How do I empty the Drupal caches:
When you are logged as an admin (obviously, not every user of the site has to power to clear the cache), there should be a page in "Administer > Site Configuration > Performance".
And, at the bottom of the page, there should be a button (something like "Clear cached data") to clear the cache
As far as I remember, there's no need for Devel to do that, and you really don't need to go to the database, nor run some home-made PHP code.
As a reference, you can take a look at How to Clear Drupal Server-side cache