**3.2 Micro gas turbine for power generation**

The numerical simulation of the gas-thermodynamic processes inside a Garrett micro gas turbine has been conducted using the commercial software ANSYS CFX. This work is part of the [12] PhD. thesis. The purpose of these numerical simulations was to validate the combination of numerical models used to simulate the turbulence process, combustion process, and liquid fuel atomization process and compare the numerical results with the functioning data of the used micro gas turbine engine.

The Garrett micro gas turbine is composed of an intake device, a single-stage centrifugal compressor, a tubular-type combustion chamber, a single-stage radial turbine, and an exhaust device (**Figure 9**).

The numerical simulation was performed only on the combustion chamber assembly. An unstructured-type computational grid, having 3.576.588 tetrahedraltype elements and 592.465 nodes, has been generated using ICEM CFD. A density was created near the injector to better capture the field near the fuel inlet (**Figure 10**).

**Figure 9.** *Garrett micro gas turbine geometry.*

**Figure 10.**

*Computational grid: (a) the exterior of the combustion chamber assembly, (b) the interior of the combustion chamber assembly, (c) the fire tube, (d) the injector.*

**Figure 6.**

**Figure 7.**

**Figure 8.**

**222**

*Temperature fields at the end of the combustor.*

*Hybrid experimental diagram for biogas combustion [10].*

*General aspect of the temperature field in the combustor.*

*Computational Fluid Dynamics Simulations*

The nominal functioning regime (20 KW load) has been considered for the simulation; thus, the fuel mass flow rate has been set at 0.0075 kg/s, and the air mass flow has been set at 0.522 kg/s. The air excess is of 4.7. Generally, for a gas turbine engine, the excess air should be between 3 and 5 [1]. The initial air temperature was set at 420 K. The used liquid fuel, Jet A, has been considered to enter the computational domain in the form of droplets. The mean diameter of the droplets has been assumed to be 30 μm, a value chosen based on the liquid droplet distribution diagram. The initial fuel temperature has been set at 300 K. The fuel spray cone angle has been set at 70°, based on the data presented in the micro gas turbine's maintenance manual [13].

In **Figure 12** the total temperature field through the micro gas turbine is

From **Figure 12(a)** it can be observed that high-temperature zone is found only inside the fire tube and does not extend into the volute that redirects the exhaust gases to the turbine stator. The average total temperature at combustion chamber

In **Figure 13** the fuel spray cone and the fuel droplet diameter distribution are

From **Figure 13** it can be observed that the fuel is completely evaporated in the primary zone of the fire tube, before reaching the walls. This confirms that the numerical models chosen to simulate the spraying and vaporization processes of the

The results presented in **Figures 13** and **14** are in good correlation. Jet A vapors obtained from the vaporization of the liquid fuel are located in the primary zone of the fire tube. They are completely consumed inside the fire tube, as it should happen in the case of a properly functioning turbo engine. The average Jet A vapor

In **Figures 15** and **16**, the CO mass fraction field and the CO2 mass fraction field,

.

presented.

presented.

**Figure 13.**

**225**

**Figure 12.**

*Fuel spray cone and droplet diameter distribution.*

exit-turbine entrance is 992 K.

liquid fuel are appropriate for the given application.

*CFD Application for Gas Turbine Combustion Simulations*

*DOI: http://dx.doi.org/10.5772/intechopen.89759*

In **Figure 14** Jet A vapor mass fraction field is presented.

mass fraction at combustion chamber assembly exit is of 7 <sup>10</sup><sup>7</sup>

*The total temperature field inside the combustion chamber assembly: (a) plan XY and (b) plan YZ.*

inside the combustion chamber, are presented, respectively.

A Reynolds averaged Navier-Stokes (RANS)-type turbulence model has been chosen, namely, the k-ε model, which is a numerically stable and robust model and very popular in the realization of technical applications numerical simulations [14–17].

