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Originally Posted by Cathar
Syscrusher. In a nutshell as to why you want the jet to be stood off slightly can be explained in the following way.
When the jet is really close to the surface, the what happens is the water just squirts out the side of the jet tube, but in the middle of the tube it doesn't really move at all. i.e. the point of central stagnation is fairly large. Imagine filling a glass with water and sticking a flat piece of something on it and turning it upside down. Now lift the glass slightly away from the surface. The water that flows out mostly flows out the small gap, but the water in the middle is barely moving at all, i.e. the stagnation region.
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I understand all that and your correct in this. To close and performance drops because of the stagnation area but that's not what I was getting at.
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Originally Posted by Cathar
Now do the same experiment but lift the glass away quickly and all the water pours out and strikes the whole area under the glass, rather than merely leaking out the sides.
By standing the jet off a certain distance we greatly reduce that central stagnation effect where the water is barely moving at all. The actual best distance to stand the jet off by is linked to the velocity of the jet stream, as one can well imagine. It gets a little more complicated in submerged jet scenarios where the jet loses power as it moves through the liquid around it, but also gains added turbulence as a bit of a bonus.
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This is why I made my last post before this. I might of misread your post but I took it that you were saying further away then what you have now was better. I don't know the cascade in it's real form but from my understanding your jet tubes are just about level with the rims of the cups which I would consider optimal as this is what I was doing with my cone shaped design - though inferior to the cup or spherical shape. There is a point were further becomes worse and there is a fine line inbetween good and bad with our flow rates/pressure. Diameter also influences the optimal distance. Being submerged complicates the situation more as the surrounding fluid acts as a restriction. In a low flow situation your about to lose the game as the surrounding warmer liquid could actually heat the incoming liquid or restricts it to the point it never reaches the base. It's a balancing act just to get some where in the middle of the two situations - low/high flow. But then, we're assumming there is no restriction after that where the restriction is greater then the block or equal to it. Though I doubt we would see that.
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Originally Posted by Cathar
The actual math of all that is the subject of a large number of research papers into impingement and its effects. I won't pretend that I could construct a mathematical answer for any scenario, and in fact I don't think that many researchers could either, but the theory that's out there does provide guidelines for good starting points, and unless your jet velocity is extremely low, having the jet really close (<2d) often turns out to be worse.
Have a poke through that paper I linked to above for more information.
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The math I'v seen has been a bit different from one study to another. But as you say, they do provide a good reference point to start with. But as you know, that's with a flat plate in mind. Throw in furniture, base plate thickness and a different heat source, all that changes. I missed the paper you linked but going to take a look after this.