With the exception of hydrogen, most gases, including air, are substances of low thermal conductivity and low heat capacity at rest. The thermal conductivity of refractory fiber is basically close to that of gas. This is because refractory fiber is interwoven with solid fibers, and the gaps are filled with gas, and the porosity reaches 90%. The filling of a large amount of air makes the refractory fiber have high temperature insulation performance. In theory, high-temperature insulation materials must meet the following two basic conditions: (1) The thermal conductivity λ should be as small as possible, λ→0, so that the heat flow from the high-temperature area to the low-temperature area is very small, that is, for two different The temperature difference AT of the position, the heat flux q should tend to 0,
q = λAT→0
(2-1)
(2) The heat capacity should be as small as possible, so that only a small amount of heat is stored in the material when the furnace temperature changes frequently. As the specific heat capacity c, and the bulk density p, the product of the heat capacity should also tend to 0,
cp.p,→0
(2-2)
The refractory fiber structure has a continuous pore phase, and is also divided into pores that are not connected to each other. There are three ways to travel hot.
Heat in refractory fiber products will be transferred by conduction, radiation and convection. Since the composition of the fiber material has at least two phases, the solid phase and the gas phase, the heat transfer takes place at the interface of these phases.
The interweaving of refractory fibers basically has no directionality, and the solid heat conduction can only be carried out along the direction of the fiber rod from high temperature to low temperature, so the solid phase heat conduction is not completely perpendicular to the hot surface. Some heat transfer paths are tortuous, which also reduces the strength of fiber solid phase heat conduction. In addition, most of the solid phases are point contacts, so the process of solid phase heat transfer is also a process of increasing thermal resistance. For the gas phase, due to the high porosity, the convective heat transfer of the gas in the pores is very little. The hot air flow entering the fiber is divided and hindered by multiple pores, and is almost in a static state. The internal pressure of the dispersed pores is constant, and the air pressure also forms a dense body shield with the solid-phase fibers, hindering the passage of hot air flow. Such a pore is also called a pore chamber, which can only accept the impact heat conduction of molecules in the gas and the radiant heat of the hot gas to it.
In the multi-phase porous refractory fiber, in addition to the solid phase heat conduction, there is also a heat conduction process in the pore chamber. The heat conduction in the gas phase is carried out through the collision process of gas molecules.