Fluid Mechanics

[Engr_Kølâ]

When calculating head loss passing through an elbow which velocity for the pipe are we to use the one that is before or after the elbow ; for example

If it’s an elbow connection between pipe A and Pipe B (I.e connecting pipe A to B)
Is the velocity of pipe A or pipe B
( Pipe A and B have different diameter)
Case 1: consider the pipe diameter A to be bigger than pipe B
Case 2 : consider the pipe diameter B to be bigger than pipe A

Fluid moves from pipe A to pipe B in all considerations

 

[pointsman]

Normally headlosses are calculated based on the velocity just upstream. One exception to this is contractions -- for contractions the higher velocity is used regardless of its position upstream or downstream.

 

[LittleInch]

Well an elbow which also changes diameter from one end to the other is rather unusual.

But I would simply use the smaller of the two diameters as there will be some effect from the reduction in size as you go round the elbow.

The difference will not be that big.

What method are you looking to use for this?

 

[fel3]

Well an elbow which also changes diameter from one end to the other is rather unusual.

Unusual, but not unheard of. Reducing elbows are sometimes used in waterworks piping to eliminate the need for two fittings. Although I have never needed them in my designs, I have seen a decent number of reducing elbows in the municipal booster pumping stations and water and wastewater treatment plants that I have visited over the years. IIRC, all of these were ductile iron. You can also buy small-diameter PVC reducing elbows at the hardware store.

 

[Snickster]

The reference you use which provides the formulas for pressure drop usually state whether the head loss is based on the velocity of the larger or smaller diameter.