**Abstract**

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 an existent gas turbine burning hydrogen are performed. The CFD simulation was done in a CFM56-3 combustor burning hydrogen and Jet A. It was intended to evaluate the viability of conversion of existent gas turbines to hydrogen, in a combustion point of view, by analyzing the emissions while burning it through ICAO's LTO cycle. The pollutant emissions (only NOx, since hydrogen combustion produce only water vapor and NOx) were evaluated through a detailed mechanism and the Ansys Fluent NOx model to get a better agreement with the ICAO's values. For this assessment, several sensibility studies were made for hydrogen burn, for example, the analysis of the air flow with/without swirl in the primary zone and different inlet temperature and pressure for fuel. In the end, it was concluded that theoretically the CFM56-3 combustor can be converted to operate with hydrogen fuel with minor changes (related to injection system). The quantity of NOx produced for each power setting when burning hydrogen is expected to be almost twice the values for Jet A.

**Keywords:** CFM56-3, combustion chamber, pollutant emissions, jet fuel, hydrogen fuel

### **1. Introduction**

The sustainable growth of aviation is important for the future of the economic growth, development, commerce, cultural exchange, and many other factors. According to some experts, by 2045, international air traffic is expected to increase by 3.3 times [1]. In 2015, international aviation consumed approximately 160 megatons (Mt) of fuel. By 2045, compared with the anticipated increase of 3.3 times growth in international air traffic, fuel consumption is projected to increase by 2.2–3.1 times

compared with 2015, depending on the advances in technology and the Air Traffic Management (ATM) scenario [1].

The emissions resulting from the combustion of fossil fuels are usually considered as the main responsible for Greenhouse Gas (GHG) emissions, which are pointed as the primary factor that leads to global warming. For climate change, the primary concerns are emissions of CO, CO2, NOx, and nvPM [2]. Also of concern are persistent contrails, which lead to cirrus clouds. Generally, it is the combination of a number of factors that determine the overall impact of the emissions on global surface temperature over a given timescale. These factors consist of quantities emitted, residence time, radiative forcing, and the temperature response profile of a particular pollutant [2].

The CO2 emissions are of particular concern because of its exceptionally long residence time (thousands of years). Aviation today accounts for 2–3% of global CO2 emissions. While at the global level, CO2 emissions are increasing by around 3% per year, aviation's emissions covered by the EU ETS have increased on average by 5% year-on-year between 2013 and 2018. By 2040, it is expected that international aviation emissions could rise by up to 150% compared with 2020. These growth forecasts take into account the incremental technology improvements that may reduce fuel consumption and emissions by 1–1.5% annually [3].

About the NOx emissions, they are evaluated in two possible scenarios, which are landing and take-off (LTO) NOx emissions, which primarily affect local air quality, and full-flight NOx emissions, which have more effect on the global climate. In 2015, LTO NOx emissions were approximately 0.18 Mt., and by 2045, they are projected to range from 0.44 to 0.80 Mt. depending on the technology and ATM scenario [1]. While, in 2015, the full-flight NOx emissions of international aviation were 2.50 Mt., by 2045, the full-flight NOx emission projection ranges from 5.53 to 8.16 Mt., which represents a 2.2–3.3 times growth compared with 2015 [1].

To mitigate this problem, there are several possible solutions. On the one hand, the fuel burn reductions through the upgrade of the technology employed in the actual aircrafts such as the airframes (aerodynamics and mass) and the engines, both with the aim of achieve higher efficiency [2]; on the other hand, the use of alternative fuels and power sources [4, 5]. According to the ICAO 2016 trends assessment, a 100% substitution of aviation fuel with SAF could reduce 63% of the baseline CO2 emissions from international flights in 2050 [4]. As referred by ATAG [5], it is possible that aviation meets netzero CO2 emissions by 2050; however, it would take an enormous effort to make it a reality. This would mean a rapid and massive transformation on aviation's energy supply through the use of SAFs, and it would require acceleration in aircraft and engine technology development, mainly: electric-, hybrid-, and hydrogen-powered aircraft.

Within this context, the conversion of the current gas turbine engines to new sustainable fuels can also be a solution. So, in this study, be analyzed the feasibility of the use of hydrogen fuel as substitute of the conventional jet fuel in a CFM56–3 combustor using a CFD approach. The NOx emissions produced while completing the ICAO's LTO cycle burning this fuel will be assessed for the standard operating conditions of the engine, as well as the influence of several operating parameters (swirl effect, temperature, and pressure of fuel) in these emissions.

#### **1.1 Brief historical review**

To date, the largest user of hydrogen in aeronautics is the space program where it is used as fuel for the rocket engines of launch vehicles. The first successful launch of a space vehicle propelled by a liquid hydrogen/liquid oxygen rocket engine took place at *Conversion of Gas Turbine Combustors to Operate with a Hydrogen-Air Mixture… DOI: http://dx.doi.org/10.5772/intechopen.106224*

Cape Kennedy on November 27, 1963. Several other rocket engine manufacturers in the United States were involved in the development of designs using LH2; for example, the General Electric Company, the Rocketdyne Division of North American Aviation (now Rockwell International, Inc.), and the Aerojet General Corporation were among the leaders. Of the designs developed by these companies, the Rocketdyne J2 engine is an example, which has been eminently successful. It was used in both the second and third stages of the Saturn V launch vehicle, used in the Apollo program, which landed U.S. astronauts on the moon. In all of the launches of the Apollo program, there has never been a failure of one of the hydrogen-fueled rocket engines.

