Natural convection refers to the heat transfer that occurs due to the movement of fluid caused by temperature differences. It is driven by buoyancy forces, where the warmer fluid rises and the cooler fluid sinks.

The correct answer is option 'D' - Cooling of billets in atmosphere. This situation pertains to free or natural convection because it involves the cooling of objects (in this case, billets) in the surrounding atmosphere.

Explanation:

Heat flow situations:
a) Air conditioning installations and nuclear reactors: These situations involve forced convection rather than natural convection. In forced convection, the fluid flow is induced by external means such as fans or pumps. In air conditioning installations, the cool air is circulated using fans, while in nuclear reactors, coolant is circulated using pumps. Forced convection is different from natural convection because it is not solely driven by buoyancy forces.

b) Flow of water inside the condenser tubes: This situation involves forced convection as well. The flow of water inside the condenser tubes is induced by pumps, and the heat transfer occurs mainly through forced convection. The water is circulated using pumps to enhance heat transfer efficiency.

c) Cooling of internal combustion engine: The cooling of internal combustion engines typically involves a combination of forced convection and natural convection. The coolant is circulated using a pump (forced convection), but the heat transfer from the engine to the coolant occurs through a combination of forced and natural convection. The coolant absorbs heat from the engine and transfers it to the atmosphere through a radiator, where natural convection plays a role in dissipating the heat.

d) Cooling of billets in atmosphere: This situation pertains to free or natural convection. When billets are cooled in the atmosphere, the heat transfer occurs mainly through natural convection. The temperature difference between the hot billets and the cooler surrounding atmosphere drives the flow of air, resulting in convective heat transfer. The warm air rises, carrying away the heat from the billets, while the cooler air replaces it, establishing a natural convection loop.

In summary, the cooling of billets in the atmosphere is the heat flow situation that pertains to free or natural convection. The other options involve forced convection, where fluid flow is induced by external means such as fans or pumps.