Owen-made Jet Impingement Cooling: Part One - The Slot

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Detailed How-To – Owen Stevens

I was intrigued by a waterblock I saw here on the forums that was said to use jet impingement cooling. I knew a little about this from my thermal/fluids engineering background. I went on the web and did some searching and found a fair amount of technical information was available.

In a nut shell, the idea behind jet impingement cooling is to spray a ‘jet’ high velocity cooling fluid onto a surface. The idea is that the fluid strikes the surface with such force as to make the boundary layer very thin and therefore heat transfer easier.

The boundary layer is a layer of fluid that forms a kind of film over the surface due to the surface’s roughness. In the boundary layer, the fluid is essentially not moving. A good analogy is what we do to a hot cup of coffee or tea. When you blow on the surface to cool the drink, you are creating a high velocity jet of air that impacts on the surface quite strongly. I’m sure most of us have done this and seen that cools the drink very well.

My next challenge was how to incorporate this concept into a waterblock that I could make. I had two ideas that I show in the pictures below. I call the first one my ‘jet block’ and the second one my ‘slot block’.

Diagram 1

Diagram 2

I thought about this a lot and decided to make the slot block first. This article is dedicated to the construction of the slot block. A future article will be dedicated to the jet block. I have also decided to make the two blocks have the same base so I can hopefully make a determination as to which type is better, although this will only be applicable to my designs.

Let’s get started!

First I drew up a crude layout on some graph paper and taped it to some 1/4″ x 2″ x 12″ (~6mm x ~50mm x ~300mm) copper bar stock.

Template

I whacked this with a metal punch like this…

Punch

to get the layout marks you can see here:

Marks

A while back my house was struck by lightning, so I am now the proud owner of a ‘dead’ NF7-S rev 1.1 motherboard, which I used to check the layout like this:

Socket

You can just barely see the dimples through the mounting holes (Thankfully, I had the whole setup attached to my UPS and the UPS manufacturer paid for a replacement motherboard – a better NF7-S rev 2.0! 🙂

The first tools were I used were a 1 ½” (~38mm) hole saw and a 5/8″ (~16mm) drill bit. The hole saw is for wood (later I’ll show why a metal one would have been better) and was purchased at Lowes for ~$5 US. My neighbor loaned me the 5/8″ (~16mm) bit – thanks Barry! Note how I sized them to minimize the metal I’d have to hacksaw later.

Diagram

I clamped the copper bar into my drill press’ cross vise and set up the hole saw first.

Drill

I lined up the pilot bit on the hole saw with my center mark and started drilling.

Start

I took awhile, but here the saw is almost ready to take a bite out of the copper. When drilling in copper, I use a slow speed, lots of WD-40 and patience. Don’t rush it or the copper may get too hot, get ‘sticky’ and may seize up on your drill bit.

Drill

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Owen Stevens aka “Owenator”

The first nibble shows that my bar stock is not quite level. With the size of hole I’m drilling, I’m not too concerned. (I call myself a shade tree machinist for a reason. 🙂

Hole

Here I drilling/cutting in deeper creating some serious ‘chippage’ (‘chip’ is another way of saying the bits of metal made while drilling or machining).

Saw

Can’t forget to cool the hole saw bit! Here is an action photo of me spraying the hole saw with WD-40 to cool and lubricate it. It also knocks off some of the smaller chips.

Spray

Back in action! It looks a bit messy but this is the way I cut copper:

Mess

The hole is beginning to take shape with this nice groove.

Groove

Almost there!

Progress

Finally I hit the wood backstop! The hole is finished! (Note: always drill with some wood below the work piece to protect your tool and vise, trust me, it’s a good idea. 🙂

Paydirt

A shot of the hole in the bar stock:

Hole

Remember this was a wood hole saw but I used it in metal. Here is the hole saw after I abused it:

Bit

I’m not sure if this hole saw will be of use in the future, but it was pretty cheap. It started out looking like this:

Before

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Owen Stevens aka “Owenator”

Here I am lining up for the first 5/8″ (~16mm) hole.

Line up

Starting out the 5/8″ (~16mm) hole. Note the small bubbles and general messy look from the initial WD-40 lubrication.

Start

This bit drills out a lot of metal so you have to back off often to clean up the area and cool and lubricate the bit.

Mess

Here the hole is done and boy, those chips are piling up!

Messier

Some words on chips: I have made a few things out of metal in my day and it is always a good idea to clean up as you go. For one thing, you don’t want a mess obscuring your view of the work piece. And you don’t want a new chip throwing the old pile all over you and your work area. Here are some of the chips I cleaned up with a small brush, aptly named a ‘chip brush’. (I wonder if anyone else knows where this pie dish is from? I do! : )

Pan

Here you can see how my careful planning (or was it luck?) paid off! The 1 ½” (~38mm) hole and 5/8″ (~16mm) holes intersect!

Intersect

I repeated the 5/8″ (~16mm) for the other side. Here I show the holes and the layout graph paper. So far, everything is proceeding according to my plan! (Insert diabolical laugh : )

Compare

Now what did I do with the little triangular bits you may ask?

