How does water travel through a modern column radiator
Hot water radiators which are connected together on both the top and bottom have no baffles or plugs. Inside the radiator is one open cave from end to end. The recommended way to pipe a cast iron radiator is hot in at the top and return (cold ), out the bottom. As the water cools in the radiator the water gets more dense and heavier causing the flow to be towards the bottom of the radiator. If you are interested in radiator construction you could buy or find an old radiator and take it apart.