Aeronautical Engineering

Understanding Thermodynamics in Jet Engine Aeronautics

In this blog I’m going to try to simplify Thermodynamics for all.
Thermodynamics is a concept or a way of application or doing things, crucially it’s more of an engineering tool used to explain processes that involve change in temperature, transformation of energy from one form to another. Thermodynamics is simply put as the relationship between heat and work. If you look at the name, Thermo – heat; dynamics – constant change in movement i.e. work.

Thermodynamics quiet literally means heat and how it operates/responds in ever changing situations environments & the relationship between the heat & work.

It can be said to be as a generalization of an enormous body of empirical evidence It is extremely general there are no hypotheses made concerning the structure and type of matter that we Engineers deal with. It is used to describe the performance of propulsion systems, power generation systems, and refrigerators, and to describe fluid flow, combustion, and many other applications.

The focus of thermodynamics in aeronautic engineering is on the production of work often in the form of kinetic/moving energy.
Basically the exhaust of a jet type engine or shaft power, from different sources of heat. For the most part the heat will be the result of combustion processes (combustion is heat & fuel mixture), but this is not always the case. You can look at it as a “propulsion chain” as shown below in my simple figure.

Above we see a progression from an energy source to useful propulsive work (thrust power of a jet engine). In terms of the different blocks, Parts I and II are mainly about how to progress from the second block to the third; Part III takes us from the third to the fourth. Below we see a practical example of Thermodynamics the progression from heat to work.

Thermodynamics Laws

I Know this is the part where it kinda gets boring and complicated so allow me simplify the three laws.

Oth Law (zerowth law)

“Temperature exists” when two items have been in contact for long period of time they will have both be of the same temperature, temperature affects the direction of heat flow,  the symbol for heat is Q.

1st law “Conservation of Energy

“the amount of heat done on a system is equal to the amount work it has received and vice versa” As a thermodynamics system works, it losses heat, when work is done on the system it gains heat, heat is converted to work & work is converted to heat, together the work & heat transfer into & out of the system represents the change in its Internal Energy (total kinetic energy & potential energy).


2nd law of Thermodynamics

“Entropy is always increasing” simply put Entropy means disorder, chaos or lack of arrangement and so it this chaos is always increasing, so for example, we all sleep in a bedroom and we try to keep it neat & tidy put everything in its place, but with time it always get disarranged and becomes a mess.

A thermodynamic example would be if you take an ice cube and drop it in a hot glass of water it will melt & there is no way they can remain separate or be reversed once mixed together (this goes for any other substance that can be mixed). So the more entropy we generate the less energy we have to do work.

3rd Law

“A perfectly crystalline solid at absolute zero has entropy of zero”

Very simply it’s as follows, the entropy (disorder) of a perfect crystal of a pure substance will approach zero as the temperature approaches zero. In Layman terms the hotter an item gets the more disorder it will have (at a molecular level) and the colder the item gets the more organized it will be, or the entropy shall be less.

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Anthony Muraya

Aeronautical Engineer.