Radiation Heat Transfer – Calculations and Explanation

Heat transfer,s a very important topic and there are lots of kinds of applications areas. Also, there are three mechanisms of heat transfer; conduction, convection, and radiation. Here, we will give in-depth information about radiation heat transfer. Thermal radiation is a very important topic in engineering.

By using the general heat transfer mechanisms in nature, engineers have developed lots of useful systems to make people’s life easier. One of these heat transfer mechanisms is radiation heat transfer. People may think radiation is related to radioactivity. But it is not like this. In radioactivity, harmful rays are emitted from radioactive matter. But in radiation heat transfer, heat is radiated or emitted from the heat source. In this article, we would like to elaborate on the radiation heat transfer mechanism. 

What is Radiation Heat Transfer?

The radiation heat transfer mechanism is not like other mechanisms which depend on mechanical situations. In the radiation heat transfer mechanism, heat is transferred via electromagnetic way with photons. So, there is no requirement for an intervening medium. Radiating thermal energy can be transferred in the vacuumed medium. The biggest example of radiation heat transfer mechanism is heat that is coming from the Son to Earth. 

In theory, all bodies emit some level of radiation if the body is above absolute zero. And radiation is a volumetric phenomenon, not a surface phenomenon. 

With the radiation heat transfer, we can understand that a particle can lose heat energy, even if we put it into a vacuum.

Radiation and Electromagnetic Spectrum

To understand the radiation heat transfer, we need to understand the working principle of the electromagnetic spectrum. You probably know from the chemistry lessons.

In the electromagnetic spectrum, there are different kinds of waves of rays. The classification and difference of waves are made according to their place in the spectrum. So, the wavelength is very important here.

The electromagnetic spectrum starts with ultraviolet waves. Infrared waves have the shortest wavelengths and highest energy-carrying capacities. Around the infrared waves, gamma rays and X-rays are the highest energy and shortest wavelength ones. In general, nuclear reactions creates gamma rays. And also, we can produce X-rays with the bombardment of the metals with electrons.

The electromagnetic spectrum has also a visible light spectrum. The human eye can sense this spectrum as colors. This spectrum starts with blue light and ends with red light. The total spectrum is between 0.40 to 0.76 micrometers.

After the visible section of the spectrum, the infrared spectrum starts. These radiation waves are lowest in energy but highest in wavelength. Around the infrared waves, microwave and radio waves. In general, we use these types of waves to transfer information between the long ranges.

Solar radiation also takes place because of the sun. Solar ways include various kinds of waves from the electromagnetic spectrum.

Thermal Radiation

After understanding the general principles of the electromagnetic spectrum, we can delve into the thermal radiation phenomenon. With electromagnetic waves, objects generally emit thermal radiation because of their temperature. All the objects emit and absorb thermal radiation up to a level if their temperature is higher than absolute zero.

For example, the temperature that comes from the Sun is because of the thermal radiation. With convection or conduction, the heat of the Suın can not come to the surface of the Earth.

Also, we assume that thermal radiation falls inside a portion of the electromagnetic spectrum. And the wavelength of the thermal radiation is between 0.1 to 100 micrometers.

Blackbodies in Radiation Heat Transfer

Blackbodies are very important theoretical assumptions in radiation heat transfer. Because they are the objects that absorb all the thermal radiation that comes to them. And also, blackbodies are ideal thermal radiation emitters. So, there are no objects that emit radiation like blackbodies. Furthermore, there are no better objects that absorb radiation better than the blackbodies.

As its name states, blackbodies appear as black to the naked eye. And also, objects that reflect all the thermal radiation coming to them appear white to the observer. But, we need to state that we can not make any assessments about the level of blackness and whiteness of the objects.

Stefan-Boltzman Law of Radiation Heat Transfer

Heat transfer mechanisms have formulations that are heavily used in engineering designs of thermal systems. Radiation heat transfer have also a formulation which is called as Stefan-Boltzman Law. With the Stefan-Boltzman Law, the maximum radiation rate that is emitted from a surface is calculated. Check the formula below; 

In this formula, ‘σ’ is the Stefan-Boltzman constant which equals 5.670×10^-8 W/m^2.K^4. or in English units, 0.1714×10^8 Btu/h·ft^2·R^4. 

‘As’ is the area of the surface where the maximum radiation is calculated. ‘Ts’ is the maximum temperature of the surface. 

This is the thermal radiation formula for blackbody radiation. As you see above, this is the maximum energy that a blackbody can emit.

Important Rules about Thermal Radiations

This information is about the thermal radiations that we need to know the understand the general working principle.

Firstly, total thermal radiation increases with the increasing wavelength of the radiation. After a level, radiation reaches its peak. And it starts to decline with the increasing wavelength.

And also, the total radiation heat transfer increases with the increasing body temperature. Think about the Sun. It sends its thermal energy through the 150 million kilometers of distance with the thermal radiation. If the Sun was not hot enough, the Earth would be very hot.

Furthermore, with the increasing temperatures, the total wavelength of the thermal radiation decreases. Because the shorter wavelengths are better at energy carrying. To carry the higher thermal energies, the wavelengths must be shorter.

