Heat transfer directly from a flame in a furnace to the furnace water walls is an example of:

The heat transfer from a flame in a furnace to the furnace water walls exemplifies radiation. This process involves the transfer of thermal energy in the form of electromagnetic waves, primarily infrared radiation, which does not require a medium (like air or water) to travel. In a furnace, the flame produces significant thermal energy that radiates outward. This energy directly heats the surfaces of the water walls without necessarily heating the air or any intervening medium.

In contrast, convection involves the transfer of heat through the movement of fluids (liquids or gases), where warmer areas of a liquid or gas rise to cooler areas. Conduction refers to heat transfer through direct contact, where heat must flow through a material, typically solids, where atoms or molecules vibrate and pass on energy. Sublimation is a phase transition where a substance transitions directly from a solid to a gas without passing through a liquid phase and is not relevant in the context of heat transfer in a furnace. Thus, understanding these processes highlights why radiation is the appropriate classification for heat transfer as it relates directly to the flame's interaction with the furnace walls.