40mm Fan Inlet Stack Noise Reducer

thingiverse

*V2 - Updated models and included more size options* These are bell inlet/velocity stacks designed for use with 40mm fans. Based on elliptical inlet bell designs for maximum discharge coefficient and maximum flow. Increases flow by matching inlet air direction with fan's axis to reduce the region of turbulence and low flow near the edge of the fan blades where the majority of airflow is generated. Minimizing this turbulence also reduces fan noise due to less turbulent flow and less vibration. In theory the big inlet should work best, but in practice I found it increases noise which is a sign that resistance and turbulence has increased. The smaller models do much better and are more likely to fit in 3D printer designs. This is likely due to the prints not having a completely smooth surface and different characteristics of axial fan flow vs what the simulated models are based on. You may notice a slight increase in pitch or different noise with the inlet on, depending on your fan. This is a result of fan speed increasing due to better flow characteristics and less resistance. There is a bit of a gray area when trying to determine how well the inlet is working by sound alone. An increase in the sound level could be from more resistance, more turbulence, or higher fan speed and higher flow which will then make more noise as it flows out the other side of the fan and goes through any ducting or parts on that side. Keep in mind that although flow and pressure output is increasing, this is not something you'll be able to feel by hand. Fusion 360 project files included so that the design can be edited for slightly different fans. The central motor housing may be bigger or smaller on your fan so the central diameter of the stack (the bullet) should be sized to match. Design inspiration: http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf Also check out my design for 30mm fans: https://www.thingiverse.com/thing:4796423