Using a Digital Twin to Reframe Aircraft Design for Sustainable Flight
In this post we will analyze the challenges faced by aerospace engineers in developing sustainable aircraft. We investigate the use of hydrogen-powered jet engines and hydrogen fuel cell technology to power next-generation propulsion systems, as well as their implications on subsystems, resulting in the need to reimagine aircraft configurations.
Simcenter™ software from Siemens Digital Industries Software supports Digital Twin technology, enabling aerospace engineering organizations to optimize aircraft performance through virtual and physical testing in the domains of fluids, thermal, mechanical and other systems related to sustainable aviation . Simcenter is part of the Siemens Xcelerator portfolio, which encompasses software, hardware and integrated services.
Sustainable Aviation
The aviation industry is responsible for nearly 5% of global greenhouse gas emissions,¹ making the transition to low-carbon propulsion systems a priority for aircraft manufacturers. However, this transition is complicated by the constant increase in passenger numbers. Currently, around 500,000 people are on flights at any given time,² and the number of air passengers is expected to double by.³
Aerospace engineers face the challenge of designing next-generation aircraft that have the capacity, speed and range of conventional jet-powered aircraft, but without the environmental impact.
Comparing Power Densities of Different Energy Sources
To understand the complexity of the task at hand, it is critical to analyze the power densities of leading energy solutions for next-generation aircraft compared to conventional kerosene.
Jet A kerosene, which powers most modern commercial and military aircraft, has a remarkable energy density of approximately 12,000 watt-hours per kilogram (Wh/kg). However, kerosene jet engines generate CO2 and non-CO2 emissions and are noisy.
A cleaner and quieter alternative is the use of battery-powered electric motors. However, current batteries used in prototype aircraft have energy densities of only 160 to 180 Wh/kg,⁴ unsuitable for long-haul aircraft. However, they are suitable for smaller aircraft, such as Bye Aerospace,⁵ specializes in electric aircraft, including light aircraft for flight training.
Figure 1. Using Simcenter , NX and Fibersim helped Bye Aerospace increase productivity, reducing engineering headcount by 66% when designing all-electric aircraft.
Hydrogen Production and Conversion into Usable Energy
There are currently two main hydrogen-based approaches to creating long-haul aircraft with zero carbon emissions. One is the use of jet engines powered by liquid hydrogen, and the other involves hydrogen fuel cells that convert hydrogen and oxygen into electricity to power electric motors.
Both liquid hydrogen and hydrogen fuel cells are being actively investigated by companies such as Siemens⁶ and Airbus⁷ as environmentally friendly alternatives for air travel. Both approaches produce water as a byproduct.
Although there are several ways to produce hydrogen,⁸ generating hydrogen is not a simple task, as it is generally present in compounds, such as water (H2O) or methane (CH4), from which it must be separated. Electrolysis is the most practical method for producing hydrogen, which involves the splitting of water into hydrogen and oxygen using an electrical current, and is considered renewable when electricity is generated from sustainable sources, such as solar and wind.
Hydrogen can be stored in gaseous or liquid form. Gaseous storage requires high-pressure tanks, while liquid storage requires cryogenic temperatures, as hydrogen boils at -252.8 degrees Celsius (°C) at atmospheric pressure.⁹
Due to the costs involved in producing, storing and transporting hydrogen, it is currently more expensive than fossil fuels. However, in terms of application as an energy source, hydrogen is conceptually simple.
Aerospace engineers dedicated to developing propulsion systems for sustainable hydrogen-powered aircraft consider three main approaches: electric engines powered by fuel cells, gas turbines powered by pure hydrogen, or hybrid solutions that combine fuel cells with gas turbines powered by hydrogen. . In the case of a hydrogen-powered jet engine, which resembles an internal combustion engine, the process involves intake of air, compression, mixing with hydrogen and subsequent ignition to generate a high-temperature flow.
