Thermal Radiation - The Third Mode of Heat Transfer

Hello friends! Hope everything is going well during these critical times. We sincerely hope that everything returns to normal soon! In the past two posts we have covered two modes of heat transfer: conduction and convection. We learned about heat transfer from one solid to another solid and from a fluid (gas or liquid) and the surface the fluid moves along. We have also learned that heat transfers more quickly when the difference in temperature of the two objects is larger.

In today's post we will cover a third mode of heat transfer: thermal radiation. Thermal radiation is a type of electromagnetic radiation that produces no heat until it reaches and is absorbed by an object. The energy carried by electromagnetic energy then becomes heat. Thermal radiation travels through air (like visible light) without heating the air.

Thermal radiation is part of the nonvisible infrared rage of the electromagnetic spectrum. Now, we can ask, do humans give off thermal radiation. Yes, the answer is that we do give thermal radiation. In fact, very object that has a non-zero can give off thermal radiation. 

Thermal radiation is absorbed by a surface, and what is not absorbed (because the surface is reflective and/or because of the type of wavelength of the radiation), bounces to other surfaces which then absorb the rest of the energy. Colder objects absorb more energy than they emit and hotter objects radiate more energy than they absorb. 

Now, how does this connect to hydronic heating systems? Well, in order for this systems to deliver heat, they use both convection (fluid that moves along a surface) and radiant energy.