Bear,

Thank you for this useful information. Is the best way to use this to measure the pressure using the manometer connected to the port just below the fan and get the vacuum measurement, then find that measurement and speed setting on your chart to get the volumetric flow rate? It appears you also did a few test with a battery charger (the 8,10, and 15v measurements). It would be good to have either a means to convert lower voltages to higher voltages in some manner or charts for other voltages. I haven't had enough coffee this morning to do the correlations yet.

Other thoughts I have is, do you account for the pressure drop in you flow meter in determining the overall pressure drop? Are you flowing the output of the fan through the hose and nozzle? Have you calculated the expected pressure drop through them? I don't have my fluid dynamics books with me at home to calculate expected pressure drops through an abrupt nozzle nor estimates of the Moody pressure drop numbers for piping.

Thanks again for all of this great work
JayM

Hi Jay,

Answering your questions:
Re: Vac->Flow - Yes, that would be the right approach. Note that I'm not fully certain about the variability of flows between fans since the tolerances are not extremely precise. So I'd use this to gain a very ballpark understanding of the flows you have. Using an orifice flowmeter would yield much more precise/accurate data.

Re: Other voltages: Yes, that could be useful. Fan blade RPM would be a good way to determine output when driving the fan at other voltages.

Re: Pressure drop: I used the standard blue tube/outlet to restrict the flow to what would be experienced normally, without having to compute the back pressures, etc. I did have the orifice flowmeter directly after the fan, the drop across the orifice would have been well under 1/4" H2O plus frictional losses in the tube, but I figured I could ignore that.

Cheers,
Bear

bear, out of curiosity, did you run any of these tests without the blue tube and nozzle on the out end? that might be why your runs found a lower vac developed than i saw when i tested the fans originally. i didn't realize you were doing the tests with the hose and nozzle on the output until i saw the above picture.

Bear,

Now that I think about it a little more, it would be interesting to correlate power (current x voltage) with flow rate. That may make comparisons with slightly different fans more accurate. In general, they will have similar losses even with small changes in geometry for the given power input. That will result in the work done on the fluid (pressure x mass flow rate) to be very similar between fans for the same power in.

JayM

I've added a modified version of the spreadsheet, "fan_test_flowmeter-DC.xls", which includes columns for the total air power (flow rate times pressure developed), and efficiency. (air power as a percentage of electrical power input to the motor). The overall efficiency is shockingly low (0-2%), which is not necessarily a bad thing BTW, but it's nice to know that there is room for improvement if it is justifiable.

Let me SWAG a calc for the pressure drop in that blue hose: if it is 1.5" i.d., then at the high end flowrates of 30m3/hr, the gas speed in the line will be 290 inches per second (7.3 m/s). This corresponds to a dynamic pressure of 32.5 Pa, which is about 0.13" H2O column.

If the hose is 8' long, that is 144 diameters. Typical pipe friction losses are one velocity head per 50 diameters, so we'd expect about three velocity heads. A straight discharge to atmosphere (i.e. no diffuser) adds one more velocity head. So all told we would expect to see a drop of "total pressure" of 4 velocity heads over the length of the blue hose and its discharge (3 heads of friction loss, plus 1 head of discharge loss). This is 4 time 0.13", or about 0.52" of water column.

This loss is very roughly the same order of magnitude as the developed suction, which is OK (if it were far bigger, that would indicate an overly restrictive discharge pipe system; if it were far smaller, it would indicate a needlessly oversized ($$$) discharge pipe).

The discharge loss ought to be added on to the suction pressure that the fan develops, in order to give it credit for what it is actually doing.

(Just a minute, I'm going to revise the spreadsheet and add this in....)

(Oops, I see I've made some errors in my previous comments; for one thing, it is 64 diameters, not 144.... ugh.).

Anyhow, I'm about to upload the second revision to my spreadsheet, which includes calculations of the discharge pressure loss...

daniel. very interesting calcs on the flow losses. thank you. but we still need to get clarification from bear. if he set up the output like it is when running on the gek, there is a 1/2" pipe nippe of 6" length at the end of the blue 1.5" hose. this is the nozzle that generates the jet into the swirl burner. i'm sure this nipple creates a bit of loss too. we need to get confirm from bear on what he had set up here.

Hi Daniel, Jim,

Thanks for the calcs Daniel. It would be interesting to slowly work on thermal, fluid, and chemical modelling of the GEK. These kinds of calculations bring us closer...
Also, the new updated spreadsheet didn't seem to be updated, though I didn't compare closely. PBwiki may have some issues with files of the same name being re-uploaded.

The tube included the standard outlet. There was also an approx. 2 foot length of 2' pipe+orifice for the flowmeter in this case.
I could run some tests just on this length of tube/outlet at different air flows to get empirical information if needed.

I'd like to look towards running tests on potential ejector designs for V3.X, but if there is a real need for other tests on the fan, I could run them.

Cheers,
Bear

I'm replying to this over on the discussion board:
http://www.gekgasifier.com/forums/showthread.php?p=90#post90