What is Heat transfer
 |
Heat transfer is a branch of thermal engineering that concerns with use of conversion, and exchange of thermal energy means heat between two physical systems. Heat transfer is classified into three types which are thermal conduction, thermal convection, thermal radiation. Transfer of energy by phase changes done by the Heat transfer. Engineers and scincetist also consider the transfer of mass with their different in the properties of two body either cold or hot, to achieve heat transfer. Use computer
Simulation of thermal convection in the Earth's mantle. Colors span from red and green to blue with decreasing temperatures. A hot, less-dense lower boundary layer sends plumes of hot material upwards, and cold material from the top moves downwards.
Heat conduction also called as diffusion, in heat conduction or diffusion, directly exchange of kinetic energy of particles through the boundary between two systems. When an one of the object is at a different temperature from another body which is in surroundings, heat flows from hotter body to the surroundings until tepmerature of body and surroundings reach the same temperature, at which point they are in thermal equilibrium. Such spontaneous heat transfer method always occurs from high temperature to lower temperature, as described in the second law of thermodynamics.
Heat convection occurs when bulk heat flow of a fluid e.g gas or liquid with the flow of matter in the fluid. The fluid flow may be forced by external processes, or sometimes in gravitational fields by buoyancy forces caused when thermal energy expands the fluid e.g in a fire plume, thus influencing its own transfer. Later process is often called "natural convection". All convection processes also move heat partly by diffusion or heat convection 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 solid body.
Thermal radiation occurs through a vacuum or any transparent medium such as solid or fluid or gas. Thermal radiation is the transfer of energy by means of photons in electromagnetic waves governed by the same laws.
Content
1) overview of heat transfer
2) modes of heat transfer
3) phase transition
4) modeling approaches
5) Engineering
6) Applications
7) cooling technique
Overview of heat transfer
Heat is defined as transfer of thermal energy into the a well-defined boundary around a thermodynamic system. Free energy of the thermodynamic is the amount of work done by thermodynamic system. Enthalpy is a thermodynamic potential, denoted by the H. Enthapy is define that is the sum of the internal energy of the system (U) and the product of pressure (P) and volume (V). Joule is a unit to internal 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. Net difference between the initial and final states of the process.
Thermodynamic system and mechanical system, heat transfer is calculated with the heat transfer coefficient. Heat thransfer coefficient is define as heat flux is directly proportional to the thermodynamic driving force for the flow of heat. Heat flux is representing of heat-flow through a surface.
In engineering contexts, heat is subtype of thermal energy. This usage has its origin in the historical interpretation of heat as a fluid (caloric) that can be transferred by various causes.
Transport equations for thermal energy such as Fourier's law, mechanical momentum or Newton's law for fluids, and Fick's laws of diffusion is used for mass transfer and these equestions are similar, and from these three transport processes or equations have been developed to facilitate prediction of conversion from any one to the others.
Thermal engineering related to the generation, use, conversion, and exchange of heat transfer. Heat transfer is involved in every sector of the economy. Heat transfer is classified into three main type, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes.
Modes of heat transfer
 |
| Types of modes of heat transfer |
Advection
Advection is modes of heat transfer of a fluid from one system to another system, and it is dependent on motion and momentum of that fluid.
Conduction or diffusion
The transfer of heat between two system that are in physical contact. Thermal conductivity is the property of a material to conduct heat form system and evaluated by Fourier's Law for heat conduction.
Convection
The transfer of heat between system and surroundings, due to fluid motion. The average temperature is take a reference for evaluating properties of the system related to convective heat transfer.
Radiation
The transfer of heat by the emission of electromagnetic radiation.
Advection
By transferring matter, heat including thermal energy is moved by the physical transfer of a hot or cold system from one place to another place. E.g placing hot water in a bottle and heating a bed, or the movement of an iceberg in changing ocean currents. A practical example of Advection is thermal hydraulics.
Conduction
In conduction mode, heat conduction occurs as hot, rapidly moving or vibrating one atoms and one molecules interact with neighboring another atoms and another molecules, transferring some of their heat to these neighboring particles. In other words, heat is transferred by conduction when adjacent one atoms vibrate against one another atom, or as electrons move from one atom to another atom. Conduction is the heat transfer within a solid or between solid body in thermal contact. Fluids specially gases are less conductive. Thermal conduction is the study of heat conduction between two solid bodies which in contact.
