Question

what provides the centripetal force in a fan

Answer 1

here's gravity pulling it downwards and its support pulling it upwards. These are pretty much balanced. 
There is a centripetal force pulling the blades towards the center of the fan. Whenever there is uniform circular motion (something going with a constant speed around and around in a circle) there will be a centripetal force equal to mv^2/r. Without this force, the blades will fly off in a straight line instead of moving in a circle. 
There is air resistance pushing the fan in the direction opposite to its rotation. 
There is the driving force (from the motor) in the direction of the rotation, making sure the air resistance doesn't slow it down. 
There is friction at the bearing, but this force is not really important

Answer 2

fans of which use impellers that can reach up to 80 m/s tip speed, i also learned that it shouldn't be tip speed but tip acceleration because the impeller is constantly changing direction it should be 80 m/s^2.

So to calculate the force should i use force = Ma or m x centripetal acceleration of which i've read is V^2/ radius?

Also can someone give me a good example how the force goes inwards because this is baffling me, i know you have probably heard this thousands of times but why does it go inwards