Search This Blog

Friday, April 13, 2018

Venturi Flow Meter Testing, Part 3

I installed the new venturi meter and re-ran the previous tests. As a reminder, the goal is to determine the flow rate and pressure drop in the fan adapters so that fans can be selected. The previous venturi had a very high pressure drop across it. My guess is that it's contraction ratio was too high and the flow was detaching in the diffuser section. I designed a new one with a smaller contraction ratio. The printer is/was still having issues when I printed it, so I had to sand the crap out of it.

Anyways, the results are better than last time, but still not good enough. The fan simply isn't developing enough flow rate. I was about 12% short according to the venturi flow meter. This made me go back and have a look at the fan curve. Turns out that, at the pressure the fan is developing (according to the fan adapter's manometer referenced to ambient), the fan should be producing about 2-3 times as much flow rate as I'm measuring with the venturi flow meter. Unfortunately, I have no way of knowing which is wrong. Either the fan is producing far less flow rate and/or pressure than it is supposed to, or the manometers are reading incorrectly. The manometers are super simple devices...the only way they could be wrong is if the static pressure taps aren't actually measuring pure static pressure, which is certainly possible since I have no way of visualizing the flow inside these parts. I think it's more likely that the fan is simply not following its pressure vs. flow rate curve. I checked the input voltage, and it's right around 12.1V when the fan is connected directly (no controller). 12V is the rated voltage and what the curve was generated with. There's some loss through the PWM controller, which is why I've been running the tests with and with out it. So if it's not a power issue, maybe it's a blockage issue. The single phi adapter has a pretty severe contraction in it...perhaps that's simply blocking some of the area. In fact, looking at it from the fan side, it looks like about 1/2 to 2/3 of the area is taken by the contraction, so that might make sense.

I printed and tested a longer version (+30mm) of the single phi fan adapter. Lengthening it makes the contraction less severe. However, this one actually resulted in a lower flow rate, meaning it had a higher deltaP than the short single phi fan adapter. This still doesn't make a lot of sense to me. The larger surface area increases viscous drag loss, but the less severe contraction should have made up for it. Perhaps the contraction is still too severe. It's not worth making it much longer because a blower type fan will be more compact, then, and if these flow rate numbers are right, then possibly more efficient as well. I didn't bother testing this one any more.

Next, I hooked up the mock phi to the single phi fan adapter. The goal of these tests was to tune the restriction plate holes so that, at the same throttle, the pressure reading at the fan adapter tap referenced to ambient (which is also the pressure developed by the fan and the pressure drop across whatever comes after the adapter) was the same as the fan adapter + real phi + pci bracket (no venturi). I had estimated the hole size and number of holes using thin perforated plate theory, so I expected to be close. Turns out it was exactly right (within measurement ability). Nice.

Then I attached the mock phi and the real phi to the dual phi fan adapter, and ran some more tests.

Dual Phi adapter, mock Phi, real Phi with exit adapter
With the real phi in the bottom position, the fan was pushing about 73% of the required flow rate according to the venturi. With the real phi in the top position, the fan was pushing about 71% of the required flow rate. (pretty good balance for first try...will tune it). However, that's through one Phi. Remember that, with the single Phi adapter, it was pushing about 88% of required flow rate. So with the dual Phi adapter, the fan is actually pushing about 70% more flow rate than with the single Phi. This partially supports my blockage theory. However, it's still about 50% under what the fan should be pushing at that pressure. Looking at the fan side of the dual phi adapter, it looks like maybe 1/3 of it is blocked, so maybe it's still a good theory. I tried taking the fan grill off the back, but that didn't seem to make any measurable difference. Taking the venturi off, the fan adapter pressure corresponds to a flow rate of about 180 m^3/hr on the fan way it's hitting that.

Now, I did not tune the mock phi for the venturi attached, so I probably should have made the mock phi more restrictive so it matches the real phi+venturi. However, if I completely taped off all of the mock phi exits (100% restrictive), then the flow rate in the real Phi only increases about 5-10%, which is still way under the flow rate required. This also supports my fan blockage theory.

I tried taping off half of the bottom half of a single phi fan adapter, and installing it with the real phi and venturi. If the flow rate was the same as with no tape, then 100% of this problem is likely blockage. However, the flow rate dropped by almost half...which actually makes sense, but not given the other results of these tests. Maybe the tape being right at the exit plane is causing more severe blade stall than the adapter with no tape.

Anyways, either the pressure taps are reading some dynamic pressure or the fan is operating below its published pressure vs flow rate curve. I need to figure out which.

