Editing Heat transfer coefficient (section)

{{Short description|Quantity relating heat flux and temperature difference}}

In [[thermodynamics]], the ''' heat transfer coefficient''' or '''film coefficient''', or '''film effectiveness''', is the [[Proportional (mathematics)|proportionality constant]] between the [[heat flux]] and the thermodynamic driving force for the [[Heat transfer|flow of heat]] (i.e., the [[Temperature gradient|temperature difference]], {{math|Δ''T''}} ). It is used to calculate [[heat transfer]] between components of a system; such as by [[convection]] between a fluid and a solid. The heat transfer coefficient has [[SI units]] in [[Watt|watts]] per square meter per [[kelvin]] (W/(m<sup>2</sup>K)).

The overall heat transfer rate for combined modes is usually expressed in terms of an overall [[Thermal conduction|conductance]] or heat transfer coefficient, {{mvar|U}}.  Upon reaching a [[steady state]] of flow, the heat transfer rate is:

:<math>\dot{Q}=hA(T_2-T_1)</math>
where (in SI units):
: <math>\dot{Q}</math>: Heat transfer rate (W) 
: <math>h</math>: Heat transfer coefficient (W/m<sup>2</sup>K)
: <math>A</math>: surface area where the heat transfer takes place (m<sup>2</sup>)
: <math>T_2</math>: temperature of the surrounding fluid (K)
: <math>T_1</math>: temperature of the solid surface (K)

The general definition of the heat transfer coefficient is:

:<math>h = \frac{q}{\Delta T}</math>
where:
: <math>q</math>: [[heat flux]] (W/m<sup>2</sup>); i.e., thermal power per unit [[area]], <math>q = d\dot{Q}/dA</math>
: <math>\Delta T</math>: difference in temperature between the solid surface and surrounding fluid area (K)

The heat transfer coefficient is the [[Multiplicative inverse|reciprocal]] of [[thermal insulance]]. This is used for building materials ([[R-value (insulation)|R-value]]) and for [[clothing insulation]].

There are numerous methods for calculating the heat transfer coefficient in different heat transfer modes, different fluids, flow regimes, and under different [[Thermal hydraulics|thermohydraulic]] conditions. Often it can be estimated by dividing the [[thermal conductivity]] of the [[convection]] fluid by a length scale. The heat transfer coefficient is often calculated from the [[Nusselt number]] (a [[dimensionless number]]). There are also online calculators available specifically for [[Heat-transfer fluid]] applications. Experimental assessment of the heat transfer coefficient poses some challenges especially when small fluxes are to be measured (e.g. {{nowrap|< 0.2 W/cm{{sup|2}}}}).<ref>{{cite journal|last1=Chiavazzo|first1=Eliodoro|last2=Ventola|first2=Luigi|last3=Calignano|first3=Flaviana|last4=Manfredi|first4=Diego|last5=Asinari|first5=Pietro|title=A sensor for direct measurement of small convective heat fluxes: Validation and application to micro-structured surfaces|journal=Experimental Thermal and Fluid Science|date=2014|volume=55|pages=42–53|doi=10.1016/j.expthermflusci.2014.02.010|bibcode=2014ETFS...55...42C |url=https://iris.polito.it/bitstream/11583/2528491/1/Sensor_ETFS-D-13-00440_v06.pdf}}<!--http://porto.polito.it/2528491/--></ref><ref>{{cite journal|last1=Maddox|first1=D.E.|last2=Mudawar|first2=I.|title=Single- and Two-Phase Convective Heat Transfer From Smooth and Enhanced Microelectronic Heat Sources in a Rectangular Channel|journal=Journal of Heat Transfer|date=1989|volume=111|issue=4|pages=1045–1052|doi=10.1115/1.3250766|url=http://heattransfer.asmedigitalcollection.asme.org/article.aspx?articleid=1440217|url-access=subscription}}</ref>