# Another question

#### sid_galt

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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#### faust9

What do you think happens?

#### sid_galt

I think the backpressure from the outer flow and the mixed flow will be too much to overcome.

#### faust9

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 occure within the smaller pipe not in the common duct.

#### Clausius2

Gold Member
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.

#### faust9

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.

#### sid_galt

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.

#### Clausius2

Gold Member
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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