Insulation materials, thermal convection

The conduction through residual gases can be reduced by the application of porous structures. The convection within a single pore is minimal if pore sizes are small. In small pores the temperature difference at the walls of the pore are negligible and no convection occurs. The convection is further reduced by the evacuation of the thermal insulating material. 

Heat is transferred by radiation, conduction, and convection. Radiation is the primary mode and can occur even in a vacuum. The amount of heat transferred for a given area is relative to the temperature differential and emissivity from the radiating to the absorbing surface. Conduction is due to molecular motion and occurs within gases, liquids, and sohds. The tighter the molecular structure, the higher the rate of transfer. As an example, steel conducts heat at a rate approximately 600 times that of typical thermal-insulation materials. Convection is due to mass motion and occurs only in fluids. The prime purpose of a thermal-insulation system is to minimize the amount of heat transferred. 

The most important property for insulation is thermal conductivity. The following transport types participate in the transmission of heat heat conduction in PS, heat conduction in the filling gas (air), radiation heat transfer and heat convection by convection flows in the closed cells. The thermal conductivity of the air in the cells contributes the most to the total heat transport. The radiation fraction depends on the diameter of the cells formed. The thermal conductivity depends on the density of the foamed PS material. Thermal conductivity decreases with increasing bulk density, reaches a minimum and then rises again (Figure 9.15). The following processes are responsible for this characteristic. 

The value of the critical radius r, is the largest when k is large and h is smalF. Noting that the lowe.st value of h encountered in practice is about 5 W/m C for the case of natural convection of gases, and that the thermal conductivity of common insulating materials is about 0.05 W/m °C, the largest value of the critical radius we are likely to encounter is... 

Solids having low k values are used as heat insulators to minimize the rate of heat flow. Porous insulating materials such as polystyrene foam act by entrapping air and thus eliminating convection. Their k values are about equal to that of air itself. Data showing typical thermal conductivities are given in Appendixes 10 to 14. 

Heat losses in residential buildings occur primarily via convection from the roof ( 25-35%) followed by air leakage and walls ( 15-25% each). Adequate insulation in roof and exterior walls is therefore a significant energy-saving investment. Table 5.7 compares thermal conductivity and environmental merits of different insulation materials (Dewick and Miozzo, 2002). The insulation efficiency is typically... 

A slab of material 1.00 m thick is at a uniform temperature of 100°C. The front surface is suddenly exposed to a constant environmental temperature of 0°C. The convective resistance is zero (Ji = co). The back surface of the slab is insulated. The thermal diffusivity a = 2.00 x lO m s. Using five slices each 0.20 m thick and the Schmidt numerical method with M = 2.0, calculate the temperature profile at = 6000 s. Use the special procedure for the first time increment. 

Farnworth also presented a theoretical model ofthe combined conductive and radiative heat flows through fibrous insulating materials and compared them with experimental values of the thermal resistances of several synthetic fibre battings and of a down and feather mixture (Table 4.9). No evidence of convective heat transfer is found, even in very low-density battings. The differences in resistance per unit thickness among the various materials may be attributed to their different absorption constants. 

The so-called apparent thermal conductivity of insulating materials depends upon four modes of heat transfer gas conduction and convection, radiation, and solid conduction. The principles of these four mechanisms of heat transfer are fairly well understood individually but their combined effect on heat transfer in insulating materials is complicated. Nevertheless, because of the additive nature of the heat transferred by these mechanisms, the conductivities assigned to each mechanism are additive. Thus, if each of these conductivities can be evaluated under various conditions of temperature and pressure, their sum stated as an apparent conductivity may be estimated. 

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