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I see no fan at all. But this model is parametric -- you can change almost anything. Just install FreeCAD, load helix_fan_duct_v7.fcstd, go to cf spreadsheet (it's a spreadsheet-like object inside source file) and modify parameters according to your geometry. This drawing explains all variables in cf spreadsheet: https://thingiverse-production-new.s3.amazonaws.com/renders/4c/a4/cf/00/e8/8eb4ad74993b5805038e597f813e8b0b_preview_featured.jpg