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Published October 15, 2024

Academic Spotlight: Illuminating the Physics of Fuel Sprays and Combustion

Author:
Elizabeth Favreau

Marketing Writing Team Lead

In today’s energy and transportation industries, sprays and combustion are at the heart of many of our most relied-upon technologies. From internal combustion engines to gas turbines to burners, better understanding the fundamental physical processes that drive these devices can help us make them more efficient and more sustainable in the future.

Professor Noah Van Dam’s Multi-Phase and Reacting Flows Laboratory at the University of Massachusetts Lowell is dedicated to studying and characterizing these processes through computational fluid dynamics (CFD) modeling. A CFD aficionado since his undergraduate days, Prof. Van Dam was introduced to CONVERGE during his postdoctoral studies at Argonne National Laboratory, where he focused on the effects of fuel properties on engine performance. When he started his own lab at UMass Lowell, he continued to use CONVERGE through the CONVERGE Academic Program, which provides licenses, training, and support for academic research. 

Starting at the Very Beginning: Simulating Spray G

CONVERGE wasn’t the only thing Prof. Van Dam carried over into his lab at UMass Lowell—he also continued his research on spray and combustion modeling. When Aman Kumar joined Prof. Van Dam’s lab in 2020 as a graduate research assistant, he began conducting detailed numerical studies of the Engine Combustion Network (ECN) Spray G injector. He was interested in understanding how the injector geometry and the location of the spray plume affected downstream conditions and overall engine performance.

“I’ve been focusing on fundamental studies, because everything starts right at the beginning with how you are injecting the fuel and how the mixture is being developed,” Aman said. “If the mixture is homogenous, the fuel-air mixture will burn at reduced combustion temperature and the engine will produce lower amounts of NOx, soot, and particulate matter emissions.”

Figure 1: Experimental and CONVERGE-predicted vapor penetration length (left) and liquid penetration length (right) for eight RANS Spray G simulations.1 (Note: roi = rate of injection; 1w = one way; flat = flat injector top; inj = counterbore injector top; co = counterbore outlet parcel initialization; no = nozzle outlet parcel initialization.)

In his studies, Aman experimented with both Reynolds-Averaged Navier-Stokes (RANS)1 and large eddy simulation (LES)2 modeling frameworks. He looked at various parameters, including having the injector tip geometry drawn in the cylinder head versus not including it, initializing the parcels at the counterbore exit versus the nozzle exit, using an experimentally derived rate of injection versus reading the injector flow parameters from a volume of fluid (VOF) simulation of the internal injector flow, and the use of a nominal versus x-ray scanned injector geometry. He compared spray penetration and other global parameters to experimental data. Figure 1 shows vapor and liquid penetration length plots for eight RANS simulation cases compared to experimental data. The different cases resulted in only slightly different penetration lengths, and the CONVERGE simulations matched well with the experimental data. Figure 2 shows a comparison of projected liquid volume fraction for the RANS and LES cases. While the RANS simulation captures the global spray behavior, the LES simulation better captures the local turbulent flow features. 

Figure 2: Projected liquid volume fraction for the RANS and LES Spray G cases using a VOF-spray one-way coupling mass flow rate, flat top geometry, and counterbore outlet initialization location.2

The Future of Energy: Simulating Ammonia Sprays

Following their Spray G studies, Aman and Prof. Van Dam turned their attention to alternative fuels, in particular ammonia. 

“Our future energy requirements need to move in a direction where we’re reducing the net greenhouse gases that we are emitting from transportation and other energy systems. Alternative fuels, such as ammonia, is one pathway that has been proposed, and it’s one that is looking more and more like it is going to be a fruitful avenue for research and actual production,” explained Prof. Van Dam.

