**3.3 Afterburning system**

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.

**Figure 17.** *Afterburning system for cogeneration: (a) partial 3D viewing and (b) assembly of the installation.*

**Figure 18.** *Experimental setup design: (a) afterburner; (b) casing; and (c) flame stabilizer.*

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

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.

assembly exit are 7.4 <sup>10</sup><sup>6</sup> and 0.0465532, respectively.

**Figure 14.**

**Figure 15.**

**Figure 16.**

**226**

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

*Computational Fluid Dynamics Simulations*

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

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

#### *3.3.1 Experimental setup*

#### *3.3.1.1 The afterburning system*

The afterburning system, shown in **Figure 18**, has the following overall dimensions: length = 304 mm, height = 228 mm, and width = 168 mm. The afterburning system is composed of a casing and flame stabilizer assembly. The casing has 240 mm in height and 304 mm in length. The casing also includes a gas fueling pipe with the following dimensions: diameter = 10 mm and height = 470 mm. The gas pipe has 20 equally spaced holes of 2 mm in diameter. The flame stabilizer assembly includes the actual, "V"-shaped flame stabilizer. The assembly also includes the ignition pipe, of the following dimensions: diameter = 16 mm and height = 115 mm. The post-combustion system presented above can raise the exhaust gas temperature up to a temperature of maximum 1800 K.

*3.3.1.5 The PLIF measurement system*

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

chamber.

stream velocity is about 35 m/s.

*Temperature profile along the centerline.*

results are presented.

**Figure 20.**

**229**

through only the fluorescent light wavelength [23].

*CFD Application for Gas Turbine Combustion Simulations*

*3.3.1.6 CFD software and numerical simulation setup*

The planar laser-induced fluorescence method is presented here. When laser radiation is tuned to specific wavelengths, it will excite certain species (molecules) within a flame to a higher energy level. Fluorescence occurs when this excited state decays and emits radiation of a longer wavelength than the incident laser radiation. In the atmospheric pressure flame created by the afterburner, quenching is negligible, and the fluorescence signal is proportional to the OH concentration. For the OH measurements presented here, the coumarin 153 dye was used. Laser light has a fundamental frequency of 1064 nm. The fluorescent light photons are captured by an intensified charge-coupled device (ICCD) camera equipped with a filter that lets

In order to evaluate the effect of the combustion model validity on the accuracy

The following results will be shown. Combustion temperature has a significant effect on NOX. NO emissions increase, but N2O emissions decrease, with increasing temperature. Velocity—the speed at which premixed laminar and turbulent flames propagate—is a fundamental parameter in many combustion applications, such as engines and gas turbines. Flame speeds influence knocking events in spark-ignited engines and play an important role in their performance and emissions. OH concentration shows flame front shape and stability. **Figures 4** and **5** present, respectively, the mean temperature and velocity components along the symmetry axis of the afterburner. The length of the recirculation region that is created in the flow by the presence of the bluff body stabilizer is of about 90 mm, and the maximum absolute value of the negative velocity reaches about 25 m/s. The far-field free

In **Figures 20**–**23**, various comparisons between numerical and experimental

domain and its axial and transversal components are presented.

The mean velocity field of the flow inside the previously defined computational

of the numerical simulation, the reference numerical simulation will use an extended EDM [27] combustion model, implemented in a fully three-dimensional numerical simulation conducted using the commercial software ANSYS CFX. In this study, the shear stress transport (SST) model has been used. The computational domain includes the post-combustion system described in the previous section and extends 350 mm downstream of the bluff body stabilizer. In the transversal direction, the extension measures 300 mm, centered on the post-combustion symmetry axis, and in the spanwise direction, it reaches the edges of the post-combustion

## *3.3.1.2 The gas turbine engine Garrett 30-67*

The gas turbine engine Garrett 30-67 [24] was fitted with a pipe that allows PIV flow seeding and transfers the seeded exhaust gas to the afterburning system (**Figure 19**).