The temperature of the Surroundings on Radiation Heat Transfer

The calculations above related to the radiation heat transfer mechanism are related to maximum and ideal conditions. But the temperature of the surroundings affects the radiation heat transfer. To add the effects of surroundings, we need to add the temperature of surroundings into the Stefan-Boltzman Law. 

As you see above, with the increasing difference between the surface and the surrounding temperatures, the total thermal radiation emission increases. In the general calculations, we use this equation in general.

Also as you see above, there is an extra notation in this equation. ϵ is the emissivities of objects that we use in the calculations.

Emissivity in Thermal Radiation

So, emissivity is a very important term in thermal radiation calculations. We use this parameter in the calculations for different objects. So, it is very important to know the emissivity and its importance.

Emissivity is the ratio of the thermal radiation emission of an object to a blackbody at a specific temperature and wavelength. So, it gives an idea about how much a body is that close to a blackbody.

So, this value is between 0 and 1. If the emissivity is 1, it means it is a blackbody. As you see the first formula that we have given above, did not include the emissivity factor. Because we use that formula for blackbodies.

Also, if the emissivity is 0, the object is a completely white body. So, the total thermal radiation is 0 which is completely impossible.

In nature, there are no complete blackbodies or white bodies that completely reflect or absorb the thermal energy.

Also, there are different types of emissivity calculations that we use in the theoric calculations.

Directional Emissivity

This is the type of emissivity that we calculate in a specific direction and specific temperature. Because the emissivities of objects can change according to the calculation directions. So, for some calculations, it can be more correct to use directional emissivity values.

So, we can say that the directional emissivity is the ratio between the emission of radiation between blackbodies and objects.

Hemispherical Emissivity

This is another theoretical assumption of the emissivity values. In general thermal radiation applications, we generally use hemispherical emissivity calculations. Because we need more average values in the general calculations.

Hemispherical emissivity values are also the values that give the average emissivity value in each direction. So in general, we use this value in our calculations.

And also, hemispherical emissivity is the ratio of the average raidatiın heat transfer of an object to the blackbody.

Absorptivity in Thermal Radiation

This is also a very important term in radiation heat transfer calculations. We generally use absorptivity instead of emissivity.

Absorbtivity is the ability to absorb of thermal radiation of an object. We generally calculate it by proportioning the total thermal radiation that comes to the body to the total thermal radiation that this object absorbs from it.

Absorptivity has also a value between 0 and 1. 0 absorptivity means, the body does not absorb any radiation heat transfer. 1 absorptivity means the body completely absorbs the thermal radiation like a blackbody.

Reflectivity

Reflectivity is also another important term that we use for the objects that make radiation heat transfer.

This is the ratio of the thermal radiation that a body reflects the total radiation heat transfer. So, it is a very important factor in the assessment of the bodies.

Also, reflectivity values take values between 0 and 1. So, if the reflectivity takes the value of 0, the body does not reflect any thermal radiation that comes to it. Also, if the reflectivity is 1, the total radiation that comes to a body is reflected like a white body.

Transmissivity

Transmissivity is also a very important term in radiation heat transfer calculations. We use the transmissivity value in the calculations of different kinds of aspects.

Transmissivity is the ratio of the total transmission of thermal radiation to the total incident radiation coming to the body. If the value is 1, all the coming thermal radiation is transmitted by the body. And also, if the value is 0, zero radiation heat transfer is transmission by the body.

Also for these three additional terms; transmissivity, reflectivity, and absorbtivity, there are directional and hemispherical values just like emissivity.

Radiosity

This is also another important term related to the total thermal radiation from a surface of an object.

They generally state that radiosity as the total radiation heat transfer from a unit surface area is the radiosity of the objects. So, this is a dependent value on the temperature of the body and other effects.

Total Heat Transfer

In the most complex and the most comprehensive heat transfer calculations, we calculate the total heat transfer by radiation, convection, and conduction. With the consideration of all these heat transfers, we can calculate the total amount of heat transfer between the systems.

So, we need to add the radiation heat transfer effect in the calculations. But sometimes, heat transfer with radiation can be very minimal if we compare it with conduction or convection. In this case, we generally omit the thermal radiation from the heat transfer calculations.

Thermal Radiation Shields

Thermal radiation shields are also very common applications to prevent heat rise due to radiation heat transfers. We generally use insulations to prevent conduction and convection heat transfers. But also, we need to apply radiation shields to prevent it.

Radiation shields are the protective surfaces between the radiation surface and the taking surface. They are made from materials that have low emissivities and high reflectivities.

The use of radiation shields is very important in space applications. Because all the thermal effects in space are radiation that comes from the Sun. And some applications, need thermal insulation to prevent these thermal radiations. So, the use of thermal radiation shields is very common in space.

Gases in Thermal Radiation

Gases also have very different characteristics on thermal radiations.

The most important feature of gases in radiation heat transfer, they emit and absorb thermal radiation on a volumetric basis. For solids, we think about the temperature and the reflectivity and absorbtivities of the surface of the objects. But in gases, we need to consider these values in all the volumes.

