Flow work in Thermodynamics

  • #1
I'm trying to solve a problem where a perfect ideal gas is entering an initially evacuated rigid vessel. The input pressure and temperature are Pi and Ti which are constant. The incoming mass Mi is an arbitrary function of time.

When i solve this, i get the temperature inside the container as (Cp/Cv)Ti which is same from the beginning till the end of the filling process.

I am unable to physically understand how the temperature in the container is supposed to be constant. Isn't the ideal gas entering the rigid vessel undergoing free expansion at the first instant and then it gradually becomes more difficult to pump the gas in? So how can the temperature inside the container be constant?

I know that some books talk of imagining a piston pushing the gas in hand hence it is doing flow work, but i cannot visualize the process especially in the context of the present problem.
 

Answers and Replies

  • #2
20,867
4,546
I'm trying to solve a problem where a perfect ideal gas is entering an initially evacuated rigid vessel. The input pressure and temperature are Pi and Ti which are constant. The incoming mass Mi is an arbitrary function of time.

When i solve this, i get the temperature inside the container as (Cp/Cv)Ti which is same from the beginning till the end of the filling process.

I am unable to physically understand how the temperature in the container is supposed to be constant. Isn't the ideal gas entering the rigid vessel undergoing free expansion at the first instant and then it gradually becomes more difficult to pump the gas in? So how can the temperature inside the container be constant?

I know that some books talk of imagining a piston pushing the gas in hand hence it is doing flow work, but i cannot visualize the process especially in the context of the present problem.
It really seems to me that you have doubts about this because it goes against your intuition, but nothing concrete. Are you saying that you are uncomfortable with the open system version of the first law of thermodynamics?

With regard to the gas entering the valve being pushed from behind, imagine that there is an imaginary membrane separating the gas entering the valve from the gas behind it. The gas behind it acts like a piston to push it into the valve.
 

Related Threads on Flow work in Thermodynamics

Replies
9
Views
835
  • Last Post
Replies
4
Views
1K
  • Last Post
Replies
2
Views
989
  • Last Post
Replies
5
Views
6K
Replies
4
Views
10K
Replies
5
Views
579
  • Last Post
Replies
1
Views
551
Replies
2
Views
1K
Replies
1
Views
386
Top