Heat transfer describes the
exchange of thermal energy, between physical systems depending
on the temperature
and pressure,
by dissipating
heat. Systems which
are not isolated may decrease in entropy. Most objects emit infrared thermal
radiation near room temperature. The fundamental modes of heat transfer are
conduction or diffusion, convection, advection and radiation.
The exchange of kinetic
energy of particles through the boundary between two systems which are at
different temperatures from each other or from their surroundings. Heat
transfer always occurs from a region of high temperature to another region of
lower temperature. Heat transfer changes the internal
energy of both systems involved according to the First Law of Thermodynamics.The Second Law of Thermodynamics defines
the concept of thermodynamic entropy, by measurable heat transfer.
Thermal equilibrium is reached when all
involved bodies and the surroundings reach the same temperature. Thermal
expansion is the tendency of matter to change in volume in response
to a change in temperature.
Overview
Earth's longwave thermal radiation intensity, from clouds, atmosphere
and surface.
Heat is defined in physics
as the transfer of thermal energy across a well-defined boundary around
a thermodynamic system. The thermodynamic free energy is the amount
of work that a thermodynamic system can perform. Enthalpy is a thermodynamic potential, designated by the
letter "H", that is the sum of the internal
energy of the system (U) plus the product of pressure (P)
and volume (V). Joule is a unit to
quantify energy, work, or the amount of heat.
Heat transfer is a process
function (or path function), as opposed to functions of state; therefore, the amount of
heat transferred in a thermodynamic process that changes the state of a system depends on how that process occurs, not
only the net difference between the initial and final states of the process.
Thermodynamic and mechanical heat transfer is calculated with
the heat transfer coefficient, the proportionality between the heat flux
and the thermodynamic driving force for the flow of heat. Heat flux is a
quantitative, vectorial representation of the heat flow through a surface.
In engineering contexts, the term heat
is taken as synonymous to thermal energy. This usage has its origin in the historical interpretation of heat as a
fluid (caloric) that can be transferred by various causes,and that is
also common in the language of laymen and everyday life.
The transport equations for thermal energy (Fourier's law), mechanical momentum (Newton's
law for fluids), and mass transfer (Fick's laws of diffusion) are similar, and
analogies among these three transport processes have been developed to
facilitate prediction of conversion from any one to the others.
Thermal engineering concerns the generation,
use, conversion, and exchange of heat transfer. As such, heat transfer is
involved in almost every sector of the economy. Heat transfer is classified
into various mechanisms, such as thermal conduction, thermal convection, thermal
radiation, and transfer of energy by phase
changes.
Mechanisms
The fundamental modes of heat
transfer are:
Advection is the transport
mechanism of a fluid
substance or conserved property from one location to
another, depending on motion and momentum.
The transfer of energy between
objects that are in physical contact. Thermal conductivity is the property of a
material to conduct heat and evaluated primarily in terms of Fourier's Law for heat conduction.
The transfer of energy between an
object and its environment, due to fluid motion. The average temperature, is a
reference for evaluating properties related to convective heat transfer.
The transfer of energy from the
movement of charged particles within atoms is converted to electromagnetic radiation.
Advection
By transferring matter,
energy—including thermal energy—is moved by the physical transfer of a hot or
cold object from one place to another. This can be as simple as placing hot
water in a bottle and heating a bed, or the movement of an iceberg in changing
ocean currents. A practical example is thermal hydraulics. This can be described by the
formula:
where Q is heat flux (W/m²), ρ is
density (kg/m³), c_p is heat capacity at constant pressure (J/(kg*K)), ΔT is
the change in temperature (K), v is velocity (m/s).
Conduction
On a microscopic scale, heat
conduction occurs as hot, rapidly moving or vibrating atoms and molecules
interact with neighboring atoms and molecules, transferring some of their
energy (heat) to these neighboring particles. In other words, heat is
transferred by conduction when adjacent atoms vibrate against one another, or
as electrons move from one atom to another. Conduction is the most significant
means of heat transfer within a solid or between solid objects in thermal
contact. Fluids—especially gases—are less conductive. Thermal contact conductance is the
study of heat conduction between solid bodies in contact.
