Afterburners: Why the Nozzle Opens Wider with Afterburner On

In this video I'll go through an example showing why the nozzle on a jet engine needs to open up wider when the afterburner is on. You can tell that an afterburner is on when you see the visible radiation in the afterburner/jet pipe.

I'll be making some more in-depth videos about converging nozzles, converging-diverging nozzles, nozzle flow, choked flow, etc. If you haven't already, subscribe so you can be notified when I post them!

===== NOTES =====
► Quick note regarding flow choking: You can increase the mass flow rate even when the flow is choked by increasing the upstream stagnation pressure. When I said you couldn't increase the mass flow rate anymore, it was for a fixed stagnation pressure, because that's what you usually have coming out of the turbine of the engine. So that plot on the board is valid for a fixed P_0, when we keep decreasing P.

===== REFERENCE VIDEOS =====
→ Turbojet Thrust Equation:
https://goo.gl/gyaalq

→ Stagnation Relations:
https://goo.gl/yOSNeL

→ Isentropic Relations:
https://goo.gl/vxhrLH

→ Speed of Sound:
https://goo.gl/3qZhII

===== ASSUMPTIONS AND NOTES =====
1) The nozzle is isentropic.
Adiabatic: no heat is added or removed.
Reversible: no friction, shocks, etc.
2) The working fluid is air. Of course the actual working fluid will be the mixture of air and combustion products. This assumption avoids extra calculations that can certainly be done, but are unnecessary for the outcome of this example.
3) The air is a calorically perfect gas (specific heats are constant).
4) The engine is fitted with a converging nozzle (not converging-diverging).
5) The direct result of turning the afterburner on results in an increase in the stagnation temperature of the fluid.
6) The flow is steady through the nozzle.
7) The flow is quasi-1D.
8) Mass flow rate and stagnation pressure do not change when the afterburner is turned on.
9) Values used are just characteristic numbers that might be valid for real engines.
10) If you do have a converging-diverging nozzle, then to get supersonic flow (higher exit velocities), you will still need to have choked flow. In order to increase subsonic flow to supersonic flow in a CD nozzle, the nozzle must be choked.

===== THUMBNAIL IMAGE =====
Photo: SAC Ben Stevenson/MOD [OGL http://www.nationalarchives.gov.uk/do...)], via Wikimedia Commons