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AVAILABILITY AND IRREVERSIBILITY 337

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M-therm\Th6-2.PM5

3. Energy is said to be degraded each time it flows through a finite temperature difference. That is, why the
   second law of thermodynamics is sometimes called the law of the degradation of energy, and energy is said
   to ‘run down hill’.


4. In non-flow systems :
   Maximum work available,
   Wmax = (u 1 – u 0 ) – T 0 (s 1 – s 0 ) – p 0 (v 0 – v 1 )
   = (u 1 + p 0 v 1 – T 0 s 1 ) – (u 0 + p 0 v 0 – T 0 s 0 )
   = a 1 – a 0 ...per unit mass
   The property a = u + p 0 v – T 0 s is called the non-flow availability function.


5. In steady-flow systems :
   Maximum work available,
   Wmax = (h 1 – T 0 s 1 ) – (h 0 – T 0 s 0 )
   = b – b 0 ...per unit mass
   The property, b = h – T 0 s is called the steady-flow availability function.


6. It may be noted that Gibb’s function g = (h – Ts) is a property of the system where availability function
   a = u + p 0 v – T 0 s is a composite property of the system and surroundings.
   Again, a = u + p 0 v – T 0 s
   b = u + pv – T 0 s
   g = u + pv – Ts
   When state 1 proceeds to dead state (zero state)
   a = b = g.


7. The actual work which a system does is always less than the idealized reversible work, and the difference
   between the two is called the irreversibility of the process. This is also sometimes referred to as degrada-
   tion or dissipation.
   Effectiveness is defined as the ratio of actual useful work to the maximum useful work.

Objective Type Questions

Choose the correct answer :

1. Gibb’s function is expressed as
   (a)(u + pv – Ts)(b)(u + pv – Tds)
   (c)(u + pdv – Tds) (d)(u + pv – sdT).


2. Availability function is expressed as
   (a)a = (u + p 0 v – T 0 s)(b)a = (u + p 0 dv + T 0 ds)
   (c)a = (du + p 0 dv – T 0 ds)(d) a = (u + p 0 v + T 0 s).


3. To increase work capacity of energy transferred by heat transfer from high temperature to low tempera-
   ture
   (a) lower temperature should be lowered keeping temperature difference same
   (b) higher temperature should be increased keeping temperature difference same
   (c) temperature difference should be increased (d) temperature difference should be decreased.


4. Helmholtz function is expressed as
   (a)(u – Ts) (b)(h – Ts)
   (c) (– sdT + vdp) (d)(u + pv).


5. If a heat source at temperature T 1 transfers heat to a system at temperature T 2 (T 1 > T 2 ), state which of the
   following statements is not true?
   (a)∆system decreases (b)∆source decreases
   (c) ()∆∆system+ source decreases (d)()∆∆system+ source increases.