Thermal conductivity of gases

Glicksman (1994) discusses three major contributions to the thermal conductivity of closed cell foams, namely the polymer structure and the thermal conductivity of the gas, as well as the radiation contribution. In most insulated foams, gas contributes most. In general, the thermal conductivity of gases decreases with increasing molecular weight (Table 18.2 from Randall and Lee (2002)). Therefore, a high molecular weight but high boiling point gas is required.

If a chlorofluorocarbon (CFC) blowing agent is forbidden, the foam is made using other gases with low thermal conductivity. Vacuum can be used for special applications (Manini, 2001), which places a large mechanical load on the open cell foam structure.

Since a typical 5 μm thick surface in PU foam is almost transparent in a portion of the infrared (IR) spectrum, some radiation can travel through the foam structure over long distances. However, the edges of the PU foam absorb IR radiation and re-irradiate; radiation scattering has a complex theory. An integrating sphere system is usually used to determine the contribution of IR to foam conductivity. Figure 18.15 compares the results of five radiation effects theory, none of which can be accurately predicted, but the data of PU form increases almost linearly with the Z mean value of cell diameter.

The thermal conductivity of the polymer structure depends on its shape and size. The model must consider the geometry of edges and vertices (Chapter 1). Ahern et al. (2005) Note that there is an important vertex contribution to the conductivity of the foam controlled by the same form. However, the thermal conductivity of air is about 10% of the thermal conductivity of solid PU and the conductivity of air is negligibly small, so the problem of heat conduction is more difficult. Therefore, this theory is semi-empirical, using the edge coefficients and surface shape coefficients, including vertex contributions. The thermal conductivity predicted by the model is compared with the thermal conductivity of the PU foam minus the radiation as shown in Figure 18.16, which shows the advantage of minimizing foam density.

The microstructure of EPS is different from the microstructure of the foam, and the polymer on the cell surface occupies the major part. This increases IR scattering from the cell surface and reduces the contribution of radiation to total conductivity. Figure 18.17 shows that the thermal conductivity of the EPS foam is minimal at a density of approximately 40 kgm - 3; the low density increase is due to the increase in radiation contribution. Higher minimum values ​​of EPS compared to PU foam reflect higher air thermal conductivity compared to hydrochlorofluorocarbon, butane or pentane.

When the thickness of the foam block exceeds about 100 mm, the measured thermal conductivity of the EPS only reaches a constant value because the contribution of radiation in the surface layer is greater than the depth inside the block (BASF, 2001 ).

In the case of gas, the theoretical prediction and experiment of gas kinetics confirms that the thermal conductivity of the gas is proportional to the square root of the temperature T and inversely proportional to the square root of the molar mass M. However, thermal conductivity is independent of the various pressures actually encountered. When the system heats up, it will store some heat and transfer the remaining heat to the other system. As we have seen, the ability of a material to transmit thermal energy is called thermal conductivity. The amount of heat stored in the material is called the heat capacity of the material. The heat capacity of a material is represented by Cp.

Light gases such as hydrogen and helium generally have high thermal conductivity. Dense gases such as helium and dichlorodifluoromethane have low thermal conductivity. The exception is sulfur hexafluoride (high density gas) having a relatively high thermal conductivity due to its high heat capacity. Argon and helium are more dense than air and are commonly used in insulating glass (double glazing) to improve heat insulation. The thermal conductivity of the bulk material in the form of porous or particulate form depends on the type of gas in the gas phase and its pressure. At lower pressures the thermal conductivity of the gas phase decreases This behavior is controlled by the number of Knudsen defined as where the mean free path of the gas molecules is located and the typical gap size of the space filled with the gas is. The particulate material corresponds to the characteristic size of the gas phase in the pores or intergranular spaces.