Jet Exhaust Velocity: Turbojet & Turbofan Engine Advantages

[vincentryan]

in turbojet and turbofan engine the hot gas exhaust velocity is less to compare with after burener turbojet and turbofan engine. in case the turbojet and turbofan enigne hot gas exhaust velocity is high without after burner, what is the advantage? assume it does not consume any additional fuel

 

[FredGarvin]

Just to make sure I understand your question, you want to know why an engine with an afterburner has a lower exhaust gas velocity than a non-afterburning engine?

 

[minger]

...assuming that the afterburner doesn't consume any additional fuel?

 

[FredGarvin]

...assuming that the afterburner doesn't consume any additional fuel?

It is an assumption...an extremely wild, unrealistic, not even on the planet kind of assumption, but an assumption nonetheless.

 

[Cyrus]

...assuming that the afterburner doesn't consume any additional fuel?

You mean you don't know about the magic afterburners?

 

[FredGarvin]

OK. We may have scared him off with the no extra fuel stuff. However, there are two reasons I can think of that would dictate a slower velocity in the reheat zones:

1) Flame stability. Even though the big A/B's usually have igniters in them, they still need flame holders to maintain the flame front. The blunt body flame holders work by inducing a hot gas recirculation zone in its wake. This works well but higher velocities tend to make them unstable. They do allow for higher velocities than the flame speed but the proper burning is a tough thing to achieve with very high velocities.

2) Thermal choking. In duct flow, the addition of heat will decrease density and increase mach number. This will happen until the M=1 limit is reached. After that you can not add any more heat to the flow. Cohen, Rogers & Saravanamuttoo have a nice plot showing various mach number lines and how they interact with the thermal choke line based on delta P of the duct and A/B temperature ratio. It's very easy to see that with a high inlet mach number, the thermal choke limit is reached very quickly with small temperature ratios (pg 123 in my edition).