[medaouarwalid]

hi , i find in the web that the equivalent diameter is not the same as the hydraulic diameter, but i did not understand the difference very well, can you guys explain to me or help me to calculate the equivalent diameter or the hydraulique diameter of this shape ?i need it to calculate Reynolds number Ansys modeler gives me only the surface Area of this sketch wich is 910 mm² and the length of the sketch

[lcarasik]

Originally Posted by medaouarwalid

hi , i find in the web that the equivalent diameter is not the same as the hydraulic diameter, but i did not understand the difference very well, can you guys explain to me or help me to calculate the equivalent diameter or the hydraulique diameter of this shape ?i need it to calculate Reynolds number Ansys modeler gives me only the surface Area of this sketch wich is 910 mm² and the length of the sketch

Based off little knowledge of your situation, I'd recommend going with the hydraulic diameter (assuming your flowing into the shape and area would be the flow area). Hydraulic diameter is just 4 times the area divided by the wetted perimeter of the shape.

[medaouarwalid]

Originally Posted by lcarasik

Based off little knowledge of your situation, I'd recommend going with the hydraulic diameter (assuming your flowing into the shape and area would be the flow area). Hydraulic diameter is just 4 times the hydraulic diameter divided by the wetted perimeter of the shape.

4 times the hydraulique diameter or 4 times the area ?

[lcarasik]

Originally Posted by medaouarwalid

4 times the hydraulique diameter or 4 times the area ?

This website should help a bit. But any standard textbook will give you an idea on why the hydraulic diameter is what you should use.

Hydraulic Diameter = 4*Area/Perimeter (I corrected the earlier post)

[medaouarwalid]

Originally Posted by lcarasik

This website should help a bit. But any standard textbook will give you an idea on why the hydraulic diameter is what you should use.

Hydraulic Diameter = 4*Area/Perimeter (I corrected the earlier post)

Thank you very much, do you have any idea about the equivalent diameter what is it ? I wrote to a professeur who deals with those things, he replied to me that the equivalent diameter of a nozzle that have that shape at the exit, is the diameter of a cercle having the same free crosse section.
In my case ( area= 910 mm2; wetted pereimeter = 203 mm
- the equivalent diameter =sqrt(( 4*area)/3.14) = 34 mm
- the hydraulic diameter = 4*area / wetted perimeter = 17 mm
What should i choose because it will cause a big change in the reynolds number ???

[lcarasik]

Originally Posted by medaouarwalid

Thank you very much, do you have any idea about the equivalent diameter what is it ? I wrote to a professeur who deals with those things, he replied to me that the equivalent diameter of a nozzle that have that shape at the exit, is the diameter of a cercle having the same free crosse section.
In my case ( area= 110 mm2; wetted pereimeter = 203 mm
- the equivalent diameter =sqrt(( 4*area)/3.14) = 34 mm
- the hydraulic diameter = 4*area / wetted perimeter = 17 mm
What should i choose because it will cause a big change in the reynolds number ???

I recommend the hydraulic diameter without knowing the specifics of your case. And in general, I wouldn't recommend using anything other than hydraulic diameter.

[LuckyTran]

There are many definitions of equivalent diameter, & many of them are derived from a laminar context and are applied to laminar problems. Some handbooks make the effort to call these laminar equivalent diameters. Equivalent diameters often have strange looking correction factors in them. You tend to find these equivalent diameters in handbooks but not simpler texts.

For turbulent flows, the hydraulic diameter (usually defined as 4*Ac/Pwet) has similar motivations. Fortunately turbulent flows are less particular about the duct shape (because the core velocity profiles are much more uniform in turbulent flows than laminar ones). This definition of hydraulic diameter serves quite well for many problems and so it was not the case that many definitions of hydraulic diameter was developed. Because of its utility, you'll often find hydraulic diameter in just about any book.

It is quite rare nowadays to see the mention of laminar equivalent diameters. Back in the empirical days, it was necessary to make all data collapse on the same curve for predictability. Nowadays with CFD readily available, it's no longer needed to fudge parameters to make your data fit a known relation.