Another important fact is that gases emit and absorb thermal radiation in a very narrow spectrum. They are not emitting like solid objects in all the spectrums. So, we need a very different approach to calculate the thermal radiation of gases.

Also, we need to know the temperature, pressure, and constituent gases to accurately calculate the thermal radiation. So, it is very important to know these values of gases.

Greenhouse Effect of Thermal Radiation

The greenhouse effect is also very important to describe in terms of radiation heat transfer. The absorption of thermal radiation leads to this greenhouse effect problem in general. We need to be deep concern about the greenhouse effect in the world.

Remember from the interioı space of cars gets very high in temperature when you leave it beneath the sun. Because the heat energy that comes from the Sun is trapped inside the car. Thermal radiation gets inside the car because of the transmissivity of the glass. And trapped inside the car because of the reflectivity.

So, the temperature in the car becomes very high if we compare it with the outside.

Also, we call this the greenhouse effect, but greenhouses are also very important examples. Farmers build a greenhouse with high transmissivity and high reflectivity materials such as transparent nylon shelters. So, the solar heat energy comes and passes through this shelter. And, a portion of the thermal radiation is trapped inside the shelter, and the temperature of the shelter increases.

Farmers are using this effect to grow vegetables and plants that are impossible to grow in the winter or cold environments.

Environment and Global Warming

Greenhouse effects are also very important on the earth. Solar radiation comes from the sun in the daytime. And a portion of this thermal radiation is trapped inside the atmosphere. This trapping effect is generally because of the carbon dioxide and moisture in the air. And, the earth can not reflect the excess thermal radiation at night because of these gases.

Normally, during the night, the earth radiates this heat energy to space. But because of the excessive carbon dioxide and other greenhouse gases, thermal radiation comes from the sun trapped in the atmosphere.

It is a very concerning issue in terms of the environment. The atmosphere gets warmer and warmer because of these effects. So, we need to decrease our carbon footprints. The less we emit carbon into the atmosphere, the more the atmosphere gets warmed because of the greenhouse effect.

Also, there are very big studies and works to decrease the use of fossil fuels in different fields to reduce carbon emissions. We are always a supporter of these studies and works.

Practical Applications of Radiation Heat Transfer

In today’s technology, we use radiation heat transfer commonly to obtain practical and useful systems. Here, you can find these engineering applications.

Coloured Clothes in Summer

People that know the thermal radiation facts will not prefer dark colors in the summer. Because the absorptivities of darker colors are higher than the bright colors. This means extra heat rises on the cloth you wear in summer. Nearly all the heat that comes from Sun is thermal radiation.

Even white shirts will be more comfortable in the summer in terms of temperature.

Car Radiators

Another important application of the radiation effect of different colors is the car radiators. If you take a look at the car radiators, you will see that the general colors of the radiators are black. So, this provides maximum heat emission from the radiator body for better cooling of the fluid inside it.

Greenhouses

We talked about the greenhouses and greenhouse effects of solar radiation. Greenhouses are very useful systems to produce extra fruits and vegetables in the winter or cold climates. They have nylon shelters that transmit solar radiation inside the greenhouse. And also, their reflectance is very good. And the thermal radiation stays trapped inside the greenhouse which increases the temperature.

Also, use the of heaters and stoves is very common in greenhouses. With the rays of the Sun and high temperature inside the greenhouse, the summer plants will grow very rapidly.

Temperature Measurement with Infrared Radiation

Also, infrared radiation measurement systems are very common in technology. These devices are measuring the thermal radiation that comes from bodies. And with this radiation, they create temperature data of bodies.

The use of temperature measurement devices is very common. You just need to hold the device on the object to measure the temperatures.

Thermal Radiation Insulators

Insulation systems are effective in conduction and convection heat transfers. But in general, they are not effective to prevent thermal radiation.

Also, they need to use coating materials that have very big reflectance values around the objects to be cold. So, these materials will reflect the thermal radiation effects to prevent the heat rise inside them.

Infrared Heaters

One of the biggest examples of radiation heat transfer mechanisms is used in infrared heaters. In these infrared heaters, a radiation heat transfer mechanism is used. 

Solar Panels

Another very important technology that the radiation heat transfer from the Sun is used solar panels. Solar panels are designed to absorb Sun’s radiation heat as much as possible. So this heat energy can be transformed into other kinds of useful energy. And it is a very clean and safe method of generation of energy compared with the use of fossil fuels. Fossil fuels are very harmful to nature and have no other alternative for us. 

Conclusion on Radiation Heat Transfer

So, radiation heat transfer is also a very important heat transfer mechanism that we need to calculate to see the outcomes. This mechanism is quite different from the other heat transfer mechanisms. Because this mechanism does not need the existence of materials.

In the calculation of thermal radiation, the assumption of the blackbody is very important. They are the bodies that absorb all the thermal radiation that comes to them. Also, they radiate the maximum value of thermal radiation from them. So, we generally make our calculations upon this idealization.

Also, there are very important terms for radiation heat transfer. And there are lots of kinds of applications in which we use the thermal radiation rule. These applications are very useful engineering systems that we use in the latest technologies.

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