[davidzo]

I actually enjoy discussing with you because you have some knowledge, i would be delighted to continue this and see what fruits come from that. Sorry for making your thread a messy discussion, i hope you don't care so much. I apologize for my bad english, i never wasn't good in any language (tried japanese and french too a little bit but that was far worse), but i estimate that you wouldn't be able to write in german too, so considering that i think i mustn't be too ashamed of my bad language skills.

Originally Posted by bobo5195

So I will ask you this question what does the Reynolds number actually mean and where does it come from?
Hint it is only partially to do with turbulence. You will from that answer that the Reynolds number only tells a bit of the story about turbulence. A flow with a Re=50000 is not necessarily turbulent.
It might take you a few thousand words to answer that. Probably by the non-dimenisonal navier-stokes equation (hint hint).

The Reynolds number describes a critical state where turbulence starts in a circular tube. The values to calculate the number are velocity, density and viscosity of the fluid and the tube diameter. Of course can't you calculate a watercooler with the formula as it is only meant for round tubes and doesn't say anything about heat transfer between the mediums. But you can use the knowledge that channel diameter and velocity are important for turbulences.
The navier-stokes equations are the right formula to calculate the flow in more complex geometry, but i would hell not calculate something with it by hand. that is what CFD is made for. But befor CFD you need some hints to create a Design. And when you basically know what the factors for heat transfer between a less conductive but high capacity Newtonian fluid and a very conductive but low capacity solid body, you can actually plan a design and adjust the fine proportions with after CFDing that. Plus the navier-stokes equation is OK for laminar flow situations but you don't know how accuarte it is for turbulent flow, you can only guess, even CFD is guessing. For turbulence you can't simplify the equation as much and DNS eats a lotta CPUpower. The Solid works plugin can't do that, you need Fluent+Gambit for that (i used Fluent once and it is a thousand times more powerful than the solidworks plugin simulations i have seen so far).

Second question what is a boundary layer?
I’ll answer this one. Its when a flow is 99% (okay so it doesn’t need to be 99% could be 90% or whatever, a large percentage) of the mean flow velocity. Actually there is the fluid boundary layer (velocity boundary) and the thermal boundary layer (temperature). You haven’t defined which one you are talking about. Common engineering language would say that boundary layer is fluid boundary layer.

I'm talking about Fluids, so i am talking about the Fluid laminar boundary layer, which is in that case a thermal isolation layer.
The boundary layer is most important to consider for watercooling and this is why:
- Water has a conductivity of 0,58 W/mk while copper has 402 W/mk. So why do we use water for heat trasportation? because copper isn't a fluid, we can circulate the water and use its unbeaten heat capacity to transport as much heat as possible.
But what is when the water doesn't circulate, when there is a dead spot of water? Isn't it as if you would isolate the copper with some low conductivity plastics when water is not flowing? That is exactly why some waterblocks with an equal surface but different flow can perform much different. When planning channels always avoid dead spots, solid copper is much better than water which is not flowing. because of the moving molecules pursuit of biggest Entrophy (Fluctuation), this Isolating effect isn't as bad as the raw numbers tell, 0,58 to 402 W/mk, but it still has the biggest impact on conductivity.
- In Conclusion: Because The Boundary layer consists of water molecules latent due to friction on the sides of the channel and some very slow moving molecules, and the bad conductivity of water, we have thin Isolation layers on our copper cooling structure which more or less effectively hinder the heat transfer between copper and water. Turbulant Flow has multiple decimal powers the Fluctuation and crossdiffusion compared to laminar flow (even if we only have a relative small pressure drop) which enhances the heat transfer significantly.

Could also argue that jets have their fluid boundary layer and I would accept that too. But in between the pins possibly not.

First one i agree, there is a boundary layer in Jets, second one is that there of course is also a boundary layer between pins. Even if its only some molecules thick between two grinding materials there are always some molecules of a boundary layer especially when one of the material is a fluid.

Pin block jets only for water distribution?
Hmm, a lot of fluid mechanics for people are trying to do other stuff and use them for cooling. Jet impingement cooling is mainly used in these situations. Get a normal heatsink that is not doing its job and add jets to make it better.

You can do an awful lot with maze blocks and a fluid mech block. Using some boundary layer stuff you could work out heat transfer rate. You may even find out that for you average maze block the thermal boundary layer would cover 10% max of the flow.

Dunno what you mean with fluid mech block.
The 10% figure depends on how you measure the boundary layer and your middle pump velocity. In laminar flow situations the fastest velocity is in the middle and it lowers linear to the sides. That is why you won't lower the boundary layer much when you increase the overall velocity because the biggest increase will be in the middle.

That is different in turbulant flow scenarios.

I was talking about blind hole (as you call it) blocks. They most definitely are connected to the baseplate. Jet hit fin, fin touches base plate.

There is no thermal connection of the jet itself to the baseplate (at least no good). Due to high velocity inside the jets and most often quite good turbulences the boundary layer there is really thin. We should use that surface of the jet entry plus the inner jet tube surface for heat dissipation.

I think you should go and read a good book on heat transfer then you might see that turbulence is only a marginally good enhancer of performance. Velocity has everything to do with it.

I fully agree with the second sentence. Velocity is the key to heat transfer between a solid body and fluids. But you didn't specify which velocity. In general when we talk about the velocity, whe mean the middle velocity, which we can measure easily by the amount of fluid which flows in a certain time. But this velocity is only important for the heat transfer time and not for the heat transfer between cooler and coolant itself. The Important velocity is the velocity directly near to the dissipating surface. Due to the laminar boundary layer we know that the velocity decreases rapidly the nearer to the dissipating surface we measure.
When we take turbulent flow instead, the velocity in the crossection is far more even to the sides (that is where the pressure drop comes from).
With turbulences the middle velocity may decrease but the velocity which is effectively making contact with the dissipating surface can be much bigger.

I was doing some CFD today on fins the correlations I came up with were heat transfer was Nu = Cl x Ra^(1/3) for lamina and Nu = Ct x Ra^(3/8) for turbulent. Not a lot of difference there. Ra = Rayliegh number its like Reynolds number for convection.

The Rayliegh number is only important for convection in a fluid, not between a solid body and a fluid. Of course it is not bad when Ra is high or rather that there is much Fluctuation (which basically is the same as Convection) and Turbulences can help that but it is not the most important figure in watercooling.
BTW: what software did you use for that, it doesn't seem too accurate

There’s much more I could post but I’m going to bed. I might add some more in the morning when this post isn't brought to you by beer .

hehe, needed a beer for the post you quoted too