In the hydrogen fuel cell scenario, hydrogen and oxygen are routed through an anode (positive terminal) and a cathode (negative terminal) in the cell, respectively. A catalyst at the anode splits hydrogen molecules into electrons and protons. Protons pass through a special membrane, while electrons power the aircraft's electric motors and other systems. Subsequently, protons, electrons and oxygen recombine at the cathode, forming water molecules.
Challenges of Hydrogen-Powered Aircraft
The main challenge in developing hydrogen-powered aircraft is their relatively unknown nature to most engineers. Designing a burner for a hydrogen gas turbine requires special structures and features, since hydrogen burns faster and hotter than kerosene.
For example, a hydrogen burner must be designed to prevent flashbacks . Furthermore, the acoustic frequencies generated by the burner and turbine need to be attenuated to minimize interaction between the flame and aircraft components.
Understanding the fluid dynamics and stresses in the thermal boundary conditions of these hydrogen-powered and electric propulsion systems, including operational phenomena such as recoil, thermoacoustics, thermal gradients, and embrittlement, is essential. ¹⁰ ¹¹ ¹² ¹³
Another challenge is that although hydrogen offers three times the energy density of kerosene per unit mass, it requires four times the volume of kerosene to produce the same result. This implies significant modifications to the aircraft structure, such as reducing cargo capacity, number of passengers or a departure from conventional designs.
Figure 2. The increased fuselage space of mixed-wing aircraft can be used to store batteries, hydrogen, or a combination of hydrogen and fuel cells, without sacrificing passenger or cargo capacity.
An alternative is the combined wing body (BWB) aircraft, such as the Airbus ZEROe BWB concept,¹⁴ where the wings and fuselage integrate into a single structure (Figure 2). This design, also called "flying wing", is responsible for all of the aircraft's lift. One of the main advantages of a flying wing configuration is the ample space in the fuselage that can be used to carry various types of payloads, including passengers, batteries, hydrogen and fuel cells.
Facing the Challenges
The complexity of the task of creating hydrogen-powered, carbon-neutral long-haul aircraft makes the evolution of physical prototypes unfeasible due to cost, time and resource constraints. The solution is to resort to multiphysics simulations to investigate the behavior of power generation systems, engines and the entire aircraft in a virtual environment.
This endeavor requires an integration of different design domains and effective collaboration between all engineering disciplines involved in aircraft development. This goes beyond propulsion systems, covering areas such as fluid dynamics, thermal, mechanics, dynamics, acoustics, among others. Engineering data from these interconnected systems must be shared efficiently across teams to enable designers to work effectively in their native development environments.
One way to achieve this effective collaboration is through the use of digitalization tools available in the Siemens Xcelerator portfolio,¹⁵ which includes integrated software, hardware and services. Simcenter test and simulation solutions , part of this portfolio, are designed to eliminate barriers between disciplines and provide an integrated design suite capable of supporting multidisciplinary aerospace engineering teams. These solutions help model, analyze and test the impact of alternative energy sources and propulsion systems.
In short, they allow the creation of a physically based digital twin (Figure 3).
Figure 3. Using Simcenter, engineers can build a digital twin to accurately predict aircraft performance, optimize designs, and innovate faster and more confidently.
Within the Simcenter environment, systems simulation modeling capabilities enable the evaluation of engine architectures, gas turbines, fuel storage, fuel cells, batteries, and other components, including their weight (Figure 4).¹⁶
Figure 4. The Simcenter Amesim model allows engineers to evaluate the thermodynamic cycle of the hydrogen-powered turbofan.
Engineers can leverage parallel fluid simulations, 3D thermal and mechanical simulations, and computer-aided design (CAD) capabilities to design each of these subsystems. In this way, they can deal with challenges such as handling cryogenic fuels, hydrogen combustion and measuring the turbine inlet temperature, as well as the durability performance and dynamic response of the system, among others. Several advanced physics are provided in robust and validated Simcenter models (Figure 5). The design workflow runs on automated workflows and design space explorations to handle conflicts between different disciplines. Components such as burners, blades, assemblies, engines, subsystems, and ultimately the aircraft as a whole can be designed in a similar way to meet different design requirements.
