Sand blasting. Any benefit?

[Volenti]

Originally posted by gmat
About sandblasted surface: i've been thinking about that for a long time... I think that there's a limit under which surface features act by 'stopping' water molecules, and not 'throwing them around' as a bigger feature does.
I think there's a limit dictated by water viscosity, or surface tension, or both. Under this limit, surface features would only thicken the boundary layer, thus reducing the heat transfer.

(personal experience): in ultra-vacuum chambers one heats the chamber walls to make them reject water & gases molecules trapped by adsorption. (Try a google on 'adsorption' to know what it's about). Because any surface is not perfect, its surface features would trap other materials molecules that come into contact.
I tend to link that 'trapping' effect to the boundary layer between a solid and a fluid in motion. Maybe i'm wrong, it's just pure speculation.
If i'm right, then mirror finished waterblock transfer surfaces would perform better than sandblasted (or rough) ones.

The limit in surface feature size would be in accordance to the minimum radius of a water drop against that surface. Does that make sense ?

perhaps a test of this would be to have 2 simple flat plate impingement blocks, one with a mirrior finish under the jet and the other bead/sand blasted?

billA?

[Since87]

There's been some work on reducing drag on boats and planes with small surface texture features. (Google "Drag Texture Americas Cup")

One speculation I've seen on that subject is that the drag reduction is due to the relative sizes of the texture features and vortices formed. (Haven't looked enough to know if there is anything more definite than speculation.) Did find, that for boats, smoother than 400 grit doesn't seem to affect drag.

I agree that at some scale roughness probably doesn't increase turbulence. (at least in a manner beneficial to cooling) What that scale is, is the question.

In looking at the info on boat hulls I saw it mentioned repeatedly that surface tension doesn't play any significant role in drag.

I'd guess the 'minimum water drop radius' is probably not relevant, but I'm way out of my league with this stuff.

[BillA]

Originally posted by Volenti
perhaps a test of this would be to have 2 simple flat plate impingement blocks, one with a mirrior finish under the jet and the other bead/sand blasted?

billA?

I'll send someone a bp if they wish to mask the sealing edges, blast it, and send it back so I can test

???

[Since87]

I can have it sandblasted, but my brief research suggests bead blasting would be better. My impression is that bead blasting does more denting than cutting, and I'd guess that's preferable.

I'll look into whether I could get it beadblasted. (As I understand it, same tools, just different media.)

[BillA]

grit blasting will be far more effective than bead blasting due to the greater surface profile angularity
- and believe me, with very coarse grit it will be rough as a cob

it must be 'brush blasted' due to copper's softness

[Since87]

What I can get done cheaply is with 4020 quartz. (The bag says 20% of the grit doesn't pass through a 40 grit mesh) Checking with McMaster - that seems to be about as coarse as it gets.

I'll see if I can find a scrap of half-hard C110 and send you a sample. If I can't find some C110, I can send you a sample made from some ultra pure, dead-soft copper sheet, but I don't know how representative that would be. (I suppose if the dead-soft stuff isn't eroded too excessively, the half-hard C110 certainly won't be.)

Anyway, I'll see what I can come up with.

[gmat]

Originally posted by Since87

In looking at the info on boat hulls I saw it mentioned repeatedly that surface tension doesn't play any significant role in drag.

Mhh what i've seen on submarines torpedoes etc. is that they use a special surface treatment, to actually *create* that boundary layer, so that surrounding water will "slip" over a layer of water instead of hitting the hard surface. That surface is like shark skin... rough and hard. There are different approaches though, with "soft skins", dolphin-like, but their surface is really soft and elastic so it's not relevant here.
For me, a hydrophilic material (alu & copper both apply) with a rough surface will trap water molecules and keep em due to viscosity / surface tension. Oh and additives such as WW and others modify those parameters indeed...

[BillA]

Since87
let me send you a 462B bp (the same smooth one tested)

[Since87]

Bill,

Ok, I'll email my address. I'll have the person doing it, do some practice runs on scrap copper to get a feel for sandblasting copper, before working on the block.

Let me know what area you want. (Same circle as the punched block?) If you draw the area on the block with a Sharpie pen, I'll mask around it.