Reducing Environmental Impact of Jet Engines by Hydrogen Co-combustion

Due to the need to mitigate global warming, there is a growing interest in alternative fuels for various means of transport, including aviation gas-turbine engines. The work aimed to check the impact of hydrogen co-combustion on the performance and emissions of aircraft engines. Zero-dimensional models of JetCat P140 RXI and DGEN 380 engines developed in the GSP (Gas Turbine Simulation Program) program were used in the research. Combustion calculations in GSP are based on the real gas model and NASA Chemical Equilibrium Applications (CEA) equations. The performance of the engines fueled by Jet A-1 and blends containing hydrogen or methane were calculated. The simulations were performed at the design point on the ground, and then in flight for selected altitudes and flight speeds. With an increase in the gas content in the blend, the thrust and temperature behind the turbine slightly increase, and the specific fuel consumption decreases, because hydrogen and methane have a higher calorific value. The performance of JetCat and DGEN 380 engines was calculated for blends of kerosene with methane or hydrogen. This knowledge will be used to convert these engines to gaseous fuels. In terms of fuels and emissions, GSP has limitations related to the set of available chemicals and the zero-dimensional model of the combustor.


Introduction
Like kerosene, hydrogen can be burned directly in gas-turbine engines to produce thrust.Work on such propulsion systems began in the 1970s as a result of the fuel crisis.Due to insufficient funding and technical difficulties, hydrogen-powered aircraft engines have not yet reached maturity, despite several successful prototype tests.Recently, the idea of direct hydrogen combustion is being seriously considered again, because other technical possibilities for reducing CO2 emissions have been exhausted for long-haul aircraft [1].
Direct combustion of hydrogen in turbines is less efficient than the use of fuel cells and electric motors, but unlike cells, it is possible to obtain high thrust and power necessary to power long-haul or large cargo planes.At the same time, it is possible to use existing turbine engines after adaptation.In practice, using pure hydrogen in conventional gas turbines is challenging, so co-combustion may be used.Gurbuz et al. [2] examined the emissions of the JetCat P80-SE micro engine powered by diesel and hydrogen gas with an energy share of up to 40%.They found that the increasing share of hydrogen raises the combustion efficiency and the local temperature in the combustion chamber by up to 350 K.At the turbine outlet, the temperature increases only by 50 K.When increasing the share of hydrogen in the fuel, CO2 and CO emissions decrease significantly, and the level of HC and NOx does not change significantly.
Design optimization must involve the analysis of engine efficiency and emission levels.Complex and computationally expensive CFD numerical models are used to design combustors with reduced emissions [3,4].However, well-known turbine modeling and simulation tools such as Gas Turbine Simulation Program (GSP) [5] can also be used for this purpose.Zero-dimensional engine models often use a simplified analytical description of combustion and tabular temperature values for fuel mixtures [6].In addition to the calorific value of the fuel, its temperature and state of matter (liquid or gaseous) must be taken into account in the calculations [7].In combustor modeling, thermodynamic equations are used to describe combustion and heat transport [8,9].
This research aimed to check the impact of hydrogen co-combustion on the performance and emissions of aircraft engines.Zero-dimensional models of the JetCat P140 RXI and DGEN 380 engines developed in GSP were used in the research.Combustion calculations in GSP were based on the real gas model and NASA Chemical Equilibrium Applications (CEA) equations.Engine performance was simulated for selected fuel blends.

Methods
Traditional engine models are based on the thermodynamic description of the engine cycle and often use compressor and turbine characteristics.The model of JetCat P140 RXI micro turbojet (Figure 1) was recently developed and fine-tuned in the GSP program, and then verified with some rig-and flight-test data [10].Due to the lack of compressor and turbine maps for the DGEN 380 engine, its simplified model was developed (Figure 2), which was only tuned in two working points: Take-off and Cruise.
Simulation studies at GSP consisted of the following steps: 1. Determination of the list of cases for simulation (fuel blends and operating points).
2. Development of the model and its tuning to design or test data 3. Calculations at the operating point, on the ground, for Jet A-1 and blends containing hydrogen or methane 4. Flight calculations for Jet A-1 and blends with hydrogen or methane content 5. Visualization and checking of results A separate case was created for each variant and fuel in the GSP project.Unless stated otherwise, JetCat simulations were performed at a maximum RPM of 125,000 rpm (ground and flight).DGEN 380 simulations were performed for n2=100% on the ground and n2=94% in flight, according to the engine specification.It is also possible to create a so-called Steady State Series and generate results depending on changes in the selected parameter (e.g.rotational speed or flight altitude).
Jet A-1, methane (CH4) and hydrogen fuels and their blends were tested.Due to GSP limitations, it was not possible to perform calculations with ammonia (NH4).It is potentially possible to simulate diesel, gasoline, alcohols and other hydrocarbon fuels [11,12].The GSP combustion model is based on the combustion reaction equations of fuels with different chemical compositions.Pre-combustion mixing or evaporation is assumed to play a negligible role in combustion (and emissions).This is a socalled well-stirred reactor that does not describe phase transitions and three-dimensional mixing effects.This model can only be a reasonable approximation if there are no large temperature gradients and if the flow is sufficiently uniform.This approach (Combustor component) was used to simulate the performance of JetCat and DGEN engines powered by fuel blends.The Fuel mixer component is designed for modeling fuel blends in GSP.It was found that equally good results can be obtained without it, by preparing the fuel blend in the 'User specified composition' tab of the Combustor component (Figure 3).Kerosene is defined as C12H23.GSP requires at least a trace amount of carbon in the blends, so pure hydrogen should be entered as 99.99% H2 and 0.01% CO2.Pure fuels defined in the 'User specified composition' tab generate the same engine performance as standard fuels (kerosene, methane, hydrogen) available in the Select fuel field.

Figure 3. Blend of 75% kerosene and 25% hydrogen -example
The calculations were made assuming that the temperature at the exit of the combustion chamber was maintained.This means that GSP selects the fuel flow so as to reach and not exceed the given temperature (Figure 4).

JetCat P140 RXI engine
The simulation inputs are consistent with the work [10].Table 1 presents selected results.The results of steady-state simulations of the JetCat P140 RXI engine depending on the content of the gas component (hydrogen or methane) in the blend with kerosene are shown in Figure 5

DGEN 380 turbofan
Data and assumptions for the DGEN380 engine are summarized in Table 2.The simulation results of the steady states of the DGEN 380 engine depending on the content of the gas component (hydrogen or methane) in the blend with kerosene are presented in Figure 9

Conclusions
Zero-dimensional modeling of combustion in GSP was carried out to predict the performance of JetCat and DGEN engines fueled with blends of kerosene with methane or hydrogen.With an increase of the gas content in the fuel, the thrust and exhaust gas temperature slightly increase, and the specific fuel consumption decreases, because hydrogen and methane have a higher calorific value.The presented results show how hydrogen co-combustion affects engine performance which is essential for planning experiments and redesigning the fuel system.Basic aircraft missions were modeled to compare fuel consumption under different conditions.However, simulation of off-design operating conditions requires compressor and turbine characteristics and model verification.Emission calculations require the use of a multireactor and the division of the combustion chamber into zones.
In terms of fuels and emissions, GSP has limitations related to the set of available chemicals and the zero-dimensional model of the combustor.Despite this, our simulation results are in line with available experimental data.

Table 1 .
Simulation of the operation of the JETCAT engine powered by various fuels.

Table 2 .
Assumptions adopted in the model Simulations for individual fuels are optimized for the temperature downstream of the combustion chamber (t = 1187.26K)when the JET A base fuel is combusted.•They are carried out for ISA conditions • Due to the lack of available data, most of the data was selected from the range provided in publications and statistical data.

Table 3 .
Results of the simulation of powering the DGEN 380 engine with various fuel blends.