#### *3.3.1.3 The PIV measurement system*

The experimental program presented here aimed at determining the instantaneous three-dimensional velocity field in the exhaust gas downstream of the postcombustion system. For the measurement a medium intensity laser beam was used, emitted by a Nd:YAG double-pulsed laser (Litron Lasers, wavelength of 532 nm and a maximum output power of 1200 mj), simultaneously with the triggering of two fast charge-coupled device (CCD) cameras that record the images thusly formed. The laser beam is passed through a light sheet optic device that converts the beam to a light sheet in the experimental zone. The time interval between two laser impulses was of 10 μs [25].

#### *3.3.1.4 Rayleigh spectroscopy*

Rayleigh scattering (RS) is a nonresonant elastic effect in contrast to the commonly used laser-induced fluorescence. RS is instantaneous and therefore completely independent of the molecules' environment [26].

**Figure 19.** *Experimental setup.*

#### *3.3.1.5 The PLIF measurement system*

*3.3.1 Experimental setup*

(**Figure 19**).

was of 10 μs [25].

**Figure 19.** *Experimental setup.*

**228**

*3.3.1.4 Rayleigh spectroscopy*

*3.3.1.1 The afterburning system*

*Computational Fluid Dynamics Simulations*

up to a temperature of maximum 1800 K.

*3.3.1.2 The gas turbine engine Garrett 30-67*

*3.3.1.3 The PIV measurement system*

The afterburning system, shown in **Figure 18**, has the following overall dimensions: length = 304 mm, height = 228 mm, and width = 168 mm. The afterburning system is composed of a casing and flame stabilizer assembly. The casing has 240 mm in height and 304 mm in length. The casing also includes a gas fueling pipe with the following dimensions: diameter = 10 mm and height = 470 mm. The gas pipe has 20 equally spaced holes of 2 mm in diameter. The flame stabilizer assembly includes the actual, "V"-shaped flame stabilizer. The assembly also includes the ignition pipe, of the following dimensions: diameter = 16 mm and height = 115 mm. The post-combustion system presented above can raise the exhaust gas temperature

The gas turbine engine Garrett 30-67 [24] was fitted with a pipe that allows PIV flow seeding and transfers the seeded exhaust gas to the afterburning system

The experimental program presented here aimed at determining the instantaneous three-dimensional velocity field in the exhaust gas downstream of the postcombustion system. For the measurement a medium intensity laser beam was used, emitted by a Nd:YAG double-pulsed laser (Litron Lasers, wavelength of 532 nm and a maximum output power of 1200 mj), simultaneously with the triggering of two fast charge-coupled device (CCD) cameras that record the images thusly formed. The laser beam is passed through a light sheet optic device that converts the beam to a light sheet in the experimental zone. The time interval between two laser impulses

Rayleigh scattering (RS) is a nonresonant elastic effect in contrast to the commonly used laser-induced fluorescence. RS is instantaneous and therefore

completely independent of the molecules' environment [26].

The planar laser-induced fluorescence method is presented here. When laser radiation is tuned to specific wavelengths, it will excite certain species (molecules) within a flame to a higher energy level. Fluorescence occurs when this excited state decays and emits radiation of a longer wavelength than the incident laser radiation. In the atmospheric pressure flame created by the afterburner, quenching is negligible, and the fluorescence signal is proportional to the OH concentration. For the OH measurements presented here, the coumarin 153 dye was used. Laser light has a fundamental frequency of 1064 nm. The fluorescent light photons are captured by an intensified charge-coupled device (ICCD) camera equipped with a filter that lets through only the fluorescent light wavelength [23].

#### *3.3.1.6 CFD software and numerical simulation setup*

In order to evaluate the effect of the combustion model validity on the accuracy of the numerical simulation, the reference numerical simulation will use an extended EDM [27] combustion model, implemented in a fully three-dimensional numerical simulation conducted using the commercial software ANSYS CFX. In this study, the shear stress transport (SST) model has been used. The computational domain includes the post-combustion system described in the previous section and extends 350 mm downstream of the bluff body stabilizer. In the transversal direction, the extension measures 300 mm, centered on the post-combustion symmetry axis, and in the spanwise direction, it reaches the edges of the post-combustion chamber.

