Heat Flux

Also know as thermal flux, thermal flow, warmefluss, warmestromdichte (German), flux thermique (French), flusso thermico (Italian).

Energy flowing through a certain imaginary surface of 1 m² in one second. Vector quantity. Usually the flow in one dominant direction is considered. Unit: W/m².

The energy can be transported by means of conduction, convection or radiation.

Usually the measurement of heat flux is done using a solid state sensor, in which case the flow is locally converted to a conductive heat flux (heat flux at the surface of a wall), or altogether interrupted (radiation measurement by means of a radiation sensor).

See also Thermal Sensors or Heat Flux.

Heat Flux Sensors

Also known as heat flux plate, heat flux microsensor, carmeflussplatte (German), capteur de flux thermique, fluxmetre, (French), flussimetro (Italian), TNO heat flux sensor (brand name), REBS Q7 (brand name), Hukseflux HFP 01 (brand name).

A sensor with which the heat flux is measured. Usually the sensor consists of a differential temperature sensor, incorporated in a medium of constant properties. The shape is usually flat. The sensor is mounted in or on the object of interest, and usually generates an output that is proportional to the local heat flux perpendicular to the sensor surface.

See also Thermal Sensors or Heat Flux & Transfer and Products.

Heat Transfer

Heat is transferred by convection, conduction and radiation.

Examples in daily life: a strong source of radiative transfer is the sun. Convection plays a large role in controlling body temperature. Convective transfer is strongly related to wind speed. Conduction transports heat through the wall of a building.

Heat Conduction

Also known as flux conductif.

Transport of heat through solid media is taking place either by radiation, if the material is transparent in the particular spectral range, or by conduction.

Convective Flux

Also know as flux convectif (French).

Transport of heat in gasses can take place by conduction, radiation or convection. The conduction is usually negligible, unless very short distances are involved. Gasses are transparent to most radiation. If energy is transported by flowing gas, this is called heat convection. Convection is often induced by density differences that are caused by temperature differences.

In conditions without forced flow, a gas exchanges energy with a solid object at a rate of 5 W/m²/C (C temperature difference between the objects).

In forced flows, the transfer becomes larger. In physics handbooks this is expressed by means of a transfer coefficient. In indoor situations, the transfer coefficient varies from 5 W/m²/C (at zero wind speed) to 9 W/m²/C at 1 m/s. The transfer coefficient can be measured using HFP01SC or TP01.

Radiative Flux

Objects of a certain temperature radiate heat. The transport between two objects of radiative heat depends on the temperatures and the spectral properties. As a rule of thumb, at a temperature of around 20 degrees C, the energy exchange between two blackbodies is 5 W/m²/C.

Thermal Conductivity

Material property, expressing the resistance of a material to energy transport in a semi-static situation. Expressed in W/m.K.

Energy per second transported through the medium per square meter, when a temperature gradient of 1K/m is forced upon the medium. See also Thermal Conductivity and Products.

Thermal diffusivity

Material property, expressing the speed of temperature change of a medium when exposed to changes in the thermal environment. The thermal diffusivity is inversely proportional to the heat capacity and proportional to the thermal conductivity. Expressed in m²/s. See also Thermal Conductivity.