Flow Through Convergent-Divergent Nozzle — Lesson 3

This lesson covers the complex process of flow through a convergent-divergent nozzle. It explains how the flow can be established either by pulling or pushing and how the flow accelerates from the inlet to the throat, then decelerates in the divergent portion. The lesson also discusses the effects of varying ambient pressure on the flow, including the creation of a weak flow, sonic speed at the throat, and the triggering of a normal shock. It further explores the concept of over-expanded and under-expanded flow, and the impact of lowering ambient pressure on these states. For instance, the lesson uses the example of a jet to illustrate how under-expanded flow swells and contracts until it eventually equilibrates with the ambient.

Video Highlights

01:35 - Establishing a weak flow in the nozzle
05:05 - Explanation of normal shock
10:24 - Explanation of design pressure and design Mach number
20:02 - Explanation of over-expanded flow and oblique shocks
23:46 - Conclusion and introduction to the next topic: flow of steam through nozzles

Key Takeaways

- The flow through a convergent-divergent nozzle is a complex process that involves acceleration and deceleration of the flow.
- Reducing the ambient pressure can lead to the flow accelerating to sonic speed in the throat of the nozzle.
- Normal shocks can occur in the divergent portion of the nozzle, leading to a loss of stagnation pressure.
- The flow can be over-expanded or under-expanded, depending on the ambient pressure. Over-expanded flow occurs when the exit pressure is less than the ambient pressure, while under-expanded flow occurs when the exit pressure is greater than the ambient pressure.
- The flow inside the nozzle remains the same when the ambient pressure is reduced beyond a certain point, but the level of over-expansion or under-expansion changes.