Understanding Systems and Control Volumes In Mechanical Engineering

When you are doing your engineering analyses such as thermodynamicsfluid mechanics, and heat transfer, you need to define a control volume in a very successful way. If not, you will have incorrect results which can cause disasters… First of all, you need to define the system, after that you need to start the calculations. Here, you can find the required information about what is the ‘system’ and control volume and similar terms, that you will use in your calculations. 

System in Thermodynamics Calculations

Illustration of open, closed and isolated systems(Image Source: https://en.wikipedia.org/wiki/Isolated_system).

A system is defined as the quantity of region in the space where the calculations are willing to be made. Outside of this space is called ‘surroundings’. The wall that separates this region from its surroundings is called a ‘boundary’. Boundaries can be static or moving. In calculations, boundaries are assumed to have zero thicknesses. 

Closed System and Isolated System

On a thermodynamics basis, a system must be defined as a closed system or isolated system. But, you need to understand what they mean. 

A ‘closed system’ is a system that does not allow mass transfer. The mass is constant and the same, and there is no mass transfer between the closed system and its surroundings. ‘Control mass’ is another description of a closed system that is also used in calculations.

Also, an ‘isolated system’ is a system where there is no mass and energy transfer between the isolated system and its surroundings. 

A closed or isolated system may have moving boundaries. 

Control Volume

Presentation of control volumes in engineering(Image Source: https://www.semanticscholar.org/paper/Control-Volume-Analysis%2C-Entropy-Balance-and-the-in-Niven-Noack/faf6a44f3e82196290d785dffd335657d561d5d7)

In engineering calculations, the term ‘Control volume‘ may be the most used one. Definition of the control volume is a very essential thing that all mechanical engineers must understand. 

‘Control volume’ is a volume where both the energy and mass transfer takes place. As you know that nearly all the essential engineering devices are control volumes. 

Think about the internal combustion engine cylinders. There is a constant intake and exhaust of fuel and burnt gases, and heat transfer between the surrounding parts of that cylinder. So, the internal space of the engine cylinder can be considered as control volume which has a moving boundary. Thermodynamical and heat transfer analyses are made according to this assumption. 

Selection of Control Volumes

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There are no strict rules to select control volumes to make calculations. But the proper selection of control volume will give much easier calculations. It depends on your experience with this issue. 


The general denominations related to thermodynamics and heat transfer systems and control volumes can be summarized like above. 

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