The process of heat transfer from one place to another place without the movement of a particles is called conduction, e.g when placing a hand on a cold glass of water, heat is conducted from the warm skin of hand to the cold glass of water. Steady state conduction is an idealized model of conduction that happens when the temperature difference betweem them is similar so that means the conduction is constant, so after, conducting of system does not change any further e.g see Fourier's law.
In steady state conduction, the amount of heat of system is equal to amount of heat of a surrounding, since the change in temperature fo a system is zero. E.g steady state conduction is the heat flow from walls of a warm house to cold inside of a house is maintained at a high temperature, and outside of a temperature is at low condition. After long time, wall of house insulated, so transfer of heat or temperature is constant . Means inside and outside of the house is in same temperature
Transient conduction e.g Heat equation occurs when the temperature within an system changes with a change in time. To Analysis of transient systems is more complicated than others modes, and solutions of the heat equation are only valid to the model systems. Applications of this system are generally investigated using numerical methods, approximation techniques.
Convection
The heat transfer may be forced by external processes, or sometimes in gravitational fields by buoyancy forces caused when heat expands in the fluid, e.g in a fire plume, the transfer of heat done by naturelly. This process is called as natural convection. All convection processes, heat transfer by diffusion mode as well. Another type of convection is forced convection. In this force convection process, the heat is transfer by forced to flow by using a pump, fan or other mechanical is called as force convections.
Convective heat transfer, or convection, is the transfer of heat from one place to another of a system by the movement of fluids. Bulk motion of fluid enhances heat transfer rate , such as e.g between a solid surface and the fluid. Convection is usually popular heat transfer in liquids and gases. Third method of heat transfer, convection mode is usually used to describe the combined effects of heat conduction within the fluid or diffusion and heat transference by bulk fluid flow. The process of transfer by fluid is known as advection. Advection is associated only with mass transfer in fluids, e.g advection of pebbles in a river. In the case of heat transfer in fluids, where transfer by advection in a fluid is always also accompanied by heat diffusion this process is also known as heat conduction. The process of heat convection is the sum of heat transport by advection and diffusion/conduction.
Free, or natural, convection occurs when bulk fluid motions e.g streams and currents are happened by buoyancy forces that result from
change in desnsity due to variation of temperature in the fluid. In forced convection, when the streams and currents in the fluid are induced by external force means such as fans, stirrers, and pumps creating an artificially induced convection current.
Nusselt number
Nusselt Number is also called as Convection cooling, is sometimes described as Newton's law of cooling:
The rate of heat loss of a body is directly proportional to the temperature difference between the body and its surroundings.
However, Newton's law of Cooling requires that the rate of heat loss from convection be a linear function ("proportional to") to the temperature difference that drives heat transfer, and in convection cooling this is sometimes not the case. In general, convection is not only dependent on temperature gradients, and in some cases is strongly nonlinear function. In these cases, Newton's law does not apply.
Convection vs. conduction
The Rayleigh number is define as the ratio of the rate of heat transfer by convection to the rate of heat transfer by conduction. This can be seen as follows, where all calculations are up to numerical factors depending on the geometry of the system.
Convection occurs when the Rayleigh number is above 1,000–2,000.
Radiation
 |
| solar flare |
Thermal radiation defined as a vacuum or any transparent medium (e.g solid or fluid or gas). It is the transfer of energy by means of photons in electromagnetic waves governed by the same laws.
Thermal radiation is energy emitted by matter as electromagnetic waves, in thermal energy, all matter with a temperature above absolute zero. Thermal radiation required matter through the vacuum of space to creat thermal radiation.
Thermal radiation is formed by random movements of atoms and molecules in matter. Since these atoms and molecules are form of charged particles (e.g protons and electrons), their movement of charged particles results in the emission of electromagnetic radiation, which carries energy away from the surface.
For radiative transfer between two objects, the equation is as follows:
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 convert into heat and power. Unlike conductive and convective forms of heat transfer, thermal radiation creat within a narrow angle i.e. coming from a source is much smaller than its distance, it can be concentrated in a small spot by using reflecting mirrors, which is used in concentrating solar power generation or a burning glass. E.g the sunlight reflected from mirrors heats the PS10 solar power tower and during the day it can heat water to 285 °C (545 °F).
The temperature of the hot source of radiation is limited . T4-law lets the reverse-flow of radiation back to the source rise. On its surface somewhat 4000 K hot sun allows to reach coarsly 3000 K at a small probe in the focus spot of a big concave, concentrating mirror of the Mont-Louis Solar Furnace in France.