Bits

I put the copper in my vise and hack sawed the triangular bits off like so:

Vise

I don’t like hack sawing, so I didn’t take many pictures of this. Here is the roughed out shape:

Rough

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Owen Stevens aka “Owenator”

Now on to the part I think is tedious, filing. Here is my assortment of files:

Tools

After quite a lot of elbow grease, I cleaned it up to this. (Note to self, BUY A DREMEL!!)

Clean

I got out my circular saw with a metal cutting blade…

Saw

And cut out these three pieces from the foot long copper bar stock I started with.

Stock

The metal saw makes a big mess, so I wear a dust mask. This sample of floor dust shows why. Yes, that is all copper dust. The saw actually grinds more than cuts. You DO NOT want to breathe in copper dust, so always wear a dust mask when dust could be made.

Dust

While we’re on the safety issue, here is the other safety gear I used. Maybe I’m being a safety nut, but like Mark Twain said “I’d rather be safe 100 times, than dead once”. Here’s some of my safety gear: I wore the heavy leather gloves only when soldering, the rest of the time I wear latex gloves. I take care around spinning objects because latex gloves can get caught in them.

Safe

A few moments on the belt sander cleans up the base and mid section nicely.

Sand

I tried something new with this block, it is called solder paste. I got it for ~$5 US at Radio Shack.

Solder

The instructions say to spread enough on the things to be soldered to form a ‘good joint’, huh? I gave it a shot.

Paste

The stuff spreads like sand mixed into in petroleum jelly, which means not very easily. I put the two pieces together and set them on top of my vise. I tried to make sure that there was a bead around the inside. (I didn’t clamp it in the vise because I thought I’d have to heat the whole vise to melt the paste).

Vise

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Owen Stevens aka “Owenator”

According to the instructions, you just heat the paste until it ‘flows into the joint’. OK, I tried this with my torch.

Vise 1

I wasn’t entirely happy with the paste’s joint, so I ended up heating the two pieces on my electric stove burner and adding solder. I always find it kind of funny to be soldering on the kitchen stove, but hey it works! Sorry no pictures of that. (Or my wife complaining about the stink. Me, I kinda’ like those “industrial” smells.)

One key element of my slot block is the idea of a slot shaped jet. All I had to do to make this was to squeeze some ½” (~13mm) ID copper tubing in my vise. I had to gradually ‘smoosh’ it down to the desired shape (yes, I know very technical jargon :). Here is a top view. You can see the ‘smoosh’ marks:

Slot

The side shows that the ‘smooshing’ was a little step wise:

Smoosh

A view of the slot from the end. One trick I learned was to stop every once and a while and pry the slot open, because the ‘smooshing’ process tends to close the center but not the edges. This is more art than science, but hey – I’m skilled. 🙂

Art

So how do I plan to get that to stay put in the top of my water block, you may ask? Well, it took some creativity and patience. First I found the center of my top piece and made some pre-drill punch marks for the slot and I also laid out the outlets.

Pre

First I drilled out the 5/8″ (~16mm) holes for the outlets. Here I show how the outlet lines up with the base’s shape:

Line up

Another shot of the two outlet holes prior to starting on the slot:

Outlet

How to make the slot was a bit of a question in my mind. What I ended up doing was drilling almost interlocking holes. Here is the setup I used:

Setup

Here are the holes. They don’t quite interlock:

Holes

To make the holes interlock, I first tried to smash the separating metal down with a chisel (brute force works – sometimes! Besides, sometimes it is very satisfying for some reason to pound on things with a hammer! But I digress.) This didn’t work very well as you can see (DUH – what a mess!).

NG

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Owen Stevens aka “Owenator”

Then I remembered my end mills! I set up the 1/8″ (~3mm) end mill and cleaned out the slot.

Mill

Not quite there yet, but a lot better.

Progress

I had to file out the hole a bit to make it fit, but it finally did!

Fit

I ‘pressed’ the copper inlet into the base by holding it in my vise and clamping it in.

Vise

Then I whacked it with a hammer to make it snug. (I already mentioned my penchant for hammering, didn’t I? 🙂 The end of the ‘smooshed’ tube projected into the block cavity, which at the time I thought would be fine.

Snug

Then I pressed in the copper inlet and outlet. The top is shown here on the right prior to soldering:

Prior

Her is how the base looked prior to soldering. I sized the holed for the pipe at 5/8″, which fits snugly with my 5/8″ (~16mm) OD ½” (~13mm) ID copper pipe.

Base

Somehow I missed the soldering pictures – the only one I have is of the inlet covered with flux. I did the same with the outlets and then soldered them together with my torch.

Inlet

You can kind of see the solder in this shot of me drilling the four mounting holes in the top. (This also shows how I messed up the bottom side of the top trying to sand around the slot.)

Drill

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Owen Stevens aka “Owenator”

The same four mounting holes being drilled in the base:

Drill

With my mounting method I need to countersink the bottom side of the holes to keep the standoffs that are attached to my motherboard from holding up the base.

