Measuring Flow Velocity in Pipe Expansion: Laminar vs. Turbulent | Explained

[davidgrant23]

Hi there,

I have a double-barreled question for you all. So I have two pipes connected to each other. The first pipe is 0.64cm and has a Re = 5000, the second pipe is 4.5cm and has a Re = 700. The two are connected so that the gas expands from the the first pipe to the second.

Now, I wish to measure the flow velocity in the 2nd, larger diameter pipe. The Re I provided for the 2nd pipe suggests it is laminar, but I cannot simply place the air velocity meter right at the expansion between the two pipes as I would think it would be turbulent (expansion causing turbulence) right at the connection. My question is, is there a way of calculating how long along the length of the 2nd pipe I should place the velocity probe to ensure that the flow is laminar and not turbulent (from the expansion)?

Also, if in the event that the flow has to be turbulent, is measuring the velocity with a hot wire anemometer in turbulent flow just a case of taking a time averaged value, to negate the effect of turbulent fluctuations?

Thanks,
Dave

 

[Greg Bernhardt]

Thanks for the post! Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

 

[gmax137]

How did you determine the Reynolds numbers without measuring velocities?

 

[davidgrant23]

How did you determine the Reynolds numbers without measuring velocities?

I know the volumetric flowrate from a flowmeter placed upstream of both. So all I do to get the Re is Re = 4Q/(πνD).

 

[RandomGuy88]

In pipe flow like this just because the Reynolds number is below about 2000 does not mean the flow will be laminar. That is just a general rule of thumb and depends on many things like the surface quality of the pipe, the presence of external disturbances and the inlet conditions. In your case the inlet conditions are probably turbulent so the flow in the second pipe is not likely to be laminar. I suppose the turbulence may decay eventually but again that really depends on many things. It is possible to relaminarize a turbulent flow if it is subjected to a sufficient strong favorable pressure gradient but that is not the case here.

The hotwire is capable of measuring the instantaneous velocity so if you take a sufficiently large number of measurements and average them you will get an accurate measurement of the mean velocity. The hotwire can also be use to determine whether your flow is laminar or turbulent.

Also keep in mind that with such a small pipe you are going to have to be very careful about the amount of blockage you introduce into the flow if you put a hotwire in the pipe. It can significantly interfere with your measurements.