Most efficient orientation of exhaust vents on top plate of chassis for air flow with regard to heatsink fins?

Question

I'm looking for some advice from an ME or anyone who understands heat transfer,convection, etc.

I'm currently laying out in Inkscape a top panel for an audio amplifier I'm building. Now, my question lies in the orientation of the exhaust fins that I will be having laser cut out of acrylic.

There is a need for these exhaust fins so the cold air from below the heatsink has a place to go.

Now, in regards to the image below, does it make more sense to have:

1. Situation 1, where the exhaust fins (green) are perpendicular to the heatsink fins?

or

2. Situation 2, where the exhaust fins are parallel to the heatsink fins?

Thank you!

Answer 1

In general, either grating configuration should permit flow equally as well provided the slot diameters are the same, and the ratio of open area is the same. By fins in the acrylic, I assume you mean slots correct? Aluminum fins on the heat sink, slots in the acrylic housing, the acrylic material between the laser cut slots are members (slang for structural beam members). Here are some design considerations:

1. From a structural standpoint number 2 will be superior because the distance each member has to span is less; this also permits you to make the slot holes in number 2 slightly larger than the slots in number 1 for the same strength over that area. (this can be accomplished by a variety of other methods)
2. I assume there is some vertical distance between the heat sink and the acrylic (A distance greater than the thickness of the acrylic members should be sufficent). If not, number 1 may be superior because the alignment of number 2 may have a few ares where members block a full channel between heat sink fins, greatly reducing performance.
3. The bottom is open? or are you putting a similar grating under the heat sink as well? Fresh cool air from the outside needs to flow in the bottom and out the top for convection to work. It has to dissipate the heat to the surroundings as a continuous process.
4. Since air is passing through the chassis, you also need to consider filtering. This is especially important if you use forced air convection, as more air is flowing through. Think of cleaning dust out of a personal computer chassis. If you put a very thin filter on cold inlet this will greatly reduce the amount of dust that ends up on the heat sink. Remember to keep the filter thin and you many need to make the area larger to reduce the restriction. You may even consider a filter on the top so dust and debris does not settle when not in operation. Simple PC Air Screens/Filters are pretty low cost, and can be hidden inside your design for visual appeal if necessary. Or don't worry about it and periodically clean the heat sink.
5. I love Inkscape, but for computer aided design you need a more powerful tool. Fusion360 is the best "free" (still free as of 7/2020) CAD software available. Just sign up as an enthusiast or startup. There are some opensource CAD programs available but they are still VERY far behind in terms of user interface and features. AutoCad Inventor is free if you are a student. And SolidWorks is low cost if you are a student.

Good luck!

Answer 2

When the slots are parallel to the aluminum fins, they promote a smooth, continuous convective stream of heat rising up and accelerating without hindrance.

Roughly the same concept as of a chiminea. It promotes the stream of heat to pick up momentum and accelerate, among other things.

Or similar to the dynamics of wildfires, they seek vertical grooves and natural re-entrant corners in the hill's face to build a rising flame.