**3. Fuels in gas turbine engines**

At the current time, almost all aviation fuel (jet fuel) is extracted from the middle distillates of crude oil (kerosene fraction), which distils between the gasoline and the diesel fractions [18]. The Jet A and Jet A-1 grades are the most used kerosene-type fuels worldwide in civil aviation.

As referred before, to mitigate the problem of the pollutant emissions in the future, there seem to be two viable solutions, the use of SAFs or the use of hydrogen fuel (through hydrogen combustion engines or fuel cells). In this work, will only be considered the hydrogen for the combustion gas turbine engines (GTEs).

#### **3.1 Hydrogen**

The hydrogen is the simplest and most abundant of the chemical elements in the universe. On Earth, under standard conditions (H2), its concentration is negligible. However, in chemically combined form, it is the third most abundant element on

*Conversion of Gas Turbine Combustors to Operate with a Hydrogen-Air Mixture… DOI: http://dx.doi.org/10.5772/intechopen.106224*

Earth. Consequently, hydrogen can be considered an energy carrier, similar to electricity, since it can be produced from other compounds, mostly from hydrocarbons and water [19]. The main reasons why hydrogen can play an increasingly significant role in meeting the worlds energy demands and addressing environmental concerns are that hydrogen meets three important criteria: a promising low-carbon alternative reducing emissions of GHG, providing energy security, and the possibility of reducing local pollutants, that is, NOx and particulates [19].

On a commercial scale, despite the several production sources, most of the hydrogen is currently produced through the process of steam reforming of methane (SRM) from natural gas [19]. As part of the energy future, the various hydrogen sources can be grouped into three types, namely fossil fuels (coal, natural gas, petroleum, oil shale, etc.), renewable sources (biofuels, water, photovoltaic, solar, algae, etc.), and nuclear (e.g., using thermal energy from nuclear reactions for water splitting). For hydrogen to be part of a sustainable energy future, renewable and nuclear sources need to play a more significant role in hydrogen production, and cost-effective carbon capture and storage technologies need to be developed and upgraded.

#### **3.2 Utilization of hydrogen in gas turbine engines**

For the use of the hydrogen fuel in the current GTEs, in theory, the minimum modifications needed are the change of the injection system and the implementation of facilities to evaporate the hydrogen, which is stored in the tanks in a liquid (or cryogenic) state. This can be accomplished by an external heat source or a heat exchanger (HE) [20]. However, to take full advantage of the hydrogen's distinct thermo-transport properties (high diffusivity, low ignition energy, wide flammability limits, and the highest laminar flame speed) that make its combustion and emission characteristics notably different from those of hydrocarbons fuels, beyond the injection system, the combustion chamber must be also changed [19]. In **Table 1**, is provided a comparison between the main properties of hydrogen and Jet A [6, 9].

When changing to hydrogen, either the combustor outlet temperature (COT) or the net thrust could be retained [20]. Because of the considerably higher heating value of hydrogen, the fuel flow to achieve the same COT or net thrust is reduced by almost


#### **Table 1.**

*Properties for hydrogen fuel and Jet A fuel.*

two-thirds. When the COT is preserved, the net thrust increases, resulting in a corresponding increased specific thrust. When it is opted to retain the net thrust, this results in a lower COT [20]. According to Boggia and Jackson [21], the performance improvements could be explained by two fundamental changes when using hydrogen: reduced mass flow and changed composition of the gases expanding through the turbine(s). While the latter improves the performance, the former deteriorates the performance. Reduced mass flow through the turbine lowers the thrust output for two reasons. First, decreasing the fuel flow implies that the exhaust mass flow decreases accordingly; hence, without any variation in gas composition, the thrust output decreases. In second, a reduced mass flow through the turbine will result in a higher total temperature drop and, thereby, also a higher total pressure drop across the turbine in order to deliver the same amount of power to the compressor. Because of the lower total temperature and pressure at the turbine exit, both the pressure thrust (thrust due to different pressure at engine inlet and exit) and momentum thrust decrease (the effect of decreased core nozzle velocity).

