Definition of thermal conductivity: Thermal conductivity is a physical quantity representing the thermal conductivity of materials, which is expressed by K, and a mathematical model is established: according to the first law of thermodynamics, the heat passing through any section per unit time is directly proportional to the temperature change rate of the section.
Deduction formula: According to Fourier law, the heat passing through unit area in unit time is directly proportional to the temperature gradient on the area. Consider the boundary conditions: on the solid boundary, the temperature gradient is zero, so the heat passing through the boundary is zero.
Using mathematical tools: using partial differential equations and definite solution conditions, the temperature distribution and thermal conductivity can be solved. Consider different materials: Different materials have different thermal conductivity, which depends on their physical properties and microstructure.
Considering the influence of temperature: Thermal conductivity usually varies with temperature, so experiments are needed to determine the thermal conductivity at different temperatures. To sum up, the derivation of thermal conductivity of chemical engineering principles is a complex process, which needs to comprehensively consider the basic principles of thermodynamics, fluid mechanics and materials science.
Different substances have different thermal conductivities:
The thermal conductivity of the same substance is related to its structure, density, humidity, temperature, pressure and other factors. When the water content and temperature of the same substance are low, the thermal conductivity is small. Generally speaking, the thermal conductivity of solid is greater than that of liquid, and that of liquid is greater than that of gas. This difference is largely due to the different molecular distances between the two states. At present, the coefficient values used in engineering calculation are determined by special experiments.
With the increase of temperature or moisture content, the thermal conductivity of five typical building materials increases. This is analyzed from the microscopic mechanism. For porous materials, when they are wetted, liquid water will replace the original air in the micropores.