The properties of ammonia, however, differ significantly from traditional hydrocarbon fuels. For example, liquid ammonia sprays are more likely to undergo flash boiling under most engine operating conditions, which could necessitate new injection strategies. Aman used CONVERGE to study how well current spray models can capture liquid ammonia spray behavior.3

He used a RANS turbulence model with two different simulation methods: a VOF approach for in-nozzle simulations and a Lagrangian-Eulerian (LE) parcel-based approach for downstream simulations. For the LE simulations, Aman also tested two different methods of initializing the spray parcels: one-way coupling using the results from the in-nozzle simulations and a prescribed rate-of-injection (ROI) method. 

Figure 3 compares experimental images with simulated ammonia sprays using the in-nozzle VOF approach at different pressure ratios. The CONVERGE simulations are able to capture the widening of the spray plume as the spray begins to undergo flash boiling at higher pressure ratios. 

Figure 3: Experimental ammonia spray images (left) and contour plots of ammonia liquid mass fraction (right) for in-nozzle VOF simulations at different pressure ratios.3

Aman found that for a non-flashing case, the two LE modeling frameworks best captured the liquid penetration lengths, whereas the VOF in-nozzle method performed the best for the flash-boiling case. A significant amount of ammonia vapor is produced inside the counterbore geometry which can be seen easily in CFD simulations but is difficult to capture in experiments. He and his lab are continuing their studies into ammonia sprays and are working to further improve the existing spray models to robustly capture ammonia’s flash boiling behavior.

Turning Up the Heat: Ammonia/Hydrogen Combustion

As mentioned earlier, fuel injection is only the beginning of the story in an IC engine. Continuing on downstream, Prof. Van Dam is also investigating the combustion of alternative fuels. For these studies, Prof. Van Dam teamed up with other researchers including Prof. Dimitris Assanis at Stony Brook University with the goal of gaining a better understanding of ammonia/hydrogen combustion.

As combustible fuels go, both ammonia and hydrogen come with some challenges. Ammonia is hard to ignite and has a very low flamespeed. On the other hand, hydrogen is very reactive and burns very rapidly. 

“By mixing hydrogen and ammonia, we can mitigate some of the issues of each individual fuel and create a blended fuel that behaves much more closely to our current hydrocarbon fuels. We have a lot of experience with hydrocarbon fuels, and so it’s much easier for us to design engines for fuels that behave similarly,” said Prof. Van Dam.

In their collaborative study,4 the researchers from UMass Lowell and Stony Brook first tested several different chemical kinetic mechanisms for ammonia/air and ammonia/hydrogen/air combustion to determine which mechanism best matched available experimental data for laminar flamespeed and ignition delay. They then took the best performing mechanism and ran 3D CFD simulations in CONVERGE to study the combustion characteristics. Figure 4 shows a visual comparison of the flame from experimental Schlieren images and the CFD results for ammonia/air combustion. The simulations show similar flame shapes as the experiment at each time step for all three equivalence ratios. 

Figure 4: Visual comparison of experimental and CONVERGE-predicted ammonia/air flames for different equivalence ratios.4

The researchers discovered that compared to ammonia/air combustion, the ammonia/hydrogen/air combustion resulted in a faster flame that was less dependent on the spark event and did not experience buoyancy effects. They concluded that ammonia/hydrogen mixtures demonstrate complementary combustion characteristics that could lead to improved performance for engine applications.4

The group from UMass Lowell and Stony Brook are continuing their research into ammonia combustion, which you can look forward to in an upcoming paper at the 2024 ICE Forward Conference.5

A Taste of Salt Air: Simulating a Swirl Burner for Marine Propulsion

Alternative fuels aren’t the only pioneering technology that Prof. Van Dam’s lab is researching—they are also helping to develop reliable propulsion systems for the next generation of unmanned surface vessels. In a collaborative project with the U.S. Office of Naval Research, Prof. Van Dam’s group is investigating how burners operating in marine environments are affected by intaking salty air.