Steady state conduction
(see Fourier's law) is a form of conduction that happens
when the temperature difference driving the conduction is constant, so that
after an equilibration time, the spatial distribution of temperatures in the
conducting object does not change any further. In steady state conduction, the
amount of heat entering a section is equal to amount of heat coming out.
Transient conduction (see Heat
equation) occurs when the temperature within an object changes as a
function of time. Analysis of transient systems is more complex and often calls
for the application of approximation theories or numerical analysis by
computer.
Convection
The flow of fluid may be forced by
external processes, or sometimes (in gravitational fields) by buoyancy forces
caused when thermal energy expands the fluid (for example in a fire plume),
thus influencing its own transfer. The latter process is often called
"natural convection". All convective processes also move heat partly
by diffusion, as well. Another form of convection is forced convection. In this
case the fluid is forced to flow by use of a pump, fan or other mechanical
means.
Convective heat transfer, or convection,
is the transfer of heat from one place to another by the movement of fluids, a process
that is essentially the transfer of heat via mass
transfer. Bulk motion of fluid enhances heat transfer in many physical
situations, such as (for example) between a solid surface and the fluid.
Convection is usually the dominant form of heat transfer in liquids and gases.
Although sometimes discussed as a third method of heat transfer, convection is
usually used to describe the combined effects of heat conduction within the
fluid (diffusion) and heat transference by bulk fluid flow streaming.The
process of transport by fluid streaming is known as advection, but pure
advection is a term that is generally associated only with mass transport in
fluids, such as advection of pebbles in a river. In the case of heat transfer
in fluids, where transport by advection in a fluid is always also accompanied
by transport via heat diffusion (also known as heat conduction) the process of
heat convection is understood to refer to the sum of heat transport by
advection and diffusion/conduction.
Free, or natural, convection
occurs when bulk fluid motions (streams and currents) are caused by buoyancy
forces that result from density variations due to variations of temperature in
the fluid. Forced convection is a term used when the streams and
currents in the fluid are induced by external means—such as fans, stirrers, and
pumps—creating an artificially induced convection current.
Convection-cooling
Convective cooling is sometimes
described as Newton's law of cooling:
The rate of heat loss of a body
is proportional to the temperature difference between the body and its
surroundings.
However, by definition, the
validity of Newton's law of cooling requires that the rate of heat loss from
convection be a linear function of ("proportional to") the
temperature difference that drives heat transfer, and in convective cooling
this is sometimes not the case. In general, convection is not linearly
dependent on temperature gradients, and in some cases is strongly nonlinear. In
these cases, Newton's law does not apply.
Convection vs. conduction
In a body of fluid that is heated
from underneath its container, conduction and convection can be considered to
compete for dominance. If heat conduction is too great, fluid moving down by
convection is heated by conduction so fast that its downward movement will be
stopped due to its buoyancy, while fluid moving up by convection is cooled by
conduction so fast that its driving buoyancy will diminish. On the other hand,
if heat conduction is very low, a large temperature gradient may be formed and
convection might be very strong.
The Rayleigh
number is a measure determining the
relative strength of conduction and convection.
where
- g is acceleration due to gravity,
- ρ is the density with being the density difference between the lower and upper ends,
- μ is the dynamic viscosity,
- α is the Thermal diffusivity,
- β is the volume thermal expansivity (sometimes denoted α elsewhere),
- T is the temperature,
- ν is the kinematic viscosity, and
- L is characteristic length.
The Rayleigh number can be
understood as the ratio between the rate of heat transfer by convection to the
rate of heat transfer by conduction; or, equivalently, the ratio between the
corresponding timescales (i.e. conduction timescale divided by convection
timescale), up to a numerical factor. This can be seen as follows, where all
calculations are up to numerical factors depending on the geometry of the
system.
The buoyancy force driving the
convection is roughly , so the corresponding pressure is roughly . In steady
state, this is canceled by the shear stress due to viscosity, and therefore
roughly equals , where V is the typical fluid velocity due to convection
and the
order of its timescale. The conduction timescale, on the other hand, is of the
order of .
Convection occurs when the
Rayleigh number is above 1,000–2,000.
