Isentropic Nozzle Behavior Explanation

In summary, the pressure at the exit of the nozzle remains the same if the nozzle is isentropic. This is due to the fact that the amount of irreversibilities remains the same, and the specific entropy of the exiting steam.
  • #1
kalamar20
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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!
 
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  • #3
kalamar20 said:
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.
 
  • #4
Chestermiller said:
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?
 
  • #5
kalamar20 said:
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.
 
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  • #6
Chestermiller said:
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.
 

1. What is an isentropic nozzle?

An isentropic nozzle is a type of nozzle used in thermodynamics and fluid mechanics to achieve a specific type of flow called isentropic flow. Isentropic flow is characterized by a constant entropy (hence the term "isentropic") and adiabatic conditions, meaning there is no heat transfer between the fluid and its surroundings.

2. How does an isentropic nozzle work?

An isentropic nozzle works by converting the internal energy of a fluid into kinetic energy, resulting in a high-velocity, high-temperature and low-pressure flow. This is achieved by gradually expanding the fluid through a diverging section of the nozzle, which allows the fluid to accelerate and reach supersonic speeds (if designed correctly).

3. What is the purpose of an isentropic nozzle?

The primary purpose of an isentropic nozzle is to efficiently convert the internal energy of a fluid into useful kinetic energy for propulsion or power generation. This is especially important in applications such as rockets, gas turbines, and steam turbines where the conversion of energy is critical.

4. What factors affect the performance of an isentropic nozzle?

The performance of an isentropic nozzle is affected by several factors, including the geometry (e.g. throat area, diverging angle), the fluid properties (e.g. density, specific heat), and the inlet conditions (e.g. pressure, temperature). Additionally, the design and manufacturing quality of the nozzle can also impact its performance.

5. How is the efficiency of an isentropic nozzle calculated?

The efficiency of an isentropic nozzle is typically calculated by comparing the actual kinetic energy of the fluid at the nozzle exit to the theoretical maximum possible kinetic energy, known as the isentropic efficiency. This isentropic efficiency can be calculated using the specific heat ratio of the fluid and the inlet and exit pressures of the nozzle.

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