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Cathar · 10-01-2004, 06:02 PM

Top plate also houses the jet intake plenum chamber, which is just a circular recess in the plate.

Basically I had spent some time studying this basic picture and thinking about JI:

Jet diameter/nozzle separation (z/d) seems to create peak Nusselt numbers at a ratio of 4 < z/d < 6 for a number of studies, due to the shearing of the jet against the surrounding liquid. The jet has a "potential core" which is roughly cone shaped and is also typically around 4-6d in length depending on the initial jet velocity from my understanding of the theory. The problem with the shearing is that it also saps jet power/velocity fairly rapidly.

Also, immediately under the jet the radial velocity of the water is zero. I was looking for a way to get the way moving radially somewhat before it even struck the base-plate, much akin to the angled impingement models some have attempted where it was theorised that a 10° angle was potentially of more benefit (can't find the link to it now). I was also looking for a way to disrupt the potential core of the jet before it reached the base-plate, and also use that disruptor as additional surface area. As such, the disruptor should not be smooth edged as we want to promote turbulence.

I was also partially inspired by this image:

However I wasn't too sure about the apparant dimensions of the pedestal used in their simulation. I tried doing just that, but it didn't make any real difference for me, and my suspicion was that the pedestal was too large. In that image my reasoning was that it was not really turbulating the core, and the base of the pedestal, as seen in the simulated thermal graph, was actually getting too hot. Basically the results I saw with that approach was about the same as without.

So I tried for even smaller, where it's no longer really a pedestal that the jet bounces off, but more of a short pin that juts up into the core of the impingement jet, attempting to disrupt it, but also not to deflect the flow so much that bulk of the jet flow is pushed away significantly. It was also apparant to me that in that image that there was a fair amount of potential for entrainment (recirculation of already heated water) around the base of the pedestal.

So basically I experimentally hunted for the better ratios of nozzle separation, pin width and pin height, along with an optimal range of cup diameters across a range of pumping scenarios. This is where it all got rather muddy trying to pick a balance.

The block's jet restriction and jet intake pressure drop was also heavily experimented with to provide what I thought was the best tradeoff of pressure drop for jet velocity over the target cooling patch area with pumps from an Eheim 1046 up to my Iwaki MD30-RZ, and while still keeping volumetric flow rates acceptable. The block's pressure drop, while very high, makes as much "use" of that pressure drop as possible to get the jet velocity typically within 85-95% of what the pump can sustain. Trying to get the jet velocity up any higher just results in absolutely killing volumetric flow rates for very minimal jet velocity gains, so overall I was pretty happy with the balance point I found here. How restrictive is it? Not overly so (IMO). Typically you'll see flow rates of around 75% of that which you'd see when using a White Water, and around 85% of what you'd see when using a Cascade.

Really I struggled with finding much of any existing theoretical/research material immediately relating to what I was attempting, and wholly admit to mostly flying blind with just experimental results to indicate if I was heading in the right direction with my thoughts.

Some References I used which were of some help:

http://widget.ecn.purdue.edu/~jmurthy/me605/wu.pdf

http://www.cape.canterbury.ac.nz/web...ata/839rev.pdf

http://elecpress.monash.edu.au/ijfd/...ijfd_v7_a1.pdf

http://www.electronics-cooling.com/h...01_may_a2.html

http://www.pressure-drop.com/

http://www.coolingzone.com/Content/D...as/fcalc10.htm

10-01-2004, 06:02 PM	#139
Cathar Thermophile Join Date: Sep 2002 Location: Melbourne, Australia Posts: 2,538	Top plate also houses the jet intake plenum chamber, which is just a circular recess in the plate. Basically I had spent some time studying this basic picture and thinking about JI: Jet diameter/nozzle separation (z/d) seems to create peak Nusselt numbers at a ratio of 4 < z/d < 6 for a number of studies, due to the shearing of the jet against the surrounding liquid. The jet has a "potential core" which is roughly cone shaped and is also typically around 4-6d in length depending on the initial jet velocity from my understanding of the theory. The problem with the shearing is that it also saps jet power/velocity fairly rapidly. Also, immediately under the jet the radial velocity of the water is zero. I was looking for a way to get the way moving radially somewhat before it even struck the base-plate, much akin to the angled impingement models some have attempted where it was theorised that a 10° angle was potentially of more benefit (can't find the link to it now). I was also looking for a way to disrupt the potential core of the jet before it reached the base-plate, and also use that disruptor as additional surface area. As such, the disruptor should not be smooth edged as we want to promote turbulence. I was also partially inspired by this image: However I wasn't too sure about the apparant dimensions of the pedestal used in their simulation. I tried doing just that, but it didn't make any real difference for me, and my suspicion was that the pedestal was too large. In that image my reasoning was that it was not really turbulating the core, and the base of the pedestal, as seen in the simulated thermal graph, was actually getting too hot. Basically the results I saw with that approach was about the same as without. So I tried for even smaller, where it's no longer really a pedestal that the jet bounces off, but more of a short pin that juts up into the core of the impingement jet, attempting to disrupt it, but also not to deflect the flow so much that bulk of the jet flow is pushed away significantly. It was also apparant to me that in that image that there was a fair amount of potential for entrainment (recirculation of already heated water) around the base of the pedestal. So basically I experimentally hunted for the better ratios of nozzle separation, pin width and pin height, along with an optimal range of cup diameters across a range of pumping scenarios. This is where it all got rather muddy trying to pick a balance. The block's jet restriction and jet intake pressure drop was also heavily experimented with to provide what I thought was the best tradeoff of pressure drop for jet velocity over the target cooling patch area with pumps from an Eheim 1046 up to my Iwaki MD30-RZ, and while still keeping volumetric flow rates acceptable. The block's pressure drop, while very high, makes as much "use" of that pressure drop as possible to get the jet velocity typically within 85-95% of what the pump can sustain. Trying to get the jet velocity up any higher just results in absolutely killing volumetric flow rates for very minimal jet velocity gains, so overall I was pretty happy with the balance point I found here. How restrictive is it? Not overly so (IMO). Typically you'll see flow rates of around 75% of that which you'd see when using a White Water, and around 85% of what you'd see when using a Cascade. Really I struggled with finding much of any existing theoretical/research material immediately relating to what I was attempting, and wholly admit to mostly flying blind with just experimental results to indicate if I was heading in the right direction with my thoughts. Some References I used which were of some help: http://widget.ecn.purdue.edu/~jmurthy/me605/wu.pdf http://www.cape.canterbury.ac.nz/web...ata/839rev.pdf http://elecpress.monash.edu.au/ijfd/...ijfd_v7_a1.pdf http://www.electronics-cooling.com/h...01_may_a2.html http://www.pressure-drop.com/ http://www.coolingzone.com/Content/D...as/fcalc10.htm Last edited by Cathar; 10-01-2004 at 06:12 PM.