Basics of Thermodynamics, Fluid Mechanics, and Heat Transfer

Thermodynamics, fluid dynamics, and heat transfer are three branches of applied science that are somehow interrelated with each other. In some cases, the principles of thermodynamics are used in fluid dynamics or heat transfer, and vice versa. However, there are distinct differences between these STEM principles.

What Is Thermodynamics?

Thermodynamics is a large part of every person’s life, whether they realize it or not. It is a branch of physics that deals with heat and how it relates to other forms of energy. Like most things in life, heat energy is composed of “vibrating” molecules that bump into others and produce measurable energy.

It’s been determined that heat energy cannot be created nor destroyed. It is merely present and may be moved from one place to another with the introduction of other molecules or particles. You can also convert it into other forms of energy, like converting heat to kinetic energy and then ultimately, electrical energy. Thermodynamics is concerned with how heat energy interacts with all these molecules ​​— whether the molecule may be a solid, liquid, or gas.

What Are Fluid Dynamics?

Fluid dynamics deals primarily with the flow of fluid. In this case, “fluid” doesn’t just refer to liquid but also to solid and gas. The study is concerned with how these different fluids interact with each other and the movement of each one depending on the environment. For example, water always flows downward while hot gas always flows upward. Since thermodynamics deals with the flow of temperature, this is where the two disciplines interact. In some cases, heat is treated as a fluid to make the calculation easier.

What Is Heat Transfer?

Heat transfer and thermodynamics are complementary branches of applied science. Thermodynamics deals with the flow of heat while heat transfer is concerned with the exchange of thermal energy through a physical system. Thermodynamics has a broader approach because it can be used to calculate the flow of heat through different states of matter while heat transfer limits itself to the physical.

How These Principles Are Used in HVAC Systems

Once you simplify their explanations, it’s pretty easy to see how these principles become relevant when it comes to solids and liquids. Heat moves through items like metal — the same way you put a pan on the stove and the handle starts to heat up after a few minutes.

An HVAC system is perhaps the best example of these principles at work: heating, ventilation, and air conditioning. Air conditioners have a refrigerant within their body that interacts with hot air to produce cool air. All rooms start off with heat and this heat is sucked into the air conditioner — hitting the refrigerant which melts the material.

This is heat transfer at work as temperature flowing through physical space produces a reaction. The melted material produces cool air which is then blown out of the air conditioner and into the space. The process works much like placing a fan in front of a block of ice to cool the space. So where do thermodynamics, fluid dynamics, and heat transfer come in? If you notice, most air coolers are placed up high. This is because thermodynamics and fluid dynamics tell us that hot air has an upward flow.

Air conditioners are positioned to “catch” the hot air from the ceiling and then blow it back into the space. This works because cold air has a downward flow, so blowing cold air from a top position guarantees that a larger area will be cooled down.

Ongoing Studies

With heat as a constant in all energy developments, it’s not surprising that thermodynamics remains to be a primary area of study in STEM research.


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Written by Harris Khan


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