However, the loss in thrust due to reduced mass flow is offset by the increased thrust owing to changed properties of the combustion products [21]. With the use of hydrogen, the combustion products contain no CO2 and a larger portion of H2O, which has a higher Cp value than CO2. Having investigated a simple turbojet engine, Boggia and Jackson [21] concluded that the Cp value has increased by 4% in the hot section of the engine when changing to hydrogen fuel. Increased Cp value through the turbine will similarly, but in the opposite direction as reduced mass flow, affect the performance. For a fixed power output, it will cause smaller total temperature and pressure drops across the turbine. Provided that the core nozzle is not choked, a larger nozzle expansion ratio will result in a larger exhaust velocity, which, in turn, will increase the momentum thrust. In total, the positive effect of increased Cp value outweighs the effect of reduced mass flow and, hence, results in an increased net thrust when switching to hydrogen and retaining the COT [20].

It should be pointed out that the energy consumption to attain a certain COT is highly dependent on the fuel-injection temperature and the location of the heat exchanger (HE) used to evaporate the liquid hydrogen. By heating the fuel more, it is possible to achieve performance benefits. The effects on engine performance are quite small, but still there are some desired features that could be exploited [20].

If the COT is kept the same, the turbine entry temperature (TET) is also about the same, thus requiring the same cooling technology [21]. On the other hand, the option of lowering the COT to preserve the net thrust will lead to a decrease in TET. So, this will require less advanced cooling technology as well as having a favorable effect on turbine blade life. Moreover, designing for a lower maximum cycle temperature will help to suppress the NOx emissions.

#### **3.3 Mechanisms of pollutant formation**

The concentration level of pollutants in gas turbine exhaust can be related directly to several factors that control the emissions in conventional combustors. These factors may be considered in terms of primary-zone temperature, equivalence ratio, degree of homogeneity of the primary-zone combustion process, residence time in the primary zone, liner-wall quenching characteristics, and fuel spray characteristics [17]. These factors vary from one combustor to another and, for any given combustor, with changes in operating conditions [17].

*Conversion of Gas Turbine Combustors to Operate with a Hydrogen-Air Mixture… DOI: http://dx.doi.org/10.5772/intechopen.106224*

For the conventional fuels, such as hydrocarbons (in this case Jet A) or even the SAFs, the pollutant emissions of most concern are CO, CO2, UHC, NOx, and PM (or smoke). From the environmental standpoint, hydrogen is nearly a clean fuel once it produces only NOx (considering that water vapor is not a major pollutant) [19]. So, in this work, only the NOx emissions will be presented.

About the NOx (NO plus NO2 emissions), in conventional gas turbine combustors, there are four main mechanisms that are responsible for the NOx formation: thermal NO, nitrous oxide (N2O) mechanism, prompt NO, and fuel NO. The last one is usually of less importance for normal fuels (there is no fuel-bond nitrogen) [17]. In the case of hydrogen burn, we must still consider the NO formation through intermediate NO2 [19].

For hydrocarbon fuels, the two main mechanisms that are responsible for the formation of NOx are thermal NO and prompt NO, while for hydrogen flames the two main mechanisms associated with NOx formation are the thermal NO and NO formation through intermediate NO2. So, in this subsection only these three mechanisms will be referred. For more information about the others, there are a good review of them in Lefebre et Ballal and Kenneth Kuo [17, 22].

The thermal NO is produced by the oxidation of atmospheric nitrogen (N2) in high-temperature regions of the flame and in the post-flame gases [22]. This endothermic process is controlled largely by flame temperature, and it proceeds at a significant rate only at temperatures above around 1850 K (it requires the breaking of the tight N2 bond) [17, 19, 22]. For the typical conditions encountered in conventional gas turbine combustors (high temperatures for only a few milliseconds), NO increases linearly with residence time, but does not attain its equilibrium value. The extended Zeldovich mechanism is utilized by the most of the proposed reaction schemes for thermal NO. The principal reactions of this mechanism are represented in Eqs. (1)– (4) [17, 19]:

$$\text{O}\_2 \Leftarrow \text{2O} \tag{1}$$

$$\text{N}\_2 + \text{O} \leftrightharpoons \text{NO} + \text{N} \tag{2}$$

$$\text{N} + \text{O}\_2 \leftrightharpoons \text{NO} + \text{O} \tag{3}$$

$$\text{N} + \text{OH} \leftrightharpoons \text{NO} + \text{H} \tag{4}$$

The prompt NO can be formed in a significant quantity in some combustion environments such as in low-temperature, fuel-rich conditions and when residence times are short. These conditions can be created in gas turbines [18].

