3. Heat Transfer Modes: Conduction, Convection, and Radiation

Heat transfer is energy transfer driven by a temperature difference. A first step in estimating the rate of heat transfer between, say, a hot mug of coffee and its environment is to identify the modes of heat transfer and the thermal channels of heat flow between two locations. There are different pathways, or thermal channels, that heat can take from hot coffee to ambient; directly from the coffee to the air above the free surface, through the side wall of the mug, or through the bottom of the mug and through the floor on which the mug rests. Within each of these pathways, heat flow encounters discontinuities in material properties, from coffee, to mug, to air. The mathematical expression of the different underlying physics associated with these discontinuities gives rise to three modes of heat transfer: conduction, convection, and radiation. In contrast, there are two mechanisms of heat transfer (conduction and radiation) that refer to the physical phenomena that give rise to the energy transfer. The mode of heat transfer by convection is based on a conduction mechanism. A thermal channel is defined as a “pathway” for heat to flow from a hot fluid or surface to a cold one, expressible in terms of a series combination of heat transfer modes. In this chapter, the practical application of a single-paned window is used to develop the engineering tools (and the thinking process behind them) used in heat transfer analysis. This example involves a single thermal channel. The basic method is applied more generally, and to the coffee/mug problem with its multiple channels, in subsequent chapters.