The chosen combustion model has been the EDM model, based on a two-step kerosene-air reaction mechanism, imported from the ANSYS library. A simple reaction mechanism has been chosen because the purpose of these numerical simulations was to see if the used numerical models give a good approximation of the turbo engine functioning as a whole. The pollutant emission level has not been of interest at this stage. Using a more complex reaction mechanism would have been more time-consuming and would have required more powerful computational resources. The EDM combustion model has been chosen because of its simplicity and robust performance in predicting turbulent reacting flows. Because of these, the model is very often used in the realization of technical application numerical simulations [18–21].

The fuel droplet atomization and evaporation processes have been simulated using the cascade atomization and breakup (CAB) model, respectively, and the liquid evaporation model, both models imported from ANSYS library.

The reference pressure has been set at 101,325 Pa.

In **Figure 11** the pressure field through the micro gas turbine is presented. The pressure levels are relative to the reference pressure.

The pressure inside the fire tube is quasi-constant, as it can be seen in **Figure 3**, thus conforming the hypothesis that the combustion process inside a gas turbine combustion chamber takes place at constant pressure. The average air absolute total pressure at compressor exit-combustion chamber entrance is 275.466 Pa, thus obtaining an overall compression ratio of 2.7:1 which is close to the reported overall compression ratio of 3:1. The obtained pressure loss through the combustion chamber assembly is of 15%.

**Figure 11.** *The total pressure field inside the combustion chamber assembly: (a) plan XY and (b) plan YZ.*

*CFD Application for Gas Turbine Combustion Simulations DOI: http://dx.doi.org/10.5772/intechopen.89759*

In **Figure 12** the total temperature field through the micro gas turbine is presented.

From **Figure 12(a)** it can be observed that high-temperature zone is found only inside the fire tube and does not extend into the volute that redirects the exhaust gases to the turbine stator. The average total temperature at combustion chamber exit-turbine entrance is 992 K.

In **Figure 13** the fuel spray cone and the fuel droplet diameter distribution are presented.

From **Figure 13** it can be observed that the fuel is completely evaporated in the primary zone of the fire tube, before reaching the walls. This confirms that the numerical models chosen to simulate the spraying and vaporization processes of the liquid fuel are appropriate for the given application.

In **Figure 14** Jet A vapor mass fraction field is presented.

The results presented in **Figures 13** and **14** are in good correlation. Jet A vapors obtained from the vaporization of the liquid fuel are located in the primary zone of the fire tube. They are completely consumed inside the fire tube, as it should happen in the case of a properly functioning turbo engine. The average Jet A vapor mass fraction at combustion chamber assembly exit is of 7 <sup>10</sup><sup>7</sup> .

In **Figures 15** and **16**, the CO mass fraction field and the CO2 mass fraction field, inside the combustion chamber, are presented, respectively.

**Figure 12.**

The nominal functioning regime (20 KW load) has been considered for the simulation; thus, the fuel mass flow rate has been set at 0.0075 kg/s, and the air mass flow has been set at 0.522 kg/s. The air excess is of 4.7. Generally, for a gas turbine engine, the excess air should be between 3 and 5 [1]. The initial air temperature was set at 420 K. The used liquid fuel, Jet A, has been considered to enter the computational domain in the form of droplets. The mean diameter of the droplets has been assumed to be 30 μm, a value chosen based on the liquid droplet distribution diagram. The initial fuel temperature has been set at 300 K. The fuel spray cone angle has been set at 70°, based on the data presented in the micro gas turbine's

A Reynolds averaged Navier-Stokes (RANS)-type turbulence model has been chosen, namely, the k-ε model, which is a numerically stable and robust model and very popular in the realization of technical applications numerical simulations

The chosen combustion model has been the EDM model, based on a two-step kerosene-air reaction mechanism, imported from the ANSYS library. A simple reaction mechanism has been chosen because the purpose of these numerical simulations was to see if the used numerical models give a good approximation of the turbo engine functioning as a whole. The pollutant emission level has not been of interest at this stage. Using a more complex reaction mechanism would have been more time-consuming and would have required more powerful computational resources. The EDM combustion model has been chosen because of its simplicity and robust performance in predicting turbulent reacting flows. Because of these, the model is very often used in the realization of technical application numerical

