Lesson 6 - Radiation and Combined

In previous lessons, we learned about two forms of heat transfer: conduction and convection. In this lesson, we will explore a third heat transfer method called radiation. We will also combine forms of heat transfer and determine an overall heat transfer coefficient for a given heat transfer process.

Any body of mass whose temperature is above absolute zero emits radiant energy.

In most practical heat transfer scenarios, all three heat transfer mechanisms (conduction, convection, and radiation) occur simultaneously. Regardless of which heat transfer mode is used, a heat transfer coefficient determines its rate. In heat transfer by conduction through a solid layer, the heat transfer coefficient is thermal conductivity over thickness, k/Δx. We also learned that thermal conductivity is dependent on properties of the materials that are in direct contact. In convection, the heat transfer coefficient is the film heat transfer coefficient, h, which is dependent on the fluid properties, geometry of the system, and flow regime.

In this lesson, we will discover that heat transfer by radiation is dependent on temperature differences as well as something called ‘emissivity’. Emissivity is a dimensionless property of the substance that is emitting radiation. We will discuss emissivity more later, but for now we can relate it to the effectiveness of emitting radiation from a surface. 

Finally, when we consider a scenario that involves multiple modes of heat transfer, we must determine the overall heat transfer coefficient that considers all forms of heat transfer inhibition. We will discuss this at the end of the lesson.