If you want to generate Fluid mesh, please use Fluent Meshing to generate volume mesh using STl files. You can check following video.
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjwrqekoLD-AhVTzqACHUWNBPkQwqsBegQICxAE&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DfzvSG1TYFxw&usg=AOvVaw3BZ8dgKp3v_F9EG3g5yFve
Regards,
Keyur
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