Phase transition
Lightning is a highly visible form of heat transfer and is an example of plasma present at Earth's surface. E.g Typically, lightning discharges 30,000 amp 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 k (27,726.85 °C or 49,940.33 °F) and electron densities may exceed 1024 m^3.
Phase transition or phase change can takes place in a thermodynamic system from one phase to another one by heat transfer. E.g the melting of ice or the boiling of water. The Mason equation explains that the growth of a water droplet based on the effects of heat transport on evaporation and condensation.
Phase transitions has a four fundamental states of matter
Solid – Deposition, freezing and solid to solid transformation.
Gas – Boiling / evaporation, recombination / deionization, and sublimation.
Liquid – Condensation and melting / fusion.
Plasma – Ionization.
Boiling
Nucleate boiling of water.
The boiling point of a substance is define as temperature and vapour pressure of the liquid equals the pressure surrounding the liquid and the liquid evaporates resulting in an change in vapour volume.
Saturation temperature is also called as boiling point. The saturation temperature is the temperature of saturation pressure at which a liquid boils into its vapour phase. The liquid can be said to be saturated with thermal energy. Any addition of thermal energy results in a phase transition.
At atmospheric pressure and low temperatures, no boiling occurs in liquid and the heat transfer rate is controlled by single-phase. As the surface temperature is increased, local boiling occurs in liquild phase and vapour bubbles nucleate, grow into the surrounding cooler fluid, and collapse. This method called as subcooled nucleate boiling, and is a very efficient heat transfer. At high vapour 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 similar atmospheric pressure and high temperatures, the hydrodynamically quieter regime of liquid film boiling is reached. Heat fluxes and the stable vapor layers are low, but rise slowly with temperature. Any contact between fluid and the surface that may be occurs extremely rapid nucleation of a fresh vapor layer e.g"spontaneous nucleation". At higher temperatures still, a maximum in the heat flux is reached (the critical heat flux, or CHF).
The Leidenfrost Effect demonstrated nucleate boiling slows heat transfer because of 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
when a vapor is cooled and changes its phase to a liquid, this process is called as condensation process. During condensation process, the latent heat of vaporization must be released. The amount of the heat absorbed during vaporization at the same fluid pressure.
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 to condesation process. when a liquid film is formed on the subcooled surface, and when the liquid wets the surface. This process called as filmwise condensation
Dropwise condensation is defined as when liquid drops are formed on the subcooled surface and when the liquid does not wet the surface.
Dropwise condensation is difficult process for condensation. therefore, industrial equipment is normally designed to operate in filmwise condensation mode.
Melting
 |
| melting of ice |
Ice melting
Melting is a thermal process which is phase transition of a substance from a solid to a liquid. The internal energy of system is increased, by using heat or pressure, resulting in a rise of its temperature to the melting point, Molten substances generally have reduced viscosity with elevated temperature, also have 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 equation
The heat equation is an important partial differential equation that describes change in heat or variation in temperature in a given region over time. In some cases, exact solutions of the equation are available and other cases the equation must be solved by numerically using computational methods such as DEM based models for thermal/reacting particulate systems as critically reviewed by Peng et al.
Lumped system analysis
Lumped system analysis method reduces the complexity of the equations to convert one first order linear differential equation, in which case heating and cooling are denoted by a simple exponential solution, this system is called as Newton's law of cooling.
The lumped capacitance model is a common approximation in transient conduction that may be used whenever heat conduction to system analyze method within an system or substance is much faster than heat conduction across the boundary of the substance. This is a method is approximation that reduces one aspect of the transient conduction system that an equivalent steady state system. This method assume that the temperature within the system is constant, although its value may be changing with time.
In this method, the ratio of the conductive heat resistance of a system to the convective heat transfer resistance across the system boundary, known as the Biot number, is calculated. For small Biot numbers, spatially uniform temperature within the object can be used. it can be presumed that heat transferred into the system has time to uniformly distribute system, due to the lower resistance to doing so, as compared with the resistance to heat entering the system.
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.
Engineering
Heat transfer has many application to the functioning of numerous devices and systems in Engineering field. Heat-transfer principles used for constant, increase, or decrease temperature in applications. Heat transfer methods are used in many branches, such as automotive engineering, thermal management of electronic devices and systems, climate control, insulation, materials processing, and power station engineerinng
Insulation, radiance and resistance
Thermal insulators are materials that used to reduce the flow of heat by limiting conduction, convection, or both. Thermal resistance is a heat property and the measurement of heat flow of a system or material, heat per time unit or thermal resistance to temperature difference.
