Gases and thermodynamics in confinement.

[Juval]

Hi.

Consider a plenum with a small restricting outlet.
Highly heated gas constantly flows through the plenum, and a build-up, or congestion of high-pressure gas exists in the plenum due to the restricting outlet pipe.
The intake flow of gas does not change.

1. A hotter plenum should increase the gas pressure since there is a small amount of
confinement due to the restricted outlet.

2. But, the hotter gas expands more, decreasing pressure/density.

Which of these is true?

 

[gsal]

No, the hotter gas is not free to expand, therefore, pressure increases and possibly pushes more gas out of the restricting outlet and/or starts offering increased opposition to whatever is feeding this plenum

 

[Juval]

No, the hotter gas is not free to expand, therefore, pressure increases and possibly pushes more gas out of the restricting outlet and/or starts offering increased opposition to whatever is feeding this plenum

But look at what you're saying. "therefore, pressure increases", and, "pushes more gas out of the restricting outlet."

If the heat causes more gas to be displaced out of the outlet (remembering that incoming gas flow does not change), the extra gas being displaced is just another way of saying that the gas is expanding, as in example 2.

And would this expansion not mean that the congestion in the plenum becomes less dense since more gas is going out without more gas being put in?

 

[Juval]

Is this a very hard question? Where are all the brains in here??

 

[alphy]

I would like to clarify that both statements are true and both contribute to the overall behavior of gases and thermodynamics in confinement.

Firstly, it is correct that a hotter plenum will increase the gas pressure. This is because the gas molecules will have more energy and move faster, leading to more collisions with the walls of the plenum and thus an increase in pressure. However, this increase in pressure may not be significant if the restricting outlet is small and the gas flow rate remains constant.

Secondly, it is also true that hotter gas expands more, which can lead to a decrease in pressure and density. This is due to the ideal gas law, which states that at a constant temperature, pressure and volume are inversely proportional. As the gas expands, the volume increases, leading to a decrease in pressure.

Therefore, in this scenario, both factors play a role in the overall behavior of gases and thermodynamics in confinement. It is important to consider both the temperature and the size of the restricting outlet when analyzing the pressure and density of the gas in the plenum. Other factors such as gas composition and flow rate may also affect the system and should be taken into account in further analysis.