% respectively.Experimental investigations were also conducted to visually observe water morphology, distribution and evolution performance in pores respectively by a CCD combined with a microscope and their affection to the test results of the thermal conductivity measuring based on these experiments were analysed. Most of the water exists as seperated liquid bridges among particles in the porous media when moisture content is less than 50%,35% and 25% for cumulate stainless steel beads, silver beads and sands respectively. Due to the sensor heating, water evaporates and transport processes diffuse in the pores near the sensor. As a result, the properties of this region become different from their originals. So the measured thermal conductivities are not the real value of the wet sample in its original state. As contrast, the water mostly exists as water masses in the porous media when moisture content is higher than the value, though the water evaporates near the heating resource, the near heating source pores are complemented as soon as the water driven back from the relatively far pores by the capillary force. So the correct thermal conductivity can be measured easily. According to the experimental results and phenomena, we can consider that morphology and distribution of pore water has effected the measurement accuracy of thermal conductivity.The reason why different porous media have different critical moisture content for thermal conductivity can be measured correctively has also been studied in this thesis. The critical moisture content is a single-value function of the water contact angle on particle surface. The contact angles of water on the stainless steel and glass surface are 72.5°and 36.6°respectively, so the largest moisture contents for water mostly exist as isolated liquid bridges are 48.6% and 35.4% indicated by the model formulated in this thesis. Comparison between experimental results and mathematical model showes that the water contact angle on porous media matrix surface determines water morphology and distribution in pores at certain moisture content, and water morphology and distribution then affect measurement accuracy of thermal conductivity. The larger the contact angle is, the higher upper water content limit that the water exists as isolated liquid bridges is, i.e. the higher the lower water content limit that the wet porous media can be easily measured correctly is.