Volumetric Properties of Fluids

The application of the energy conservation Law to a system of interest requires the knowledge of the properties of the system and how these properties are related. Thus, property relations are relevant to engineering thermodynamics. The state of a closed system at equilibrium is described by the values of its thermodynamic properties. From observation of many systems, it is known that not all of these properties are independent of one another, and the state can be uniquely determined by giving the values of the independent properties. Values for all other thermodynamic properties can be determined once this independent subset is specified.

For most engineering applications, we are interested about the behaviour of the intensive and extensive properties of systems. Of particular interest are systems of commonly encountered substances, such as water, air, or a uniform mixture of nonreacting gases. These systems are denominated as simple compressible systems. Experience shows that the simple compressible system model is useful for a wide range of engineering applications.

For example, in the case of a gas, temperature and another intensive property such as specific volume might be selected as the two independent properties. The state principle then affirms that pressure, specific internal energy, and all other pertinent intensive properties could be determined as functions of T and v:p = p(T, v), u = u(T, v), and so on. The functional relations would be developed using experimental data and would depend explicitly on the chemical identity of the substances making up the system.

Thus, for a given amount of substance contained in a system, the temperature, volume, and pressure are not independent quantities; they are connected by a relationship of the general form

F (p, V, T) = 0

An equation used to model this relationship is called an equation of state.

Further reading on equation of state.

Pure substances

This section is concerned with the thermodynamic properties of simple compressible systems consisting of pure substances. A pure substance is one of uniform and invariable chemical composition.

Please, take a look at the following video: Thermal Equation of State

It deals with the properties of pure, simple compressible substances and the relations among the properties pressure, specific volume, and temperature. From experiment we will learn that temperature and specific volume can be regarded as independent and pressure determined as a function of these two: p = p(T, v). The graph of such a function is a surface, the p–v–T surface.

Self-assessment

Once you have watched the full video, maybe you are interested in checking if the main concepts are clear. Here you have some self-assessment questionnaire. If you want to remember some specific aspects, you can see the part of the video corresponding to each level of the questionnaire.

Are you interested in reading more carefully the concepts developed in the video?

Please, open the corresponding e-book

Multicomponent systems (Mixtures)

However, many of the engineering applications from thermodynamics to engineering are made in systems where multi-component gases or liquids experience changes in their composition as a result of mixing or separating processes, by transferring species from one phase to the other, or by some chemical reaction. The properties of these systems depend on composition as well as temperature and pressure.

When the equilibrium between liquid and gas (or vapour) phases is established in these multi-component systems, the so-called vapour-liquid equilibrium is discussed

Further reading on vapour-liquid equilibrium.