Figure 5. This multidisciplinary design exploration rendering of a hydrogen-burning hybrid cryogenic propulsion system was generated using the Simcenter 3D , Simcenter STAR-CCM+ , Simcenter Amesim , and HEEDS software tools, accurately representing the aeroelasticity of the design.
Simcenter models – including those developed in conjunction with Siemens partners – are generated and run with real-world fidelity to enable aerospace companies to design and deliver real-world systems (figure 6). Simcenter results can be combined with the Siemens Xcelerator portfolio to also take into account the manufacturing capacity of components and systems.
Figure 6. This multi-physics design exploration of an H2 micromix burner leverages NX CAD , Simcenter STAR-CCM+, and Simcenter 3D driven by the HEEDS automated optimization tool . (source: B&B AGEMA, RWTH Aachen and Kawasaki)
Conclusion
Companies such as Siemens Energy,¹⁷ Rolls-Royce¹⁸ and Airbus¹⁹ are carrying out comprehensive evaluations and, in some cases, designing prototypes of hydrogen-powered and hydrogen-hybrid aircraft.
However, it is crucial to understand that the transition to sustainable energy sources goes beyond simply modifying aircraft. This transition marks the beginning of a decades-long journey to reimagine aircraft configurations and address challenges that include supply chains, energy production, distribution and logistics networks, airport fueling systems, and more (Figure 7).
Figure 7. Ditching fossil fuels requires modernizing energy production and logistics networks, including fuel distribution systems at airports.
The Siemens Xcelerator portfolio and Simcenter tools are focused on supporting the digitalization efforts needed to scale the aviation industry toward a sustainable future.
At CAEXPERTS (Siemens technology partner specializing in multiphysics computer simulation), we recognize the urgency of the transition to sustainable aviation. The development of hydrogen-powered aircraft and other low-carbon propulsion systems is crucial to addressing the environmental challenges facing our society. With a team of CAE (Computer Aided Engineering) experts and high-performance cloud capabilities, we are ready to lead this revolution in the aerospace industry.
Our computer simulation and advanced engineering services are prepared to face the complexity of sustainable aircraft projects. We help industries increase their level of innovation, increase their competitiveness and achieve more efficient operations.
If you are committed to innovation and seek solutions to the challenges of sustainable aviation, contact us. Schedule a meeting with CAEXPERTS and discover how our services can boost your projects and accelerate the transition to the aviation of the future. Let's build a cleaner and more sustainable future together.
References
https://www.spikeaerospace.com/how-many-passengers-are-flying-right-now/
https://www.bbc.com/future/article/20210401-the-worlds-first-commercial-hydrogen-plane
https://aerospaceamerica.aiaa.org/features/faith-in-batteries/
https://www.plm.automation.siemens.com/global/en/our-story/customers/bye-aerospace/78928/
https://www.siemens-energy.com/global/en/offerings/renewable-energy/hydrogen-solutions.html
https://www.plm.automation.siemens.com/global/en/our-story/customers/siemens-energy/93022/
https://www.plm.automation.siemens.com/global/en/our-story/customers/b-b-agema/98716/
https://webinars.sw.siemens.com/en-US/simulation-for-digital-testing-with-bb-agema/
https://webinars.sw.siemens.com/en-US/aerospace-defense-aircraft-propulsion-system-simulation
https://www.airbus.com/en/innovation/zero-emission/ hydrogen/zeroe
https://www.plm.automation.siemens.com/global/en/products/simcenter/
https://www.siemens-energy.com/global/en/offerings/renewable-energy/hydrogen-solutions.html
https://www.rolls-royce.com/innovation/net-zero/decarbonising-complex-critical-systems/hydrogen.aspx