I wanted to share with you all a feature within SOLIDWORKS Flow Simulation that doesn't get a lot of publicity but really came in handy a couple weeks ago for me. The challenge I faced was I needed to somehow characterize a heat exchanger within a system. As I'm sure you know, the way you make a heat exchanger efficient is to create as much surface area to convect heat as possible. Well that concept and meshing with simulation definitely don't play well together; for all that increased surface area you have to mesh it and your problem becomes exponentially larger very quickly. This leaves us with the problem of having to try and characterize the heat exchanger in a system without actually analyzing the solid geometry. There are two options in Flow Simulation we can do this with, one is the feature called a Heat Sink Simulation and the other is a Porous Media. Both of these however require a curve that defines the pressure drop across the heat exchanger at different flow rates. Unless we can get lucky enough that the manufacturer has this data or we have time to physically test the heat exchanger, this is something that is hard to characterize. One option is to run a separate analysis on just the heat exchanger at different flow rates to get the pressure drop across it. With hundreds of tubes that have very small thicknesses to optimize heat transfer, this becomes very impractical really quick; not to mention if the heat exchanger has fins on the tubes!
So how do we solve this problem?? Well the solution lies with this little known feature called Periodicity. What this allows us to do is analyze only a small section of geometry and make the assumption that flow geometry and flow characteristics around that geometry are repeated in the direction we define. As you can see from the above images, we can assume that in two directions we have repeating geometry, the top view and end view. We can take a small slice from the top view and repeat the length of the tubes and the end view we can take one set of "zigzagged" tubes and assume that set it repeated in both directions. The assumption that is made here is that any direction you define using Periodicity has to be able to be assumed to repeat infinitely in that direction. Now nothing we analyze is truly infinite, but if we can assume that there is enough repeating that the flow effects at the end do not affect those at the center then we can safely use this tool.
When we define this in Flow Simulation all we did is set the size of the computational domain to areas that needs be repeated. For example, in the X direction, we defined the computation domain right to edge of the tube so that when it's repeated it will treat it like there is a solid tub in that direction. Similarly, in the Z direction we set the computation domain to the midpoint between the repeating sections on either side of the geometry we are analyzing. I extended the Y direction so that I could get the flow to develop before taking my pressure drop readings. Once the computational domain is defined to the right size, you define that domain as periodic by clicking the drop down to the right of the computational domain values and choosing Periodicity.
The final thing I did before running the analysis was model a couple plates at the top and bottom of the computation domain that I apply goals to determine the pressure drop. You just need to remember to disable those components in the flow simulation study.
Make sure to remember to keep this feature in mind next time you run into a problem like this. It's not the most used feature in the world, but it really helped me out in this case!
By: Chris Olson, Simulation Applications Engineer