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I cannot understand how afterburner fuel nozzles are not crating a lot of backpressure and allow engine to work when afterburner is not in use? They close almost 50% of the exhaust area. Is there any efficiency loss during normal operation? Can nozzles move out of the stream when afterburner is not in use? enter image description here

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  • $\begingroup$ The gas in the afterburner tube is relatively slow. Pressure isn't converted to velocity until the very end of the nozzle. $\endgroup$ Oct 9 at 3:09
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    $\begingroup$ "allow engine to work when afterburner is not in use" because the engine is specifically designed to work with them there. You can't just add an afterburner to any old engine and call it a day... $\endgroup$
    – FreeMan
    Oct 11 at 15:46

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I don't believe that those are the fuel nozzles (more correctly called the fuel spray bars). Here's a schematic of a turbojet with and without afterburner.

Afterburner

image from aerospaceweb.org

The fuel spray bars are very thin. The afterburner flame holder is much thicker, because its purpose is maintain combustion by slowing the air and mixing the fuel and air so complete combustion can occur within the engine.

I believe the object in the picture is the flame holder, which needs to slow airflow to do its job. Counterintuitively, the engine wouldn't make as much power without the flame holder because the flame would blow out of the engine before the air and fuel finished mixing and burning.

I am surprised that it doesn't interfere with airflow when the afterburner is not in use. However, notice that in the diagram (and suggested in your picture) the engine gets wider. It doesn't take a big increase in diameter to double the area of the engine. That means that the open space around the flame holder is not that much smaller than the unobstructed width at the spray bars. I can't find a source to prove that explanation is correct though.

You can see these parts in this YouTube video: Afterburner Fuel System

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Is there any efficiency loss during normal operation?

Yes, indeed a couple of percentage points of dry thrust is lost due to the afterburner assembly.

But everything visible in that picture is needed and is needed in that position, mainly for one simple reason: the primary limiting factor of a combustion (everywhere it happens, either in a combustion chamber or in an afterburner) is its speed. Combustion is an extremely slow process, so slow that burning velocity is normally measured in cm/s (inches/s). The main components of an afterburner are therefore designed taking into account this limitation. Starting from the low pressure turbine and moving downstream, we can find:

  • a diffuser (i.e. an area-increasing section); this is the easiest way to slow down the flow which has normally a value of a couple of hundreds of m/s just after the turbine;
  • a row of simple annular rings with holes; these are the fuel injectors supplying fuel downstream; several are needed with different radius and in different positions in order to have a smooth combustion; due to the still relative high airflow speed, fuel is broken apart as soon as it exists the holes, vaporises and mixes with the air-fuel mixture coming out of the turbine; this mixture has still a lot of air in it since the fuel in the combustion chamber upstream is burned in excess of air to limit temperature on the turbine's blades; so, air is here, atomised fuel is here, temperature is high enough, there is nothing left to do but further reducing the speed down till the burning velocity, as said before; this is achieved by a;
  • flame holder; this is a simple row of v-shaped bodies which create a lot of recirculation behind them and stabilise the flame i.e. give time to the flame to develop and burn.

The following cutaway shows a Pratt & Whitney F100:

Pratt & Whitney F100

I have highlighted in blue the diffuser, in green two of the seven rings of fuel injectors and in purple one of the v-shaped flame holders.

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