Radiance or spectral radiance are measured of a radiation that passes through or is emitted. Radiant barriers are materials that are reflect radiation, and used to reduce the flow of heat from radiation sources. That means Good insulators are not necessarily good radiant barriers, and vice versa. E.g 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. So 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. E.g Vacuum flasks, or Dewars, are silvered to approach this ideal. In a space, satellites use multi-layer insulation, for insulation many layers of aluminized mylar are used to greatly reduce radiation heat transfer and control satellite temperature.
Devices
A heat engine is a system that performs the convert of thermal energy or heat into mechanical energy to perform mechanical work.
A thermocouple is a temperature measuring device which is widely used type of temperature sensor for measurement. Thermocouple used to control temperature, and can also be used to convert heat into electric power.
A thermoelectric cooler is a solid state electronic device that transfer heat from one side of the device to the other of a device when electric current is passed through it. It is based on the Peltier effect.
A thermal diode or thermal rectifier is a device that causes heat to flow in one direction.
Heat exchangers
A heat exchanger is used for more efficient heat transfer or to dissipate heat. Heat exchangers are used in refrigeration, air conditioning, space heating, power generation, and chemical processing, these are the application of the heat exchanger. For example of a car's radiator, in which the hot coolant fluid is cooled by the flow of air over the radiator's surface.
Types of heat exchangers are parallel flow, counter flow, and cross flow. In parallel flow, fluids move in the same direction during transferring heat. In counter flow, the fluids move in opposite directions during heat transfer and in cross flow, the fluids move at right angles to each other. Other types of heat exchangers include shell and tube, double pipe, extruded finned pipe, spiral fin pipe, u-tube, and stacked plate, these the heat exchanger types. Every type has advantages and disadvantages over other types.
A heat sink is a heat transfer device that transfers heat solid medium to fluid medium, such as air or a liquid. For example, 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 medium.
Applications
Architecture
Climate engineering
Climate engineering consists of carbon dioxide removal and solar radiation management, these are the problem to solve in climate engineering. Since the amount of carbon dioxide can be solved by the radiative balance of Earth atmosphere, carbon dioxide removal techniques can be applied to reduce the radiative forcing. Solar radiation management is absorb less solar radiation to offset the effects of greenhouse gases.
Cooling techniques
Evaporative cooling
when water vapor is added to the surrounding air, this process called as Evaporative cooling. In evaporate, the water is taken from the air in the form of sensible heat and converted into latent heat, during this process, air remains at a constant enthalpy. In Latent heat, the amount of heat that is needed to evaporate the liquid, this heat come out from the liquid itself and the surrounding gas and surfaces. Maximium the difference between the two temperatures, maximium the evaporative cooling effect. When the diffirence temperatures is zero, no net evaporation of water in air occurs; thus, there is no cooling effect.
Laser cooling
Laser cooling is used in quantum physics to achieve temperatures of near absolute zero (−273.15 °C, −459.67 °F) of atomic and molecular, to observe unique quantum effects that can only cause at this heat level.
Doppler cooling is the most common method used in the laser cooling.
In Sympathetic cooling process, particles of one type cool particles of another type. By this
process 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, they cooled by this method or process.
Magnetic cooling
Magnetic refrigeration and Magnetic evaporative cooling
Magnetic evaporative cooling is a process for minimize temperature of a group of atoms, which already cooled by methods such as laser cooling. Magnetic refrigeration cools below 0.3K, by making use of the magnetocaloric effect.
Radiative cooling
In Radiative cooling process, a body loses heat by radiation. Outgoing system is an important effect in the Earth's energy budget. In the case of the Earth atmosphere system, the process by which long wave radiation is emitted to balance the absorption of short-wave (visible) energy from the Sun. Convective transfer of heat and evaporative transfer of latent heat both remove heat from the surface and redistribute it in the atmosphere.
Thermal energy storage
This type of technologies used to collecting and storing energy for later purpose. For example, it may be used to balance energy demand between day and nighttime. The thermal reservoir may be maintained at a temperature above or below that of the ambient environment. Applications include space heating, domestic or process hot water systems, or generating electricity
1 Comments
demek girma
ReplyDelete