Thermal systems engineering typically requires the use of **three thermal science disciplines**:

- Thermodynamics,
- Fluid mechanics, and
- Heat transfer.

Figure 1 shows the roles of these disciplines in thermal system engineering and their relationship to one another.

**Thermodynamics** provides the analysis of thermal systems through the conservation of mass and conservation of energy principles, the second law of thermodynamics and property relations. Fluid mechanics and heat transfer provide additional concepts, including the empirical laws necessary to specify, for an instance, material choices, component sizing, and fluid medium characteristics. For example, thermodynamic analysis can tell you the final temperature of a hot workpiece quenched in oil, but the rate at which it will cool is predicted using a heat transfer analysis.

**Fluid mechanics** deals with the behavior of fluids at rest or in motion. In Fig. 1, two fundamentals that play central roles in our discussion of fluid mechanics are the conservation of momentum principle that stems from Newton’s second law of motion and the mechanical energy equation. Principles of fluid mechanics allow the study of fluids flowing inside pipes (internal flows) and over surfaces (external flows) with consideration of frictional effects and lift/drag forces. The concept of similitude is used extensively in scaling measurements on laboratory-sized models to full-scale systems.

**Heat transfer** deals with energy transfer as a consequence of a temperature difference. There are three modes of heat transfer. They are:

- Conduction refers to heat transfer through a medium across which a temperate difference exists.
- Convection refers to the heat transfer between a surface and a moving or still fluid having a temperature difference.
- Thermal radiation represents the net exchange of energy between surfaces at different temperatures by electromagnetic waves independent of any intervening medium.

For these modes of heat transfer, the heat transfer rates depend on the transport properties of substances, geometrical parameters, and temperatures. Many applications involve more than one of these modes; this is called multi-mode heat transfer.