Reducing turbulence through 90 degrees prob

Main Question or Discussion Point

hello

I wanted to know what happens to the level of turbulence when air pass through a 90 degrees pipe bend (where in the bend turbulence is greatest). I want to design a pipe to reduce turbulence through 90 degrees. I know its obvious that level of turbulence increases but i am puzzled as to what influences it. I am designing bends inside the pipe which the air can flow to reduce turbulence. Wanted to know if anyone has any ideas or can help me?



Thank you

Engineeing
 

Answers and Replies

Astronuc
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Turbulence is a consequence of the viscosity of a fluid and mechanical interaction with the boundaries, and in the case of elbows and fittings, the tumbling caused by abrupt changes in direction (acceleration).
 
I am only dealing with gases at the moment I assume your definition is still valid. My problem is reduction of turbulence in terms of air flowing through a 90 degrees elbow. Would introductions of internal bends be a way of reducing turbulence? Please view my sketch I apologize in advance for the sketch its v poor.



Thank you for response


Engineering
 

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russ_watters
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I'm not sure "turbulence" is really what you are looking for since fully developed flow is largely turbulent and "turbulence" isn't really a quantifiable property anyway. What engineers really concern themselves with is pressure drop across such a fitting. Have you studied much fluid dynamics?

In any case, the question you are asking is a critical one for HVAC engineering and as such it is well-researched and well understood. If you can, get a copy of the SMACNA duct design guidelines, they have highly detailed tables for predicting pressure loss through different duct configurations. This includes the use of the type of structures in your sketch: they're called "turning vanes".
 
Thank you for your reply

I have studied fluid dynamics. Why would pressure changes in the pipework be of importance? (by that i mean what happens to the flow of air at different pressures?) I think it may affect the density of the air traveling through bend but i am not sure. I understand that it maybe something to do with bernoulys equation, please elaborate

greatful for any help



thank you

engineering
 
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stewartcs
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Thank you for your reply

I have studied fluid dynamics. Why would pressure changes in the pipework be of importance? (by that i mean what happens to the flow of air at different pressures?) I think it may affect the density of the air traveling through bend but i am not sure. I understand that it maybe something to do with bernoulys equation, please elaborate
Yes, as the pressure changes the density changes. In compressible fluid flow this is more apparent.

CS
 
FredGarvin
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I am only dealing with gases at the moment I assume your definition is still valid. My problem is reduction of turbulence in terms of air flowing through a 90 degrees elbow. Would introductions of internal bends be a way of reducing turbulence? Please view my sketch I apologize in advance for the sketch its v poor.



Thank you for response


Engineering
You're not looking at turbulence. You need to reduce recirculation inside the elbow. The two zones of recirculation are mostly responsible for the pressure losses. Usually guide vanes are put inside bends to help the fluid around the bend.

http://www.freepatentsonline.com/6668580-0-display.jpg [Broken]
 
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russ_watters
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Why would pressure changes in the pipework be of importance? (by that i mean what happens to the flow of air at different pressures?)
The pressure loss through elbows and fittings is the primary source of energy loss in most ductwork systems. It's what determines the energy required to move the required amount of air through the duct. When properly designed, it isn't much worse than the skin friction losses, but when a mistake is made (say, the contractor picks a bad aspect ratio for the elbow or doesn't use turning vanes), the energy loss is a real killer.

This past year, I did some troubleshooting on an HVAC system that wasn't meeting its required airflow. I found that fully half of the pressure lost in the entire system was lost in a single poorly conceived fitting.
 
FredGarvin
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Here is a scan from my text by Munson, Young and Okiishi. Look at the second page and note the areas of separation and the secondary flow induced. There is also another picture of an elbow with guide vanes installed.
 

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Thanks for all the help guys much appreciated.

also cheers fred ill look over these two pages looks like interesting stuff thanks for taking the time to locate this information for me.




Engineering
 

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