Radiation
Red-hot iron object, transferring
heat to the surrounding environment primarily through thermal radiation
Thermal
radiation occurs through a vacuum or any transparent medium
(solid or fluid). It is the
transfer of energy by means of photons in electromagnetic waves governed by the same
laws. Earth's radiation balance depends on the
incoming and the outgoing thermal radiation, Earth's energy budget. Anthropogenic
perturbations in the climate system, are responsible for a positive radiative
forcing which reduces the net longwave radiation loss out to Space.
Thermal
radiation is energy emitted by matter as electromagnetic waves, due to the
pool of thermal energy in all matter with a temperature
above absolute
zero. Thermal radiation propagates without the presence of matter through
the vacuum of
space.
Thermal radiation is a direct
result of the random movements of atoms and molecules in matter. Since these
atoms and molecules are composed of charged particles (protons and electrons),
their movement results in the emission of electromagnetic radiation, which carries
energy away from the surface.
The Stefan-Boltzmann equation, which
describes the rate of transfer of radiant energy, is as follows for an object
in a vacuum :
For radiative transfer between two
objects, the equation is as follows:
where Q is the rate of heat
transfer, ε is the emissivity (unity for a black body),
σ is the Stefan-Boltzmann constant, and T is the
absolute temperature (in Kelvin or Rankine). Radiation is typically only
important for very hot objects, or for objects with a large temperature
difference.
Radiation from the sun, or solar
radiation, can be harvested for heat and power. Unlike conductive and
convective forms of heat transfer, thermal radiation can be concentrated in a
small spot by using reflecting mirrors, which is exploited in concentrating solar power generation. For
example, the sunlight reflected from mirrors heats the PS10 solar power tower and during the day it
can heat water to 285 °C (545 °F).
Phase transition
Lightning is
a highly visible form of energy transfer and is an example of plasma present at Earth's
surface. Typically, lightning discharges 30,000 amperes at up to 100 million
volts, and emits light, radio waves, X-rays and even gamma rays. Plasma
temperatures in lightning can approach 28,000 Kelvin (27,726.85 °C) (49,940.33
°F) and electron densities may exceed 1024 m−3.
Phase transition or phase change,
takes place in a thermodynamic system from one phase or state
of matter to another one by heat transfer. Phase change examples are the
melting of ice or the boiling of water. The Mason
equation explains the growth of a water droplet based on the effects of
heat transport on evaporation and condensation.
Types of phase transition
occurring in the four fundamental states of matter, include:
- Solid - Deposition, freezing and solid to solid transformation.
- Gas - Boiling / evaporation, recombination / deionization, and sublimation.
- Liquid - Condensation and melting / fusion.
- Plasma - Ionization.
Boiling
.
The boiling
point of a substance is the temperature at which the vapor pressure of the
liquid equals the pressure surrounding the liquid and the liquid evaporates
resulting in an abrupt change in vapor volume.
Saturation temperature means
boiling point. The saturation temperature is the temperature for a
corresponding saturation pressure at which a liquid boils into its vapor phase.
The liquid can be said to be saturated with thermal energy. Any addition of
thermal energy results in a phase transition.
At low temperatures, no
boiling occurs and the heat transfer rate is controlled by the usual
single-phase mechanisms. As the surface temperature is increased, local boiling
occurs and vapor bubbles nucleate, grow into the surrounding cooler fluid, and
collapse. This is sub-cooled nucleate boiling, and is a very efficient
heat transfer mechanism. At high bubble generation rates, the bubbles begin to
interfere and the heat flux no longer increases rapidly with surface
temperature (this is the departure from nucleate boiling, or DNB).
At high temperatures, the
hydrodynamically-quieter regime of film boiling
is reached. Heat fluxes across the stable vapor layers are low, but rise slowly
with temperature. Any contact between fluid and the surface that may be seen
probably leads to the extremely rapid nucleation of a fresh vapor layer
("spontaneous nucleation"). At higher temperatures still, a maximum
in the heat flux is reached (the critical heat flux, or CHF).
The Leidenfrost Effect demonstrates how nucleate
boiling slows heat transfer due to gas bubbles on the heater's surface. As
mentioned, gas-phase thermal conductivity is much lower than liquid-phase
thermal conductivity, so the outcome is a kind of "gas thermal
barrier".
