Home-Barista.com

the no-slip condition is described in fluid mechanics where the fluid will have zero velocity relative to the boundary. what does this mean for espresso? an espresso basket is a cylinder shape like a small section of a pipe and with coffee, there is an added porous medium. The best conditions would produce a laminar flow where the flow next to the surface of the walls is 0 and the highest velocity would be in the center. because irregular coffee particles are thrown into the mix and packed together the no-slip condition applies to each coffee particle or groupings and can create a turbulent flow as contrasted in the picture below. The better we get at puck prep the closer we get to perfect flow which would produce laminar flow.



the best we can achieve is a transitional flow (basically a turbulent laminar flow) because we obviously can never get rid of the coffee and we would work at decreasing the Reynolds number which brings us closer to perfect laminar flow. if it was always laminar flow we would never get donut shots so obviously there are other things influencing the flow. but is laminar flow the best goal to aim for and are we not just asking for forms of channeling at the point of highest velocity (the center) and under-extracted coffee at the walls? the flow near the surface of the walls would be extracting through diffusion only and diffusion is going to be slower than the flow, so wouldn't it be better to have an even flow through the entire puck and work at eliminating the no-slip condition altogether?

The no-slip condition doesn't just work on stationary walls but also for moving walls. if the walls are moving (and there is no other influence) the flow relative to the surface of the walls would be zero (so would be moving with the walls) and the fluid next to those would slip a little bit more, then next to those a little bit more, etc... producing a reverse laminar flow. So at first, I thought how could you make a PF with moving walls to match the flow at the highest velocity and counteract the no-slip condition by controlling it? but a PF like that would be a bit ridiculous, essentially a long pipe that would be pressed through the PF and act as moving walls. Perhaps it could work as a sort of kick-starter hand press gizmo but I think an easier solution would be a superhydrophobic coating on the PF walls which would do a good job at eliminating the no-slip condition. Water wouldn't stick to the surface but rather bead up on the walls and immediately exit.

if there was no resistance at the walls it will have an effect at increasing the flow and probably have higher EY as the sides would be extracted at the same rate as the centre (no laminar flow effect). Typically this is an excuse to dose down and grind finer but in this case it may just be dosing down which would result in the same TDS in the cup as the extraction would be more efficient.

is the no-slip condition a problem in espresso that we just deal with and should we be looking at ways to reduce the no-slip? would a superhydrophobic coated PF be a solution to this? what do you think?

civ wrote:Hello:

What do I think?

Well ...
I think that you are definitely over-thinking it. 8^D!

Check the bottom line: does your espresso taste as you'd like it to taste?

Cheers,

CIV

This is why I posted it in the knockbox section. but it's not a question if I like my espresso... it's a question of how can we improve the efficiency of the extraction. the largest potential benefit of higher EY is not so we all can sip on 30% EY espresso but that we can produce the same results with less coffee. with climate change coffee's future is changing with it, and more efficient methods will reduce the industry's overall footprint. If a busy cafe that serves 200 espresso drinks a day dosed down 1 gram it would be a savings of 73 kilos of roasted coffee every year which would be in the ball park of 90 kilos of green coffee.

Channeling aside, I'm of the belief that the fluid dynamics of the bed overwhelm those of the boundary at the basket. This is an intuitive belief, with no rigorous support.

The velocities are already seemingly slow. 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.
(Please let me know if I got this calculation wrong)

Edit: The goal of more drinks of comparable quality with less coffee is a noble one. I've got some related thoughts that I'll post once we get power back.

I have the same intuitive feeling about this. At the typical flow rate wouldn't an empty 58mm "pipe" not even achieve the full no-slip condition because it's too short in the case of an espresso basket? Meanwhile the dynamics of the flow around the coffee powder will be very complicated, combining likely turbulence with changing viscosity and mass transfer.

But I'll bet a couple of shots of espresso would increase attention and concentration while studying fluid mechanics.

Marcelnl wrote:forget flow ever being laminar or even needing to be laminar. It's hard enough to do real world calculations on more or less undisturbed pipes, believe me I've done a few...
Length, shortness is a factor as it adds flow resistance. So a longer pipe with the same flow and pressure and viscosity will see turbulent flow sooner than a shorter pipe.

Ever flushed without a PF locked in? I pretty much guarantee that is NOT laminar flow. It might at some point become laminar when flow is restricted, but with everything going on, extraction-increasing viscosity-particles swelling, extract being a mix of fluid and gas, flow increasing over time, you probably need some serious modelling to say anything worthwhile.

there's still is the no-slip condition happening at the walls regardless of the quality or velocity of the source going into the PF. at the walls the flow is going to be 0 because the fluid nearest the walls will stick and as you move away from the wall the flow will increase. What would be happening is not a perfect laminar flow but a transitional flow or some hybrid of laminar/turbulent flow. also, the parabolic arc of the laminar flow would probably be far less pronounced and may appear more flattened because of the low velocity and small section. putting a superhydrophobic coating over the basket walls would have an effect of the fluid not sticking to the walls (removing the no-slip). It would increase the flow and change the no-slip condition but how much impact it actually would have is maybe a different story.

I don't have a decent machine to play with, and you would need something that can objectively measure flow like the DE1 to really know the impact You can buy a nano hydrophobic spray and I wonder would happen if you just sprayed the basket with this stuff (although I'm not certain if you can get food-grade stuff). you would have to pull a bunch of shots with a stock basket and a bunch of shots with a stock spayed basket to observed what sort of changes this actually does.

Before that how about modeling it? Put some numbers to it and see if it's even close to measurable.