Isothermal Gas Emissivity — Lesson 7

This lesson covers the complex structure of radiation bands in gases, focusing on the role of vibration-rotation. It explains how vibrational transitions, along with rotational transitions, lead to three branches: Q, R, and P. The lesson also discusses the impact of high gas temperatures on these transitions, leading to overtones. It further explores the absorption bands in different gases, such as oxygen, nitrogen, carbon monoxide, carbon dioxide, nitrous oxide, water vapor, ozone, and methane. The lesson also delves into the challenges of solving problems in radiation due to the rapid variations in absorption. It concludes by discussing the concept of emissivity and absorptivity in gases.

Video Highlights

00:56 - Explanation of the three branches of vibrational transitions: the Q branch, the R branch, and the P branch.
08:31 - Discussion on the concept of Doppler broadening and Lorentz broadening.
16:37 - Comparison of band strength v/s wavelength for different gases.
27:10 - Explanation on the concept of emissivity and emissivity chart.
41:40 - Discussion on the concept of band absorptance and how it can be used to calculate the emissivity of gases.

Key Takeaways

- Vibration-rotation plays a significant role in creating the complex structure of radiation bands in gases.
- High gas temperatures can lead to overtones, which are important in high-temperature environments like furnaces.
- Different gases have different absorption bands, which are generated based on either symmetric stretching, asymmetric stretching, or bending.
- The absorption in gases is a strong function of frequency or wavelength, and variations are very rapid.
- Emissivity and absorptivity in gases can be very different, making the treatment of gases as a gray gas not a good approximation.