[Cathar]

Another thing to consider here is the width of the jetted area.

The WW design doesn't really care how wide a heat source is that straddles the fins. It effectively "slices" that heat source up, and treats it as a series of elongated strips that run the length of the channels. This is a simplistic view of things, but given the thin base-plate, isn't too bad of an approximation. This basically means that a heat source under a channel can be effectively viewed as a 8-12mm long strip that needs to be cooled. This means that our useful jet impingement region needs to hopefully be around the 12mm wide mark within the channel.

Now myv65 was onto something when he talks about what I call diminishing returns with respect to making the nozzle smaller than a small number of millimeters. For a 3PSI pump like an Eheim 1250, the smaller the nozzle is made, the higher the water velocity, but to a point. Eventually you reach a stage where the water velocity goes up VERY slowly as the nozzle is decreased in size. A bit like plotting a hyperbola.

Now there are a few things going on here that need to be balanced. As the nozzle is reduced, so does volumetric flow. It's pretty easy to plot the raw C/W of water vs volumetric flow. Short of phase changing the water, it's impossible to get a better C/W than that line - you can only ever hope to be as close to it as possible. So as we reduce volumetric flow rate through a smaller nozzle, we can reach a point where the volumetric flow decrease overwhelms the gain from the marginally higher water velocity.

There's a second effect that happens here too. The width of the nozzle also dictates the width of the jet impingement region. The higher the volumetric flow rate, the wider the jet region. Also, the wider the nozzle, the wider the region - to an extent - if the nozzle is made too wide then no real jet region forms and the water just mashes out the side and never really hits the base hard where we want it.

So here it can be seen that there is again a fine balance point. I spent a lot of time, theory and practise behind the scenes away from any public discussions tracking down what I believed to be the optimal nozzle width for standard pump applications with the White Water.

It's easy to think that making the nozzle smaller will boost velocity and hence performance, but the reality is that it's not that simple. There are many things to consider here, and it's not all that obvious at first glance.