Fan Slip Factor

[Cnoj]

Hi,

I'm looking for some guidance on simulating manufactured fans, especially re. Slip Factor. The pics below show a quick simulation using a manufacturers dimensions and fan curve but at different slip factors and the flow patterns vary considerably. 

Assuming everything else in my model is OK (.CFZ attached), how do I determine what slip factor will give the most realistic flow pattern? 

Also, the Adv5 example looks a little odd. Should I avoid using Adv5 with internal fans?

Fan is the pink bit. The blue air masses have the ends and cylindrical surfaces set as zero pressure boundaries. 

Flow is left to right in all of these:

Here's the adv5 with odd looking inlet flow:

Thanks,

Jon

[Jon.Wilde]

Hi Jon, I have a couple of comments:

1. The air volumes need more mesh, right now they are too coarse to capture the flow well
2. We must ensure that flow either enters or exits over a Boundary Condition (BC). It must not recirculate over them or the results will be off. Here you have P=0's applied to all walls and this is a problem. We should only have P=0 at the ends of the tubes and they need to be long enough so that there is no recirculation over them. This is not an ideal setup to ever achieve this though. Once you have a fan in a real assembly, it will likely perform better

[Cnoj]

Here's another simulation that gives varying results depending on fan slip factor. The goal was to predict flow distribution through six heat sinks fed by a single fan.

The model consists of an inlet, fan and a duct leading to six heat sinks (pink) each with their own exhaust duct. The heat sinks are modelled as resistance regions with head curves matching the manufacturer's. Mesh was approx 1 mil. elements.

At fan slip factor 0.5 the distribution was pretty even and there was a pressure gradient through the sinks as expected:

(plane at right hand exhaust end shows velocity, the other two planes show pressure)

Now here's the same simulation with the fan slip factor at 0.7. It shows significant flow variation between the sinks and an odd pressure gradient through them:

So I'm still left with the problem: Assuming my model is set up correctly, how do I determine which slip factor will give me the most realistic results?

I've attached a support file and put the big 'Archive' support file here: http://a360.co/1llgQAD

Thanks,

Jon.

[srhusain]

Are you designing the fan itself or trying to use commercially available fans. Assuming the latter, the manufacturer should be able to provide the slip-factor values. As an alternative, here is a link to the wiki on how to estimate slip factors:

http://en.wikipedia.org/wiki/Slip_factor

You can experiment with the different methods of estimating fan slip factors.

[yathuru]

Hello Everyone,

Sorry for restarting the thread. I am facing the same problem. Was there a solution for this post? Is there an optimum slip factor which represent the manufactured fan for electronic cooling application (for example 80 mm*80 mm)?

regards

Naresh