Porous heat insulation materials refer to materials with high porosity, small bulk density, low thermal conductivity, etc., which can effectively block the transmission of heat insulation. Porous refractory products achieve thermal insulation by forming a large number of pores inside the material, so the formation of pores is the most important link in the preparation process of porous thermal insulation materials. In recent years, while focusing on low thermal conductivity, porous thermal insulation materials have continuously developed toward high quality, high efficiency, multiple varieties, and serialization. The research work mainly involves the following three aspects:
(1) Development of new porous insulation materials;
(2) Research on thermal insulation mechanism and influencing factors;
(3) Preparation and process optimization of porous insulation materials.
The thermal conductivity of porous insulation materials is mainly explained from the perspective of heat transfer, including the heat transfer mechanism of insulation materials, the way to reduce the thermal conductivity of insulation materials and the influencing factors of insulation performance.
Honeycomb regenerator
The thermal insulation behavior of thermal insulation materials is an extremely complicated process, and its fundamental lies in reducing its thermal conductivity. Due to the existence of a large number of pores in the porous insulation material, its heat transfer is through the solid phase and the gas phase. When the heat is transferred inside the material, the transfer methods are divided into heat transfer in the solid phase bypassing the pores and gas phase heat transfer through the pores, wherein the gas phase heat transfer through the pores includes the conduction of the gas itself, convection heat transfer) and radiation Heat transfer. In the heat transfer process of heat insulation materials, various heat transfer mechanisms may work together, resulting in non-linear behavior of refractory heat transfer.
For thermal insulation materials, the heat conduction is mainly completed by the solid phase, so the porosity of the material determines the amount of solid heat transfer; the literature indicates that the thermal conductivity of porous thermal insulation materials can be based on the following formula (1) Make a simple statement:
(1)
Among them, the effective thermal conductivity of λe-material, W / (m · k);
λg gas phase thermal conductivity, W / (m.k);
λs effective thermal conductivity between solid and gas phase, W / (m · k);
V-apparent porosity of the material, (%).
Convective heat transfer is the displacement generated by the convective movement of air in the insulation material, transferring heat from one place to another. In addition to the thermal conduction of phonons, thermal insulation and refractory materials also have radiant heat transfer. At low temperatures, the radiation energy is negligible and can be ignored; but at high temperatures, the radiation energy is significantly increased. If any research indicates that the radiant energy E of the material is proportional to the fourth power of the temperature T, it must be considered.
(2)
In the formula, Eb a radiant energy, W / (m · k);
σ-Boltzmann constant, 566.88 × 10-10W / (m · k);
T-thermodynamic temperature, (K)
Based on the above heat insulation mechanism, it can be seen that conduction is the most important heat transfer method of heat insulation materials; in addition, in heat insulation materials, solid phase heat transfer is also much higher than gas phase heat transfer. Therefore, in order to reduce the thermal conductivity of thermal insulation materials, the content of pores in the materials should be increased, the pore size structure should be optimized, and the complexity of solid phase crystallization should be increased.
High aluminum insulation brick
Porous thermal insulation materials are composed of aggregates of multiphase materials. Therefore, there are many factors that affect the thermal conductivity of thermal insulation materials. The main conclusions are: the content and distribution of gas phase, chemical mineral composition and crystalline state.
(1) Influence of gas relative heat transfer
Generally speaking, the porosity of light-weight insulation materials is generally above 45%, so the light-weight insulation materials are mainly in the gas phase. Because the thermal conductivity of the solid phase is higher than that of the gas phase, at low temperatures, the thermal conductivity of the material decreases as the porosity increases. However, in the range of pore size of general insulation materials (1 micrometer to several millimeters), the increase in the porosity of the insulation material, the increase in the pore size and the reduction in the number of pores will cause the number of gas-solid interfaces to decrease Radiative heat transfer. And as the temperature increases, the proportion of radiant heat transfer in the entire heat transfer process will increase. Therefore, for samples of the same material, the higher the porosity, the faster the thermal conductivity increases as the temperature increases. Each insulation material will have an optimal porosity. At this specific temperature, too high or too low porosity will increase its thermal conductivity. Figure 2 shows the optimal porosity of alumina foam ceramics at different temperatures.
The best average porosity of alumina foam ceramics at different temperatures
(2) Influence of pore size
When the porosity is kept constant, the thermal conductivity of the insulation material mainly depends on the shape, size and communication between the pores inside the material. The smaller the pore size, the lower the thermal conductivity of the material. The main reasons are: (1) the reduction of the pore size reduces the convective heat transfer efficiency; (2) the increase of the gas-solid interface increases the solid conduction distance and reduces the material Heat conduction. Therefore, while keeping the porosity of the material constant, reducing the pore size helps to reduce the thermal conductivity. Loeb model entry = 4γdδεT3 gives the relationship between the pore shape factor γ, pore size d, radiation constant δ, thermal emissivity ε, and absolute temperature T. But the Loeb model is only suitable for thermal insulation materials with pore diameters> 1um; when the pore size is nano-scale, its thermal conductivity cannot be predicted. A lot of research and theoretical derivation prove that: when the pore size is less than 50nm, the pores are basically in a vacuum state, that is, the gas is adsorbed on the pore wall, and the air in the pores is in a static state.
(3) The effect of solid relative heat transfer
The heat transfer of the solid phase accounts for 70% of the heat transfer of the insulation material. From the perspective of phase composition, the solid phase in thermal insulation materials can be divided into glass phase and crystalline phase. Since the atoms or ions in the glass phase are arranged randomly, and the atoms or ions are arranged in order, the thermal conductivity of the glass phase is generally lower than that of the crystalline phase; however, as the temperature increases, the viscosity of the glass phase decreases, The resistance to particle movement is small, which in turn increases its thermal conductivity. For crystalline phase gastritis, the crystalline phase is different. As the temperature increases, the thermal vibration and non-resonance of atoms or ions increase, which can also reduce the thermal conductivity of the material.