In hydrocarbon flames, prompt NO occurs in the earliest stage of combustion and its formation is associated with the reaction of molecular N2 with radicals, such as C, CH, and CH2, which are fragments derived from fuel, through a complex series of reactions and many possible intermediate species. Some of these reactions are represented in Eqs. (5)–(9):

$$\text{CH} + \text{N}\_2 \rightleftharpoons \text{HCN} + \text{N} \tag{5}$$

$$\text{N} + \text{O}\_2 \rightleftharpoons \text{NO} + \text{O} \tag{6}$$

$$\text{HCN} + \text{OH} \rightleftharpoons \text{CN} + \text{H}\_2\text{O} \tag{7}$$

$$\text{CN} + \text{N}\_2 \rightleftharpoons \text{NO} + \text{CO} \tag{8}$$

$$\text{CH}\_2 + \text{N}\_2 \leftrightharpoons \text{HCN} + \text{NH} \tag{9}$$

For hydrocarbon flames, the major contribution is from CH and CH2 species, as shown in Eqs. (5) and (9). The products of these reactions could lead to formation of amines and cyano compounds that subsequently can react with species such as N, O, or OH to form NO by reactions like those occurring in oxidation of fuel nitrogen or oxidation of other nitrogen species. At present, the prompt NO contribution to total NOx from stationary combustors is small. However, as NOx emissions are reduced to very low levels by employing new strategies that tend to reduce the flame temperature (such as burner design or geometry modification), the relative importance of the prompt NO can be expected to increase [18].

For hydrogen flames, the second mechanism that is relevant for the NOx emissions corresponds to the NO formation through intermediate NO2. This mechanism may be represented by the **Eqs. (10)** and **(11)** [19]:

$$\text{NO} + \text{HO}\_2 \rightarrow \text{NO}\_2 + \text{OH} \tag{10}$$

$$\rm{NO}\_2 + \rm{H} \rightarrow \rm{NO} + \rm{OH} \tag{11}$$

#### **3.4 Influence of temperature/pressure in NOx formation with hydrogen fuel**

Owing to its high adiabatic flame temperature, hydrogen combustion produces significant NOx. Therefore, by increasing the strain rate (as), which is pressure-dependent, the flame temperature can be lowered and NOx emissions are reduced. So, the pressure can be also a controlling parameter for NOx formation with hydrogen flames.

This way, the thermal NO mechanism is dominant at low pressures, whereas NO formation *via* intermediate NO2 becomes important at moderate pressures once in that condition, the flame temperature decreases for a given strain rate, as, due to enhanced recombination by the reaction [19]:

$$\rm H + O2 + M \rightarrow HO2 + M \tag{12}$$

Consequently, the maximum NO formation decreases for moderate pressures [22].

At higher pressures, the net effect of reactions (10) and (11) is to H + HO2! 2OH, through which radicals H, OH, and O are produced [19]. This enhances the formation of thermal NO. Some studies refer that under specific conditions, the formation of NO can be dominant over NO2.

Still considering the influence of pressure, but now for hydrocarbon fuels, the N2O and prompt mechanisms dominate at low temperature and are independent of pressure, whereas the higher NOx levels associated with higher combustion temperature are primarily due to thermal NO, which exhibits a square-root dependence on pressure [22].

#### **3.5 Chemical kinetic mechanisms**

Chemical kinetics is a capital point when modeling a combustion problem. In this case, the fuel combustion kinetics is extremely important in order to develop a model that allows a good emission prediction from the engine. Without proper kinetics all the attempts will go in vain. The development of detailed chemical kinetic models is extremely challenging once typical fuels (such as gasoline, diesel, and jet fuels) derived from different sources can be composed of hundreds to thousands of compounds. Therefore, detailed kinetic models for such fuels cannot contain all the compounds due to the limitation of current computational resources [23]. For that reason, a simplified mixture called surrogate mixture must be defined and used to develop a kinetic model. For the emission predictions, the kinetic model development can be even more difficult once the NOx chemistry must be developed together with fuel chemistry making realistic chemistry getting even more complicated. In this section, will be introduced the Jet A and hydrogen combustion kinetic models applied in this study.

