ANS:D - radiation

Heat transfer to the water wall in a high-pressure water wall type boiler furnace is mainly by radiation. In water wall boilers, the furnace walls are lined with tubes filled with water, known as water walls. These water walls absorb heat from the combustion process occurring within the furnace and transfer it to the water flowing through the tubes. The primary mode of heat transfer in this scenario is radiation. Radiation occurs when heat energy is transferred through electromagnetic waves, without the need for direct physical contact between the heat source and the object being heated. In the context of a boiler furnace, radiation occurs as thermal energy is emitted from the hot gases and flames in the combustion zone and absorbed by the surfaces of the water walls. While conduction and convection also play roles in heat transfer within the boiler system, especially in transferring heat from the water walls to the water within the tubes, radiation is the dominant mode of heat transfer from the combustion zone to the water walls themselves. Therefore, the correct answer is: radiation.