There are three ways of heat transfer: solid conduction heat transfer, gas or liquid convection heat transfer, and radiation heat transfer. The thermal conductivity of ceramic fiber board is the sum of three heat transfer effects: conduction heat transfer inside the solid fibers and at the fiber contact area, convective heat transfer in the pores, and radiation heat transfer between the pore walls composed of solid fibers. Therefore, it is also called the equivalent thermal conductivity or apparent thermal conductivity.
The thermal conductivity of ceramic fiber board mainly depends on its usage temperature, porosity, pore structure and properties, volume density, slag ball content, fiber diameter, fiber humidity, usage atmosphere, and fiber direction. Below is a brief analysis of the 8 factors that affect the thermal conductivity of ceramic fiber boards.
1 Operating temperature
The thermal conductivity of general ceramic fiber boards increases with increasing temperature. The reason is that the heat transfer between the pore walls, the convective heat transfer of air in the pores, and the heat transfer inside the solid fibers and at the fiber contact areas all increase proportionally due to the increase in temperature and the enhanced thermal motion of gas and solid molecules. When the temperature rises above 800 ℃, the heat transfer inside the ceramic fiber board is mainly based on the radiation resistance, and the higher the temperature, the greater the proportion of radiation resistance heat transfer.
2 Pore rate, pore structure and properties
Porosity refers to the ratio of the volume of pores in a ceramic fiber board to the total volume of the ceramic fiber board, expressed as a percentage. The pores of ceramic fiber board are filled with air, and the thermal conductivity of air at room temperature is only 0.025w/(m.k), which is much lower than the conduction heat transfer of ceramic fiber solid. Ceramic fiber board is a mixed structure composed of solid fibers and air, with a porosity of over 80%. A large amount of low thermal conductivity air fills the pores and destroys the continuous network structure of solid molecules, thereby achieving excellent insulation performance. The above analysis indicates that the insulation and energy-saving function of ceramic fiber board mainly utilizes the insulation effect of air in the pores.
The structure and properties of pores mainly affect the convective heat transfer of air in ceramic fiber boards. The larger the pore size of pores, the smaller the corresponding volume density of ceramic fiber boards, while the convective heat transfer of air in pores is greater. Moreover, the thermal conductivity coefficient of ceramic fiber boards is more affected with increasing temperature. There are three types of pores inside ceramic fiber boards: continuous pores (open), semi continuous pores (open and closed), and isolated pores (closed). The isolated pores (closed) structure has the lowest thermal conductivity.
3 Volume density
Volume density refers to the ratio of the weight of ceramic fibers to their volume. After research, it has been found that the relationship between the thermal conductivity and density of ceramic fibers follows a certain pattern, as shown in Figure 1:
Figure 1: The variation of thermal conductivity of ceramic fibers with density and temperature
From the above analysis, the following two rules can be drawn:
a The thermal conductivity of ceramic fibers decreases with increasing density, but the magnitude of the decrease gradually decreases, so that when the density exceeds a certain range, the thermal conductivity no longer decreases and tends to increase.
b At different temperatures, there is a minimum thermal conductivity and corresponding minimum density value. The density corresponding to the minimum thermal conductivity increases with increasing temperature.
4 Shot Content
Slag balls are spherical particles that cannot form fibers during the fibrosis process of high-temperature molten liquid. The slag ball content refers to the percentage of non fibrotic substances in refractory ceramic fibers and products that have passed through a 75 micron standard sieve, as a percentage of the total sample size. As the content of slag balls increases, it will lead to a decrease in the amount of solid fibers and a decrease in the density of the fibers themselves. Therefore, it will lead to an increase in the thermal conductivity of the fiber products, deteriorate the insulation performance of the fiber products, and reduce the strength and elasticity of the fiber products. The effect of slag ball content on the thermal conductivity of fiber products increases with increasing temperature.
5 Fiber diameter
When the density of fiber products is the same, the finer the fiber diameter and the smaller the pore size, the greater the damping effect on heat transfer; Secondly, the finer the fiber, the longer the total length of the fiber, and the greater the damping of heat conduction, resulting in a decrease in thermal conductivity. On the other hand, the finer the diameter of ceramic fibers, the greater the shrinkage of the heating line of the product, and the decrease in heat resistance indicators. To achieve optimal comprehensive technical performance, fibers should have an appropriate fineness (diameter).
6 Fiber humidity
At 0 ℃, the thermal conductivity of water is 0.522w/(m.k), which is more than 20 times higher than the thermal conductivity of air under the same conditions of 0.0247w/(m.k). Therefore, an increase in fiber humidity or moisture content will inevitably lead to an increase in the thermal conductivity of fiber products. If the water inside the pores in the fiber freezes into ice, the thermal conductivity of ice under the same conditions is 2.32w/(m.k), which is close to 100 times the thermal conductivity of air under the same conditions. Therefore, for pipeline insulation engineering, it is necessary to control the humidity of the pipeline insulation material to the minimum content, and there should be corresponding strict moisture-proof requirements on the outer protective layer material and structure of the pipeline to ensure the insulation performance of the fiber insulation structure.
7 Use atmosphere
Ceramic fiber products are usually used in atmospheric environments, and the gas present in pores is air. Therefore, the role played by the gas phase in ceramic fiber products is actually the insulation effect of air. However, in some cases, ceramic fiber products are used under various conditions such as vacuum, protective atmosphere, or controlled atmosphere, such as using hydrogen, carbon monoxide, carbon dioxide, hydrocarbons, and inert gases. At this time, the thermal conductivity value of ceramic fibers changes. The thermal conductivity of a gas is related to its composition and structure. Generally speaking, the smaller the molecular weight of a gas, the simpler its structure, and the greater its thermal conductivity. The thermal conductivity value of ceramic fiber products used in gas A can be corrected according to the following formula:
8 Fiber direction
When the material and volume density are the same, the thermal conductivity when the heat flow direction is perpendicular to the fiber is smaller than the thermal conductivity when the heat flow direction is parallel to the fiber. Generally, the heat flow direction of a layered structure is close to perpendicular to the fiber direction, and the thermal conductivity of fiber products is small; while the heat flow direction of a stacked structure is close to parallel to the fiber direction, and the thermal conductivity of fiber products is large. Under the same conditions of material and volume density, the thermal conductivity of laminated structural fiber products is 20% to 30% higher than that of laminated structural fiber products.
this article is from 找耐火材料网平台 (163.com)