Simulating spray atomization and combustion within large marine engines
Andrea Di Matteo defended his PhD thesis at the Department of Mechanical Engineering on May 9th.
Large ships, such as container carriers and cruise liners, depend on powerful engines. For both environmental protection and economic viability, it’s important to enhance the efficiency of the engines in these ships and reduce their emission from fuel combustion. Concurrently, the exploration of novel, sustainable fuels is essential. Direct studies of the complex processes of fuel spray atomization and combustion within these massive engines under real-world conditions are crucial for this. However, significant challenges such as high costs and practical limitations make these studies hard to do. Andrea Di Matteo focuses in his PhD research on the development and testing of a detailed computational model that can be used as a virtual laboratory to investigate these processes in large marine engines.
The objective of Andrea Di Matteo’s research was to accurately replicate the conditions found inside the engines in massive ships while maintaining computational efficiency. Before simulating the marine-sized lab facility, the accuracy of the computational model was tested on smaller, well-understood and documented Engine Combustion Network (ECN) sprays. Researchers often use these fuel sprays as benchmarks for their studies.
Turbulence and combustion
These initial tests focused particularly on the simulations of turbulence and combustion. Turbulence is mainly modeled in two ways: the LES is highly detailed but computationally expensive, and RANS has a simpler and more efficient approach. Andrea Di Matteo found that RANS provided sufficient accuracy and practicality for his research. Next to that combustion was pre-calculated and stored in look-up tables. During the simulation, the software retrieves the necessary species corresponding to the local thermophysical conditions from these tables. This significantly reduces computational cost while maintaining high fidelity, as the pre-calculation can utilize highly-refined chemical mechanisms.
The combustion process
Further investigation using the ECN sprays delves into the initial moments of the combustion process, specifically examining the influence of the injector size and the initial temperature on the ignition delay. This delay is the time lag before combustion begins. The modeling revealed that the microscopic mixing rate between fuel and air, and its interaction with turbulence, plays a critical role. As shown by this research, low-to-medium levels of mixing rate are the most beneficial for the first onset of combustion at all temperatures and for both injector sizes.The successful validation using ECN sprays provided confidence in applying these computationally efficient yet accurate models to simulate larger, marine-scale sprays.
Full-scale simulation
Recognizing that marine injectors are significantly larger than the benchmark ECN injectors, Andrea Di Matteo examined the effect of nozzle size on ignition timing and flame stabilization location with a dedicated study. This analysis yielded clear mathematical relationships, enabling the prediction of combustion behavior for significantly larger nozzles. The models outlined and tested in the preceding steps were then used to simulate the full-scale marine combustion chamber. Simulation predictions of spray behavior and key combustion parameters for both single- and multi-nozzle configurations were validated against experimental data. The results of this confirm the effectiveness of the models used and extends their applicability to the large scales.
Simulations with ammonia combustion
Finally, Andrea Di Matteo studied ammonia as a promising alternative carbon-free fuel with the developed computational model. Burning ammonia is tricky, particularly without help from a pilot injection of traditional fuel like diesel. The simulations show this clearly, but they are instrumental in identifying the most favorable engine-like conditions that allow an auto-ignition of ammonia. Under these conditions, stable ammonia combustion is predicted, which is a positive step towards using this fuel in future marine engines. In short, this research provides a trustworthy and efficient numerical model to investigate marine sprays. This enhances the understanding of combustion processes in marine engines, supports efforts to improve their performance and efficiency, and facilitates the exploration of cleaner alternative fuels such as ammonia.
Title of PhD thesis: . Promotor: Associate Prof. Bart Somers and Prof. Jeroen van Oijen.