Heat flux density

Physical size
Surname	warmth
Formula symbol	${\displaystyle {\dot {q}},q}$
Size and unit system unit dimension SI W / m 2  = kg · s -3 M · T −3

The heat flux density , also called, is a physical parameter for the quantitative description of heat transfer processes . It is a power density . ${\displaystyle {\dot {q}}}$ ${\displaystyle q}$

(Area-related) heat flux density

The heat flux density is defined as the change in thermal energy across the boundaries of a homogeneous system , based on a certain cross section and a certain time interval. One can therefore also speak of the heat flow per area in analogy : ${\displaystyle Q}$ ${\displaystyle {\dot {Q}}}$ ${\displaystyle A}$

${\displaystyle {\dot {q}}={\frac {Q}{A\cdot \Delta t}}={\frac {\dot {Q}}{A}}}$

It describes the transferred heat per transfer area and time interval or the thermal power per area.

The SI unit of heat flux density is W / m ² (watts per square meter ).

Often the heat flux is also used as a vector . The direction of the vector indicates the direction of the heat flow. This also enables cases in which the heat flow does not necessarily go vertically through a surface.

Volumetric heat flux density

The volumetric heat flow density is used to calculate some problems . It describes the heat flow per volume : ${\displaystyle {q}_{V}}$ ${\displaystyle {\dot {Q}}}$ ${\displaystyle V}$

${\displaystyle {\dot {q}}_{V}={\frac {Q}{V\cdot \Delta t}}={\frac {\dot {Q}}{V}}}$

The SI unit of the volumetric heat flow density is W / m ³ (watts per cubic meter ).

context

The relationship between volumetric and area-related heat flux density takes place via the heat conduction path ${\displaystyle l={\frac {V}{A}}:}$

${\displaystyle \Rightarrow {\dot {q}}=l\cdot {\dot {q}}_{V}.}$

Heat flux density of the earth

The heat flux density in the geosciences describes the transport of heat from the interior to the surface of a planetary body (earth, moon, Mars etc.). Sources of heat in the earth's interior are the residual heat of the earth's core and the radiogenic heat production of the rocks, mainly in the upper 20 to 40 km of the earth's crust. The radiogenic heat production is the result of the decomposition processes in rocks and is mainly controlled by the concentration of the elements thorium , uranium and potassium . In general terms, the heat flux density is higher, the more the crust is enriched with rocks of high radiogenic heat production or the thinner the earth's crust is. The heat production in the lower crust and mantle is rather low; the heat flux density oceanic rather higher than continental.

The heat flux density can be calculated from the product of the rock thermal conductivity and the temperature gradient of an interval. The most reliable values ​​are obtained in boreholes with properties measured on drill cores and continuous borehole temperature measurements recorded in the borehole that are in thermal equilibrium. The heat flow on the earth's surface that is unaffected by any local processes, such as climate, advective heat transport, groundwater flow, topography, geological structure, etc., is called the terrestrial heat flow density and is required for the interpretation of geodynamic processes as well as the consideration of the lithosphere - astenospheric boundary.

Global heat flow values ​​are collected and made available by the International Heat Flow Commission (IHFC).

See also

• Conduction
• Thermal conductivity

Individual evidence

1. ^ Karl-Heinrich Grote and Jörg Feldhusen (Hrsg.): Dubbel: Pocket book for mechanical engineering . 23rd edition. Springer, Berlin Heidelberg 2011, ISBN 978-3-642-17305-9 , pp. D34 .
2. ^ Kai Schild and Wolfgang Willems: Thermal protection . Vieweg + Teubner, 2011, ISBN 978-3-8348-8145-8 , pp. 40 .
3. ↑ Global heat flow database. International Heat Flow Commission (IHFC), accessed September 22, 2019 .