Gasturbines

A gas turbine is a type of internal combustion engine which has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in between.The basic operation of the gas turbine is similar to that of the steam power plant except that air is used instead of water. Fresh atmospheric air flows through a compressor that brings it to higher pressure. Energy is then added by spraying fuel into the air and igniting it so the combustion generates a high-temperature flow. This high-temperature high-pressure gas enters a turbine, where it expands down to the exhaust pressure, producing a shaft work output in the process. The turbine shaft work is mainly used to drive an electric generator that is coupled to the shaft but it also has to drive the compressor and other devices. For this case generally fuel and air are premixed. For propulsion the energy that is not used for shaft work comes out in the exhaust gases, so these have either a high temperature or a high velocity. In this case fuel (frequently liquid) and air are not premixed. The purpose of the gas turbine determines the design so that the most desirable energy form is maximized. Gas turbines are used to power electric generators, aircrafts, trains and ships.For producing electricity often a combined cycle is used,including a steam cycle (CCGT) or for heavy heavy fuels a gasification step is used before, integrated gasification combined cycle, IGCC.

Fig 1. GE H series power generation gas turbine: in combined cycle configuration, this 480-megawatt unit has a rated thermal efficiency of 60%.

Thermodynamic Cycle

Thermodynamically the cycle is given by a Brayton-Joule cycle, see the figure below.

Fig 2. PV and TS diagram of a Brayton-Joule cycle.

De cycle starts at point 1 with isentropic compression after which an  isobaric heating (combustion) follows and than expansion and cooling.

Efficiency and Emissions

Gas turbines produce the major electricity power in the world. Therefore the efficiency and the emissions, associated with the combustion process are of major importance for the operation of gas turbines. This together with combustion stability and potential failure and loss of the machine. Purchase and installation costs are typically of the order of one billion € for a single turbine.

Fuel Flexibility and Part Load

With changes in fuel gas composition (think of addition of hydrogen by processing biomass and mixing of gas of different quality, or the result of power to gas , P2G, strategies to store energy), one has to investigate the phenomena that occur. Also the increasing supply of energy (which might be fluctuating largely) from different sources requires smart grids and fluctuating power requirements for gas turbine operation which can be a serious problem.

Our Contribution

Our group’s expertise lies in the computational prediction of the turbulent combustion process. Here we use the FGM (described in another section of this website) combustion model supplemented by a PDF chance distribution on the occurrence of a progress variable in the unclosed areas supported by only a few moments (typically 2). For these moments transport equations are solved. For an example of an outcome of these calculations see the movie below.

Fig 3. Temperature.

Fig 4. Progress variable.