Condensation
Condensation occurs when a vapor
is cooled and changes its phase to a liquid. During condensation, the latent heat of vaporization must be
released. The amount of the heat is the same as that absorbed during
vaporization at the same fluid pressure.
There are several types of
condensation:
- Homogeneous condensation, as during a formation of fog.
- Condensation in direct contact with subcooled liquid.
- Condensation on direct contact with a cooling wall of a heat exchanger: This is the most common mode used in industry:
o Filmwise
condensation is when a liquid film is formed on the subcooled surface, and
usually occurs when the liquid wets the surface.
o Dropwise
condensation is when liquid drops are formed on the subcooled surface, and
usually occurs when the liquid does not wet the surface.
Dropwise condensation is difficult
to sustain reliably; therefore, industrial equipment is normally designed to
operate in filmwise condensation mode.
Melting
Ice melting
Melting is a physical process that
results in the phase transition of a substance from a solid to a liquid. The internal
energy of a substance is increased, typically by the application of heat or
pressure, resulting in a rise of its temperature to the melting
point, at which the ordering of ionic or molecular entities in the solid
breaks down to a less ordered state and the solid liquefies. An object that has
melted completely is molten. Substances in the molten state generally have
reduced viscosity with elevated temperature; an exception to this maxim is the
element sulfur,
whose viscosity increases to a point due to polymerization
and then decreases with higher temperatures in its molten state.
Modeling approaches
Heat transfer can be modeled in
the following ways.
Climate models
Climate
models study the radiant heat transfer by using quantitative
methods to simulate the interactions of the atmosphere, oceans, land surface,
and ice.
Heat equation
The heat
equation is an important partial differential equation that
describes the distribution of heat (or variation in temperature) in a given
region over time. In some cases, exact solutions of the equation are available;
in other cases the equation must be solved numerically using computational methods.
Lumped system analysis
Lumped system analysis often
reduces the complexity of the equations to one first-order linear differential
equation, in which case heating and cooling are described by a simple
exponential solution, often referred to as Newton's law of cooling.
System analysis by the lumped capacitance model is a common
approximation in transient conduction that may be used whenever heat conduction
within an object is much faster than heat conduction across the boundary of the
object. This is a method of approximation that reduces one aspect of the
transient conduction system—that within the object—to an equivalent steady
state system. That is, the method assumes that the temperature within the
object is completely uniform, although its value may be changing in time.
In this method, the ratio of the
conductive heat resistance within the object to the convective heat transfer
resistance across the object's boundary, known as the Biot
number, is calculated. For small Biot numbers, the approximation of spatially
uniform temperature within the object can be used: it can be presumed that
heat transferred into the object has time to uniformly distribute itself, due
to the lower resistance to doing so, as compared with the resistance to heat
entering the object.
Engineering
Heat exposure as part of a fire
test for firestop products
Heat transfer has broad
application to the functioning of numerous devices and systems. Heat-transfer
principles may be used to preserve, increase, or decrease temperature in a wide
variety of circumstances. Heat transfer methods are used in numerous
disciplines, such as automotive engineering, thermal management
of electronic devices and systems, climate control, insulation, materials processing, and power
station engineering.
Insulation, radiance and
resistance
Thermal insulators are materials specifically
designed to reduce the flow of heat by limiting conduction, convection, or
both. Thermal resistance is a heat property and the
measurement by which an object or material resists to heat flow (heat per time
unit or thermal resistance) to temperature difference.
Radiance or
spectral radiance are measures of the quantity of radiation that passes through
or is emitted. Radiant barriers are materials that reflect radiation, and therefore reduce the
flow of heat from radiation sources. Good insulators are not necessarily good
radiant barriers, and vice versa. Metal, for instance, is an excellent
reflector and a poor insulator.
The effectiveness of a radiant
barrier is indicated by its reflectivity, which is the fraction of
radiation reflected. A material with a high reflectivity (at a given
wavelength) has a low emissivity (at that same wavelength), and vice versa. At
any specific wavelength, reflectivity = 1 - emissivity. An ideal radiant
barrier would have a reflectivity of 1, and would therefore reflect 100 percent
of incoming radiation. Vacuum flasks, or Dewars, are silvered to
approach this ideal. In the vacuum of space, satellites use multi-layer insulation, which consists of
many layers of aluminized (shiny) Mylar to greatly reduce radiation heat transfer and control
satellite temperature.
