Gas thermal engines
Introduction
A gas turbine is a turbomachine whose working fluid is a gas. As the compressibility of gases cannot be neglected, gas turbines are thermal turbomachines.[1] Gas turbines are commonly spoken of separately from turbines since, although they work with substances in a gaseous state, their design characteristics are different, and, when in these terms we speak of gases, a possible phase change is not expected, whereas when we speak of vapors it is.
Gas turbines are used in power cycles such as the Brayton cycle and in some refrigeration cycles. It is common in everyday language to refer to aircraft engines as turbines, but this is a conceptual error, since these are turbojets which are machines that, among other things, contain a gas turbine.
The basic operation of the gas turbine is similar to the steam engine, except that air is used instead of water. Fresh air from the atmosphere flows through a compressor that raises it to high pressure. Energy is then added by dispersing fuel into it and burning it so that the combustion generates a high temperature flow. This high temperature and pressure gas enters a turbine, where it expands, decreasing the outlet pressure, producing the movement of the shaft during the process. The job of this turbine shaft is to move the compressor and other devices such as electrical generators that may be attached to it. The energy that is not used for work comes out in the form of gases, so they will have either a high temperature or a high speed. The purpose of the turbine determines the design that maximizes this form of energy. Gas turbines are used to power aircraft, trains, ships, electrical generators, and even battle tanks like the M1 Abrams.
Thermodynamic analysis
During the passage of the working fluid through a gas turbine, the first delivers energy to the second, and during this process the fluid expands and its temperature decreases. A thermodynamic analysis of this process can be done by doing an energy balance:
This equation is the first law of thermodynamics in specific properties, but unlike other nomenclatures, the work is considered positive if it leaves the control volume, which in this case contains the fluid as it passes through the turbine; It is the speed, it is the internal energy, it is the pressure, it is the height, it is the heat transferred per unit of mass and it is the specific volume. The subscripts refer to the output and refer to the input. To simplify the analysis, the following considerations are made: