Thermal engineering

Physics

Thermal engineering often concerns itself with three subdisciplines of physics: thermodynamics, fluid mechanics, and heat transfer. Principles and concepts from these subdisciplines are often useful in the analysis of thermal systems.

Thermodynamics

Thermodynamics deals with the relationship between heat, work, and other properties among systems in equilibrium. It is the science of energy. There are four laws of thermodynamics.

• The zeroth law of thermodynamics states that if two bodies are in thermal equilibrium with a third body, then they are also in thermal equilibrium with each other.
• The first law of thermodynamics, also known as the law of conservation of energy, states that energy can neither be created nor destroyed. The only ways that the total energy in a closed system may change is through work or heat transfer.
• The second law of thermodynamics, addressing the limitations of the first, accounts for the quality and quantity of energy and dictates the flow of energy between systems. According to Clausius' statement, the second law of thermodynamics states that it is impossible for any system to act in such a way such that the sole result would be an energy transfer by heat from a cooler body to a hotter body. The Kelvin-Planck statement is also commonly cited for the second law. It states that it is impossible for a system that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. Two statements imply the same result, and violating one would be violating the other.
• The third law of thermodynamics states that the entropy of a pure crystalline structure at absolute zero is zero. This law provides a reference point to calculate the absolute entropy of substances.

Fluid mechanics

Fluid mechanics can be divided into fluid statics, the study of various fluids at rest; and fluid dynamics, the study of the effect of forces on fluid motion. Common applications of fluid mechanics include plumbing systems for water, gas, and sewage in both households and cities, refrigerant flows and heat exchangers in refrigerators, transportation of fuel and other fluids in automobiles, etc.

Heat transfer

Heat transfer is one of the two forms of energy interactions in a closed system. Heat transfer refers to interactions where the driving force is the temperature difference within the system. Heat transfer occurs through one of the three following methods: conduction, convection, and radiation. Conduction occurs when heat transfers between two parts of the substance through physical contact, without the significant displacement of molecules making up the substance. Convection occurs when heat is transferred due to the mixing of fluids, which, in contrast to conduction, depends on the displacement of the molecules in the fluid. Lastly, radiation is heat transfer through space or matter not by conduction or convection.