Confused about how combustor pressure is lower than compressor exit pressure in turbomachinery?

Question

I understand the compressor exit pressure is the highest pressure in all of the turbomachinery, and the combustor is a constant pressure process, and since the combustor P is lower than compressor exit P, the working fluid is directed down towards the turbine and nozzle, but What I dont fully understand is this:

how/why does pressure drop from compressor exit to combustor ?? Especially since the combustor is releasing so much extra energy something has to compensate for that and then some to achieve the lower pressure.

It seems to me that it must be the extra volume of the combustor causing this pressure drop. This is where I get fuzzy because this seems to be contradictory to the basic function of a diffuser, where volume expansion causes increase in pressure not decrease.

Any clarification would be appreciated.

Answer 1

The combustion process is taken as ideally constant pressure. Real life intrudes and losses occur. Mattingly has a simple analysis (reference below) that indicates the inlet Mach number is very important. He models the flow as having a drag on the walls. The compressor design team probably does not care how fast the flow is, but it needs to be compatible with the combustion section.

Mattingly, Jack W. Element of Gas Turbine Propulsion 1996 McGraw-Hill, p. 822ff. (Probably in 2nd edition, other books he coauthored)

Answer 2

This is not an answer. This is only my attempt to organise my thoughts and try to explain it using (simplistic) physical principles (and in the meantime improve my own understanding). All constructive criticism (even accompanied by downvoting) is welcome.

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I suspect what you mainly have in mind is something like the following image.

figure: Jet engine Turbomachinery stages (source: A python approach to multi-code simulations: CHIMPS)

Basically the compression stage is responsible for increasing the air pressure (which means that its pulling more air from the surrounding environment and compressing it). The combustor stage is responsible for igniting and expanding it, and the turbine stage is responsible for converting some of the energy to rotation to power up --mainly-- the compressor stage.

The way I understand it is that although the combustor indeed releases the fuel's thermal energy and increase the volume and temperature of the flue gases (not entirely sure the term flue applies in this context here), the pressure that the combustion takes place is atmospheric. So the way I understand it is, that if the turbine stage was not there, then the temperature would have been significantly lower and probably the pressure also). So, the turbine stage is responsible for keeping the pressure at the exit stage of the combustor so high, because its putting a barrier (which is required to extract the energy).

Obviously, I expect that if the turbine stage is not properly designed (if flow is too much restricted) then there could be a pressure buildup.

So although the compressed air comes in and there is combustion, because its an open process to the atmospheric pressure there can be no further build up of pressure.

Answer 3

This is how the design must work

1. The compressor must be a higher pressure than the combustor or else the gas flow wouldn't get into the combustor.

2. The exit must be lower pressure than the combustor or else the gas wouldn't flow out of the device.

3. We control this be designing the exit stage to give us the right pressure in the combustor.

As @NMech's answer's diagram shows, there is a lot of compression going on in the inlet. Without it, the device just won't function. These are very complex machines; our chance of creating one on our own without access to others' designs is probably about zero.