Have you ever tried food coloring? It can make cooking a whole lot more fun and provides a great example of how two fluids can mix well – or not very well at all.
Add a small droplet to the water and you might see it slowly dissolving into the larger liquid. Add a few more drops and you might see a wave of color spreading, the colored droplets spreading and breaking apart to diffuse more fully. Add a spoon and start stirring quickly, and you’ll probably find that the water completely changes color, just as you want it to.
Researchers at the USC Viterbi School of Engineering, led by Ivan Bermejo-Moreno, assistant professor of aerospace and mechanical engineering, investigated a similar phenomenon with high-velocity gases, with a view to more efficient mixing for support supersonic scramjet engines. In the study published in Fluid physics, Ph.D. USC Viterbi. Jonas Buchmeier, along with Xiangyu Gao (USC Viterbi Ph.D. ’20) and former M.Sc. student Alexander Bußmann (Technical University of Munich), developed a new tracking method that zoomed in on the fundamentals of how mixing occurs. The study helps to understand, for example, how the injected fuel interacts with the surrounding oxidants (air) in the engine to make it work optimally, or how interstellar gases mix after a supernova explosion to form new stars. . The method focuses on the geometric and physical properties of the turbulent swirling motions of gases and how they change shape over time as they mix.
Scramjet engines – super-fast experimental engines with no moving parts – have already set the air speed record for jet aircraft at Mach 9.6, allowing a potential trip from Sydney to London in around 90 minutes.
“The dynamics of these individual flow structures and the geometric changes they undergo have not been tracked over time,” Bermejo-Moreno said, “because we previously did not have the computational techniques to do so, especially in a turbulent propulsion system (like in a jet engine).Now we can observe thousands or hundreds of thousands of these flow structures simultaneously and track for each how the shape of the structure changes and how it mixes and interacts with surrounding structures.
The value, Bermejo-Moreno said, is that once you can identify the most useful patterns to speed up the mixing process, you can replicate those specific conditions, since you can see how the structures (of the fuel and of the oxidant, for example) at every moment.
“In a supersonic combustion engine, you want fuel mixing to happen as quickly as possible so the fuel is completely used up before it exits the engine,” he said. “To do that, we need to understand how mixing happens at different times.”
Shapeshifting and Shockwaves
When fuel is injected into a rocket or jet engine, it begins a diffusion process, Bermejo-Moreno said.
“The injection process will typically break the fuel into small, nearly spherical structures, which are then carried and mixed by the turbulent airflow inside the engine. The turbulence will continue to break up the fuel structures and change their shapes.”
The figure above shows an “ideal” case, where the fuel is far from the walls of the engine, and essentially there are no limits. But in a real-world scenario, the engine walls will also impact the mixture. The new study focuses on isolating the effects of shock waves as a key part of compressing fuel — limiting its volume — and breaking it down, Bermejo-Moreno said. A shock wave is a disturbance that travels faster than the speed of sound and causes an abrupt and discontinuous change in the pressure, temperature and density of the medium it impacts. In this case, a shock wave flattens the shape of the fuel structures and creates more surface area for the fuel to be broken up by the turbulence inside the engine.
Understanding the effects of compression – via a shock wave, for example – on turbulent mixing processes is very important for advancing aerobic super and hypersonic propulsion systems.
These systems are characterized by a forced influx of air into the engine, which is heated and evacuated through an exhaust. Such systems also have compressed time requirements for mixing to occur. Knowing exactly how injected fuel is broken down can help researchers identify exactly what conditions favor the most advantageous mixture scenario for these engines to operate efficiently.
Previous research by Bermejo-Moreno identified shock waves as a beneficial force for accelerating fuel mixing, but this research did not benefit from the tracking methodology algorithm implemented in the new study. Although multiple events can be tracked manually, trying to find an accurate representation and recommendation of how fuel will mix under different conditions relies on the presence of a large enough sample showing a similar result.
This new tracking methodology provides a clearer picture of the structural change in injected fuel from moment to moment, better informing aerospace engineers on how to replicate the conditions that will most benefit supersonic and hypersonic engines.
“Once you have this tracking algorithm, you can apply it to any stream to get a graph that encapsulates the interactions of all structures found in the stream over time,” Bermejo-Moreno said. “You can query the graph and look for patterns that move similarly over time. You can see how often these patterns repeat and collect statistics about the physical processes involved to say, for example, ‘This is a common behavior in the fuel injected rupture process.'”
Bermejo-Moreno said the impact of a shock wave is particularly significant in cases with larger spherical structures rather than smaller spherical structures, because larger spheres are more susceptible to “splitting events” where the fuel breaks into more and more pieces.
“If you think of larger structures,” he said, “you think they will take longer to diffuse, but the turbulent mixing they experience will benefit more from the shock interactions, causing them to will break more quickly into smaller structures.”
If you think back to the case of food coloring, the more small drops of food coloring, the easier it is for the coloring to dissolve in water and combine with it to form a new solution.
“If you can get a better mix, it will help improve the performance of your propulsion systems,” Bermejo-Moreno said.
Inform future recommendations
Bermejo-Moreno said the next steps include studying what happens when you get closer to the engine walls and in the mixing layers – two fluid streams moving at two different speeds. “We will follow turbulence structures over time to understand how viscous shear affects mixing processes from a structural dynamics perspective,” he said.
For now, Bermejo-Moreno said there are additional factors that will ultimately impact propulsion performance that will be considered before providing real-world recommendations, but that’s a step forward. .
Shockwaves could deliver the jolt needed to reach Mars
Jonas Buchmeier et al, Geometry and dynamics of passive scalar structures in compressible turbulent mixing, Fluid physics (2021). DOI: 10.1063/5.0068010
Provided by University of Southern California
Quote: New tracking method in high-powered jet engines paves way for optimal combustion (November 19, 2021) Retrieved January 30, 2022 from https://phys.org/news/2021-11-tracking-method- high-powered-jet-paves.html
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