Assuming that blockage is the cause of the fan not operating on it's pressure vs. flow rate curve, how can I improve the efficiency of these adapters? Intuitively, smoother transitions/contractions should help, but as the extended single phi tests showed, they do not. Perhaps if the adapters were made much longer, and the area changes very gradual, then it would help, but there isn't room for that in a desktop computer, and the blower fans, which already have a compact exit area, would be better then. When I was doing initial fan selection, I assumed that the fan adapters would be fairly efficient. That may not actually be the case, particularly for a 80x80mm axial fan to a single phi. Assuming the fan is under performing, the efficiency seems to be ok for a dual Phi.

I can do a fairly easy check by hooking the fan directly up to the venturi. I was too impatient to wait for something to print, so I created a long paper cone to adapt from the fan to the tube before the venturi. Lots of tape for sealing.

It should be good enough...essentially no blockage. The result at full power was about 82 m^3/hr according to the venturi, which is about the same as with the dual Phi card setup. This kind of makes sense because only half of the flow is going through the lossier tubes+venturi with the dual Phi setup, and all of it is going through the tubes + venturi here (higher velocity = higher dP), which just happens to result in a deltaP similar to the dual Phi setup. I poked/cut a small hole in the side of the paper near the fan and put the other manometer tube in, leaving the other end ambient. Wiggling it around some (making sure it's not pointed into the flow), this gave me a pressure of about 186 Pa, which corresponds to about 162 m^3/hr on the fan curve. With the adapter and single Phi at this pressure, the flow rate is about 50 m^3/hr, so perhaps blockage is partially to blame, but clearly there is something else going on. Either the venturi is reading about 1/2 the flow rate, or the fan is outputting about half of its spec flow rate. I doubled checked all of my math...I'm multiplying the manometer height change by 2, the venturi equation and conversion factors are all correct, etc. Without the venturi the pressure is about 155 Pa, which corresponds to about 170 m^3/hr on the fan curve. So the venturi seems to be fairly low loss, which is good. But what is causing the discrepancy between the fan curve and what I'm measuring?

I'd need a hot wire anemometer or a pitot tube rake in order to measure velocity directly, so that's out. I can do a static stall test, though. I took the venturi off and taped off the end of the tube that leads to the venturi. I also added more tape to fan -paper cone interface, taped off the hole I cut there, and checked for any other holes. I used the pressure tap in the end of the tube (the original venturi high pressure port) to measure static pressure, with the other end open to ambient. I got about 470 Pa. I felt around for any leaks...there probably were some very minor ones, but wrapping my hands around various parts didn't move the manometer more than about 0.5mm. Besides, the published spec is 607 error would have to be huge to be off by that much. Plus, the static pressure tap is basically a perfect pressure tap at this point because the flow rate over it at the end of the sealed tube is 0. This test lends support behind the motor under performing theory. In fact, some of the measurement points seem to fall just below the next fan in the series' curve, so perhaps the variant Nidec made for supermicro is actually closer to that variant. That said, the current drawn at 12 V agrees more with the first variant...

Anyways, I think my static ports and manometers are probably fine. For whatever reason, the fan is under performing by about half. Blockage from the Phi adapters makes this problem worse, but I've already minimized that, so there's not much more I can do about that. I'll take another look at blowers for single Phi setups. IIRC, the smallest blower that could meet the required flow rate and pressure drew about 18W, which is right around what this Nidec axial fan is drawing now.

Another thing worth mentioning: This required flow rate is worst case scenario, which is 45 C inlet temperature and running the Phi at full power. Properly cooled desktops will not have 45 C air inside them. The measured flow rates achieved with the current dual Phi setup are good for ~39 C inlet air. The measured flow rate achieved with the current single Phi setup is good for ~43 C inlet air. I'm really close to where I wanted to be. These adapters should work fine.

The test goals were partially met. I was able to estimate the pressure drop in the adapter + pci bracket for the single Phi setup. Since my measurement error is fixed, the dual phi setup had relatively more error for this because the dual adapter's dP is lower than the single phi adapter. However, I was unable to determine primary vs. secondary flow due to all of the holes in the secondary flow path of the Phi. I've simply scaled the area of the secondary flow inlet assuming a uniform velocity profile, which is not a good assumption, but it's the best I can do. Overall, I can size fans based on these results. However, this is all based on the partially supported but unconfirmed assumption that the fan I was using was under performing significantly, and that I didn't have another test setup error. If I had a test setup error and the fan was performing correctly, then it means all of my flow rate measurements were low, which means that this fan and these adapters are more than sufficient. I need to try another fan to see if it's just this one under performing. If that one also under performs, then either it's common to embellish the published fan curves and I need to add some serious safety factors to the fan sizing formula, or both fans are actually correct and I had some unknown measurement error that gave extremely conservative results.

Actual Phi testing to commence next week...

No comments:

Post a Comment