How to compensate for asymmetric flow in a square duct?

[hollerg]

I am trying to establish how to install a sensor so that the installed accuracy is at least +/- 5% (would like +/-3%) in square/rectangular duct with less than the normal L/D availble. I have an FCI thermal mass meter, but would be willing to buy a new multipoint meter, if it is unlikely to work

So far I haven’t found data validated L/D requirements for flow meters mounted in rectangular ducts for my complex geometry (details below). I've also had no luck finding validated rectangular duct data on converting a highly asymmetric flow profile to a flatter profile, except for material on wind tunnels. [There they use and antiswirl (honeycomb or tubes) screens (flattening) AND a specially designed converging section.]

I will admit my experience is with instruments in circular ducts/pipes but given the data and practices of AGA and ISA, I wonder if the L/D requirements Vs the achieved accuracy quoted for multipoint arrays in rectangular ducting are too optimistic.
IE - Ebtron meter.

IF someone has links to such data, or a referral to a rectangular duct flow expert (wind tunnels?), I would be grateful.

Lastly, it has occurred to me that if I used elbows with turning vanes, would I consider the total straight run as being nearly the same as one straight run? I imagine that the flow out of the elbows would be fingers of flow, but less likely to be skewed to the wall. Any idea how poor an approximation that is?

Thanks in advance for your help
Gary

Background:

I performed PITOT traverses @ the meter location for the current configuration using the T-rule:

Centrifugal blower (side outlet @ bottom elevation,
-- a divergent section (~8 deg half angle),
-- a straight section 6D (equiv) of 18x18 duct (meter 2.4D upstream 3.5 D downstream)
-- a double elbow section. The elbows (R=1) form a U w/ 1.5D between them, directing the duct back under the blower
-- a final straight section of 6D before I enter the process equipment

Five sets of 5x5 PITOT traverses established that the profile at the current meter location is highly inconsistent. Sometimes flow hugged the duct so tightly the traverse (per the equal area method) misses the highest flow against the wall. Other times I see the velocity is relatively flat along one axis and parabolic along the other axis.

The vendor for the thermal "mass" meter (FCI) and a competitor (Kurtz) both agree the current location is a tough place for a meter. They think moving it to the last straight section and calibration from traverse data is more than adequate to make a single point meter accurate. However my Pitot traverse makes me believe they trivialize the impact of square/rectangular duct geometry, especially in light of the fact both would recommend installing flow conditioners to reach rated accuracy, if it were circular duct.

Even with the sensor in final straight run, I fear that the skewed flow, the short straight runs and the double elbow will generate a profile too inconsistent to establish a known accuracy, without some other mechanical change.

 

[danw2]

I can't help you, but your efforts appear to border on square duct flow research.

You've probably already stumbled onto TUV NEL's site and their list of papers,

http://www.tuvnel.com/tuvnel/technical_papers/

largely oriented towards energy gas/oil and multiphase flows, not air in square ducts, but flow nonetheless.

Maybe an email to them citing your efforts might garner a response from a empathetic flow guy who knows someone who knows someone . . . who's has dabbled in such and can point to a paper or two.

A non-Facebook approach to narrowing down that 6 degrees of social separation to one or two, eh?

 

[hollerg]

I hadn't, but I will try contacting them too. Thanks

 

[danw2]

Don't give up on checking back here, it's always (slow) over a weekend and someone might pop up with advice.

 

[KiwiMace]

ASHRAE recommends 7.5D downstream of any disturbance and a clear 3D downstream, so you are not in any ideal spot there. You could try to calibrate your fixed meter based on some duct traverse data at different flowrates - check out the difference between the two and linearize the readings on the DCS/PLC/BMS/whatever computer.

 

[hacksaw]

the reason there is no for large ducts of any shape, regardless of the L/D is that it is nearly impossible to insure a velocity profile and average that can be consistently inferred from a single point measurement.

you are generally forced into 2-d pitot arrays or if not pitot, a 2-d array of velocity measurements, to get meaningful measurements.

Even with flow re-distribution, you still have to allow for the "turbulent flow profile" to get re-established.