In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. [1] It is a measure of the rate of heat transfer inside a material and has SI units of m2/s. It is an intensive property. Thermal diffusivity is usually denoted by lowercase alpha (α), but a, h, κ (kappa), [2] K, [3] ,D, are also used.
The formula is: [4] where
Together, ρcp can be considered the volumetric heat capacity (J/(m3·K)).
As seen in the heat equation, [5] one way to view thermal diffusivity is as the ratio of the time derivative of temperature to its curvature, quantifying the rate at which temperature concavity is "smoothed out". Thermal diffusivity is a contrasting measure to thermal effusivity. [6] [7] In a substance with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its volumetric heat capacity or 'thermal bulk'.
Thermal diffusivity is often measured with the flash method. [8] [9] It involves heating a strip or cylindrical sample with a short energy pulse at one end and analyzing the temperature change (reduction in amplitude and phase shift of the pulse) a short distance away. [10] [11]
Material | Thermal diffusivity (mm2/s) | References |
---|---|---|
Pyrolytic graphite, parallel to layers | 1,220 | |
Diamond | 1,060 - 1,160 | |
Carbon/carbon composite at 25 °C | 216.5 | [13] |
Helium (300 K, 1 atm) | 190 | [14] |
Silver, pure (99.9%) | 165.63 | |
Hydrogen (300 K, 1 atm) | 160 | [14] |
Gold | 127 | [15] |
Copper at 25 °C | 111 | [13] |
Aluminium | 97 | [15] |
Silicon | 88 | [15] |
Al-10Si-Mn-Mg (Silafont 36) at 20 °C | 74.2 | [16] |
Aluminium 6061-T6 Alloy | 64 | [15] |
Molybdenum (99.95%) at 25 °C | 54.3 | [17] |
Al-5Mg-2Si-Mn (Magsimal-59) at 20 °C | 44.0 | [18] |
Tin | 40 | [15] |
Water vapor (1 atm, 400 K) | 23.38 | |
Iron | 23 | [15] |
Argon (300 K, 1 atm) | 22 | [14] |
Nitrogen (300 K, 1 atm) | 22 | [14] |
Air (300 K) | 19 | [15] |
Steel, AISI 1010 (0.1% carbon) | 18.8 | [19] |
Aluminium oxide (polycrystalline) | 12.0 | |
Steel, 1% carbon | 11.72 | |
Si3N4 with CNTs 26 °C | 9.142 | [20] |
Si3N4 without CNTs 26 °C | 8.605 | [20] |
Steel, stainless 304A at 27 °C | 4.2 | [15] |
Pyrolytic graphite, normal to layers | 3.6 | |
Steel, stainless 310 at 25 °C | 3.352 | [21] |
Inconel 600 at 25 °C | 3.428 | [22] |
Quartz | 1.4 | [15] |
Sandstone | 1.15 | |
Ice at 0 °C | 1.02 | |
Silicon dioxide (polycrystalline) | 0.83 | [15] |
Brick, common | 0.52 | |
Glass, window | 0.34 | |
Brick, adobe | 0.27 | |
PC (polycarbonate) at 25 °C | 0.144 | [23] |
Water at 25 °C | 0.143 | [23] |
PTFE (Polytetrafluorethylene) at 25 °C | 0.124 | [24] |
PP (polypropylene) at 25 °C | 0.096 | [23] |
Nylon | 0.09 | |
Rubber | 0.089 - 0.13 | [3] |
Wood (yellow pine) | 0.082 | |
Paraffin at 25 °C | 0.081 | [23] |
PVC (polyvinyl chloride) | 0.08 | [15] |
Oil, engine (saturated liquid, 100 °C) | 0.0738 | |
Alcohol | 0.07 | [15] |
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This page provides supplementary data to the article properties of water.
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The laser flash analysis or laser flash method is used to measure thermal diffusivity of a variety of different materials. An energy pulse heats one side of a plane-parallel sample and the resulting time dependent temperature rise on the backside due to the energy input is detected. The higher the thermal diffusivity of the sample, the faster the energy reaches the backside. A laser flash apparatus (LFA) to measure thermal diffusivity over a broad temperature range, is shown on the right hand side.
Kenneth Claughan Mills, was head of the Slags group at the National Physical Laboratory and a visiting professor in the Department of Materials at Imperial College London.
The transient hot wire method (THW) is a very popular, accurate and precise technique to measure the thermal conductivity of gases, liquids, solids, nanofluids and refrigerants in a wide temperature and pressure range. The technique is based on recording the transient temperature rise of a thin vertical metal wire with infinite length when a step voltage is applied to it. The wire is immersed in a fluid and can act both as an electrical heating element and a resistance thermometer. The transient hot wire method has advantage over the other thermal conductivity methods, since there is a fully developed theory and there is no calibration or single-point calibration. Furthermore, because of the very small measuring time there is no convection present in the measurements and only the thermal conductivity of the fluid is measured with very high accuracy.
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