The following results will be shown. Combustion temperature has a significant effect on NOX. NO emissions increase, but N2O emissions decrease, with increasing temperature. Velocity—the speed at which premixed laminar and turbulent flames propagate—is a fundamental parameter in many combustion applications, such as engines and gas turbines. Flame speeds influence knocking events in spark-ignited engines and play an important role in their performance and emissions. OH concentration shows flame front shape and stability. **Figures 4** and **5** present, respectively, the mean temperature and velocity components along the symmetry axis of the afterburner. The length of the recirculation region that is created in the flow by the presence of the bluff body stabilizer is of about 90 mm, and the maximum absolute value of the negative velocity reaches about 25 m/s. The far-field free stream velocity is about 35 m/s.

In **Figures 20**–**23**, various comparisons between numerical and experimental results are presented.

The mean velocity field of the flow inside the previously defined computational domain and its axial and transversal components are presented.

**Figure 20.** *Temperature profile along the centerline.*

Below the variation of the mean OH concentration along the axis of symmetry is presented. The reaction mechanism used in numerically simulation did not allow the concentration of hydroxyl (OH) to be captured so that it is presented experimentally and not numerically validated. It must be noted from the beginning that the OH radical is a very fast radical, which is created and destroyed rapidly in the combustion process. For this reason, its presence can be detected in the flame front only, being a very precise indication on its position. As seen in **Figure 24**, the position of the mean flame front coincides to the recirculation region that forms downstream of the flame stabilizer. The turbulent flame brush, clearly visible in **Figure 24**, determines a significant increase of the mean flame front, as compared to its instantaneous thickness. The turbulent flame brush is an effect of the turbulent intermittency, which causes, through the effect of the turbulent fluctuation of the flame, a given point in space in the flame front region to be part of the time inside the flame front and part of the time outside it. Therefore, the averaging

process leads to a region much thicker than the very thin flame front characteristic to a laminar flame, where the mean fields have characteristics corresponding partially to the flame front, partially to the preheating region, and partially to the oxidation zone [28]. In the axial direction in **Figure 25**, the maximum OH concentration, of about 5000 ppm, is reached at about 50 mm from the flame stabilizer trailing edge, and the turbulent flame brush extends between 0 and 100 mm with

Hydrogen is studied as a possible fuel in gas turbines due to its high calorific value and promising results in the field of environmental protection. More, hydrogen become actual again, since new ways for producing and transporting it developed lately. One interesting idea is to produce the hydrogen on site by electrolysis, using wind power or solar energy, and to transport it using the existing natural gas distribution network. Combining hydrogen with natural gas strongly influences the combustion parameters, due to the different properties of the mixture. Using the existing equipment would face new problems, like the modification of the flame

By numerical simulation on the combustion chamber of a small gas turbine, for 100% CH4 and 100% H2, a clear difference can be observed in **Figure 26**, indicating probable working problems and a possible installation's component damage.

Thus, the idea of searching for a new solution was born, by changing the type of

In the process of designing of the new injector solution, numerical simulations were used, testing and comparing different types, in order to obtain an optimized variant to be produced and experimentally tested later on. For example, in

**Figure 27**, two types of geometries for swirl injectors were compared in respect to

respect to the same axial coordinate origin.

*The variation in the mean OH concentration along the axis of symmetry.*

*CFD Application for Gas Turbine Combustion Simulations*

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

front, the risk of flashback, and higher temperatures.

injector and part of the geometry of the combustion chamber.

the flow characteristics and temperature and velocity fields.

**3.4 Hydrogen use in gas turbines**

**Figure 25.**

**231**

**Figure 24.**

*Mean OH axial concentration.*

**Figure 21.** *Axial velocity profile along the centerline.*

#### **Figure 22.**

*Left: numerical mean axial velocity field. Right: PIV experimental mean axial velocity field (the same velocity color scale).*

#### **Figure 23.**

*Left: numerical mean transversal velocity field. Right: experimental mean transversal velocity field (the same velocity color scale).*

