The circulation in a natural circulation boiler is an example of which type of heat transfer?

In the context of a natural circulation boiler, the correct answer is natural convection. This process occurs due to the differences in density of the fluid caused by temperature variations. As water is heated in the boiler, it becomes less dense and rises, while cooler, denser water sinks. This movement creates a continuous circulation pattern without the need for mechanical assistance, such as pumps.

Natural convection is primarily reliant on buoyancy forces, which stem from the temperature gradient within the fluid. In contrast, forced convection involves the movement of fluid caused by external forces like fans or pumps, which is not the case in a natural circulation boiler.

Conduction, another option, refers to heat transfer through a material without movement of the material itself, such as heat transferring through the walls of the boiler. Lastly, radiation involves the transfer of heat through electromagnetic waves, which occurs in various situations but is not the primary method of heat transfer within a natural circulation boiler. Thus, natural convection accurately describes the heat transfer mechanism in this scenario.