# How can flow in the compressor of a jet engine go back to the inlet?

In aerodynamic lessons and books about jet engines, it is always said that there is a risk that air flow in the combustion chamber or compressor can go back to reverse direction, towards the inlet if something goes wrong, for instance a compressor error, surge.

What I don't understand here is that when we imagine the engine in flight, the engine cruises very fast into the air, which means that there is a remarkable amount of air rammed into the inlet towards the combustion chamber.

Even though the compressor doesn't work properly and push the air to the combustion chamber, there is still much air coming into the inlet and pushing the air in the compressor towards the combustion chamber. How can the air in the combustion chamber go back while another air volume is coming to it from the inlet side continuously?

• Commented May 25, 2023 at 16:28
• I used to have the opposite question, why don't the gases expanding in the combustion chamber force their way out the front all the time... But I think these answers clarify that question as well. Commented May 25, 2023 at 21:45

Note that the flow speed past the diffusor (which sits between the compressor and the combustion chamber) is very low in order to allow air and fuel to mix, the fuel to evaporate and the mixture to ignite. Little energy is needed to let the compressed air change direction and accelerate into the compressor.

Burning the fuel will heat up the air, so it expands. A lot. Now where should that added volume go? At the front the compressor pushes new air into the diffusor and at the back the turbine is blocking the flow, albeit less than the compressor.

When the compressor stalls, the pressure it creates drops, so the resistance which before prevented the hot air from expanding into the compressor is gone. Now the turbine will block the flow more and the hot air expands into the compressor.

While the ram pressure raises the pressure even before the compressor, the pressure in the combustion chamber is much higher because the compressor was working as intended until the surge occurred, raising the pressure far above the total pressure of the air.

The reason is the ram air mass flow entering the front of the compressor isn't that strong relative to the other forces happening within the engine. You have a system that depends on mass and inertia moving air from front to rear to keep the cycle going, and most of the mass and inertia packing air into the combustor, and forcing it to expand out the back, is generated by the compressor.

When the compressor stalls, the pressure and velocity of the air flow into the combustion chamber drops off sharply. The pressure in the combustion chamber from expanding gases, normally forced to expand rearward by the air piling in from the front, is able to expand both rearward and forward.

This creates an intense pressure wave of hot expanding gases forward, and so this blast of hot air from the combustion chamber overpowers the relatively weak ram air flow from the engine's forward movement, sometimes with still burning fuel in it making orange flames, out the front cowl.

If the compressor blades stall, the pressure inside the combustion chamber then "shows up" on the inlet side of the compressor stage. That pressure will block the incoming airflow which then spills out and around the inlet lip instead of entering the engine.