#### *3.5.1 Jet A kinetic mechanisms*

Although kinetic models of jet fuel are still underdeveloped, significant progress has been made in this area in the recent decades. Jet fuels are kerosene-type cut of petroleum containing C-10–C-18 hydrocarbons, including alkanes, cycloalkanes, and aromatic compounds. The literature reviews show that there are several kinetic models available for jet fuel combustion and some of these models are listed (and briefly descripted) by Mostafa [23].

However, only few of them are suitable to satisfy our current needs. Based on our objective to predict aircraft engine emission (specifically NOx) using CFD simulation, we need at first a jet fuel kinetic mechanism that allows to simulate the combustion process and, then, one that fairly predicts NOx formation in this combustion chamber. As CFD with kinetic models is computationally highly expensive, the number of species in the kinetic scheme needs to be limited. For that reason, after evaluating the kinetic models available, it was opted by the option presented by Kundu et al. [24].

Kundu et al. [24] proposed a simplified kinetic mechanism with NOx chemistry based on 17 species and 26-step reaction for Jet A (17 steps for Jet A reaction and nine steps for the sub-mechanism for NOx prediction). This mechanism has been developed specifically to predict NOx formation during combustion of aviation kerosene. However, the mechanism does not cover the entire range of pollutant species, once to limit the number of species, the mechanism does not include NO2. Despite the limitation of this kinetic model, it was used in this work. This was not the best option, but among the available ones was the least bad.

#### *3.5.2 Hydrogen kinetic mechanisms*

The hydrogen oxidation chemistry represents the most fundamental and important building block in the hierarchy of hydrocarbon chemistry. Consequently, its chemistry has been extensively investigated, and a large number of detailed mechanisms (that can be found in the literature), including H2/O2 kinetics, have been developed and validated using different combustion configurations [19, 25]. Some of these mechanisms have been optimized for the combustion of pure hydrogen, but most of them are dedicated to the combustion of hydrocarbons including submechanisms for H2/O2 chemistry. However, the accuracy of the H2/O2 subset is also essential for the overall performance of a hydrocarbon mechanism [19].

To choose the kinetic model more adjusted for this case, some reviews available in the literature [25, 26] were analyzed. For instance, Ströhl et al. [25] made an evaluation of detailed reaction mechanisms for hydrogen combustion under gas turbine conditions. That study shows that the mechanisms of Li et al. or Ó Conaire accurately represent H2/O2 kinetics under gas turbine conditions. However, it suggests that the Li et al. mechanism is best suited for the prediction of H2/O2 chemistry since it includes more up-to-date data for the range of interest [25]. Also, the Li et al. mechanism has been found to provide the best match with measurements over a wide range of equivalence ratio and pressure, using various targets, including shock tube ignition delay and laminar flame speed data [26].

So, it was concluded that the Li et al. mechanism should perform better than the others, and for that reason it will be used in this work. For the NOx prediction (of hydrogen burn), was used the NOx sub-mechanism based on the study by the Glarborg group that is available in the database of ANSYS Fluent 2020R2 together with the mechanism of Li et al.

## **4. Materials and methods**

#### **4.1 Geometry**

For this study, was used a CAD design of the CFM56–3 combustor made in the study [14], based on the CFM56-3 combustor of **Figure 1**. Due to the existent symmetry in the CFM56-3 combustor and in order to decrease the simulation time and effectively represent the four fuel injectors (in the 20) that supply a richer mixture, it will be used only a quarter section of the combustor for simulation purposes; that is, there will be one fuel injector for every five injectors present in each quarter section of the model combustor that supplies an even richer mixture. In this study, the rich fuel injector is the middle one (element 24 of the **Figure 2b**). All the details present in the combustor geometry are represented in **Figure 2**, including the combustor walls, dome, dilution holes, fuel injectors, and primary/secondary swirlers.

*Views of the CAD combustor model section used in the simulations: (a) outside view; (b) top view; (c) side view; and (d) inner view.*

*Conversion of Gas Turbine Combustors to Operate with a Hydrogen-Air Mixture… DOI: http://dx.doi.org/10.5772/intechopen.106224*

#### **4.2 Modifications in the original geometry**

As told by Oliveira [14], there were some parts of the combustor such as the swirlers that the exact geometry was not achieved. Furthermore, after analyzing some documentation of the CFM56-3 engine, photos of the combustor and doing the preliminary simulations, there were found some problems related with the fuel injectors position, the shape of the exit of the secondary swirlers, and the connection of this exits with the cooling walls of the dome. All of these problems were affecting the results. For that reason, some changes have been attempted to try to correct these small problems. All the changes in the model were performed in CATIA V5 R20. In **Figure 3** are presented two cut-view images of the first swirler, in which the first shows the original CAD model received and the second the final CAD model with the modifications made during this work.