However, the space applications are relatively recent, if we look at history, the first reported use of hydrogen in aeronautics was a long time ago. According to Brewer [6], hydrogen was first employed as lifting medium when, in France, a small silk balloon was constructed by the Roberts brothers, under the direction of physicist J.A.C. Charles, and it was flown in Paris on August 27, 1783. This balloon rose to a height of 3000 ft. (914.4 m) and traveled a distance of 15mi (24.14 km). In that year, on December 1, a larger hydrogen-filled model, which carried two passengers, the physicist Charles and one of the Roberts brothers, was launched. This flight traveled 25mi (40 km) from Paris in less than 2 hours.

Later in history, airships came into being as a result of man's desire to control the direction and speed of flight. Numerous attempts were made to achieve such control with balloons without measurable success until 1852, when a Frenchman, Henri Giffard, constructed an airship on which he mounted a steam engine of his own design. Giffard flew this hydrogen-filled airship from the Hippodrome in Paris on September 24, 1852, attained an estimated speed of 6 mph, and demonstrated the first appreciable control of a "lighter-than-air craft." In 1872, Paul Haenlein developed and flew an airship powered by an internal combustion engine, which was fueled by gaseous hydrogen that was drawn from the lifting cells of the airship envelope [6]. A significant step leading to the use of hydrogen in commercial air transportation occurred in 1900 when the first rigid airship designed by Count Ferdinand von Zeppelin, the LZ-1, made a successful flight. In 1911, commercial air operations were started by a German transportation company (DELAG), using five Zeppelin airships. In October 1924, the Zeppelin factory at Lake Constance, in Germany, completed the construction of the LZ-126, inflated it with hydrogen, and delivered it to the United States by a transatlantic flight.

In 1955, a report by Silverstein and Hall of the (then) NACA-Lewis Flight Propulsion Laboratory was published in which the potential of liquid hydrogen as a fuel for use in both subsonic and supersonic aircraft was explored. According to it, theoretically, the use of hydrogen fuel could significantly improve the maximum range [6]. As a result of this study, an experimental program with a U.S. Air Force B-57 twin-engine medium bomber was initiated to demonstrate the feasibility of burning hydrogen in a turbojet engine at high altitude. The modified aircraft was first flown in 1956.

From 1954 to 1955, Lockheed Aircraft Corporation made a series of conceptual design studies of hydrogen-fueled aircraft in cooperation with Pratt & Whitney Aircraft and the Rex Division of AiResearch Corporation. In 1956, the U.S. Air Force awarded a contract to Lockheed's Advanced Development Projects organization to build two prototype aircrafts (known as CL-400), which would be capable of cruising at Mach 2.5 at 100000 ft. (30,480 m) altitude. This aircraft was to carry a two-man crew, and the main objective was related to long-range reconnaissance missions.

Also in 1956, at the same time the U.S. Air Force contracted with Lockheed for the development of the CL-400 airplane, and the Pratt & Whitney Aircraft Division of United Aircraft Corporation was awarded a contract to investigate the feasibility of using LH2 as a fuel in aircraft engines. The work at Pratt & Whitney covered a broad spectrum ranging from applied research efforts such as heat transfer and materials investigations, to development testing of a J57 engine modified to operate on LH2. It also included the design, construction, and test of a new design of engine (the Model 304). Conversion of the J57 to operate on LH2 was accomplished in just 5 months, and the first tests were performed in the fall of 1956 [6]. The work with the J57 showed that conventional jet engines could be readily adapted to use LH2 fuel. In this research, after examining many possible cycles, the Hydrogen Expander cycle (this cycle is well explained by Brewer [6]) was selected for experimental evaluation to create the Model 304 engine. This was a unique cycle developed specifically to take advantage of the properties of hydrogen and to meet the performance requirements of the CL-400 airplane. The first demonstration test of a complete 304 engine was accomplished in September 1957.

In spite of the success in developing practical solutions to the problems encountered with handling the cryogenic liquid fuel, the CL-400 aircraft was never built due to performance and logistics limitations. So, in 1957, the program was terminated by mutual agreement between the Air Force, Lockheed, and Pratt & Whitney. However, the CL-400 design and development program showed that it was entirely feasible to build a hydrogen-fueled airplane.

In the 1970s, Lockheed performed studies on different liquid hydrogen-fueled subsonic cargo and passenger transport jets for NASA Langley Research Center. The results are presented in the NASA-reports NASA CR-132558, NASA CR-132559, and NASA CR-144935. The main conclusions from these and furthers studies have been summarized by Daniel G. Brewer in [6]. The studies showed that hydrogen propulsion is especially beneficial in terms of energy use for long-range aircraft with internal hydrogen tanks.

