Before we dive in, a hard truth: COMSOL does not offer a perpetually free, fully-featured commercial license. You cannot download the full software suite forever for zero cost without strings attached. However, COMSOL AB (the developer) heavily subsidizes access for
used to run simulation applications created by others through a COMSOL Server™. While you cannot models from scratch, you can run pre-made apps for free. Student Versions comsol multiphysics free license