# What causes air to move to combustor?

I know the pressure is highest at end of compressor. But what forces air to move low to higher pressure? Am i missing out something? I have heard seals are used to prevent leakage of air from compressor to forward section of engine.

• Your question is confusing. You acknowledge that pressure is highest at the final stage of the compressor, yet seem to think that the airflow could somehow reverse direction, or flow from low to high. If the pressure is lower in the combustion section then that's where air will flow... – Michael Hall Aug 21 '20 at 20:53
• And any seal preventing forward flow would also restrict aft flow, right? – Michael Hall Aug 21 '20 at 23:37

What forces air to move from lower to higher pressure?

In the compressor, the pressure continuously increases from first stage to last stage, that's the purpose of the whole thing. But your question is valid: Why would air move from the first stage to the second stage where the pressure is higher?

The reason is indeed the rotating blades of a stage push air to the next stage. Energy extracted by the turbines is used to rotate the compressor blades and create a relative wind. Air is accelerated over the blade and like for a wing, a downwash force is created. This accelerated air is then slowed down by the next stator vanes to convert the additional velocity into additional pressure, and maintain velocity nearly constant in the compressor (else efficiency would decrease), as visible in tsg's answer.

Stop the blades and air goes back until pressure equalizes in all stages.

I have heard seals are used to prevent leakage of air from compressor to forward section of engine

Correct, there are seals. But while the air moves towards the combustor within the compressor annular duct, under the effect of blades rotation, seals are here to prevent it from getting back to previous stages by escaping the annular duct, and leaking below the stator vanes (between the rotor drums and the inner shrouds of the vanes).

The fixed vanes plane is actually the plane where pressure increase occurs. So air will try to return on the front side of the vanes, towards the rotor blades using the interstices (orange flow), but it will be blocked by seals.

The seals are highlighted in yellow on the picture below, they rub on the inner shroud of the stator vanes (actually for the engine shown, a honeycomb ring fixed inside the vanes shroud provides an abradable track to the labyrinth seal:

HP compressor interstage labyrinth seals (CFM56-7B), source

These seals are required because a small gap exists between the rotor and the stator platforms forming the inner wall of the duct.

The compressor, of course.

The pressure inside the combustor is lower than at the end of the compressor. How is that achieved? By making the combustor wider than the compressor's final stages. That's why, on a cross-section, you see the gas path flare-up from the compressor to the combustor.

The combustor does not increase the pressure. After the compressor, the pressure only ever drops; this loss of pressure is what propels the air from the front of the engine to the nozzle.

So the pressure decreases throughout the combustor. What the flames do is increase the gases' energy, which is stored as temperature and velocity. Once the hotter gases reach the turbines, then, their pressure is "worth more" than for cold air. This way, the torque they put into the turbines as they expend their pressure is greater than the torque the compressor required to compress cold air.

Finally, some of the energy remains as velocity to help propel the aircraft.

Diagram shamelessly stolen from here. It was also used in this question, which is useful for additional detail.

Just remember what pressure is - force divided by area - so a difference in pressure forces the air (or other fluids) to flow in its gradient.

You do need seals to prevent flow in an unwanted gradient - but they mostly just "choke" the flow, not completely prevent it.

• What prevents air to leak completely? – Auberron Aug 21 '20 at 16:13
• @Auberron that's a separate question – Federico Aug 21 '20 at 16:24
• @Auberron, air doesn't "leak" forward or backwards, it flows through the engine continuously. – Michael Hall Aug 21 '20 at 20:55
• @Auberron speed of sound prevents air leaking completely – Abdullah Sep 21 '20 at 5:36