ANS:D - radiation

The predominant mode of heat transfer which takes place from the boiler furnace to the water walls of a boiler is radiation. In a boiler, the furnace generates heat through combustion of fuel. This heat is emitted in the form of electromagnetic waves, primarily in the infrared spectrum. These waves travel through the space within the furnace and are absorbed by the surfaces of the water walls, which are typically made of metal tubes filled with water. The absorption of radiant heat by the water walls causes the temperature of the walls to increase, transferring heat energy to the water circulating within the tubes. This process is known as radiation heat transfer. While conduction and convection also play roles in transferring heat within the boiler system, radiation is the primary mode of heat transfer from the furnace to the water walls due to the high temperatures involved in the combustion process.