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I was going through a review of rotary compressor and i understood that the stagnation pressure and stagnation temperature both change in the rotor stage (as energy is being added).I also read that in the stator stage the stagnation temperature remains a constant but there is loss of stagnation pressure. If there is a loss in stagnation pressure shouldn't the stagnation temperature increase (I am guessing here that the loss is converted into heat)? (I am guessing this has to do something with expansion but I am not sure)

additional info: https://physics.stackexchange.com/questions/292589/why-does-a-diffuser-increase-pressure

https://www.physicsforums.com/threads/why-does-the-total-temperature-not-change-in-adiabatic-flow.812811/post-5113056

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  • $\begingroup$ Can someone pleas explain this to me? $\endgroup$
    – GRANZER
    Aug 12, 2019 at 11:19

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If the total temperature increased, energy would need to have been added to the flow via work or heat transfer. A change in stagnation temperature is directly related to work being taken or added to the flow (see energy equation/Euler work equation). Since the stator does not put work into the flow or extract work out, the total temperature will stay the same. Things become more nuanced when phenomena such as friction or secondary flows are involved. However, unless you have a crazy amount of friction, the total temperature of the flow will remain essentially the same in the stator. I am assuming you believe the temperature will increase from decreasing pressure because of the Ideal Gas Law. However, remember in compressible flow that the density is a variable and changes with pressure. Hopefully my answer is helpful for you.

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  • $\begingroup$ Thank you. Yes, I was expecting the total temperature to increase with decreasing total pressure as the loss is total pressure is due to friction. I am guessing this is due to the dynamic pressure loss, while static pressure increases but not enough to compensate for the dynamic pressure loss. This results in the total pressure to decrease. Am I right? $\endgroup$
    – GRANZER
    Aug 14, 2019 at 15:30
  • $\begingroup$ The function of the stator is to convert the kinetic energy of a flow (related to dynamic pressure) into static pressure (engineering.stackexchange.com/questions/12277/…). In a frictionless stator row, the total pressure would remain constant and the stator would redistribute the amounts of dynamic and static pressure comprising the total pressure. When friction is added, velocity is lost which reduces the dynamic pressure. Thus, there is a decrease in total pressure from the loss of velocity that cannot be converted into static pressure. $\endgroup$
    – mechcad
    Aug 15, 2019 at 16:52
  • $\begingroup$ Not really, even in frictionless stator blades, there is a loss of total pressure due to increase in entropy web.mit.edu/16.unified/www/FALL/thermodynamics/notes/… $\endgroup$
    – GRANZER
    Aug 15, 2019 at 17:20
  • $\begingroup$ You are totally correct, frictionless was not the correct word to use. I suppose I should have said "ideal". Thank you for the clarification $\endgroup$
    – mechcad
    Aug 15, 2019 at 17:29

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