Concentric Pipe Air Ejection: Effects on Flow Mixing and Pressure

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Discussion Overview

The discussion revolves around the behavior of air flows ejected from two concentric pipes into a common duct, specifically examining the effects of pressure and temperature differences on flow mixing and potential backpressure. The inquiry includes theoretical considerations and the need for numerical simulations to predict outcomes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that the backpressure from the outer flow may prevent the inner flow from exiting the pipe.
  • Others argue that the mixed flow will occur within the smaller pipe rather than in the common duct.
  • One participant notes that the outcome is complex and may depend on the inertia of both streams, proposing that numerical simulation could be necessary to analyze the situation accurately.
  • There is a suggestion that if the velocity of the low-pressure stream is sufficiently low, it could lead to a backward flow from the high-pressure stream due to inadequate inertia to convect downstream.
  • Another participant raises the possibility of supersonic flow dynamics within the inner pipe, speculating on the behavior of pressure waves and shock formation based on differing speeds of sound in the two flows.
  • Concerns about the absolute inertia of each stream are mentioned, emphasizing the need for numerical simulations to clarify the dynamics.

Areas of Agreement / Disagreement

Participants express differing views on the effects of backpressure and flow mixing, with no consensus reached regarding the outcomes of the described scenario. The discussion remains unresolved with multiple competing perspectives presented.

Contextual Notes

Limitations include the dependence on assumptions about flow inertia and the need for numerical simulations to fully understand the interactions between the two air streams.

sid_galt
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There are two concentric pipes. The outer pipe is ejecting air at 10 atm and 1200K. The inner pipe is ejecting air at 1 atm and 600K.Mass flow rate is the same and the air is being ejected into a common duct. What will happen?

Will the the two flows mix together in the common duct producing a medium pressure and medium temperature or will the outer pipe exert backpressure on the inner one preventing the air from getting out?
 
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What do you think happens?
 
I think the backpressure from the outer flow and the mixed flow will be too much to overcome.
 
sid_galt said:
I think the backpressure from the outer flow and the mixed flow will be too much to overcome.

Yes, but the 'mixed' flow will occur within the smaller pipe not in the common duct.
 
sid_galt said:
There are two concentric pipes. The outer pipe is ejecting air at 10 atm and 1200K. The inner pipe is ejecting air at 1 atm and 600K.Mass flow rate is the same and the air is being ejected into a common duct. What will happen?

Will the the two flows mix together in the common duct producing a medium pressure and medium temperature or will the outer pipe exert backpressure on the inner one preventing the air from getting out?

To my understanding the answer is not an easy one. It would depend on the inertia of both streams. Maybe to answer your question it would be needed a numerical simulation. If the velocity of the low pressurized stream is small enough, then it could be a backwards flow from the high pressurized stream because the first one wouldn't have the needed inertia to convect the flow downstream.
 
Clausius2 said:
To my understanding the answer is not an easy one. It would depend on the inertia of both streams. Maybe to answer your question it would be needed a numerical simulation. If the velocity of the low pressurized stream is small enough, then it could be a backwards flow from the high pressurized stream because the first one wouldn't have the needed inertia to convect the flow downstream.

The mass flow rates of the outer and inner fluid are the same though so inertial differences are negligible.
 
Clausius2 said:
To my understanding the answer is not an easy one. It would depend on the inertia of both streams. Maybe to answer your question it would be needed a numerical simulation. If the velocity of the low pressurized stream is small enough, then it could be a backwards flow from the high pressurized stream because the first one wouldn't have the needed inertia to convect the flow downstream.

Or perhaps supersonic flow again with a converging diverging nozzle shape of the inner pipe. Although I don't have much idea how that will play out as the speed of sound is different in both the flows. My best guess is that oblique shock will not occur as the velocity of sound in the outer flow is much higher than in inner flow (assuming the mach numbers in the inner flow are very low but above 1) but as the flow inside the inner pipe is still supersonic, the pressure waves won't travel upwards and the flow wouldn't be blocked.
 
faust9 said:
The mass flow rates of the outer and inner fluid are the same though so inertial differences are negligible.

I don't care the inertia difference. I am talking about absolute inertia of each stream. Numerical simulation is needed here.
 

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