Heat moves between objects and systems through three distinct physical mechanisms: conduction, direct transfer through a material by contact; convection, transfer carried along by a moving fluid; and radiation, transfer through electromagnetic waves that needs no material medium at all. A mechanical engineer analysing heat transfer in a real system, an engine block, a heat exchanger, an electronics enclosure, almost never gets to isolate just one of these mechanisms; a system's actual thermal behaviour typically depends on more than one mode operating simultaneously, and the engineering challenge is modelling how they interact.
Conduction and convection are usually coupled at a system's surfaces
Heat conducted through a solid component, say, a metal casing surrounding a hot engine part, eventually reaches that solid's outer surface, where it doesn't simply stop; it then transfers into whatever fluid, air or a liquid coolant, is in contact with that surface, a process governed by convection rather than conduction. This means a mechanical engineer modelling a real component's temperature can't treat conduction through the solid as a self-contained problem: the rate of heat leaving the solid's surface into the surrounding fluid, itself dependent on the fluid's flow characteristics, directly affects the temperature gradient driving conduction within the solid in the first place, coupling the two modes together in a single combined problem.
Radiation adds a further layer that becomes critical at high temperatures
Radiation, unlike conduction and convection, doesn't require any physical contact or intervening fluid at all, and its contribution to a system's total heat transfer grows sharply as temperature rises, since radiative heat transfer scales with temperature raised to the fourth power rather than linearly. In high-temperature mechanical systems, combustion chambers, furnaces, certain aerospace components, radiation can end up dominating a system's total heat transfer even while conduction and convection continue operating alongside it, meaning a mechanical engineer working at high temperatures generally has to model all three mechanisms together rather than safely ignoring any one of them.
What we're still unsure about
The three basic heat transfer mechanisms, and the physical principles governing each individually, are precisely defined, well established engineering thermodynamics, extensively validated across an enormous range of applications. What's more genuinely an ongoing area of applied engineering challenge is accurately modelling combined heat transfer in geometrically complex real systems, where conduction, convection and radiation interact in ways that are computationally expensive to simulate precisely — engineers continue to refine numerical simulation methods and simplified approximations for these harder combined-mode problems, without every real-world system's thermal behaviour yet being predictable from first principles with full precision at reasonable computational cost.
This sits inside Thermodynamics & Heat Transfer in Systems, one of eight topics in Mechanical Engineering, one of four domains in Engineering, one of seventeen subjects the app can quiz you on.