ANS:C - forced convection and conduction.

In pipe flow, heat transfer from a hot wall to the liquid occurs primarily through forced convection and conduction. Here's a breakdown of these mechanisms:

1. Forced Convection: The primary mode of heat transfer in pipe flow is forced convection. This occurs as the fluid flows over the heated wall, it picks up heat through convection. The movement of the fluid (forced by a pump or other means) enhances the heat transfer process by increasing the rate at which heat is carried away from the wall.
2. Conduction: Heat is also transferred through the solid wall of the pipe via conduction. As the wall is in direct contact with the hot fluid inside the pipe, heat is conducted through the wall material from the hot side to the cold side, contributing to the overall heat transfer process.