Should we care about the no-slip condition with flow? - Page 3

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wkmok1

#21: Post by wkmok1 »

Good point. The super simplified model would assume a restriction that results in said flow rate located many entrance lengths down the pipe. Definitely says nothing about microscopic effects of how water swirls around coffee particles.
Winston

jrham12

#22: Post by jrham12 »

I believe you are over-thinking it... Bear in mind that in order for a fluid to develop into a natural laminar or turbulent profile, you have to have a certain entrance length of undisturbed pipe. That's not going to happen in the pathways of an espresso machine. The constant elbows, orifices, distribution devices (shower screen) will probably produce some wild swirl and a distorted velocity profile which works against development of a laminar flow profile.

Also bear in mind that the "no-slip condition" (ie: boundary layer) will also exist even in a fully turbulent flow profile when the Reynolds number is > 4000. But is it viscosity dependent so it will be exceedingly small on a fluid with the viscosity of water. I think the fact that the coffee particles are literally in contact with the inside of the basket will negate this concern.

Just my 2 cent opinion... :)

Josh

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DamianWarS (original poster)

#23: Post by DamianWarS (original poster) »

jrham12 wrote:I believe you are over-thinking it... Bear in mind that in order for a fluid to develop into a natural laminar or turbulent profile, you have to have a certain entrance length of undisturbed pipe. That's not going to happen in the pathways of an espresso machine. The constant elbows, orifices, distribution devices (shower screen) will probably produce some wild swirl and a distorted velocity profile which works against development of a laminar flow profile.

Also bear in mind that the "no-slip condition" (ie: boundary layer) will also exist even in a fully turbulent flow profile when the Reynolds number is > 4000. But is it viscosity dependent so it will be exceedingly small on a fluid with the viscosity of water. I think the fact that the coffee particles are literally in contact with the inside of the basket will negate this concern.

Just my 2 cent opinion... :)

Josh
I actually contacted a scientist whose expertise is in adhesion, surfactants, nano coatings and solubility science. He was a really nice guy and told me a lot of people have big dreams for these nano coatings but they simple don't work for a lot of these applications. He is quoted here:
Many years ago I made (literally) kilometres of superhydrophobic surfaces and have worked with them a lot. I can assure you that they are completely irrelevant to your needs. The core problem is that under the slightest pressure and/or oily contaminant (and you have both!) they cease to be anything other than slightly rough, fully wetted surfaces. The academic literature has been full of credulous papers on how amazing it would be if they did X with a superhydrophobic surface, but the total number of useful applications remains pretty close to zero. We had warned the people who wanted us to make lots of superhydrophobic film that it was a waste of time, but they insisted, then tested, then abandoned the project. You can't fight the laws of physics.
So I don't think a nano coating spray is going to do much but no slip is still happening however it may not be the problem I'm making it out to be. I wonder if there is a way to isolate the extractions from the walls to the center (without adding walls to the PF) and see how they differ to determine even if it's worth improving extraction along the walls to begin with. You would have to look at the puck and separate it out to see how coffee closest to the walls is any different to the rest of the coffee.