| Thermal conductance | |
| Unit: | watt per kelvin (W/K) |
| Baseunits: | kg⋅m2⋅s−3⋅K-1 |
| Dimension: | L2MT-3\Theta-1 |
| Thermal resistance | |
| Unit: | kelvin per watt (K/W) |
| Baseunits: | kg-1⋅m-2⋅s3⋅K |
| Dimension: | wikidata |
In heat transfer, thermal engineering, and thermodynamics, thermal conductance and thermal resistance are fundamental concepts that describe the ability of materials or systems to conduct heat and the opposition they offer to the heat current. The ability to manipulate these properties allows engineers to control temperature gradient, prevent thermal shock, and maximize the efficiency of thermal systems. Furthermore, these principles find applications in a multitude of fields, including materials science, mechanical engineering, electronics, and energy management. Knowledge of these principles is crucial in various scientific, engineering, and everyday applications, from designing efficient temperature control, thermal insulation, and thermal management in industrial processes to optimizing the performance of electronic devices.
Thermal conductance (G) measures the ability of a material or system to conduct heat. It provides insights into the ease with which heat can pass through a particular system. It is measured in units of watts per kelvin (W/K). It is essential in the design of heat exchangers, thermally efficient materials, and various engineering systems where the controlled movement of heat is vital.
Conversely, thermal resistance (R) measures the opposition to the heat current in a material or system. It is measured in units of kelvins per watt (K/W) and indicates how much temperature difference (in kelvins) is required to transfer a unit of heat current (in watts) through the material or object. It is essential to optimize the building insulation, evaluate the efficiency of electronic devices, and enhance the performance of heat sinks in various applications.
Objects made of insulators like rubber tend to have very high resistance and low conductance, while objects made of conductors like metals tend to have very low resistance and high conductance. This relationship is quantified by resistivity or conductivity. However, the nature of a material is not the only factor as it also depends on the size and shape of an object because these properties are extensive rather than intensive. The relationship between thermal conductance and resistance is analogous to that between electrical conductance and resistance in the domain of electronics.
Thermal insulance (R-value) is a measure of a material's resistance to the heat current. It quantifies how effectively a material can resist the transfer of heat through conduction, convection, and radiation. It has the units square metre kelvins per watt (m2⋅K/W) in SI units or square foot degree Fahrenheit–hours per British thermal unit (ft2⋅°F⋅h/Btu) in imperial units. The higher the thermal insulance, the better a material insulates against heat transfer. It is commonly used in construction to assess the insulation properties of materials such as walls, roofs, and insulation products.
Thermal conductance and resistance have several practical applications in various fields:
Understanding thermal resistance helps in designing energy-efficient buildings with effective insulation materials to reduce heat transfer.
Thermal resistance is crucial for designing heat sinks and thermal management systems in electronic devices to prevent overheating. Calculating thermal conductance is crucial for designing effective heat sinks and cooling systems in electronic devices.
Automotive engineers use thermal resistance to optimize the cooling system and prevent overheating in engines and other vehicle components. Evaluating thermal resistance helps in designing engine components and automotive cooling systems.
In industries like HVAC and chemical processing, heat exchangers use thermal conductance to efficiently transfer heat between fluids.
In spacecraft and aircraft, thermal resistance and conductance are critical for managing temperature variations in extreme environments. Designing spacecraft and aviation systems require considerations of thermal conductance and resistance to manage temperature extremes.
Understanding thermal properties is vital for the design of cryogenic systems used in superconductors and medical applications.
Thermal management is crucial for medical equipment like magnetic resonance imaging (MRI) machines and laser systems to maintain precise operating temperatures. Ensuring proper thermal management is crucial for the safety and performance of medical devices and laser systems.
The food industry uses knowledge of thermal conductance to optimize processes like pasteurization and cooking and design equipment for food processing, such as ovens and refrigeration units.
Researchers use thermal conductance data to develop new materials for various applications, including energy storage and advanced coatings.
Thermal resistance is considered in climate studies to understand heat transfer in Earth's atmosphere and oceans. Evaluating thermal resistance is useful in studying soil temperature profiles for environmental and agricultural research.
Assessing thermal conductance is important in geothermal heat exchangers and energy production.
Infrared cameras and thermal imaging devices use principles of thermal conductance to detect temperature variations.
Absolute thermal resistance is the temperature difference across a structure when a unit of heat energy flows through it in unit time. It is the reciprocal of thermal conductance. The SI unit of absolute thermal resistance is kelvins per watt (K/W) or the equivalent degrees Celsius per watt (°C/W) – the two are the same since the intervals are equal: ΔT = 1 K = 1 °C.
The thermal resistance of materials is of great interest to electronic engineers because most electrical components generate heat and need to be cooled. Electronic components malfunction or fail if they overheat, and some parts routinely need measures taken in the design stage to prevent this.
See main article: analogical models and Onsager reciprocal relations.