The fuel droplet atomization and evaporation processes have been simulated using the cascade atomization and breakup (CAB) model, respectively, and the

In **Figure 11** the pressure field through the micro gas turbine is presented. The

The pressure inside the fire tube is quasi-constant, as it can be seen in **Figure 3**, thus conforming the hypothesis that the combustion process inside a gas turbine combustion chamber takes place at constant pressure. The average air absolute total pressure at compressor exit-combustion chamber entrance is 275.466 Pa, thus obtaining an overall compression ratio of 2.7:1 which is close to the reported overall compression ratio of 3:1. The obtained pressure loss through the combustion

liquid evaporation model, both models imported from ANSYS library.

*The total pressure field inside the combustion chamber assembly: (a) plan XY and (b) plan YZ.*

The reference pressure has been set at 101,325 Pa.

pressure levels are relative to the reference pressure.

maintenance manual [13].

*Computational Fluid Dynamics Simulations*

simulations [18–21].

chamber assembly is of 15%.

**Figure 11.**

**224**

[14–17].

*The total temperature field inside the combustion chamber assembly: (a) plan XY and (b) plan YZ.*

**Figure 13.** *Fuel spray cone and droplet diameter distribution.*

**3.3 Afterburning system**

*CFD Application for Gas Turbine Combustion Simulations*

*DOI: http://dx.doi.org/10.5772/intechopen.89759*

**Figure 17.**

**Figure 18.**

**227**

The afterburning system is a component that is added to aviation engines (gas turbine engines), usually military ones, in order to maximize the thrust force of the planes. But it also has industrial applications like cogeneration. Cogeneration is a modern solution which allows simultaneous production of electricity and heat. Due to cogeneration units, not only it is possible to lower the costs associated with heating and electricity producing, but it can also generate it in a way that is efficient and environmentally friendly. The fact that the combustion process in the gas turbine consumes only a small part of the oxygen from the intake air flow makes possible the application of a supplementary firing (afterburning) for increasing the steam flow rate of the heat recovery steam generator. A new patented afterburner installation was proposed, for use in cogenerative applications (**Figure 17**) [22]. This study focuses on Stage I of the afterburner (**Figure 18**) for which a special

experimental installation was designed. Here experimental measurements and numerical results of mean velocity and temperature are presented. The velocity measurements are carried out using particle image velocimetry (PIV), and the temperature measurements are performed using Rayleigh spectroscopy. Supplementary, flame front position measurements are presented, obtained with the planar laser-induced fluorescence (PLIF) technique [23]. The experimental setup, closely reproduced by the numerical simulations, consists of a post-combustion system, designed and manufactured at COMOTI and installed behind a Garrett 30–67 gas turbine engine serving as a gas generator. The flame is stabilized by means of a V-shaped flame holder, placed in the gas generator exhaust flow. Methane is injected into the flow upstream of the flame holder and ignited downstream of it, at a location where premixed conditions are reached.

*Afterburning system for cogeneration: (a) partial 3D viewing and (b) assembly of the installation.*

*Experimental setup design: (a) afterburner; (b) casing; and (c) flame stabilizer.*

**Figure 14.**

*Jet A vapor mass fraction field: (a) plan XY and (b) plan YZ.*

**Figure 15.** *The CO mass fraction field: (a) plan XY and (b) plan YZ.*

**Figure 16.** *The CO2 mass fraction field: (a) plan XY and (b) plan YZ.*

The highest CO and CO2 concentrations are found inside the fire tube. This is in good correlation with the temperature field (**Figure 12**) and Jet A vapor field (**Figure 14**), suggesting that the combustion reaction takes place and is completed inside the fire tube. The average CO and CO2 mass fractions at combustion chamber assembly exit are 7.4 <sup>10</sup><sup>6</sup> and 0.0465532, respectively.

Based on the obtained results, the temperature and pressure fields, the fuel vapor, and CO and CO2 mass fraction fields, it has been concluded that the numerical models used for the numerical simulation of the gas-thermodynamic processes inside the combustion chamber are appropriate for the given application, the results being consistent with actual functioning data of the Garrett micro gas turbine.