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

**Figure 24.** *Mean OH axial concentration.*

Below the variation of the mean OH concentration along the axis of symmetry is presented. The reaction mechanism used in numerically simulation did not allow the concentration of hydroxyl (OH) to be captured so that it is presented experimentally and not numerically validated. It must be noted from the beginning that the OH radical is a very fast radical, which is created and destroyed rapidly in the combustion process. For this reason, its presence can be detected in the flame front only, being a very precise indication on its position. As seen in **Figure 24**, the position of the mean flame front coincides to the recirculation region that forms downstream of the flame stabilizer. The turbulent flame brush, clearly visible in **Figure 24**, determines a significant increase of the mean flame front, as compared to its instantaneous thickness. The turbulent flame brush is an effect of the turbulent intermittency, which causes, through the effect of the turbulent fluctuation of the flame, a given point in space in the flame front region to be part of the time inside the flame front and part of the time outside it. Therefore, the averaging

*Left: numerical mean axial velocity field. Right: PIV experimental mean axial velocity field (the same velocity*

*Left: numerical mean transversal velocity field. Right: experimental mean transversal velocity field (the same*

**Figure 22.**

**Figure 21.**

*Axial velocity profile along the centerline.*

*Computational Fluid Dynamics Simulations*

*color scale).*

**Figure 23.**

**230**

*velocity color scale).*

**Figure 25.** *The variation in the mean OH concentration along the axis of symmetry.*

process leads to a region much thicker than the very thin flame front characteristic to a laminar flame, where the mean fields have characteristics corresponding partially to the flame front, partially to the preheating region, and partially to the oxidation zone [28]. In the axial direction in **Figure 25**, the maximum OH concentration, of about 5000 ppm, is reached at about 50 mm from the flame stabilizer trailing edge, and the turbulent flame brush extends between 0 and 100 mm with respect to the same axial coordinate origin.

## **3.4 Hydrogen use in gas turbines**

Hydrogen is studied as a possible fuel in gas turbines due to its high calorific value and promising results in the field of environmental protection. More, hydrogen become actual again, since new ways for producing and transporting it developed lately. One interesting idea is to produce the hydrogen on site by electrolysis, using wind power or solar energy, and to transport it using the existing natural gas distribution network. Combining hydrogen with natural gas strongly influences the combustion parameters, due to the different properties of the mixture. Using the existing equipment would face new problems, like the modification of the flame front, the risk of flashback, and higher temperatures.

By numerical simulation on the combustion chamber of a small gas turbine, for 100% CH4 and 100% H2, a clear difference can be observed in **Figure 26**, indicating probable working problems and a possible installation's component damage.

Thus, the idea of searching for a new solution was born, by changing the type of injector and part of the geometry of the combustion chamber.

In the process of designing of the new injector solution, numerical simulations were used, testing and comparing different types, in order to obtain an optimized variant to be produced and experimentally tested later on. For example, in **Figure 27**, two types of geometries for swirl injectors were compared in respect to the flow characteristics and temperature and velocity fields.

The second type of injector shows better volume flame repartition and more intense recirculation zones, benefic characteristics for flame stability, and pollutant emission formation/inhibition. Also the higher swirl number resulted for type 2 is promising, influencing the inlet turbulence, with the mention that other authors of [30] suggest that it is not the case that a rise in the swirl number leads always to an increase in combustion efficiency, but there is an optimum angle for swirling vanes at which the combustion efficiency, temperature, and radiation heat transfer of the flame stand at their maximum. In the same time, the higher local temperatures can

lead to thermal NOx formation, a disadvantage that can be controlled by flame

choosing the right geometry and leading to the optimized version to be

manufactured and tested (**Figure 29**) [31].

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

*CFD Application for Gas Turbine Combustion Simulations*

the danger of flashback phenomenon.

*The swirl injector and working principle [29].*

*Isometric section for 0% H2 and 20% H2 (airflow 0.04 kg/s, excess air 3.5).*

The CFD results show a clear picture of the differences between the two types, with very similar results to the later conducted experiments (**Figure 28**), helping in

The design of the new swirl injector was patented, considering some innovative ideas. For example, the convergent shape of the nozzle avoids the uneven velocities between the base section and tip section at the exit of the channel and leads to higher velocity in the exit section than the burning velocity of the fuel, eliminating

The specific numerical simulation methods also helped for studying the main subject of the work, mentioned above, focusing on the different aspects of combustion of the mixtures CH4-H2, with various volumetric proportions (**Figure 30**). The results show relevant data, with very similar results as in the experiments,

cooling technics.

