Example Problem with Complete Solution

8A-3 : Entropy Production of Mixing Two Liquids at Different Temperatures
8 pts
An isolated system of total mass m is formed by mixing two equal masses of the same liquid initially at the temperatures T1 and T2. Eventually, the system attains an equilibrium state. Each mass is incompressible with constant specific heat C.
                   
a.) Show that the amount of
entropy produced is :
Eqn 1
             
b.) Demonstrate that Sgen must be positive.  
                   
Read : For part (a) Perform an entropy to determine an equation for Sgen. Then perform an energy balance to determine an expression for the final temperature and substitute the expression into Sgen and simplify.
Given : Initial State: Final State :
Chamber 1 : T1 Chamber 1 : Teq
Chamber 2 : T2 Chamber 2 : Teq
Incompressible fluids with CP = CV = C.
 
Find : Part (a) Show that the amount of
entropy generated is:
Eqn 1
Part (b) Demonstrate that Sgen must be positive.
Diagram :
Assumptions :
1 - The system consists of the total mass of liquid in the entire tank.
2 - The system is isolated (adiabatic and closed).
3 - The liquid is incompressible with constant specific heat, C.
4 - No work crosses the system boundary.
Equations
Part a.) Let's begin with the defintion of entropy generation: Eqn 2
We can solve Eqn 2 for Sgen : Eqn 3
Since the system is isolated, there is no heat transferred: Eqn 4
We can use Eqn 4 to simplify
Eqn 3, yielding :
Eqn 5
The change in the entropy of
the system is :
Eqn 6
Eqn 7
We can rearrange this equation to show that the total change in entropy for the system is the sum of the changes in entropy of each of the two fluids.
 
Eqn 8
The entropy change for an incompressible fluid depends
only on temperature.
Eqn 9
   
Because the heat capacity in this problem is a constant, it is relatively easy to integrate Eqn 9 to get: Eqn 10
Next, apply Eqn 10 to determine
the entropy change of each fluid
in this process and substitute the result into Eqn 8 :
Eqn 11
Properties of logarithms let us rearrange Eqn 11 to : Eqn 12
Combining Eqn 12 with
Eqn 5 gives us :
Eqn 13
To complete this derivation, we must elimnate Tfinal from Eqn 13.  We can determine Tfinal in terms of T1 and T2 by applying the 1st Law to this process.
Eqn 14
No work or heat crosses the sytem boundary, so Eqn 14 becomes : Eqn 15
Now, use the constant specific heat of the incompressible fluid to determine ΔU : Eqn 16
Eqn 17
Now, solve Eqn 17 for Tfinal : Eqn 18
Now, we can use Eqn 18 to
eliminate Tfinal from Eqn 13 :
Eqn 19
Simplify Eqn 19 algebraically : Eqn 20
Finally : Eqn 21
Part b.) Entropy generation is non-negative when : Eqn 22
The  values of m and C must be positive so, Sgen is non-negative when : Eqn 23
   
Simplify Eqn 23 by algebraic manipulation, as follows : Eqn 24
Squaring both sides of
Eqn 24 yields :
Eqn 25
( This is OK because T1> 0 K and T2> 0 K )
Expand the left-hand side of Eqn 25 : Eqn 26
Eqn 27
Finally, we get : Eqn 28
The inequality in Eqn 28 is satisfied for either T1>T2 or T2>T1.
The equality in Eqn 28 is satisfied only when T1 = T2.
Verify : The assumptions made in this solution cannot be verified with the given information.
 
Answers : Part a.)
Part b.) when : which is ALWAYS true !
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