Insulation materials, thermal heat transfer

In the section illustrated, the surface 1-4-7 is insulated. The convection heat-transfer coefficient at surface 1-2-3 is 28 W/m2 - °C. The thermal conductivity of the solid material is 5.2 W/m - °C. Using the numerical technique, compute the temperatures at nodes 1, 2, 4, and 5. 

Porosity in ceramic materials may have a dramatic influence on thermal conductivity under most circumstances, increasing the pore volume results in a reduction of the thermal conductivity. In fact, many ceramics used for thermal insulation are porous. Heat transfer across pores is typically slow and inefficient. Internal pores normally contain stiU air, which has an extremely low thermal conductivity—approximately 0.02 W/m K. Furthermore, gaseous convection within the pores is also comparatively ineffective. 

Origins of the science associated with thermal insulations coincide with the development of thermodynamics and the physics associated with heat transfer. These technical subjects date to the eighteenth century. Early obseiwations that a particular material was useful as thermal insulation were not likely guided by formal theoi y but rather by trial and error. Sawdust was used, for example, in the nineteenth centui y to insulate ice storage buildings. 

Transparent or translucent insulating materials (TIMs) can provide light or solar gains without view. TIMs typically have thermal properties similar to conventional opaque insulation and are thicker than conventional insulating glass units, providing significant resistance to heat transfer. 

A pipeline of 100 mm outside diameter, carrying steam at 420 K, is to be insulated with a lagging material which costs 10/m3 and which has a thermal conductivity of 0.1 W/m K. The ambient temperature may be taken as 285 K, and the coefficient of heat transfer from the outside of the lagging to the surroundings as 10 W/m2 K. If the value of heat energy is 7.5 x 10 4 /MJ and the capital cost of the lagging is to be depreciated over 5 years with an effective simple interest rate of 10 per cent per annum based on the initial investment, what is the economic thickness of the lagging ... 

If a layer of insulating material 25 mm thick, of thermal conductivity 0.3 W/m K, is added, what temperatures will its surfaces attain assuming the inner surface of the furnace to remain at 1400 K The coefficient of heat transfer from the outer surface of the insulation to the surroundings, which are at 290 K, may be taken as 4.2. 5.0, 6.1, and 7.1 W/m K, for surface temperatures of 370, 420, 470, and 520 K respectively. What will he the reduction in heat loss ... 

Though short fiber-reinforced mbber composites find application in hose, belt, tires, and automotives [57,98,133,164] recent attention has been focused on the suitability of such composites in high-performance applications. One of the most important recent applications of short fiber-mbber composite is as thermal insulators where the material will protect the metallic casing by undergoing a process called ablation, which is described in a broad sense as the sacrificial removal of material to protect stmcrnres subjected to high rates of heat transfer [190]. Fiber-reinforced polymer composites are potential ablative materials because of their high specific heat, low thermal conductivity, and ability of the fiber to retain the char formed during ablation [191-194]. 

Thermal conductivity is a physical property of the solid through which the heat is being transferred. It is a measure of the material s ability to conduct heat. Insulators have a low thermal conductivity and conductors have a high thermal conductivity. The rate of heat transfer has magnitude and direction. This is represented mathematically by the negative sign that appears in Fourier s law of heat conduction. 

Heat Transfer in Insulated Pipes Solve case (b) of Problem 2.22 for a composite tube made of material of thermal conductivity kj for / , < r < Rm and of material of thermal conductivity k0 for Rm < r < R0. 

Film conductances are also often defined for the impedance to thermal conduction when two solid conductors are placed in mechanical contact. A significant contact resistance is often observed when, on a microscopic scale, heat transfer involves an air-gap between the materials. Under such conditions, phonon propagation must be replaced by the kinetic interaction amongst gaseous atoms and then back to phonon heat transfer in the next solid. Fibrous and foam insulation axe effective thermal insulators because of the numerous contact resistances involved in the transfer of heat. 

The wall of an insulated container can be modeled as a 5-cm wide vertical enclosure with an aspect ratio of 5. The enclosure is filled with a low grade insulating material that has a permeability of 10"8 m2 and an apparent thermal conductivity of 0.04 W/m-K. The inner wall of the enclosure is at 5°C. Plot a curve showing how the heat transfer rate across the enclosure varies with outer wall temperature. 

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. 

For a highly evacuated (on the order of 1.3 x 10" Pa) multilayer insulation, heat is transferred primarily by radiation and solid conduction through the spacer material. The apparent thermal conductivity of the insulation material under these conditions may be determined from... 

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. 

A layer of material of known thickness and area can be heated from one side by an electric resistance heater of known output. If the outer surfaces of the heater are well insulated, all the heat generated by the resistance heater will be transferred through the material whose conductivity is to be determined. Then measuring the two surface temperatures of the material when steady heal transfer is reached and substituting them into Bq. 1-21 togelher with other known quantities give tlie thermal conductivity (Fig. 1-26). 

A common way of measuring the thermal conductivity of a material is to sand-v/ich an electric thermofoil heater between two identical samples of the material, as shown in Fig. 1-30. The thickness of the resistance heater, including its cover, v/hich is made of thin silicon rubber, is usually less than 0.5 mm. A circulating fluid such as tap water keeps the exposed ends of the samples at constant temperature. The lateral surfaces of the samples are well insulated to ensure that heat transfer through the samples is one-dimensional. Two thermocouples are embedded into each sample some distance L apart, and a... 

Note that heal transfer through the urettiane material is less than the heat transfer through the air determined in (a), although the thermal conductivity of the insulation is higher than that of air. This is because the insulation blocks the radiation whereas air transmits it. 

When two such surfaces are pressed against each other, the peaks form good material contact but the valleys form voids filled with air. As a result, an interface contains numerous air gaps of varying sizes that act as insulation because of the low thermal conductivity of air. Thus, an interface offers some resistance to heat transfer, and this resistance per unit interface area is called the thermal contact resistance, R. The value of is determined experimentally using a setup like the one shown in Fig. 3-15, and as expected, there is considerable scatter of data because of the difficulty in characterizing the surfaces. 

When w c analyze heat transfer ifi a medium consisting of two or more layers, the first thing we need to know is whether the thermal contact resistance is significant or not. We can answer this question by comparing the magiii-tudes of th thermal resistances of the layers with typical values of thermal contact resistance. For example, the thermal resistance of a 1-cm-thick layer of an insulating material per unit surface area is... 

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