Thermodynamics and Propulsion

1.1 What it's All About

Thermodynamics is a science and, more importantly, an engineering tool used to describe processes that involve changes in temperature, transformation of energy, and the relationships between heat and work. It can be regarded as a generalization of an enormous body of empirical evidence^1.1. It is extremely general: there are no hypotheses made concerning the structure and type of matter that we 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 phenomena.

The focus of thermodynamics in aerospace engineering is on the production of work, often in the form of kinetic energy (for example in the exhaust of a jet engine) or shaft power, from different sources of heat. For the most part the heat will be the result of combustion processes, but this is not always the case. The course content can be viewed in terms of a ``propulsion chain'' as shown in Figure 1.1, where 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, and a chapter in Part IV takes us from the first to the second. We will start with the progression from heat to work, examples of which are given in Figure 1.2.

Figure 1.1: The propulsion chain

Figure 1.2: Examples of heat engines

[Converting heat to useful work] [Propulsion system] [Power generator] [Refrigerator] [Converting work to useful heat]

Practice Questions

1. Describe the energy exchange processes in (blank) (fill in the blank, e.g. a nuclear power plant, a refrigerator, a jet engine).
2. Given that energy is conserved, where does the fuel+oxidizer energy that is used to power an airplane go?
3. Describe the energy exchange processes necessary to use electricity from a nuclear power plant to remove heat from the food in a refrigerator.
4. Describe the energy exchange processes necessary for natural gas to be used to provide electricity for the lights in the room you are in.