Holes

I decided to try to make the exit tube entrances as smooth as possible, to I used a conical grinding stone and did this:

Top

Here’s a close up of the whole. The idea it to take away and sharp edges to make the outgoing flow move more smoothly.

Close

I had decided earlier that I wanted to join the top and bottom with something other than solder. I finally settled on bolting them together with a gasket. I had to be sure that the holes I drilled left enough metal around the edges of the internal cavity for a good seal. I used an old method to transfer the cavity shape to my layout graph paper. I rubbed my dirty fingers around the edge to get this impression. (The center is torn from the punch I used earlier when making the slot.)

Xfer

I worked backwards by attaching the layout graph paper to the top.

Graph

And then I picked my hole locations. I tried to make them equally spaced around the perimeter and also not to close to the edges. Here the marks are shown in pencil prior to punching it:

Vise

I needed to make small holes just big enough for my mounting hardware. I’ve had so much trouble in the past drilling small holes with my drill press, so I tried a different technique. I used my hand drill with the clutch set very low, so if the bit bound up, the clutch would keep the drill from turning. My drill press, on the other hand, has so much torque it routinely snaps of bits when the bind up. Here I am starting off with the top in my vise:

Top

To transfer the holes to the base, I bolted the top and bottom together and the drilled using the top as my guide. (You can also see the soldering job on the top here. I like to make a thick solder joint to withstand the abuse of installing tubing.)

Vise

The holes transferred like a charm!

Vise 1

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Owen Stevens aka “Owenator”

Here is a shot of the bottom showing how I countersunk the holes for the bolts I used (shown also):

Bolts

It was a tight fit with the mounting holes but all the eight bolts fit just fine 🙂

8

Here is a test fit of the top. I didn’t like how short the bolts were, though.

Test

I had initially made a paper gasket and assembled the lot with car gasket sealer. That leaked during my ‘bubble test’. While at Home Depot to get longer bolts, I asked about rubber gaskets. The plumbing guy showed my this orange stuff:

Gasket

It is a 8″ x 8″( ~200mm x ~200mm) sheet of 1/16″ thick (~1.5mm) rubber. Thank you Home Depot! This stuff was just what I was looking for – terrific!

Sheet

I marked and cut off a 2″ x 4″ (~50mm x ~100mm) piece.

Trace

Making the holes for the bolts was tedious but had to be done. I used a very small razor knife for this.

Mark

I laid this out on my block and cut out the center for my slot. (Note, that I gave up and sanded off the part of the slot that projected into the base – I may redo this later.)

Layout

These are the bolts and nuts I used:

Bolts

And don’t forget the lock washers to keep the thing tight. Of course, if you tighten small bolts too tight, I’ve heard that they snap off. (Not that I know from personal experience or anything. OK, OK – I had to return to the store to get a second bag of bolts after I ‘killed’ a few, nobody’s perfect! 🙂

Washers

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Owen Stevens aka “Owenator”

Here is how the gasket fit with the base. (See anything that looks wrong? See my post script at the end of the article.):

Fit

A side shot of the top bolted on. This worked quite well and it passed my bubble test just fine.

Mount

Then I put it into my system, as seen here:

PC

A close up showing the ‘Y’ connector (I thought I had a shot of these, but I guess I don’t. I think they are the same as the ½” (~13mm) ones Danger Den sells.) and the 3″ (~76mm) bolts use to attach it. I use long bolts so I can initially attach it without compressing the springs I use for force. Then I just dial some nuts down to compress the block to the CPU.

Bolts

A extreme close up shot showing the new tubing I am using:

Close

New tubing you say? What do I mean? I found that my Home depot now stocks ½” (~13mm) ID by ¾” (~19mm) OD clear tubing. Why do I use this stuff? The walls are 1/8″ (~3mm) thick so it is very hard to kink. Here is a comparison of ‘standard’ 5/8″ (~16mm) OD vs the new ¾” (~19mm) OD tubing. It doesn’t look it, but they are both the same ID just the one on the left has a thicker wall. Using this tubing allowed me to ditch my old elbows and go to a much freer flowing system.

Tubing

I’ve only had it in my box for a little while, but the performance specs are Ambient Temp 23C, Water Temp 26C, Idle CPU Temp 36C, Load CPU Temp 40C. This may get better as the thermal grease settles in, but I wasn’t that patient in writing this up. I hope you don’t mind. 🙂 Overall another fun project for me, I hope you enjoyed it.

A parting composite shot of my system:

Complete

PS: I said I would tell you what was wrong with the gasket. OK, I’ll come clean. I forgot to cut out the gasket for the exit holes, take a look.

Look

No, that orange stuff is not copper, it’s the gasket! I actually had this mounted in my system and was running it before I figured it out. At first I thought it was just a bad design but when going through the pictures for this article, I caught it. The amazing part to me is that I was actually able to install this and cool my system without frying my CPU! Apparently the water was able to squeeze around the slot gasket opening and back to the pump. In this configuration the temps were more like 42C idle – ouch!

Here is how I fixed the gasket – I just cut out the entire center.

Gasket

So now it looks like this re-assembled:

Done

This was the final version that I quoted temps for above.

Owen Stevens aka “Owenator”

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