During the early phases of the work, it was concluded that even so, the simulation time for the quarter section would be very large. So, to test new sets of modifications needed in the geometry, mesh, or the used models, a geometry where only one injector was represented was developed. Thereby, some sets of modifications could be excluded without spending the total time of the simulation in the quarter section.

#### **4.3 Mesh generation**

In this work, several meshing software (HELYX-OS, SnappyHexMesh, Simscale and Fluent Meshing) were tried in order to get the best mesh possible with the computational resources available (mainly the quantity of RAM). Due to the complexity of the geometry, the only software that provided a good quality mesh was the Fluent Meshing after using the set of tips provided by ANSYS in [27] about the best practices for gas turbine combustion meshing. This way, it was possible to create a good enough mesh for the simulation, where all the features of the geometry were correctly represented with the available computational resources.

After analyzing several meshes, the independency test was performed using three meshes, with coarse, medium, and fine refinement, having 11,830.638 cells, 16.318.327 cells, and 22.602.875 cells, respectively. The data collected relative to numerical/experimental data for Jet A fuel (no experimental data relative to H2 combustion in this GTE was available) were presented by Ribeiro [28] and ICAO [12], and

**Figure 3.** *Cut view of the models: (a) original model and (b) modified model.*

these documents contain the exit temperature of the combustion chamber (only for the operating condition of 100%) and the NOx emissions of the engine, respectively. For that reason, the parameters used to analyze the independency of the mesh were:


All the independency tests were performed for the operating condition of 100%, since it is the only operating condition where the outlet temperature of the chamber is known. The maximum difference occurred for the average static temperature in the plane of the cut view of the swirler, and it was in the order of 2.16% between the values of the coarse and the intermediate meshes; however, between the values of the intermediate and the fine meshes, the difference was only 0.1%. The difference between the values of the average static temperatures for the outlet was nearly 0% between the coarse and the intermediate meshes as well as between the intermediate and the fine meshes. For the velocity magnitude, in the cut-view plane, the difference between the values was approximately 0.71%, between the values of the coarse and intermediate meshes, and 0.49%, between the values of the intermediate and the fine meshes.

#### **4.4 Numeric simulation**

The software used to perform this study was ANSYS Fluent 2020R2 [29]. Doubleprecision option was enabled, once a small error in this case can influence largely the results of the models. For the setup, the energy model was enabled. This model must be activated as this regards the energy related to the temperature change within the combustion process or heat transfer. For the viscous model, as was utilized a step-bystep solution, the first step was made with the realizable k-? and then was used the RSM in the other steps. For the radiation model, the P1 radiation model was chosen to simulate the heat transfer by radiation. This model was chosen for this study because it is accurate enough and reduces the computational cost in relation to the other models. Concerning the species, two models were used. At first to calculate an initial guess, the non-premixed combustion and species transport with one equation were used, for Jet A and hydrogen fuel simulations, respectively. And then, after obtaining a first initial solution converged (or almost converged), the detailed mechanisms referred before (Kundu et al. for Jet A and Li et al. for hydrogen) were imported, and the simulations were resumed until obtaining completely converged solutions. In this case, the used Turbulence-Chemistry Interaction was Eddy-Dissipation and the Chemistry Solver was the Relax to Chemical Equilibrium.

To evaluate the NOx emissions, two approaches were followed. The first one was to use the sub-mechanisms provided in the detailed mechanisms. These submechanisms presented before can cause some problems once; for instance, the submechanism for NOx produced by Jet A does not include the NO2 species, which clearly will affect the results. The second approach was to use the NOx model provided in ANSYS Fluent. This model must be enabled to ANSYS Fluent display information regarding NOx formation during the solution calculation, or it can be calculated in post-processing (the approach chosen in this work). In the end, an

*Conversion of Gas Turbine Combustors to Operate with a Hydrogen-Air Mixture… DOI: http://dx.doi.org/10.5772/intechopen.106224*

assessment must be made to reveal which approach is in better agreement with the ICAO's database values (for the Jet A), to be used in the work.

