How is 'heat transfer' defined?

Heat transfer is defined as the movement of thermal energy from one object to another. This process occurs due to a temperature difference between the two objects, where thermal energy naturally flows from the hotter object to the cooler one until thermal equilibrium is reached. Understanding heat transfer is essential in power engineering as it impacts the efficiency of systems such as boilers, heat exchangers, and other thermal devices.

The other options focus on different concepts: the amount of energy consumed during combustion pertains to the chemical energy transformation rather than the physical transfer of thermal energy. Circulation of coolant relates specifically to the mechanisms used to transfer heat away from hot areas in a system, which is more of a process than a definition of heat transfer itself. Finally, the release of gas during a chemical reaction addresses a separate aspect of thermodynamics dealing with reactions and gas laws instead of the movement of thermal energy.