# I Fluid flow: larger diameter vs. smaller diameter

1. Jul 26, 2016

### 12Element

Hello everybody,

It is normally said that fluid flow in a larger diameter pipe is easier (less energy losses) than in a smaller
diameter pipe. This indeed appears to be true in relatively high flowrate applications however, I'm struggling to
understand why drinking from a normal straw (small diameter) is much easier than drinking from say a 1"
diameter pipe (large diameter). The only explanation I could come up with is that even though fluid through a
smaller pipe will have higher velocity (given the same flowrate) in comparison to a larger pipe however, it will
also be exposed to less area and thus, lower losses due to friction. Conversely, as flowrate increases the flow
through the smaller pipe will become turbulent earlier than the larger pipe due to the higher velocity and thus
more energy losses will result whilst the larger diameter pipe will maintain a laminar flow for higher flowrates. I
don't know whether I'm right and I would appreciate it if someone could sheds some light on this. I hope my
question is clear.

2. Jul 26, 2016

### Orodruin

Staff Emeritus
The principle behind drinking using a straw is to vacate the straw and thus pull the fluid up using the negative pressure differential (essentially, the atmosphere is pressing the fluid up). A wider straw would mean more volume to vacate.

3. Jul 27, 2016

### 12Element

Perhaps I wasn't clear. I understand how a straw works. My question is will fluid flow encounter more
energy losses in a small diameter pipe or in a larger diameter pipe. The typical answer normally given
is: for the same flowrate a larger diameter pipe will be more efficient (less energy losses) due to the
smaller velocity resulting from the larger cross sectional area.

This is as I said the typical answer which is indeed true when you have a significant flowrate. But, in
many application you see smaller diameter pipes favored. For instance a car intake manifold may
employ a variable geometry design where the intake runner used at lower rpm (and hence lower flowrate)
will be smaller and the larger runner will be retained for higher rpm (higher flowrate). Even old carburetored
engine will normally use a multi barrel design where the low rpm barrels will be of a smaller size. Also, the
example I cited in my opening post which I think is also related to my question, which is it is easier to drink
using a straw instead of a 1" diameter pipe. In another word, given the same vacuum you can develop in your
mouth, a smaller diameter pipe will result in lower energy losses (you can drink more easily using the straw).

why at low flow rate the typical answer (larger diameter = less energy losses) appears to not hold
even though, the velocity will still be lower in the larger diameter pipe and hence, the head losses
will be lower.

As I already said above can someone explain to me why at lower flowrates smaller diameter pipes
appear to be more efficient.

The only explanation I could come up with (which I don't know if it's right or wrong or I'm simply conflating
things together) is that at smaller flowrates the smaller diameter pipe will be more efficient due to
the smaller area (wall area) and hence less friction. And even though the velocity at the smaller diameter pipe
will be higher however, it is still very low at low flowrates and head losses due to velocity will be negligible.
However, if flowrate increases the losses due to velocity will become more significant and thus the larger diameter
pipe will be more efficient (due to the lower velocity given the same flowrate).

I hope I was clear enough this time.

4. Jul 28, 2016

### jbriggs444

To drink through a one inch pipe, you must use your lungs to suck the air out of the pipe, then close the opening to your trachea and suck up a mouthful of water with your mouth. Nobody likes to suck with their lungs and time the closing of the trachea properly. Inhaling water by accident is a learning experience. One then swallows the water, using the tongue to form a boundary between low pressure water at the lips and normal pressure water going down the gullet. It is somewhat easier to do this without a one inch pipe in your mouth. In addition, one inch diameter pieces of pipe are clumsier than straws and take more room in the drawer.

Have you ever tried to drink through one of those cheap coffee stirrers? Large diameter is easier up to a point.

Edit: It's also harder to slurp up a 12 ounce tumbler of milk when it takes 8 ounces just to fill the straw.