There are three ways of heat transfer: heat conduction Q1, convection heat transfer Q2 and radiation heat transfer Q3. Among them, heat conduction refers to the heat transfer between objects in contact through the thermal motion of free electrons, molecules, atoms and other microscopic particles. Convection heat transfer refers to the relative displacement between fluids, causing the mutual penetration between cold and hot fluids to achieve heat transfer. Radiation heat transfer refers to the object's own temperature emitting electromagnetic waves or photons to transfer heat. What are the factors that affect the thermal conductivity of insulating refractory bricks?
The physical phase of the insulation material is usually composed of solid phase and gas phase, so its insulation mechanism is closely related to the phase composition. Heat energy is conducted from the high temperature surface to the low temperature surface. First, there is only solid phase conduction. When the heat energy passes through the pores in the solid phase, there are two modes of solid phase conduction and gas phase conduction. Therefore, the length of the solid phase conduction route is extended; secondly, it is related to thermal radiation. When the temperature is low, the effect of thermal radiation on the insulation effect is negligible and can be ignored. When the temperature is high, the conduction of heat energy will be affected by radiation, thus playing a role in heat insulation.
Usually, thermal insulation materials are multiphase materials, and each phase has its own unique structure. Therefore, the thermal conductivity of is affected by many factors such as the composition, content, and internal structure of the phase. Therefore, there are many factors affecting the thermal insulation mechanism and its thermal insulation effect. The thermal conductivity of thermal insulating refractory bricks is affected by the volume density, porosity, phase composition, etc. of the material.
(1) Pore size: The number of pores and the size of the pores are generally presented at the same time. Under the premise of keeping the total amount of pores unchanged, reducing the pore size can increase the number of pores, thereby reducing the thermal conductivity of the material; when the number of pores increases, the specific surface area of the thermal insulation material will increase, and the radiation conductivity will decrease.
(2) Volume density: The thermal conductivity of solids is higher than that of gases. The decrease in volume density means that the gas phase in the material increases, so the thermal conductivity decreases. However, when the volume density is too low, the gas phase heat transfer effect in the material will be enhanced, and the thermal conductivity will increase. Therefore, if you want the thermal insulation material to have excellent thermal insulation performance, the lower the volume density, the better. At a specific temperature, each material will have an appropriate thermal conductivity.
(3) Material composition: The emissivity of the thermal insulating refractory bricks is related to the material, temperature and particle size. Therefore, in the thermal insulation material ratio, you can appropriately increase or add Al2O3, MgO, CaO, ZnO and other components with low emissivity, while avoiding the addition of transition element oxides such as Fe, Ni, Cr, etc.
