Jacqueline put a kettle of water on the stove to make tea, and turned the stove on. What is the direction of heat flow?

The direction of heat flow is "downward to upward".

Explanation

When Jacqueline puts a kettle of water on the stove to make tea, and turned the stove on, then it is obvious that the kettle and the burner are in touch with each other. Hence convection is playing a role because the heat passes from the burner into the kettle, thus heating the water at the bottom. Heat flows from hot to cold within the solids.

The motion triggered by the ability of hotter and thus less dense substance to rise within a fluid and the cooler, more dense material to fall under the influence of gravity, resulting in heat transfer, thus this whole process is understood as convection.