Should we care about the no-slip condition with flow? - Page 2

Want to talk espresso but not sure which forum? If so, this is the right one.
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#11: Post by Marcelnl »

My model is:
grounds go in, hot water goes through, espresso comes out :mrgreen:
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#12: Post by Jeff »

Power's back, and I apologize that I've lost some of my thoughts.

I think you're on a right track to getting more, good coffee out of the same weight of grinds. Uneven extraction seems to not only reduce the quality of the cup, but potentially lead to greater coffee usage.

I could do some hand-wavy arguments about the volume of grinds impacted by the basket boundary conditions, but I'd have to be even more hand-wavy about the volume impacted by a channel and how many channels there are in the puck.

Home-barista prep can be insanely time consuming compared to the demands on a barista in a busy cafe. What that obsessiveness has shown is that through better puck prep and, to some extent, more uniform grinders (either or both shot-to-shot, as well as particle-size distribution), the EY can be raised significantly from the "loosey-goosey" prep. Figuring out how to take some of this into the world of cafes seems, to me, a more fruitful path than basket coatings that probably won't survive the rigors of use and cleaning in a commercial environment. While the OCD is an interesting step, at least as I understand it, it was motivated primarily by the rules of barista competitions, not that it necessarily improved "believable" in-cup testing. The idea that grind homogenization and leveling could be done effectively, in a couple seconds, "provably" increasing either or both quality or cups/kg, seems ripe for exploration. Might even be able to sell me on a "distributor" that could be convincingly shown to do more for shot quality than some acupuncture needles in a cork and 30 seconds of time.

I've just started playing around intentionally with the high-flow techniques suggested by Hendon, et. al. I've had unintentional gushers in the past that surprised me in their drinkability and am looking forward to exploring the approach in a bit more structured way. (Now if I could only justify $300 for an Atago, then $150 for a centrifuge, ...)


#13: Post by wkmok1 »

Are the fluid flow figures in post #1 of a 58mm diameter pipe the correct starting point for this analysis? As member Jeff pointed out, the slug of water is travelling forward at approx 0.5mm per second. To me, this sounds really slow; like move ahead by 1mm in the time it takes to say One-Mississippi twice, kind of slow. In such a scenario, is there sufficient "shear" between the water touching the wall vs water a short distance away, so as to cause turbulence?


#14: Post by jpender »

That figure is just an "any pipe" diagram that you would find in a textbook. It isn't quantitative.

I think Jeff estimated 1mm/s flow which is just ballpark for the water above the puck. I don't know what happens with such a short section of pipe but I don't think it's long enough for the top diagram of laminar flow to fully develop. And in any case the coffee grounds change things significantly. Within the puck the water velocity will be higher than 1mm/s as the effective cross sectional area will be smaller due to the grounds.

What's it all mean? Probably just a COVID fantasy.


#15: Post by wkmok1 »

I think Jeff was estimating half an mm per sec. If one had a 58mm pipe, and water inside is moving at this rate, how significant is the length with respect to the dynamics of the flow?


#16: Post by jpender »

Jeff wrote:The area of a 58 mm basket is around 27 cm^2 so a bulk velocity of 1 mm/s would be around 2.7 ml/s, a bit higher than typical 36 g in 25 s shots flow.
He actually said 1mm/s but we can do math ourselves. 36g in 25s is an average flow rate of about 1.4cm^3/s. With a diameter of 5.8cm that means the average velocity is 1.4cm^3/s / (pi * (5.8cm/2)^2) = 0.05cm/s, or 0.5mm/s. So you're right. He estimated 0.5mm/s -- even though he didn't say that explicitly.

Of course the flow rate isn't constant over time so the peak velocity, averaged over the area of the cylinder, will be higher.

I never studied fluid dynamics. There are some basic equations you can find online, wikipedia or whatever. You can calculate the Reynolds number from the information we have and from that determine whether or not to expect laminar flow, at least in just a pipe. You can also calculate the "entrance length", which is a measure of how far down the pipe from the entry that fully laminar flow really begins. It's not hard to do. Does your computer have a calculator app?


#17: Post by wkmok1 »

Thanks, John. Never heard of Reynold's number before. I found a Reynolds number calculator on line though. Used :

1000 kg/m^3 for density
0.0005 m/s for velocity
0.058 m for diameter
0.00032 Ns/m^2 for viscosity at 90 degrees C

The calculator gave a Reynolds number of Re=91 and declared the flow as laminar. Entrance Length is given by Le=0.05*Re*D = 0.05*91*58mm=263mm.

My understanding is after Entrance Length the flow is faster in the middle and slower towards the wall, and this velocity profile will be maintained indefinitely for any length of pipe beyond that point. Since the basket is around 10% of Le, we can safely assume we have not achieved laminar flow. The water column stays as a column with essentially the same velocity over the entire circular cross-section. The static layer of water at the basket wall has not been able to spread it influence to encompass the rest of the column yet.

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#18: Post by Marcelnl »

think you need to look up flow dynamics in filtration units or something similar. You are forgetting the 9 bar that drops over time to 4 or so and a bunch of other things. A prepped basket is very different from a straight section of pipe.

I predict that doing the experiment is faster than plowing through a load of complex fluid dynamics. ... sAllowed=y
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#19: Post by jpender »

I don't think we've forgotten about the coffee. :-)

The OP wants to make hydrophobic basket walls (or moving walls?) to minimize friction but he hasn't bothered to calculate what the expected velocity distribution in an unmodified empty basket even looks like, never mind one with coffee in it.

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#20: Post by Marcelnl »

even when you change something that makes the wall resistance different, will that affect extraction in any way?
I reckon that if you sand the wall of the basket so water molecules stick to it, like a race sailboat you optimise flow more than with any other compound as the shear rate of water on water is lowest.
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