The boundary conditions were defined through an iterative process that has three phases. In the first one, through an iterative process (simulation and result analysis) and the available data present by Ribeiro [28] regarding the conditions of each stage of the GTE (mainly air mass flow, fuel flow, operating pressure, and oxidizer temperature) for 100% power, it was possible to determine the percentage of the air flow that enters the combustion chamber through the swirlers and all the boundary conditions for 100% power.

In phase 2, also through an iterative process (simulation and result analysis), the available data presented by Ribeiro [28] regarding the operating conditions obtained in the test-bed charts presented (mainly the operating pressure and oxidizer temperature), the values of the ICAO's database [12], and the percentage of the air flow that enter the combustion chamber through the swirlers calculated in phase 1, it was possible to define the boundary conditions for 7% power. Finally, after definition the boundary conditions for the conditions of 7% and 100% power, the values of the operating pressure, oxidizer (air) temperature, overall AFR, and primary zone AFR for the other operating conditions were calculated through a linear regression. This allowed the obtention of the boundary conditions for the 30 and 85% power conditions.

Knowing the overall AFR and fuel flow for each condition, it was possible to determine the total air mass flow and then through the primary zone AFR, it is possible to calculate the air mass flow that enters the combustion chamber through the swirlers and the air mass flow that enters through the other entries (mixers and dilution holes). Once these steps were concluded, the boundary conditions for the air mass flow inlets were defined for each power condition of the LTO cycle.

### **5. Results and discussion**

In this section, the results of the simulations are presented. It should be noted that all the results presented are only for one-fourth of the CFM56-3 combustor.

#### **5.1 Combustor exit temperature**

The first set of results was obtained during the process of estimation of the boundary conditions where the outlet average static temperatures to the simulations with Jet A fuel were calculated, as shown in **Figure 4**. Those are important values once we need them, first to compare with the reference temperature of 1649.94 K obtained by Ribeiro [28] to the condition of 100% power, and then to get reference values for the outlet temperature to allow the calculation of the mass flow of hydrogen fuel for each power condition.

The quantity of fuel for the simulations with hydrogen was calculated through the mass of Jet A fuel for each power condition and the ratio between the lower heating values, LHV, of the fuels, as shown in Eqs. (13) and (14).

$$
\dot{m}\_{\text{Hydrogen fuel}} = \dot{m}\_{\text{jetA fuel}} \times \frac{LHV\_{let-A}}{LHV\_{Hydrogen}} \tag{13}
$$

$$
\dot{m}\_{\text{Hydrogen fuel}} = \dot{m}\_{\text{jetA fuel}} \times \mathbf{0}, \text{3597} \tag{14}
$$

**Figure 4.**

*Combustor outlet average static temperature throughout ICAO's LTO cycle, while burning Jet A and Hydrogen fuel; and the reference values for 100% throttle.*

Since the LHV changes from author to author, this ratio was calculated and then adjusted for the condition of 7% power in order to get the same value for the outlet average static temperature for the simulations with the Jet A and hydrogen fuel.

**Figure 4** shows the most relevant outlet average static temperatures obtained in this work. The values for the sensibility tests are not represented here.

Through **Figure 4** it is possible to conclude that the values of outlet temperature obtained through the simulations with the input data from ICAO and *GasTurb* values (blue line and yellow x, respectively) are quite similar to the reference value for the full power condition (to Jet A).

Through the same figure, but now considering the simulations with hydrogen, it is possible to conclude that:


In this study, it was opted to fix only one value for the ratio, but as we can see, the best approach seems to be to calculate the specific ratio between the fuels to get the same exact outlet temperatures for each power condition.

#### **5.2 NOx emissions**

In the emission analysis, it is possible to study the emissions of all the pollutants, mainly CO, CO2, UHC, and NOx. However, since in this work the only objective related to the pollutants is to compare the pollutant emissions between the Jet A fuel and hydrogen fuel, the only emissions analyzed were the NOx, once H2O is not assumed as a pollutant.

