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Therefore, many insulating materials (e.g.polystyrene) function simply by having a large number of gas-filled pockets which prevent large-scale convection. Air and other gases are generally good insulators, in the absence of convection. The effect of temperature, pressure, and chemical species on the thermal conductivity of a gas may be explained in terms of the kinetic theory of gases. In gases, the thermal conduction is caused by diffusion of molecules from higher energy level to the lower level. In liquids, the thermal conduction is caused by atomic or molecular diffusion. The thermal conductivity of gases and liquids is therefore generally smaller than that of solids. Because the intermolecular spacing is much larger and the motion of the molecules is more random for the fluid state than for the solid state, thermal energy transport is less effective. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids. In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress. Thermal Conductivity of Fluids (Liquids and Gases) Special reference: Thermophysical Properties of Materials For Nuclear Engineering: A Tutorial and Collection of Data.
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In almost all thermal power stations (coal, gas, nuclear), water is used as the working fluid (used in a closed loop between boiler, steam turbine and condenser), and the coolant (used to exchange the waste heat to a water body or carry it away by evaporation in a cooling tower). Water and steam also reacts with metals commonly found in industries such as steel and copper that are oxidized faster by untreated water and steam. It is especially effective to transport heat through vaporization and condensation of water because of its very large latent heat of vaporization.Ī disadvantage is that water moderated reactors have to use high pressure primary circuit in order to keep water in liquid state and in order to achieve sufficient thermodynamic efficiency. It used due to its availability and high heat capacity, both for cooling and heating. Water and steam are a common fluid used for heat exchange in the primary circuit (from surface of fuel rods to the coolant flow) and in the secondary circuit.