Entropy_Production gas mixing

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Two well-insulated rigid tanks A and B have the same volume (VA=VB=1 m3 ) and both are filled with air with the following thermodynamic states:

Tank A: pA = 1 bar ; TA = 500 K

Tank B: pB = 18 bar ; TB = 500 K

Now these two tanks are connected with each other through a pipe, so that air from tank A and tank B can mix with each other and reach the same pressure in the end. During this mixing process, no heat is transferred.

Determine the entropy production after this mixing process.

Assume that in this case, air is an ideal gas and its specific gas constant is R=287 J/kg∙KEP5


Index 1: state before mixing

Index 2: state after mixing

At first, we calculate the mass of gas in both tanks before mixing: EP1

In order to determine the entropy production, pressure and temperature p2 and T2 after mixing should also be obtained.

The boundary of the control volume chosen in this case encloses both tanks, tank A and tank B. Since both tanks are well-insulated and rigid, there are no heat transfer and work for volume change. Hence we get the simplified equation of the 1st Law of thermodynamics for closed system and the final temperature T2:


→ mA·cv·(T2-TA)+ mB·cv·(T2-TB)=0

→ T2=TA= TB=500K

So the final pressure p2 is: EP2

Then the entropy production is: EP3 EP4


We can obtain an important conclusion:

The entropy after gas mixing and expansion will be produced (entropy production >0 ) and therefore this process is irreversible.