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In physics and engineering, heat flux or thermal flux, sometimes also referred to as heat flux density [1], heat-flow density or heat-flow rate intensity, is a flow of energy per unit area per unit time. Its SI units are watts per square metre (W/m 2). It has both a direction and a magnitude, and so it is a vector quantity.
The rate of heat flow is the amount of heat that is transferred per unit of time in some material, usually measured in watts (joules per second). Heat is the flow of thermal energy driven by thermal non-equilibrium, so the term 'heat flow' is a redundancy (i.e. a pleonasm). Heat must not be confused with stored thermal energy, and moving a hot ...
The heat transfer coefficient is the heat transferred per unit area per kelvin. Thus area is included in the equation as it represents the area over which the transfer of heat takes place. The areas for each flow will be different as they represent the contact area for each fluid side.
This is quantified in terms of a heat flux, which gives the rate, per unit area, at which heat flows in a given direction (in this case minus x-direction). In many materials, q {\displaystyle q} is observed to be directly proportional to the temperature difference and inversely proportional to the separation distance L {\displaystyle L} : [ 1 ]
The radiant exitance (previously called radiant emittance), , has dimensions of energy flux (energy per unit time per unit area), and the SI units of measure are joules per second per square metre (J⋅s −1 ⋅m −2), or equivalently, watts per square metre (W⋅m −2). [2] The SI unit for absolute temperature, T, is the kelvin (K).
For most materials, the amount of heat conducted varies (usually non-linearly) with temperature. [1] Thermal conductivity is often measured with laser flash analysis. Alternative measurements are also established. Mixtures may have variable thermal conductivities due to composition.
In the field of heat transfer, intensity of radiation is a measure of the distribution of radiant heat flux per unit area and solid angle, in a particular direction, defined according to d q = I d ω cos θ d A {\displaystyle dq=I\,d\omega \,\cos \theta \,dA}
Heat flux sensor according to ASTM C1041-10 Standard Practice for In-Situ Measurements of Heat Flux, picture shows model IHF01. Heat flux measurements of thermal insulation are applied in laboratory and industrial environments to obtain reference or in-situ measurements of the thermal properties of an insulation material. Thermal insulation is ...