Path followed by steam when total friction loss occurs in divergent section

Q.  Consider the following (h-S) diagram of steam passing through a nozzle. Which is the correct path followed by steam when total friction loss occurs in divergent section.

Where,
p1 = pressure at the inlet of nozzle
p2 = pressure at the exit of nozzle
pt = pressure at the throat of nozzle
- Published on 16 Sep 15
(h-S)-diagram-with-friction.png


a. 1 – 2 – 3
b. 1 – 2 – 3'
c. 1 – 2 – 3''
d. None of the above

ANSWER: 1 – 2 – 3''

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    Discussion

  • Pankaj   -Posted on 13 Oct 15
    Consider the above (h-S) diagram of a convergent-divergent nozzle with friction. It is assumed that the friction losses in convergent section are negligible and the whole of the friction loss occurs in the divergent section.

    If we do not consider any friction losses, the process will be ideal and isentropic. This process follows the path 1 – 2 – 3'. But considering the friction losses in the divergent section, the enthalpy of the steam increases due to the friction. Therefore this process follows the path 1 – 2 – 3''.

    Therefore,
    Nozzle efficiency is given by,

    ηn = (h1 – h3'') / (h1 – h3')

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