In the 1980s, Tupolev developed the Tu-155 that was based on the medium-range transport aircraft TU-154B. Moreover, the TU-155 was built and successfully tested without any serious incidents, and it first flew burning hydrogen in one of its three engines in April 1988. The modified engine was also able to be run with natural gas. The TU-155 was followed by the TU-156 that could be run with natural gas or kerosene [7].

At the beginning of the twenty-first century, the Cryoplane Project comprised of 36 European research partners from industry, universities, and research institutions. They contributed to this project covering aircraft configuration, systems and components, propulsion, safety, environmental compatibility, fuel sources and infrastructure, transition. The total project time was 26 months and started in April 2000. During this project, several conventional and unconventional overall aircraft design studies and detailed investigations of hydrogen fuel systems and components were performed [7, 8].

More recently, in July 2010, Boeing unveiled its hydrogen-powered Phantom Eye UAV that uses two converted Ford Motor Company piston engines. Nowadays, governments and companies are investing again in hydrogen's potential. For instance, the ENABLEH2 (ENABLing cryogEnic hydrogen-based CO2 free air transport) consortium was given such a hand, almost 20 years after the European Commission's last attempt to ramp up LH2 research and development under the Cryoplane project. The project's objective is to demonstrate that switching to hydrogen is feasible and must

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

complement research and development into areas such as advanced airframes, propulsion systems, and air transport operations [9]. Another example is the project named ZEROe, announced by Airbus in September 2020, which has the ambition to develop the world's first zero-emission commercial aircraft. This project consists of the development of three concept planes (powered by hydrogen combustion through modified gas turbine engines or hybrid systems), which Airbus says that could be ready for deployment by 2035 [10].

### **2. Basic engine principles**

#### **2.1 CFM56-3 general specifications**

The CFM56-3 is a high bypass, dual-rotor (or dual-shaft), axial flow turbofan engine, and this particular variant of CFM56-3 has a bypass ratio of 5.1:1 and a dry weight of 1966 kg [11, 12]. Its dual-shaft design consists of a fan and booster (low-pressure compressor), high-pressure compressor, annular combustion chamber, and a high- and low-pressure turbine section. The two shafts respectively connect the low- and highpressure sections using a five-bearing system (three roller, and two ball bearings) [13].

#### **2.2 Combustor**

First of all, it is important to understand the difference between the combustor and the combustion chamber [14]. The combustor includes all of the combustion systems, that is, the diffuser, the combustion chamber, the inner and outer casing, the spark plugs, and the fuel injectors, whereas the combustion chamber refers to the exact place in which combustion takes place.

The main purpose of a gas turbine combustor is to introduce heat energy into the mass of air previously compressed (in the compressor) [15], by burning fuel in it so that the products of combustion can be expanded to get useful work output (absorbed by the turbines) and then, on their discharge to atmosphere, provide a propulsive jet [16]. Due to space limitations and requirements of energy and momentum, the volume flow rate as well as rate of heat release is very high in a gas turbine combustion chamber and the residence time of fuel is very small, of the order of a few milliseconds [15]. In gas turbines, the combustion is a continuous process that takes place at high pressure in a smaller space and usually at a very high temperature [16]. Thus, continuously high combustion temperatures, large continuous flow, and high heat energy release make the design and development of a gas turbine combustor rather difficult [15].

A gas turbine combustor must satisfy a wide range of requirements. However, for the aircrafts, the priorities are the reliability, the low fuel consumption, low pollutant emissions, engine size, and weight [17].

The choice of a particular combustor type and design is determined largely by the overall engine design and by the need to use the available space as effectively as possible [16]. Overall, the combustors may be subdivided into three main types: tubular or can, tubo-annular, and annular [16]. The CFM56-3 has an annular combustor, and **Figure 1** shows it during the digitalization process to obtain the model of geometry used in this work.

The annular configuration is used by most modern jet engines because of its lighter design. This type of combustor represents the ideal configuration for combustors since

#### **Figure 1.** *CFM56-3 combustor photograph [14].*

its "clean" aerodynamic layout results in compact dimensions [16]. This configuration consists of an annular liner that is mounted concentrically inside an annular casing. Among the advantages, the annular combustion chambers have the least pressure drop due to larger volume per unit surface area and are more efficient than can-type chambers. It also requires about half of the diameter of can-type chambers for the same mass flow [15].

Its main drawback stems from the heavy buckling load on the outer liner [17]. Moreover, any change in the flow velocity profile can result in significant change in the temperature distribution of the outlet gases, and distortion of inner annular chamber is critical because it disrupts the flow of cooling air and also changes the outlet temperature distribution. This is because of lower degree of curvature of the chamber surfaces [15]. Another drawback is related to experimental tests of this type of combustion chamber. At full-load conditions, the tests of large annular combustion chambers supplying air at the levels of pressure, temperature, and flow rate required are extremely difficult, and the cost is very high [17]. Nowadays, there are very few facilities worldwide that can supply air in those conditions [18].
