Conditions are adiabatic and reversible about a turbine

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Discussion Overview

The discussion revolves around the conditions of adiabatic and reversible processes in relation to turbines, specifically questioning why these conditions lead to the assumption of isentropic behavior. The scope includes theoretical aspects of thermodynamics and entropy, as well as the implications for turbine operation.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants question why adiabatic and reversible conditions imply isentropic behavior, suggesting that not all adiabatic processes are isentropic.
  • One participant points out that the relation dS = dQ/T applies under quasi-static conditions, which may not hold true for the dynamic processes in turbines.
  • Another participant emphasizes the need to consider additional terms in the entropy equation, challenging the simplification of dS = dQ/T.
  • A participant reiterates the thermodynamic definition of entropy, indicating that dS is defined as δQ/T in reversible processes.

Areas of Agreement / Disagreement

Participants express disagreement regarding the application of the isentropic assumption to turbines under adiabatic and reversible conditions. There is no consensus on whether all adiabatic processes can be considered isentropic.

Contextual Notes

The discussion highlights limitations in the assumptions made about the conditions of the processes, particularly the quasi-static nature required for the isentropic assumption to hold. The implications of dynamic behavior in turbines are also noted as a point of contention.

scott_for_the_game
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Why is it when the conditions are adiabatic and reversible about a turbine, the assumption is its isentropic?
 
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scott_for_the_game said:
Why is it when the conditions are adiabatic and reversible about a turbine, the assumption is its isentropic?
If dQ = 0, then dS = dQ/T = 0.

This would seem to imply that all adiabatic processes are isentropic (constant entropy - ie dS = 0) which is not true. The relation: dS = dQ/T assumes a quasi static process in which the system is always at equilibrium. If the process is quasi-static and adiabatic, the process is isentropic.

I don't see how this would apply to a turbine, however. The expanding gas is necessarily dynamic (in order to drive the turbine), not quasi-static/reversible.

AM
 
you are missing a few terms in your entropy equation. You can't simply assume that dS=dQ/T.
 
sicjeff said:
you are missing a few terms in your entropy equation. You can't simply assume that dS=dQ/T.
I am not assuming that dS = dQ/T. That is the thermodynamic definition of dS.

Wikipedia said:
"[URL
Quantitatively, entropy, symbolized by S, is defined by the differential quantity dS = δQ / T, where δQ is the amount of heat absorbed in a reversible process in which the system goes from one state to another, and T is the absolute temperature.[3][/URL]

AM
 
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