Trying to wrap my head around gas turbine thermodynamics

[jsh111]

TL;DR Summary

after air is heated from a source, is the thermal energy converted into pressure, velocity, or both?

I created a crayon drawing to aid the discussion below:

Basically if you have a blower of low pressure, and you blow it through a tube which has a very hot center, when the heat is added to the air, does the pressure of the air increase after passing by the fire, or is that impossible since this is an Open end and no "trapped volume" , and then maybe the heat energy is transformed into velocity, and not pressure as a result. I know in a normal gas turbine the focus is on pressurizing the air prior to heating through combustion. Suppose you instead focused on a high velocity of air, and heated that. I'm sure there is a reason pressure is focused on but I don't understand. My idea is to pass a low pressure but high velocity quantity of air through such a tube, absorb a much thermal energy into it as possible and use it to drive a "Hillbilly turbine". I know this wouldn't be very efficient, or powerful, for now I'm just trying to understand the concepts.

Another question related to this is, if one was to take a two piston system, where one piston is pushing air through a cylinder, past a heat source, and directly into an opposed cylinder which is being driven, I know this is an isochoric process since when one piston is compressing, the other is expanding. When the air is pushed into the heat source energy is added to it, which should pressurize it, but would the pressure not work on both pistons (compressor and expander piston) equally? A picture of this scheme is also below. I believe this is the ericcson cycle, and assumes that the pistons in the below drawing have what I believe is 180 degree phase angle (when one piston begins expanding, the other begins compressing




assumed

 

[jack action]

What you are describing is a ramjet and they do work as you expect in particular circumstances. The design of the inlet and outlet ducts - together with the inlet velocity - will determine if the added energy will be converted to pressure, velocity, or both.

When the air is pushed into the heat source energy is added to it, which should pressurize it, but would the pressure not work on both pistons (compressor and expander piston) equally?

There is a check valve in your diagram to prevent backflow to the compressor.

 

[jsh111]

What you are describing is a ramjet and they do work as you expect in particular circumstances. The design of the inlet and outlet ducts - together with the inlet velocity - will determine if the added energy will be converted to pressure, velocity, or both.

View attachment 340346​

There is a check valve in your diagram to prevent backflow to the compressor.

Thank you for your reply, I am going to go off and investigate ram jets much more deeply.

If you don't mind me asking another question, suppose in the above drawing the check valve were removed and back flow was now possible. Essentially what I would have now, I believe, is a piston version of the above ram jet. If the pistons were driven slowly, my suspicion is that the heat added would expand equally to both the compressor and expansion piston.

I suspect in order for this scheme to do anything useful you would need a very fast cycle time (and therefore rpm), perhaps unrealistically fast for a piston driven heat engine, but if this is so, what would going faster gain you?

Is it possible that if the pistons were driven fast enough, the delay in heating up the gas would impact the expansion piston more than the compressor piston to get net work out of the thermal system?

 

[jack action]

suppose in the above drawing the check valve were removed and back flow was now possible. Essentially what I would have now, I believe, is a piston version of the above ram jet

What you would have is a turbojet:

You have a compressor and an expander (turbine) with a combustion chamber in between - which is just a duct. Once the pressure increases in the combustion chamber, it can go back into the compressor which, obviously, is at a lower pressure. And it sometimes does in certain conditions known as pressure surge:

Axi-symmetric stall, more commonly known as compressor surge; or pressure surge, is a complete breakdown in compression resulting in a reversal of flow and the violent expulsion of previously compressed air out through the engine intake, due to the compressor's inability to continue working against the already-compressed air behind it.

but if this is so, what would going faster gain you?

Momentum. The faster the air goes in, the harder it is to slow it down, or possibly reverse its flow direction. Every compressor has a characteristic known as a surge line that relates the maximum pressure ratio across the compressor versus the mass airflow within the compressor. You can find it on a compressor map:

You could make such a compressor map for your engine with pistons but your pressure ratio and mass flow would most likely be very low without any kind of valves (in the 1.0; 0.00 region on the previous graph).

 

[GAS]

I am writing a scientific article, the question is: how does thermal efficiency affect a gas turbine? Could it be that with high thermal efficiency, the turbine blades will collapse due to an increase in temperature?

 

[russ_watters]

I am writing a scientific article, the question is: how does thermal efficiency affect a gas turbine? Could it be that with high thermal efficiency, the turbine blades will collapse due to an increase in temperature?

What? Can you elaborate on your logic?