# Why is there a pressure drop in the combustion section of a jet engine?

This image, referenced in this question, shows in green the pressure value in a jet engine:

The combustion section between compressor and turbine is where fuel is injected, mixed with air, and burnt. Why is there a pressure drop while combustion occurs and temperature suddenly increases by 1,500°C (to be compared with pressure increase after ignition in a reciprocating engine).

## 3 Answers

The air moves¹ in direction of decreasing pressure except where forced by the compressor. So the pressure must be decreasing to maintain desired flow. If the pressure was increasing, the flow would stop and reverse and the engine would stop operating.

In fact, that's exactly what happens if fuel is added too fast: the energy quickly raises, but the still slow-spinning turbine provides too much resistance, so pressure will increase above what the compressor can provide and the compressor will stall, the engine will emit loud bang and some flames from both ends and likely flame out as it runs out of oxygen for a moment.

As already explained, in the normal flow the released energy increases the velocity of the flow instead of pressure.

¹ More precisely accelerates. If the pressure was constant, it would keep moving. But it must not raise. It decreases slightly due to friction.

• Ideally in brayton Cycle the process is constant pressure heat addition and the fact that combustion chamber is left open allows it to expand thus pressure doesn't increase.But I have also heard that combustion chambers in gas turbine engines are designed by considering the flow as Rayleigh flow.And in a Rayleigh flow for subsonic case the static and stagnation pressure decreases during heat addition.So is the flow Rayleigh flow? Apr 30 '20 at 11:49
• @AbhishekPallipparagopakumar: It seems, in first approximation: "The combustion chambers inside turbojet engines usually have a constant area and the fuel mass addition is negligible. These properties make the Rayleigh flow model applicable for heat addition to the flow through combustion". In annular combustor, the pressure drop is about 2%, mostly due to (cold) friction. Source (p. 66)
– mins
Apr 30 '20 at 15:39
• @mins yes you are right. I had found from another Q&A that Rayleigh flow is indeed applicable and the fact that mass addition being very small clearly supports the assumption. Although I was actually confused about the pressure drop occurring in combustion chamber being solely attributed to frictional drops or does the pressure drop during Rayleigh flow has a part in it. From the Q&A it does have a part. Apr 30 '20 at 15:59
• @AbhishekPallipparagopakumar: That's my understanding. Same document, p.70: "The heat addition causes a decrease in stagnation pressure which is known as the Rayleigh effect and is critical in the design of combustion systems". According to Farokhi, the pressure loss range for heat release is 0.5-1%, the "friction" (cold) loss is 4-7% for some combustion chamber.
– mins
Apr 30 '20 at 17:11
• @AbhishekPallipparagopakumar: Another interesting paper.
– mins
May 1 '20 at 13:33

The question you linked to contains the answer to your question:

the small pressure drop in the combustor is caused by friction

• @mins: You have a restricted volume in a piston engine, while the volume inside the combustor is open at the rear end. Granted, there are some turbine blades in the way, but in general jet engine design is about restricting flow more at the front and less at the rear, so the air knows which direction to flow. Nov 20 '16 at 16:13

In a reciprocating engine, the gases are trapped until the exhaust valve opens, unlike a jet engine where it's an open exhaust. In a cylinder the gases are not accelerated. So, v rise in a jet, p rise in an engine cylinder. Temperature rise in both.