Some of the results presented in this section were obtained through the emission index, EI. This value can be obtained using **Eq. (13)**:

$$\text{EI}[\text{g/kg}] = \frac{\text{Emission flow rate}[\text{kg/s}] \times 1000}{\text{Inlet } \dot{m}\_{fuel} \text{ [kg/s]}} \tag{15}$$

*Conversion of Gas Turbine Combustors to Operate with a Hydrogen-Air Mixture… DOI: http://dx.doi.org/10.5772/intechopen.106224*

In **Eq. (13)**, the emission flow rate was obtained by reporting the *Flow rate* of the desired pollutant in the outlet plane in Ansys Fluent, and this value is given in kg/s. The values used for the *m*\_ *fuel* (obtained from ICAO [12]) at the inlets are given in kg/s. The results are presented in the form g[Emissions]/kg[fuel], which allows the comparison with the ICAO's reference data.

In the first analysis (reference standard), regarding Jet A fuel simulations, the emission index is used since the quantity of fuel burned is the same for each operating condition, and it can be easily interpreted by the reader. However, in the comparison between the pollutant emissions of Jet A fuel and hydrogen fuel, as the quantities of fuel are very different (the mass flow of hydrogen is almost a third of the mass flow of Jet A), the use of the EI may give a wrong perception of the emissions difference to the reader. So, to simplify the analysis, it was opted for the use of the flow rate of NOx, in grams per second, for a fourth of the chamber.

#### *5.2.1 Control simulation and emission comparison between Jet A and hydrogen fuel*

The results for the control simulations are presented in **Figure 5a**, which shows the EI(NOx) in the outlet of the quarter of the combustion chamber for the Jet A fuel.

As expected, and previously referred, for both approaches, the NOx emissions are lower at low power settings and attain maximum values at the highest power condition, where the temperatures are higher.

Considering **Figure 5a**, it is possible to conclude that:


#### **Figure 5.**

*Results for the NOx emissions for one-fourth of the combustion chamber throughout ICAO's LTO cycle: (a) EI results of NOx for the combustion of Jet A obtained through the use of two approaches, NOx sub-Mechanism and Ansys NOx model; and (b) comparison of the flow rates of NOx for the combustion of Jet A and Hydrogen fuel obtained through the use of two approaches, NOx sub-Mechanism and Ansys NOx model.*

quantity close to the ones of ICAO's database; however, for the higher power conditions (85% and 100% power), this model also overpredicts the NOx emissions, in relation to the reference values (ICAO's database), approximately 2.4 times more for 85% power and 2.9 times more for 100% power.

• The NOx model from ANSYS Fluent can predict the NOx emissions better than the other approach, and these values will be used in the next step.

In a second phase, with the same boundary conditions (air mass flow rate, temperature, and pressure) for each operating condition, the simulations were repeated, but now for the hydrogen fuel. **Figure 5b** shows the comparison between the reference values (from ICAO and from the better model of the previous phase for Jet A fuel) and the NOx emission analysis made through two different approaches for hydrogen fuel, the NOx sub-mechanism, and the NOx model from ANSYS Fluent. As referred before, those analyses were made in terms flow rate, with the units of grams per second.

Looking at **Figure 5b**, considering the models used for NOx forecast, it is possible to take two principal conclusions:


#### *5.2.2 Sensibility tests*

In this work, several sensibility tests were carried out, namely the presence of the swirl effect on the swirler's inlets and the influence of the fuel injection pressure and temperature. In this work, the initial value of fuel temperature (used in the study) for the Jet A and the hydrogen fuel was 298.15 K, once this is the fuel temperature used by the software *Gasturb*. The results of these tests are presented in **Figure 6**.

As the represented values are similar, they are also presented in **Table 2**.

About the influence of the swirl effect, whose results for the NOx emissions are presented in **Figure 6a**, if one considers the same forecast approach (only the values of ANSYS model or the values of the sub-mechanism), it is possible to conclude that for this specific case the presence of this phenomenon helps to reduce slightly the quantity of NOx emissions for the high power conditions, while for lower power conditions the NOx values are closer.

