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Saturday, April 7, 2018

Venturi Flow Meter Testing, Part 2

I completed the test setup. I'll let the pictures do most of the talking:

Drying after washing after sanding ports flush to inside

Trick from rocketry: coat edges of cardboard tubes with superglue and sand.

Test fit

Screwed together and test fit for cables

PWM controller and ghetto ammeter wiring

Ignore shitty chinese fan cable wire colors

Powering from desktop PSU

It lives!

Making ruler parallel to gravity

Manometers filled and working
Filling the manometers was a pain. I filled them with another tube + straight connector because I didn't have a funnel small enough. The ID of the tubing is so small that the water's surface tension allows bubbles to stay in the tubing. I had to flick the tubes a lot to get the bubbles to float out.

Anyways, it works, but there are two major problems. 1. The assumption about the downstream components not affecting the Phi's airflow is definitely violated. I can feel significantly higher flow rate out of the holes/slots in the Phi with the venturi attached. 2. The venturi causes so much pressure loss that the fan is at a point in its operating curve where it's only pushing about 60% of what I need it to. Without the venturi, and the fan on full throttle, the pressure drop across the Phi is significantly higher than spec, which means the fan is probably pushing a significantly higher flow rate than spec. That's not surprising considering the fan should be way over powered. When the venturi is attached, the pressure drop across the phi is significantly lower than spec, which supports the low flow rate measurements. *sigh...

I can't do anything about #1 other than tape or partially tape over the holes. I can't increase the fan's input voltage because I don't have an adjustable power supply here, and the fan is already at about 75% of max power. I checked, and the pipes don't cause much pressure drop, which makes sense considering the flow is fairly slow. I'm guessing that the venturi diameter ratio (beta =~0.38) is too extreme, which is resulting in separation in the diffuser and high pressure loss instead of recovery. The only thing I can do is redesign the venturi with a larger diameter restriction, but then I lose accuracy. For example, increasing the diameter of the restriction from 20 to 30 mm (beta from 0.38 to 0.59) reduces the expected total change in fluid height (and thus pressure difference) by more than 80%. It will still be readable, so that's probably what I'll do. I gotta fix the 3D printer first, though.

Backup plan: print best guestimate designs for the adapters and test them on powered Phi's. If the Phi's don't overheat, then they're probably ok. 

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