mechanical engineering-basics part-3

Thermodynamic Working Substances and their Properties

Thermodynamics is a study of the relationship between heat (or the lack thereof) and machines.
The most obvious example being that of an engine that produces power by the combustion of Fuel. Many more varied examples can be found in industry but one constant exists between them all.
That constant is…….

All Thermodynamic systems require a working substance to operate

Working Substances are typically fluids such as air or steam but any fluid that can fulfill these requirements can be used

• Can be compressed
• Can be expanded
• Can partake in an Energy Transfer

Working Substance Properties

Dependent Property

The best example of a dependent property is probably the boiling point of water. The temperature that is required for water to boil changes as the atmospheric pressure changes.
This makes the temperature dependent on pressure to complete the process.

Independent Property

Properties that can be independently altered are extremely important for testing and analysis. Examples are temperature and pressure that can be increased or decreased without having to alter any other system variables.
This then allows the engineer or scientist to observe the natural changes in the other system variables as they react to the change.

Extensive Property

Properties that are dependent on mass are regarded as extensive properties. Volume is the best example of a mass dependent property.
Further use of extensive properties will also be used specific quantities, which is basically a specific variable at a specified mass.

Internal Property

Internal properties are vital to the analysis of a working substance.
These properties are derived from the chemical structure of the fluid itself. This makes them perfect to determine the state of the fluid.
Determining the state of a working substance is the focus of most thermodynamic theory and practice. Thus the internal properties remain the most used in theory.
The main internal properties are Pressure, Volume and Temperature.

Pressure

 Pressure can loosely be compared to simple stress in the sense that the formula used to calculate both are extremely similiar.
Its components are


And the equation

Its important to realize that the pressure that gets induced in the working substance gets transmitted in all directions. This means that every surface that the substance is in contact with will be experience the same pressure.
This is unlike the simple stress experienced in mechanical components where the stress is only applied in the direction of the Force.

Pressure in a Working Substance gets transmitted in all directions by the substance on its surroundings

Other notables

Volume

The Volume of a working fluid helps us determine whether there has been a compression or expansion of the fluid.

If the volume of a fluid has increased then an expansion has occurred and vice versa.

Temperature

The degree of hotness or lack thereof is essentially temperature.

Heat exists not cold, cold is simply an absence of heat.

Three main temperature scales are used worldwide

• Fahrenheit
• Celsius
• Kelvin

The Fahrenheit and Celsius Temperature Scales are the most widely used scales by ordinary human beings but because engineers are special the Kelvin scale was adopted.

How these scales relate to each other

The Kelvin scale was developed to help engineers work from a new point of reference regarding Temperature.
The absolute zero of temperature is effectively where all the chaos in the working fluid ceases. This is not at the melting point of pure ice but at zero Kelvin.
Hence its importance.
It is duly called the Absolute Thermodynamic Temperature Scale.

Summary

Anyone that needs to work with Thermodynamic processes needs to be pro-efficient with working substances and their properties.

• Pressure
• Volume
• Temperature

are the main culprits and must be fully understood.
Lastly, the Kelvin scale is the basis for most Thermodynamic temperature readings, so knowing how to convert is important.