Heat penetration coefficient

The heat penetration coefficient is a material-specific characteristic value from thermodynamics , which is specified in the unit : ${\displaystyle b}$ ${\displaystyle \mathrm {\frac {J}{K\,m^{2}\,{\sqrt {s}}}} }$

${\displaystyle b={\sqrt {\lambda \cdot \rho \cdot c}}.}$

With

• ${\displaystyle \lambda }$ Thermal conductivity in W / ( m · K )
• ${\displaystyle \rho }$ Density in kg / m³
• ${\displaystyle c}$ specific heat capacity in J / (kg K).

If two halbunendlich expanded bodies with different initial temperatures and brought into perfect thermal contact, it can contacting temperature (the temperature of the contact surface. E.) With the heat penetration coefficient , and the two substances are determined: ${\displaystyle T_{1}}$ ${\displaystyle T_{2}}$ ${\displaystyle T_{m}}$ ${\displaystyle b_{1}}$ ${\displaystyle b_{2}}$

${\displaystyle T_{m}=T_{1}+{\frac {b_{2}}{b_{1}+b_{2}}}(T_{2}-T_{1})={\frac {b_{1}\cdot T_{1}+b_{2}\cdot T_{2}}{b_{1}+b_{2}}}.}$

This formula can also be used to estimate the initial contact temperature of finite bodies.

The heat penetration coefficient can be practically experienced if you touch different substances at the same temperature with your bare hand:

• Substances with a high heat penetration coefficient (e.g. metals ) are perceived as particularly cold if their temperature is below that of the skin.
• Substances with a low heat penetration coefficient (e.g. insulation materials or wood ), on the other hand, are perceived as warmer at the same temperature.

The same applies when the body is warmer than the skin.

Heat penetration coefficient of some substances

material	${\displaystyle b\;\left(\mathrm {in\;{\frac {kJ}{K\,m^{2}\,{\sqrt {s}}}}} \right)}$
Insulation material ( mineral fibers )	0.06
cork	0.10
Wood	0.4 ... 0.5
rubber	0.6
human skin	1.0 ... 1.3
Glass	1.3 ... 1.5
water	1.6
concrete	1.8 ... 2.2
steel	14.0
copper	36.0

A related quantity is the thermal diffusivity .

literature

• Wolfgang M. Willems, Kai Schild: Thermal protection. Basics - calculation - evaluation, 2nd edition, Springer Fachmedien, Wiesbaden 2013, ISBN 978-3-658-02570-0 .

Individual evidence

1. ↑ Hans Dieter Baehr, Karl Stephan: Heat and mass transfer . 4th edition Springer, Berlin 2004, p. 172, ISBN 3-540-40130-X

Web links

• Heat penetration coefficient (accessed January 3, 2020)
• In-situ analysis of sediment properties in steam generators (accessed January 3, 2020)
• Building physics basics of an astonishing temperature measurement (accessed on January 3, 2020)
• Development of thermally optimized wall elements made of solid wood panels with milled air pockets (accessed on January 3, 2020)
• Development of composite insulation materials based on milling chips from wood (accessed on January 3, 2020)