Impact of Spectral Variation of Surface Properties on Radiative Heat Transfer — Lesson 6

This lesson covers the concept of radiative heat transfer, focusing on the impact of spectral variation of properties on heat transfer. It discusses the behavior of gray and non-gray surfaces, and how their emissivity and absorptivity functions affect heat transfer. The lesson uses the example of a Dewar flask to illustrate how the spectral variation of properties can lead to unusual situations. It also discusses the Newtonian cooling law and its limitations in certain scenarios. The lesson further explores the concept of selective surfaces and their use in temperature control in space. Finally, it discusses the design of tungsten filament lamps and the challenges faced due to the non-gray nature of the surface.

Video Highlights

02:45 - Explanation of the Newtonian cooling law and its application in heat transfer.
12:58 - Discussion on the impact of emissivity on temperature in the Dewar flask.
23:16 - Explanation of the use of selective surfaces in temperature control in space.
45:18 - Discussion on the design of tungsten filament lamp and the challenges faced due to the non-gray nature of the surface.

Key Takeaways

- Gray and non-gray surfaces behave differently in terms of heat transfer due to their varying emissivity and absorptivity functions.
- The Newtonian cooling law, which states that heat transfer is proportional to the temperature difference, may not always hold true, especially in cases involving radiative heat transfer.
- Selective surfaces, which have varying emissivity at different wavelengths, are used in temperature control in space.
- The design of tungsten filament lamps is challenging due to the non-gray nature of the surface and the need to balance high temperature for light emission with material longevity.