Electrical engineers are familiar with Ohm's law and so often use it as an analogy when doing calculations involving thermal resistance. Mechanical and structural engineers are more familiar with Hooke's law and so often use it as an analogy when doing calculations involving thermal resistance.
| type | structural analogy[1] | hydraulic analogy | thermal | electrical analogy[2] | ||
|---|---|---|---|---|---|---|
| quantity | impulse J | volume V | heat Q | charge q | ||
| potential | displacement X | pressure P | temperature T | potential V | ||
| flux | load or force F | flow rate Q | heat transfer rate
| current I | ||
| flux density | stress \sigma | velocity v | heat flux q | current density j | ||
| resistance | flexibility (rheology defined) [1/Pa] | fluid resistance R | thermal resistance R | electrical resistance R | ||
| conductance | ... ... | fluid conductance G | thermal conductance G | electrical conductance G | ||
| resistivity | flexibility 1/k | fluid resistivity | thermal resistivity [(m·K)/W] | electrical resistivity \rho | ||
| conductivity | stiffness k | fluid conductivity | thermal conductivity k | electrical conductivity \sigma | ||
| lumped element linear model | Hooke's law \DeltaX=F/k | Hagen–Poiseuille equation \DeltaP=QR | Newton's law of cooling \DeltaT=
R | Ohm's law \DeltaV=IR | ||
| distributed linear model | ... ... | ... ... | Fourier's law q=-k\boldsymbol{\nabla}T | Ohm's law J=\sigmaE=-\sigma\boldsymbol{\nabla}V |
The heat flow can be modelled by analogy to an electrical circuit where heat flow is represented by current, temperatures are represented by voltages, heat sources are represented by constant current sources, absolute thermal resistances are represented by resistors and thermal capacitances by capacitors.
The diagram shows an equivalent thermal circuit for a semiconductor device with a heat sink.
From Fourier's Law for heat conduction, the following equation can be derived, and is valid as long as all of the parameters (x and k) are constant throughout the sample.
R\theta=
| \Deltax | |
| Ak |
=
| \Deltaxr | |
| A |
where:
R\theta
\Deltax
k
r
A
In terms of the temperature gradient across the sample and heat flux through the sample, the relationship is:
R\theta=
| \Deltax | |
| A\phiq |
| \DeltaT | |
| \Deltax |
=
| \DeltaT | |
| q |
where:
R\theta
\Deltax
\phiq
| \DeltaT | |
| \Deltax |
A
\DeltaT
q
A 2008 review paper written by Philips researcher Clemens J. M. Lasance notes that: "Although there is an analogy between heat flow by conduction (Fourier's law) and the flow of an electric current (Ohm’s law), the corresponding physical properties of thermal conductivity and electrical conductivity conspire to make the behavior of heat flow quite unlike the flow of electricity in normal situations. [...] Unfortunately, although the electrical and thermal differential equations are analogous, it is erroneous to conclude that there is any practical analogy between electrical and thermal resistance. This is because a material that is considered an insulator in electrical terms is about 20 orders of magnitude less conductive than a material that is considered a conductor, while, in thermal terms, the difference between an "insulator" and a "conductor" is only about three orders of magnitude. The entire range of thermal conductivity is then equivalent to the difference in electrical conductivity of high-doped and low-doped silicon."
The junction-to-air thermal resistance can vary greatly depending on the ambient conditions.[3] (A more sophisticated way of expressing the same fact is saying that junction-to-ambient thermal resistance is not Boundary-Condition Independent (BCI).[4]) JEDEC has a standard (number JESD51-2) for measuring the junction-to-air thermal resistance of electronics packages under natural convection and another standard (number JESD51-6) for measurement under forced convection.
A JEDEC standard for measuring the junction-to-board thermal resistance (relevant for surface-mount technology) has been published as JESD51-8.[5]
A JEDEC standard for measuring the junction-to-case thermal resistance (JESD51-14) is relatively newcomer, having been published in late 2010; it concerns only packages having a single heat flow and an exposed cooling surface.[6] [7] [8]
When resistances are in series, the total resistance is the sum of the resistances:
R\rm=RA+RB+RC+...
Similarly to electrical circuits, the total thermal resistance for steady state conditions can be calculated as follows.
The total thermal resistance
Simplifying the equation, we get
With terms for the thermal resistance for conduction, we get
It is often suitable to assume one-dimensional conditions, although the heat flow is multidimensional. Now, two different circuits may be used for this case. For case (a) (shown in picture), we presume isothermal surfaces for those normal to the x- direction, whereas for case (b) we presume adiabatic surfaces parallel to the x- direction. We may obtain different results for the total resistance
{Rtot}
{q}
{|kf-kg|}
Spherical and cylindrical systems may be treated as one-dimensional, due to the temperature gradients in the radial direction. The standard method can be used for analyzing radial systems under steady state conditions, starting with the appropriate form of the heat equation, or the alternative method, starting with the appropriate form of Fourier's law. For a hollow cylinder in steady state conditions with no heat generation, the appropriate form of heat equation is [9]
Where
{k}
{k}
Where
{A=2\pirL}
{kr(dT/dr)}
{r}
{qr}
In order to determine the temperature distribution in the cylinder, equation 4 can be solved applying the appropriate boundary conditions. With the assumption that
{k}
Using the following boundary conditions, the constants
{C1}
{C2}
{T(r1)=Ts,1}
{T(r2)=Ts,2}
The general solution gives us
{Ts,1=C1lnr1+C2}
{Ts,2=C1lnr2+C2}
{C1}
{C2}
The logarithmic distribution of the temperature is sketched in the inset of the thumbnail figure.Assuming that the temperature distribution, equation 7, is used with Fourier's law in equation 5, the heat transfer rate can be expressed in the following form
| {Q |
r={2\piLk(Ts,1-Ts,2)\overln(r2/r1)}}
Finally, for radial conduction in a cylindrical wall, the thermal resistance is of the form
{Rt,cond={ln(r2/r1)\over2\piLk}}
{r2>r1}
10. K Einalipour, S. Sadeghzadeh, F. Molaei. “Interfacial thermal resistance engineering for polyaniline (C3N)-graphene heterostructure”, The Journal of Physical Chemistry, 2020. DOI:10.1021/acs.jpcc.0c02051
There is a large amount of literature on this topic. In general, works using the term "thermal resistance" are more engineering-oriented, whereas works using the term thermal conductivity are more [pure-]physics-oriented. The following books are representative, but may be easily substituted.