Radiation Heat Transfer Explained in Easy Language - Core Chemical

Radiation Heat Transfer is the third and one of the most important modes of heat transfer we encounter in the real world. Radiation Heat Transfer has wide applications from high-temperature furnaces to solar energy-harnessing equipment. So learning about Radiation Heat Transfer becomes important for any engineer. 

In this article, we will learn the basics of Radiation Heat Transfer. We will start with the definition, then we will learn about working, then about governing equations and at last we will learn about applications of Radiation Heat Transfer. 

Radiation Heat Transfer

Definition of Radiation Heat Transfer

Radiation heat transfer is a method of heat transfer that occurs through the emission, absorption, and transmission of electromagnetic waves, or photons, without the need for a material medium. This type of heat transfer can take place in a vacuum also. For example, Solar Radiation is one type of Radiation Heat transferred from the Sun reaches Earth as there is no material in the space between Earth and the sun. 

This process is distinct from conduction and convection, as it can take place in a vacuum and involves the exchange of thermal energy between objects at different temperatures.  In short, Conduction and Convection require a medium between hot and cold objects for heat transfer, but in Radiation, no medium is needed. 

Three Important Terminologies Related to Radiation Heat Transfer

Radiation heat transfer takes place through the emission, absorption, and transmission of electromagnetic waves, or photons. Here's a breakdown of how it occurs:

• Emission and Emissivity (ε): Any object with a temperature above absolute zero emits thermal radiation. This emission is a result of the thermal motion of particles within the object, causing them to release photons. The amount and type of radiation depend on the temperature and emissivity of the object. Emissivity is a measure of how efficiently an object can emit radiation.
• Absorption and Absorptivity (α): When radiation encounters another object, it can be absorbed if the material has the appropriate properties. The ability of a material to absorb radiation depends on factors such as its composition and colour. Dark, matte surfaces typically absorb more radiation than light, reflective surfaces. Absorptivity is a measure of how efficiently an object can absorb radiation.
• Transmission and Transmisivity (τ): Radiation can also pass through a material if it is transparent to the specific wavelengths of radiation. Materials that allow radiation to pass through are said to be transparent to those wavelengths. For example, glass is transparent to visible light but may absorb infrared radiation. Tramissivity is a measure of how efficiently an object can transmit radiation.
• Reflection and Reflectivity (ρ): Reflection refers to the process by which thermal radiation encounters a surface and bounces back into the medium it originated from. Reflectivity is a property that quantifies the ability of a surface to reflect thermal radiation.

Formula of Radiation Heat Transfer

Stefan-Boltzmann Law for Black Body

The Stefan-Boltzmann Law is an equation that can be used to find out energy radiated by a perfect black body (emissivity: 1) as a function of the area of the black body and temperature. The law is named after the physicists Josef Stefan and Ludwig Boltzmann, who independently formulated it in the late 19th century.

The radiation heat transfer between two black body bodies by the Stefan-Boltzmann Law can be expressed by the following equation:

$Q = \sigma ·A·({T_1}^4 - {T_2}^4)$

Where: 

• Q is the rate of heat transfer (energy per unit time),
• σ is the Stefan-Boltzmann constant (5.67×10−8 W m−2 K−4)
• A is the surface area of one of the black bodies (assuming both bodies have the same area),
• T1​ and T2​ are the absolute temperatures of the two black bodies in Kelvin.

Stefan-Boltzmann Equation for Real Body of Emissivity (ε  < 1)

This equation can be written for the real body of emissivity (ε  < 1) as: 

$Q = \epsilon ·\sigma ·A·({T_1}^4 - {T_2}^4)$

Application of Radiation Heat Transfer

Radiation heat transfer finds numerous industrial applications due to its ability to transfer heat through electromagnetic waves without the need for a material medium. Here are some notable industrial applications:

• Industrial Furnaces: In a furnace, thermal radiation heat transfer is the process by which high temperatures generated within the furnace lead to the emission of electromagnetic waves (thermal radiation). This radiation is crucial for heating materials uniformly and facilitating various industrial processes. The furnace walls and any materials inside absorb this radiation, causing an increase in temperature. This process is fundamental to heat treatment, melting, and smelting applications. 
• Drying Processes: In industries such as food processing, paper manufacturing, and textiles, radiation heat transfer is used in drying processes. 
• Ceramic and Glass Manufacturing: Radiation is crucial in the production of ceramics and glass. Kilns and furnaces utilize thermal radiation for heating and shaping raw materials, helping to create a wide range of products from glassware to ceramics.
• Solar Energy Systems: Solar thermal systems harness radiation from the sun to generate heat for various industrial processes, including steam production and power generation. Concentrated solar power (CSP) systems focus sunlight to create high-temperature heat for electricity generation.
• Medical Sterilization: Radiation heat transfer is utilized in medical sterilization processes. Equipment such as autoclaves and sterilization chambers use thermal radiation to kill bacteria and pathogens on medical instruments.