Basic Heat Transfer Anthony F Mills (Author), Carlos F. M. Coimbra
Basic heat transfer is the process by which thermal energy is transferred from one body to another. The rate of heat transfer is determined by the difference in temperature between the two bodies and the properties of the intervening medium. In most cases, heat transfer occurs via conduction, convection, or radiation.
Conduction is the transfer of thermal energy through direct contact between molecules. When two objects are in contact with each other, their molecules interact and energy is transferred from the hotter object to the cooler object. The rate of heat transfer by conduction depends on the conductivity of the material and the area of contact between the two objects.
Convection is the transfer of thermal energy by means of fluid flow. When a hot fluid comes into contact with a cold surface, heat is transferred from the fluid to the surface. The rate of convection depends on both the properties of the fluid and on how it is moving relative to the surface.
Radiation is electromagnetic waves emitted from a hot body that are absorbed by a colder body. Radiation can occur even in a vacuum, making it an important mode of heat transfer in space applications.
What is the Basic Mechanism of Heat Transfer
There are three primary mechanisms of heat transfer: conduction, convection, and radiation. All three methods involve the transfer of energy from a warmer object to a cooler one.
-Conduction is the direct transfer of heat between two objects that are in physical contact with each other.
The molecules in the warmer object vibrate more rapidly than those in the cooler object, and these vibrations are transferred directly to the cooler object. Metals are particularly good conductors of heat, which is why metal pans are often used for cooking.
-Convection is the transfer of heat via fluid motion.
When a fluid (liquid or gas) is heated, it becomes less dense and rises. This movement creates convection currents that circulate warm fluids away from the heat source and colder fluids back towards it. Convection can be natural, like when warm air rises and cold air sinks in Earth’s atmosphere, or it can be manmade, like when water is heated in a radiator.
-Radiation is the transfer of heat via electromagnetic waves. All objects emit radiation (even our bodies!) but some objects emit more than others. The hotter an object is, the more radiation it emits.
Radiation can travel through empty space–that’s how we get sunlight on Earth!–but it can also be absorbed or reflected by other materials.
What are the Three Modes of Heat Transfer
The three modes of heat transfer are conduction, convection, and radiation. All three mechanisms involve the transfer of thermal energy from a warmer object to a cooler object.
Conduction is the transfer of heat through direct contact between particles.
When two objects are in contact with each other, heat will flow from the warmer object to the cooler object until both objects reach the same temperature. The rate of heat transfer depends on the material properties of the conductor (such as thermal conductivity), as well as the magnitude of the temperature difference between the two objects.
Convection is the transfer of heat via fluid motion.
When a fluid (liquid or gas) is heated, it expands and becomes less dense than its surroundings. This causes it to rise upwards while cooler, more dense fluid flows in to take its place. The result is a large-scale circulation pattern known as convection currents.
Convection can occur naturally (e.g., wind) or be induced by mechanical means (e.g., fans).
Radiation is the transfer of heat via electromagnetic waves. Unlike conduction and convection, which require matter to be in physical contact with each other, radiation can occur through a vacuum—making it an important mechanism for heat transfer in space!
Radiation transfers energy by absorbing and emitting electromagnetic waves; these waves carry energy away from their source (in this case, a hot object). The amount of radiation emitted by an object depends on its surface area and temperature; hotter objects emit more radiation than cooler ones.
What are the Differences between Conduction, Convection, And Radiation
Conduction is the transfer of heat between two objects that are in physical contact with each other. Convection is the transfer of heat between an object and a fluid (liquid or gas) that surrounds it. Radiation is the transfer of heat energy through electromagnetic waves.
How Do You Determine the Rate of Heat Transfer for a Given System
In order to determine the rate of heat transfer for a given system, one must first understand the meaning of heat transfer. Heat transfer is defined as the movement of thermal energy from one place to another. The three main types of heat transfer are conduction, convection, and radiation.
Conduction is the most common type of heat transfer and occurs when there is a difference in temperature between two objects that are in contact with each other. The hotter object will transfer its thermal energy to the cooler object until both objects reach equilibrium. The rate at which this happens depends on the materials involved, as well as the surface area in contact and the distance between them.
Convection occurs when hot fluid (liquid or gas) transfers its thermal energy to a colder substance by moving past it. This can happen either naturally, due to density differences (such as hot air rising), or artificially, such as with a fan. The rate at which convection occurs is determined by the speed of fluid movement, as well as the difference in temperature between the two substances.
Radiation is less common than conduction and convection but can be just as important in certain situations. It occurs when electromagnetic waves carry thermal energy from one place to another without requiring any physical contact between them. Radiation can travel through a vacuum, making it different from both conduction and convection.
The rate at which radiation occurs is determined by its wavelength; shorter wavelengths radiate more quickly than longer ones.
Now that we have reviewed all three modes of heat transfer, we can return to our original question: how do you determine the rate of heat transfer for a given system? As we now know, there are three factors that affect this: material properties, surface area (in contact), and temperature difference.
In most cases, determining an accurate value for all three of these variables is difficult if not impossible; however, there are some general guidelines that can be followed.
For solids: good conductor –> high rates of heat transfer; poor conductor –> low rates conductivity (insulator) –> very low rates
For fluids: high flow velocity –> high rates natural convection –> moderate rates forced convection –> high rates
What Factors Affect the Efficiency of Heat Transfer
There are three primary factors that affect the efficiency of heat transfer: conduction, convection, and radiation.
Conduction is the transfer of heat through solid matter, and is governed by the laws of thermodynamics. The rate of heat transfer is proportional to the temperature difference between the two objects in contact.
In other words, if two objects are at different temperatures, heat will flow from the hotter object to the colder object until they reach equilibrium. The efficiency of heat transfer via conduction can be increased by increasing the surface area of contact between the two objects, or by using a material with a high thermal conductivity (such as metal).
Convection is the transfer of heat through fluid motion (liquid or gas), and also follows the laws of thermodynamics.
When fluids are heated, they expand and become less dense than cooler fluids. This causes them to rise up while cooler fluids sink down, creating convection currents. The hot fluid then cools off as it transfers its heat to cooler surroundings, and the cycle repeats.
The efficiency of convective heat transfer can be increased by using a fluid with a high specific heat capacity (such as water), or by increasing turbulence (which makes it easier for hot fluid to mix with cold fluid).
Radiation is the transfer of energy via electromagnetic waves, and does not require any physical contact between objects.Hotter objects emit more radiation than colder objects, so radiation always flows from hot to cold. The efficiency of radiative heat transfer can be increased by using materials that are good at absorbing and/or emitting radiation (such as black paint), or by increasing the surface area exposed to radiation.
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Conclusion
This blog post covers the basics of heat transfer, discussing the three main types of heat transfer (conduction, convection, and radiation) and providing examples of each. The post also explains how different materials can affect heat transfer rates, and offers some tips for reducing heat loss in your home.
