Do to convection you do get some heating of the
radiator. The warm fluid moves up and the cold
fluid moves in to take it's place and as it cools
it drops and the cycle goes on.

It's just not driven by a pump. Speaking of you could
make some system for quick disconnect and pump
heated coolant into the engine. The hard part would
be a valve that does not loose fluid in the connect
disconnect process.

Sean