Undergraduate researcher Colin Wildman began working on this project when he joined Prof. Van Dam’s lab in 2022. The first step for the project was to test different diesel fuel surrogates in a swirl burner to determine which one most accurately represented the flame shapes and emissions of the experimental setup. Using the SAGE detailed chemistry solver and LES turbulence modeling, Colin tested five different diesel fuel surrogates (Surrogate A, Surrogate B, T15, T15 + CH5, and T20).6 He also tested a more computationally efficient RANS turbulence model, using Surrogate A, to see if that would give them reasonable results in a shorter amount of time. Figure 5 shows the temperature contours of the resulting flames for the different surrogates they tested. The RANS model produced a smoother, more cylindrical flame shape compared to the LES simulations, which more accurately captured the intricate flame structures. Because of this, they decided to stick with LES modeling with diesel Surrogate A. The next step in this project is to introduce salt into the flame and see how that affects combustion and emissions production. 

Figure 5: Temperature contours of the swirl burner flames for different diesel surrogates.6

Growing as Researchers

When Colin joined Prof. Van Dam’s lab, he had no experience with CFD software. By watching our on-demand training courses, getting hands-on experience with CONVERGE, and working with our support engineers, he was able to become a proficient and independent CFD user. 

“The CONVERGE Academic Program, with the training videos and support, has helped me grow into a student that’s independent. It’s kind of a happy memory for me thinking that when I started, I had no idea what I was doing. And now I’m independent, running cases on my own. Now when we have new students join the lab, I’m the one that shows them the ropes,” Colin said. 

The goal of the CONVERGE Academic Program is to equip students and other academic researchers with the tools and skills they need to succeed in academia and beyond. Academic users get access to the full-featured CONVERGE package, which helps prepare them for a smooth transition to a career in industry after graduation. Stories like Colin’s emphasize that with the right resources and support, learning an advanced CFD software and conducting impactful, cutting-edge research is well within your reach. 

To learn more about the CONVERGE Academic Program, visit our webpage or fill out this form to get in touch with our academic specialists!

References

[1] Kumar, A. and Van Dam, N., “Study of Injector Geometry and Parcel Injection Location on Spray Simulation of the Engine Combustion Network Spray G Injector,” Journal of Engineering for Gas Turbines and Power, 145(7), 2023. DOI: 10.1115/1.4062414

[2] Kumar, A., Boussom, J.A., and Van Dam, N., “Large-Eddy Simulation Study of Injector Geometry and Parcel Injection Location on Spray Simulation of the Engine Combustion Network Spray G Injector,” Journal of Engineering for Gas Turbines and Power, 146(8), 2024. DOI: 10.1115/1.4063957

[3] Kumar, A. and Van Dam, N., “Liquid Ammonia Sprays for Engine Applications,” ILASS-Americas 34th Annual Conference on Liquid Atomization and Spray Systems, Ithaca, NY, United States, May 19–22, 2024.

[4] Shaalan, A., Nasim, M.N., Mack, J.H., Van Dam, N., and Assanis, D., “Understanding Ammonia/Hydrogen Fuel Combustion Modeling in a Quiescent Environment,” ASME 2022 ICE Forward Conference, ICEF2022-91185, Indianapolis, IN, United States, Oct 16–19, 2023. DOI: 10.1115/ICEF2022-91185

[5] Mathai, J.R., Rana, S., Shaalan, A., Nasim, M.N., Trelles, J.P., Mack, J.H., Assanis, D., and Van Dam, N., “Numerical Study of Buoyancy and Flame Characteristics of Ammonia-Air Flames,” 2024 ASME ICE Forward Conference, ICEF2024-141569, San Antonio, TX, United States, Oct 20–23, 2024. (Forthcoming)
[6] Wildman, C., Fernandez, J., and Van Dam, N., “Low-Pressure Swirl Burner for Marine Propulsion Applications,” 2023 CONVERGE CFD Conference, Online, Sep 26–28, 2023.

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