Radiation Heat Transfer

By Mohit Uniyal|Updated : June 16th, 2022

Before we can understand radiation heat transfer, we must first understand the concept of heat and heat transfer. The transfer of thermal energy across a well-defined boundary around a thermodynamic system is characterized as heat in physics. The amount of work that a thermodynamic system can perform is defined as its thermodynamic free energy. Heat transfer, unlike state functions, is a process function (or path function); thus, the quantity of heat transferred in a thermodynamic process that changes the state of a system depends on how that process occurs, not just the net difference between the initial and final states of the process. We will see the characteristics of radiation heat transfer based on this property as well.

The transfer of energy via thermal radiation, i.e. electromagnetic waves, is known as radiation heat transfer. It can happen in a vacuum or any transparent substance (solid or fluid or gas). At temperatures above absolute zero, thermal radiation is emitted by all objects due to the random movement of atoms and molecules in matter. Because these atoms and molecules are made up of charged particles (protons and electrons), their movement causes electromagnetic radiation to be emitted, which transfers energy away. In most engineering applications, radiation is only relevant for very hot items or objects with a huge temperature difference. Let's look more closely at the fundamentals of radiation heat transfer, as well as the properties.

Table of Content

What is Radiation Heat Transfer?

The process of transferring heat through Electromagnetic Radiation, which is generated by the thermal motion of particles in matter, is known as thermal radiation. This particular electromagnetic radiation is found in the infrared area of the spectrum, which is invisible to most bodies on Earth. Thermal Radiation is emitted by every particle having a temperature greater than absolute zero. The mobility of particles inside the body is what causes thermal radiation. This type of motion is completely stopped at absolute zero, which is why a body at absolute zero does not admit any radiation whereas anything above absolute zero does.

Radiation heat loss is the difference between the energy state that existed before and the energy that is radiated out after it. Thermal radiation is largely responsible for the glowing quality of hot objects, with iron being referred to as "red hot" since most of the thermal energy emitted by the body at that temperature falls in the red band of the spectrum. It will begin to emit a distinct colour at even higher temperatures. It's an interesting story about how heat radiation was discovered. William Herschel, an English astronomer, discovered it. When he moved a thermometer from one aperture of a prism spectrum to another.

Properties of Radiation Heat Transfer

Radiation is the energy emitted by matter in the form of electromagnetic waves as a result of changes in the electronic configurations of atoms and molecules. Thermal radiation or Radiation heat transfer is defined by four key characteristics.

  • At any temperature, a body's thermal radiation emits a wide variety of frequencies. Planck's equation of black-body radiation gives the frequency distribution for an idealized emitter.
  • As the temperature of the emitter rises, the primary frequency (or colour) range of the emitted radiation moves to higher frequencies. A red hot item, for example, radiates primarily in the visible band's long wavelengths (red and orange). It begins to produce detectable amounts of green and blue light as it heats up further, and the dispersion of frequencies over the visible range causes it to seem white to the human eye; it is white-hot. Even at a white-hot temperature of 2000 K, 99 per cent of the radiation's energy is still infrared. This is determined by Wien's displacement law.
  • As the temperature rises, the total amount of radiation of all frequencies increases sharply; it rises as T4, where T is the absolute temperature of the body.
  • Reciprocity describes how the rate of electromagnetic radiation released at a particular frequency is related to the quantity of absorption experienced by the source. As a result, a surface that absorbs more red light emits more red light when heated.

A Black Body Thermal Radiator's Subjective Color

William Herschel discovered that the temperature changed. The greatest temperature recorded fell below the visible light spectrum's red band. As a result, the term "infrared" was coined. Infrared waves, on the other hand, should not be mistaken for heat waves. The subjective colours in relation to temperature are as follows:

Temperature °C

Subjective Color

480 °C

faint red glow

580 °C

dark red

730 °C

bright red, slightly orange

930 °C

bright orange

1100 °C

pale yellowish-orange

1300 °C

yellowish-white

>1400 °C

white (When viewed from afar through the atmosphere, it appears yellowish.)

Stefan-Boltzmann Law

The entire amount of radiation (all wavelength range radiations) emitted by a body per unit area and time is known as the emissive power. The emissive power of a black body, according to Stefan- Boltzmann, is proportional to absolute temperature to the fourth power. 

Eb = σAT4

Where σ = Stefan-Boltzmann constant 

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FAQs on Radiation Heat Transfer

  • The Sun's heating of the Earth is an example of energy transfer by radiation. Another example is the use of an open-hearth fireplace to heat a room. The heat is radiated directly to the things in the room by the flames, coals, and hot bricks, with little of it absorbed by the intervening air.

  • The transfer of energy via electromagnetic waves is known as radiation heat transfer. Light, infrared, ultraviolet, radio waves, microwaves, and gamma rays are all examples of electromagnetic waves. Infrared radiation is the most common type of radiation we encounter in our daily lives when it comes to heat transfer.

  • Radiation is the process of releasing energy in the form of particles or waves. It can take the form of sound, heat, or light in general. Most people, on the other hand, use it to refer to electromagnetic radiation, which includes everything from radio waves to visible light and gamma waves.

  • Radiation is now employed to assist humanity in medical, academia, and industry, as well as to generate power. Radiation is also useful in agriculture, archaeology (carbon dating), space exploration, law enforcement, geology (including mining), and a variety of other fields.

  • Black, in particular, is faster than white at both absorption and emission of heat. The color of a person's skin has no bearing on their thermal emissivity. Thermal Radiation is absorbed and emitted perfectly by the skin. We would all appear black if we saw a human and passed him or her through infrared wavelengths. Even though the surrounding atmosphere is not that much cooler than our bodies, we tend to lose half of our energy through radiation. This is why people prefer to dress in white during the summer and darker colors during the winter.

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