Devices
Heat engine diagram
- Heat engine is a system that performs the conversion of heat or thermal energy to mechanical energy which can then be used to do mechanical work.
- Thermocouple is a temperature-measuring device and widely used type of temperature sensor for measurement and control, and can also be used to convert heat into electric power.
- Thermoelectric cooler is a solid state electronic device that pumps (transfers) heat from one side of the device to the other when electrical current is passed through it. It is based on the Peltier effect.
- Thermal diode or thermal rectifier is a device that causes heat to flow preferentially in one direction.
Heat exchangers
A heat
exchanger is used for more efficient heat transfer or to dissipate heat.
Heat exchangers are widely used in refrigeration,
air
conditioning, space heating, power
generation, and chemical processing. One common example of a heat exchanger
is a car's radiator, in which the hot coolant fluid is
cooled by the flow of air over the radiator's surface.
Common types of heat exchanger
flows include parallel flow, counter flow, and cross flow. In parallel flow,
both fluids move in the same direction while transferring heat; in counter
flow, the fluids move in opposite directions; and in cross flow, the fluids
move at right
angles to each other. Common constructions for heat exchanger include shell
and tube, double pipe, extruded finned pipe, spiral fin pipe,
u-tube, and stacked plate.
A heat sink
is a component that transfers heat generated within a solid material to a fluid
medium, such as air or a liquid. Examples of heat sinks are the heat exchangers
used in refrigeration and air conditioning systems or the radiator in a car. A heat pipe
is another heat-transfer device that combines thermal conductivity and phase
transition to efficiently transfer heat between two solid interfaces.
Examples
Architecture
Efficient energy use is the goal to reduce the
amount of energy required in heating or cooling. In architecture, condensation
and air
currents can cause cosmetic or structural damage. An energy
audit, can help to assess the implementation of recommended corrective
procedures. For instance, insulation improvements, air sealing of structural
leaks or the addition of energy-efficient windows and doors.
- Smart meter is a device that records electric energy consumption in intervals.
- Thermal transmittance is the rate of transfer of heat through a structure divided by the difference in temperature across the structure. It is expressed in watts per square meter per kelvin, or W/m²K. Well-insulated parts of a building have a low thermal transmittance, whereas poorly-insulated parts of a building have a high thermal transmittance.
- Thermostat is a device to monitor and control temperature.
Climate engineering
An example application in climate
engineering includes the creation of Biochar through
the pyrolysis
process. Thus, storing greenhouse gases in carbon reduces the radiative forcing
capacity in the atmosphere, causing more long-wave (infrared)
radiation out to Space.
Climate engineering consist of carbon dioxide removal and solar radiation management. Since the
amount of carbon dioxide determines the radiative
balance of Earth atmosphere, carbon dioxide removal techniques can be
applied to reduce the radiative forcing. Solar radiation management is
the attempt to absorb less solar radiation to offset the effects of greenhouse
gases.
Greenhouse effect
A representation of the exchanges
of energy between the source (the Sun), the Earth's surface, the Earth's atmosphere, and the ultimate sink outer space.
The ability of the atmosphere to capture and recycle energy emitted by the Earth
surface is the defining characteristic of the greenhouse effect.
The greenhouse
effect is a process by which thermal radiation from a planetary surface is
absorbed by atmospheric greenhouse gases, and is re-radiated in all directions.
Since part of this re-radiation is back towards the surface and the lower
atmosphere, it results in an elevation of the average surface temperature above
what it would be in the absence of the gases.
Heat transfer in the human body
The principles of heat transfer in
engineering systems can be applied to the human body in order to determine how
the body transfers heat. Heat is produced in the body by the continuous
metabolism of nutrients which provides energy for the systems of the body. The
human body must maintain a consistent internal temperature in order to maintain
healthy bodily functions. Therefore, excess heat must be dissipated from the
body to keep it from overheating. When a person engages in elevated levels of
physical activity, the body requires additional fuel which increases the
metabolic rate and the rate of heat production. The body must then use
additional methods to remove the additional heat produced in order to keep the
internal temperature at a healthy level.