To analyze the influence of the inlet fuel temperature, the sensibility tests were made with the hydrogen fuel at 600 K, and the results for the NOx emissions are presented in **Figure 6b**. From this figure, it is possible to conclude that:

• For the simulations with the hydrogen temperature of 600 K, the error between the two approaches used to forecast the NOx is considerable for the higher power *Conversion of Gas Turbine Combustors to Operate with a Hydrogen-Air Mixture… DOI: http://dx.doi.org/10.5772/intechopen.106224*

#### **Figure 6.**

*Flow rates of NOx emissions for the sensibility tests for one fourth of the combustion chamber throughout ICAO's LTO cycle burning hydrogen: (a) analysis of the influence of swirl effect through the use of two approaches, NOx sub-mechanism and Ansys NOx model; and (b) analysis of the influence of the hydrogen fuel temperature through the use of two approaches, NOx sub-mechanism and Ansys NOx model.*


#### **Table 2.**

*NOx flow rates in [g/s] obtained for hydrogen fuel (as in Figure 6).*

conditions (85 and 100% power); that is, if we look to the values obtained, the predicted emissions for the NOx model of Ansys are almost twice the value predicted with the NOx sub-mechanism.


About the injection pressure tests, the modification of this value did not make any changes to the results. For that reason, the results are not presented once they do not allow to analyze the influence of this parameter.

### **6. Conclusion**

In this work, an overview of the use of hydrogen in aviation, the modifications needed to adapt an existent gas turbine to use hydrogen, and a CFD simulation of the CFM56-3 combustor burning hydrogen is provided. During this work, it was demonstrated theoretically that the CFM56-3 can work with hydrogen fuel with minor changes (related only to the injection system).

Regarding the results, starting with the control simulation (reference standard), there are several possible reasons that can be pointed out for the differences between the simulations and the ICAO's database values. For instance, the fact that the fuel was considered in the gaseous state when injected into the combustion chamber simulates "perfect" atomization, increasing the combustion efficiency and creating a higher temperature inside the combustion chamber. Other reason can be the fact that the chosen mechanism/sub-mechanism does not represent the combustion of the Jet A fuel or the NOx production in the best way. The choice of the radiation model can also influence this result, once the radiation representation is more important in the hydrocarbon fuel burn than in the hydrogen fuel burn. Other possible reason could be the chemical model used. However, due to the limited computational resources, it was not possible to use more complex models.

Regarding the other results, comparing the NOx emissions obtained for the simulations (for both Jet A and hydrogen), it was shown that for this geometry of combustor and injector, the quantity of NOx produced when burning hydrogen is almost twice of the NOx emissions for Jet A. Once we are using the same swirlers and injector geometry (single hole) for both fuels, these results are in agreement with the results of C. J. Marek et al. [30], who concluded in their job that using similar injection geometries, the minimum NOx levels for hydrogen fuel were twice than for Jet A fuel.

Finally, regarding the sensibility tests (changing the swirl effect, fuel injection pressure, and temperature), only the changes in swirl effect and the fuel temperature produced relevant changes in the results. The fact that the changes in fuel injection pressure did not produce major changes in the NOx emission results could be explained as the fact that for these tests, the geometry of the injectors was always considered the same (the area of the inlets did not change), and the fuel mass flow rate was the same for each power condition. For that reason, the pressure changes will not affect the behavior of the fuel jet that much. In practice, in gas turbine engines, the pressure is usually used to control the quantity of fuel injected.

Through the results obtained for the tests with the swirl effect, it was demonstrated that without the swirl effect, the NOx emissions increased. After analyzing the recirculation zone (position and form), it was concluded that without the swirl effect, the quality of the recirculation zone was reduced and the temperature across the combustion chamber was slightly increased (also increasing the emissions). So, the presence of the swirl effect helps to stabilize the recirculation zone, reducing the presence of hot spots in the flame.

About the influence of the fuel temperature, it was expected that an exponential raise in this value could affect largely the temperature in the outlet of the chamber, increasing the efficiency. However, that did not happen, and not only the outlet temperature changed just a small percentage (1–2%) for the double of the initial fuel temperature, but also cause malfunctions across the chamber, with greater evidence in the higher power conditions, where the analysis of the flame shows a great deterioration of the recirculation zone and the presence of a phenomenon that seems to be the

*Conversion of Gas Turbine Combustors to Operate with a Hydrogen-Air Mixture… DOI: http://dx.doi.org/10.5772/intechopen.106224*

occurrence of autoignition or flashback inside the swirlers. These malfunctions were associated with the velocity of hydrogen fuel flow, which has almost doubled when the temperature was raised. The most credible reason for this phenomenon consists of the density reduction of the hydrogen fuel due to that change in temperature.
