Thermal conductivity Saylor（热导率塞勒）.pdf

Thermal conductivity 1 Thermal conductivity In physics, thermal conductivity, , is the property of a materials ability to conduct heat. It appears primarily in −1 −1 Fouriers Law for heat conduction. Thermal conductivity is measured in watts per kelvin-meter (W·K ·m , i.e. −1 −1 −1 W/(K·m) or in IP units (Btu·hr ·ft ·F , i.e. Btu/(hr·ft⋅F). Multiplied by a temperature difference (in kelvins, K) 2 and an area (in square meters, m ), and divided by a thickness (in meters, m), the thermal conductivity predicts the rate of energy loss (in watts, W) through a piece of material. In the window building industry thermal conductivity [1] is expressed as the U-Factor , which measures the rate of heat transfer and tells you how well the window insulates. U-factor values are generally recorded in IP units (Btu/(hr·ft⋅F)) and usually range from 0.15 to 1.25. The lower the U-factor, the better the window insulates. The reciprocal of thermal conductivity is thermal resistivity. Measurement There are a number of ways to measure thermal conductivity. Each of these is suitable for a limited range of materials, depending on the thermal properties and the medium temperature. There is a distinction between steady-state and transient techniques. In general, steady-state techniques are useful when the temperature of the materi