Heat transfer by convection is driven by
the movement of fluids over the surface of the body. This convective fluid can
be either a liquid or a gas. For heat transfer from the outer surface of the
body, the convection mechanism is dependent on the surface area of the body,
the velocity of the air, and the temperature gradient between the surface of
the skin and the ambient air. The normal temperature of the body is
approximately 37°C. Heat transfer occurs more readily when the temperature of
the surroundings is significantly less than the normal body temperature. This
concept explains why a person feels “cold” when not enough covering is worn
when exposed to a cold environment. Clothing can be considered an insulator
which provides thermal resistance to heat flow over the covered portion of the
body. This thermal resistance causes the temperature on the surface of the
clothing to be less than the temperature on the surface of the skin. This
smaller temperature gradient between the surface temperature and the ambient
temperature will cause a lower rate of heat transfer than if the skin were not
covered.
In order to ensure that one
portion of the body is not significantly hotter than another portion, heat must
be distributed evenly through the bodily tissues. Blood flowing through blood
vessels acts as a convective fluid and helps to prevent any buildup of excess
heat inside the tissues of the body. This flow of blood through the vessels can
be modeled as pipe flow in an engineering system. The heat carried by the blood
is determined by the temperature of the surrounding tissue, the diameter of the
blood vessel, the thickness of the fluid, velocity of the flow, and the heat
transfer coefficient of the blood. The velocity, blood vessel diameter, and the
fluid thickness can all be related with the Reynolds
Number, a dimensionless number used in fluid mechanics to characterize the
flow of fluids.
Latent heat
loss, also known as evaporative heat loss, accounts for a large fraction of
heat loss from the body. When the core temperature of the body increases, the
body triggers sweat glands in the skin to bring additional moisture to the
surface of the skin. The liquid is then transformed into vapor which removes
heat from the surface of the body. The rate of evaporation heat loss is
directly related to the vapor pressure at the skin surface and the amount of
moisture present on the skin. Therefore, the maximum of heat transfer will
occur when the skin is completely wet. The body continuously loses water by
evaporation but the most significant amount of heat loss occurs during periods
of increased physical activity.
Cooling techniques
Evaporative cooling
Evaporative cooling happens when water vapor is
added to the surrounding air. The energy needed to evaporate the water is taken
from the air in the form of sensible heat and converted into latent heat, while
the air remains at a constant enthalpy. Latent heat describes the amount of heat that is
needed to evaporate the liquid; this heat comes from the liquid itself and the
surrounding gas and surfaces. The greater the difference between the two
temperatures, the greater the evaporative cooling effect. When the temperatures
are the same, no net evaporation of water in air occurs; thus, there is no
cooling effect.
Laser cooling
In Quantum
Physics laser cooling is used to achieve temperatures of near absolute
zero (−273.15°C, −459.67°F) of atomic and molecular samples, to observe
unique quantum effects that can only occur at this heat
level.
- Doppler cooling is the most common method of laser cooling.
- Sympathetic cooling is a process in which particles of one type cool particles of another type. Typically, atomic ions that can be directly laser-cooled are used to cool nearby ions or atoms. This technique allows cooling of ions and atoms that cannot be laser cooled directly.
Magnetic cooling
Magnetic evaporative cooling is a
process for lowering the temperature of a group of atoms, after pre-cooled by
methods such as laser cooling. Magnetic refrigeration cools below 0.3K, by
making use of the magnetocaloric effect.
Radiative cooling
Radiative
cooling is the process by which a body loses heat by radiation. Outgoing energy is an important effect in the
Earth's energy budget. In the case of the
Earth-atmosphere system, it refers to the process by which long-wave (infrared)
radiation is emitted to balance the absorption of short-wave (visible) energy
from the Sun. Convective transport of heat and evaporative transport of latent
heat both remove heat from the surface and redistribute it in the atmosphere.
Thermal energy storage
Thermal energy storage refers to
technologies used to collect and store
energy for later use. They can be employed to balance energy demand between
day and nighttime. The thermal reservoir may be maintained at a temperature
above (hotter) or below (colder) than that of the ambient environment.
Applications include later use in space heating, domestic or process hot water,
or to generate electricity.
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