passive flow bench for fan testing

[raronson]

Hi all,

New to this forum but it seems like a very useful place to visit so figured I would post

I'm trying to measure the flow characteristics of a fan using a flow bench. Most of the online resources I've found are for active flow benches to measure flow rate at a specified pressure, which only somewhat apply to what I'm doing. I figured I should reach out and see if anyone has any advice/experience/suggestions for other resources I should look at.

It's a small fan for electronics cooling, with approximate maximum pressure drop of 300 Pa (1.2 in. wc) and flow rate of 65 scfm. My basic plan is to get a measurement unit (I've been looking at the Flow Performance one for no real reason other than I found it first) and several different orifices to vary the flow resistance and acquire data on different points on the curve. I'm somewhat worried about the low system pressure, as well as the general setup, so I was hoping someone could give me a few pointers.

Thank you!
Reuben

[Tony]

Welcome to the Forum Reuben.

You are right, the unusually low back pressures are going to present an interesting measurement problem.

If you wish to measure airflow volume, probably the simplest way would be to get yourself a small hand held anemometer, and place it into a duct of suitable diameter, and measure air velocity.
From average velocity, flow volume can be easily calculated.
The advantage of doing it this way, is you can measure down to very low airflow rates with high resolution, at very low back pressure, and do it quite accurately.
Something like this perhaps.



That, with an adjustable air restriction in the duct, and a water manometer to measure back pressure would enable you to plot some fan curves.

These small fans are often bolted direct onto a large heat sink, and knowing the total final thermal resistance of the fan/heat sink combination is often much more useful than knowing the actual airflow.

In that case, a metal clad resistor bolted to the heat sink will enable a direct measurement of heat sink temperature rise, with a known number of heating watts going in. Dead easy to do, and very accurate.

[raronson]

Thanks for the reply! The current plan is to set it up pretty much as you said, with some 3" pvc as a duct, a butterfly valve for a variable flow restriction, and an inline (hot-wire) anemometer for flow speed.

You're right that the heatsink/fan combination is more useful, but for this test we're trying to get our CFD flow model to match up with our actual test setup and we're hoping it's the fan that's off. Though the 5-minute plastic tubing/water/duct tape test says that's not it, so we might be back to the drawing board.

Thanks!
Reuben

[Tony]

A sloping water manometer will give good resolution, but a magnehelic gauge may be more convenient.

[vic_dahn]

I work at EMP I have tested and been in and around fan development for 10+years. this is how to test them... you must use a dia adjustment ring on the end to increase head pressure (an IRIS)

[Darcy R]

I was just reading about Iris duct restrictions this week, and was wondering if they had any aplications in flow testing. Is there a source for a precision Iris?

[vic_dahn]

mcmaster carr I think

precision dia on the iris is not the critical with fan testing.
you will need a flow straitener inside the tube and about 10 or more Dia in length before the pitot's

there is a spec for fan testing (I think SAE)