**Figure 29.**

**Figure 30.**

**233**

**Figure 26.**

*Temperature field distribution for CH4 and H2 combustion.*

**Figure 27.**

*CFD analysis for two different types of swirled injectors [29]. (a) Type 1 and (b) Type 2.*

**Figure 28.**

*Comparison of CFD results and visualization during the experiment. (a) Temperature field (combustion simulation); (b) velocity fields (cold simulation); and (c) experimental images.*

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

The second type of injector shows better volume flame repartition and more intense recirculation zones, benefic characteristics for flame stability, and pollutant emission formation/inhibition. Also the higher swirl number resulted for type 2 is promising, influencing the inlet turbulence, with the mention that other authors of [30] suggest that it is not the case that a rise in the swirl number leads always to an increase in combustion efficiency, but there is an optimum angle for swirling vanes at which the combustion efficiency, temperature, and radiation heat transfer of the flame stand at their maximum. In the same time, the higher local temperatures can

**Figure 26.**

**Figure 27.**

**Figure 28.**

**232**

*Temperature field distribution for CH4 and H2 combustion.*

*Computational Fluid Dynamics Simulations*

*CFD analysis for two different types of swirled injectors [29]. (a) Type 1 and (b) Type 2.*

*Comparison of CFD results and visualization during the experiment. (a) Temperature field (combustion*

*simulation); (b) velocity fields (cold simulation); and (c) experimental images.*

lead to thermal NOx formation, a disadvantage that can be controlled by flame cooling technics.

The CFD results show a clear picture of the differences between the two types, with very similar results to the later conducted experiments (**Figure 28**), helping in choosing the right geometry and leading to the optimized version to be manufactured and tested (**Figure 29**) [31].

The design of the new swirl injector was patented, considering some innovative ideas. For example, the convergent shape of the nozzle avoids the uneven velocities between the base section and tip section at the exit of the channel and leads to higher velocity in the exit section than the burning velocity of the fuel, eliminating the danger of flashback phenomenon.

The specific numerical simulation methods also helped for studying the main subject of the work, mentioned above, focusing on the different aspects of combustion of the mixtures CH4-H2, with various volumetric proportions (**Figure 30**). The results show relevant data, with very similar results as in the experiments,

**Figure 29.** *The swirl injector and working principle [29].*

**Figure 30.** *Isometric section for 0% H2 and 20% H2 (airflow 0.04 kg/s, excess air 3.5).*

validating the chosen CFD methods and input values, as part of a PhD. thesis [32] and part of a Romanian Research Authority-funded project [33].

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[16] Petcu AC, Sandu C, Berbente C. Numerical simulations of jet-A

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[3] The jet engine, Rolls Royce 1996

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[10] Popescu J, Vilag V, Petcu R, Silivestru V, Stanciu V. Researches Concerning Kerosene-to-Landfill Gas Conversion for an Aero-derivative Gas Turbine, ASME Turbo Expo 2010: Power for Land, Sea and Air; 14–18 Iunie 2010; Glasgow, UK. NEW YORK, USA: ASME. ISBN: 978-0-7918-3872-3

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For all these RANS simulations, k-ε model turbulence model was used. As for combustion model, the flamelet probability density function (FPDF) model was chosen, because of the CFX available kinetic reaction library, which provided a fast and easy way to mix the two gaseous fuels. This mathematical model has some known drawbacks too, like slightly higher temperature estimations and the absence of NOx calculation, but this case did not require high precision; the only purpose was just getting a correct image of the combustion process for different cases, in order to optimize the solution and to have a clear preview to the experimental phase.

For this specific case, the combustion of CH4-H2 mixtures, there is also an interesting important issue that has no solution for the moment: the existing mathematical models do not take into consideration the very different reaction times of the two fuels in this mixture and thus cannot capture and validate the hypothesis that the hydrogen from the mixture burns faster and consumes at a higher rate from the oxygen required to burn the entire mixture, thus resulting in incomplete combustion of methane.

Considering all these aspects, depending on the studied case and on the purpose of the research, different CFD methods should be chosen, considering the resources, the allocated time, and the requested detail level of the results.
