Isentropic Nozzle Behavior Explanation

[kalamar20]

Hello. I was working on a sample problem on my thermodynamics book about isentropic nozzles. Pressures, velocities, and temperatures of steam entering to a non isentropic nozzle and exiting it are known. The question asks me to find the velocity of the steam exiting, if the nozzle were isentropic.

I find the amount of irreversibilities using entropy data and then find the specific entropy of the exiting steam. Here is my question: The solution says the exiting pressure remains the same and the temperature decreases if the nozzle were isentropic. What is the reason for that? Couldn't the pressure change and temperature remain the same?

I hope you understand my question and thank you very much for your attention!

 

[SteamKing]

Or, why couldn't both temperature and pressure change?

All I know about nozzles is here:
http://www.grc.nasa.gov/WWW/BGH/isentrop.html

 

[Chestermiller]

Hello. I was working on a sample problem on my thermodynamics book about isentropic nozzles. Pressures, velocities, and temperatures of steam entering to a non isentropic nozzle and exiting it are known. The question asks me to find the velocity of the steam exiting, if the nozzle were isentropic.

I find the amount of irreversibilities using entropy data and then find the specific entropy of the exiting steam. Here is my question: The solution says the exiting pressure remains the same and the temperature decreases if the nozzle were isentropic. What is the reason for that? Couldn't the pressure change and temperature remain the same?

I hope you understand my question and thank you very much for your attention!

Maybe the pressure is fixed at the downstream end at some specific value.

 

[kalamar20]

Maybe the pressure is fixed at the downstream end at some specific value.

How can the pressure be fixed? Also, isn't it necessary to state that in the problem?

 

[Chestermiller]

How can the pressure be fixed? Also, isn't it necessary to state that in the problem?

I haven't seen the statement of the problem, so I don't know. But, if the outlet of the nozzle is into the atmosphere (or into some other large region of prescribed pressure) that's going to determine the pressure at the exit of the nozzle. The pressure at the exit of the nozzle does not change discontinuously.

 

[kalamar20]

I haven't seen the statement of the problem, so I don't know. But, if the outlet of the nozzle is into the atmosphere (or into some other large region of prescribed pressure) that's going to determine the pressure at the exit of the nozzle. The pressure at the exit of the nozzle does not change discontinuously.

OK, thank you very much for your help.