In physics, a fluid is a substance that continually deforms (flows) under applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas, and, to some extent, plastic 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. The thermal conductivity of gases and liquids is generally smaller than that of solids. In liquids, thermal conduction is caused by atomic or molecular diffusion. In gases, thermal conduction is caused by the diffusion of molecules from a higher energy level to a lower level. Thermal Conductivity of Gases  The thermal conductivity of gases is directly proportional to the density of the gas, the mean molecular speed, and especially to the mean free path of a molecule. The mean free path also depends on the diameter of the molecule, with larger molecules more likely to experience collisions than small molecules, which is the average distance traveled by an energy carrier (a molecule) before experiencing a collision. Light gases, such as hydrogen and helium, typically have high thermal conductivity, and dense gases such as xenon and dichlorodifluoromethane have low thermal conductivity. In general, the thermal conductivity of gases increases with increasing temperature. Thermal Conductivity of Liquids As was written, in liquids, the thermal conduction is caused by atomic or molecular diffusion, but physical mechanisms for explaining the thermal conductivity of liquids are not well understood. Liquids tend to have better thermal conductivity than gases, and the ability to flow makes a liquid suitable for removing excess heat from mechanical components. The heat can be removed by channeling the liquid through a heat exchanger. The coolants used in nuclear reactors include water or liquid metals, such as sodium or lead. The thermal conductivity of nonmetallic liquids generally decreases with increasing temperature.
Thermal conductivity is a material property that describes ability to conduct heat. Thermal conductivity can be defined as "the quantity of heat transmitted through a unit thickness of a material - in a direction normal to a surface of unit area - due to a unit temperature gradient under steady state conditions" Thermal conductivity units are [W/(m K)] in the SI system and [Btu/(hr ft °F)] in the Imperial system. See also thermal conductivity variations with temperature and pressure, for: Air, Ammonia, Carbon Dioxide and Water Thermal conductivity for common materials and products:
1) Asbestos is bad for human health when the tiny abrasive fibers are inhaled into the lungs where they can damage the lung tissue. This seems to be exacerbated by cigarette smoking and the resulting diseases are mesothelioma and lung cancer. Example - Conductive Heat Transfer through an Aluminum Pot versus a Stainless Steel PotThe conductive heat transfer through a pot wall can be calculated as q = (k / s) A dT (1) or alternatively q / A = (k / s) dT where q = heat transfer (W, Btu/h) A = surface area (m2, ft2) q / A = heat transfer per unit area (W/m2, Btu/(h ft2)) k = thermal conductivity (W/mK, Btu/(hr ft °F)) dT = t1 - t2 = temperature difference (oC, oF) s = wall thickness (m, ft) Conductive Heat Transfer Calculator k = thermal conductivity (W/mK, Btu/(hr ft °F)) s = wall thickness (m, ft) A = surface area (m2, ft2) dT = t1 - t2 = temperature difference (oC, oF)
Note! - that the overall heat transfer through a surface is determined by the "overall heat transfer coefficient" - which in addition to conductive heat transfer - depends on
Conductive Heat Transfer through an Aluminum Pot Wall with thickness 2 mm - temperature difference 80oCThermal conductivity for aluminum is 215 W/(m K) (from the table above). Conductive heat transfer per unit area can be calculated as q / A = [(215 W/(m K)) / (2 10-3 m)] (80 oC) = 8600000 (W/m2) = 8600 (kW/m2) Conductive Heat Transfer through a Stainless Steel Pot Wall with thickness 2 mm - temperature difference 80oCThermal conductivity for stainless steel is 17 W/(m K) (from the table above). Conductive heat transfer per unit area can be calculated as q / A = [(17 W/(m K)) / (2 10-3 m)] (80 oC) = 680000 (W/m2) = 680 (kW/m2) |
What is the thermal conductivity of a gas?
Related Posts
Copyright © 2024 ShotOnMac Inc.
