Firefighting in challenging conditions

Stopping forest fires can make a big contribution to reducing CO₂ emissions

At present, the mainstream of combustion science is concerned with the "use of combustion," such as how to burn fuel efficiently to extract energy. This can be seen in efforts to reduce fuel consumption in automobile engines, for example. However, Prof. Nakamura is focused on "combustion suppression" in the field of combustion science. Why does he focus on researching phenomena that are so difficult to reproduce, such as fires in special environments such as on a space station?

"Research on the use of combustion is of course an important topic, but researchers have worked to improve combustion efficiency for many years, so these days it is difficult to increase efficiency by even 1%. So, focusing on this is not very effective. On the other hand, the damage caused by forest fires, for example, is enormous. It varies from year to year, but the amount of CO₂ emitted due to forest fires is said to account for about 10% of the world's CO₂ emissions. In other words, if we can stop the spread of even one forest fire, it would be more effective than creating more efficient combustion technology.

As well as its role in reducing CO₂, preventing fires is also essential for creating a safe and secure society. Fires are not limited to general buildings, but occur also in facilities such as spacecraft, submarines, or nuclear power plants, where they can cause significant damage. Of course, such facilities are fully equipped with fire protection measures, but the risk is not zero. Suppressing any fires that do occur can have a great impact on society," says Prof. Nakamura, describing the significance of his research.

A significant challenge is that the results of fire research differ greatly depending on the environment and conditions, but the conditions considered by Prof. Nakamura's work are impossible to reproduce exactly in the laboratory. Furthermore, any research that cannot be reproduced runs the risk of being considered unscientific. In fact, when Prof. Nakamura told his academic advisor that he wanted to do basic research on fires in space as the topic for his Ph.D., he was advised against it because it was not an academic topic.

"I thought, if my academic advisor, who is an authority on basic combustion science, cannot do it, I bet no one can do it. So I decided to do it! (laughs) If I was going to do something, I wanted to do something that nobody else was doing."

This sentiment captures the pioneering spirit of Prof. Nakamura, which has remained unchanged since his student days.

"Similarity rules" that help reproduce phenomena at different scales

Prof. Nakamura introduced an experimental method based on models in order to study difficult-to-reproduce fires in space. Simply put, he uses a model to try to reproduce the same conditions as space without actually going to space.

"One of the biggest problems I encountered when researching fires on space stations as a research topic was that I couldn't simulate zero gravity. Dropping a capsule to create weightlessness allows you to conduct an experiment for a few seconds and costs millions of yen each time. So that is far too expensive. Flames become round in zero gravity, but despite a great deal of thinking about the problem, I couldn't find a way to make a round flame. Just around that time, I encountered a phenomenon while studying abroad in the United States in which the shape of a flame becomes round as you make it smaller. It came as a great shock when I found out that just reducing the flame size could create a situation that would be less susceptible to buoyancy," says Prof. Nakamura.

Just by changing the size of the flame, it is possible to reproduce zero gravity flames. Thinking about why this is the case, he discovered that he just needed to satisfy certain laws.

"Those are the similarity rules. If you follow the similarity rules, you can predict that if you reduce the air pressure, you can approach weightlessness. So, when I burned a flame in a partial vacuum, it really did become round. In other words, you can reproduce the same phenomenon as you would find in zero gravity if you make the flame small or reduce the surrounding air pressure and weaken the relative buoyancy. In this way, I was convinced that I could study fires in space even on earth by conducting appropriate scale model experiments."

Actually, this principle is exactly the same as the trick used to make special effects look realistic in movies. For example, in scenes of the movie "Godzilla" that include special effects, a miniature Godzilla of about 1:25 scale is used, and in scenes where Godzilla smashes or drops things, the film is shot at 4 to 5 times faster than normal speed and then reproduced in slow motion. By showing the film at a slower speed than the actual movement, you can represent movement at a different scale. Prof. Nakamura explains that the mathematical expressions for this are known as "similarity rules."

"Taking Godzilla as an example, if we assume that we must satisfy the simplest ‘similarity rule,' we can easily calculate that we should shoot at 5x normal speed. Professional special effects engineers know from experience that this speed is suitable for making the movement look realistic."

Similarity rules are convenient, but it is not easy to derive them. Of course, there are many established similarity rules, and they are used in various situations such as vibration experiments for models of bridges. However, similarity rules have not been determined for all physical phenomena in the world, and similarity rules for combustion in space cannot be found in any book.

"In the case of combustion, it is particularly difficult to derive similarity rules because there are many factors that affect the phenomenon such as heat, airflow, and reactions. I thought that if I could find a way to easily derive similarity rules for specific situations, it would be of great help when solving various problems that people had given up on. It would mean that we would be able to use a laboratory to reproduce fires that occur in certain special environments, such as space, or even large-scale wildfires."

A vacuum fire extinguisher that makes it easy to return a spacecraft to normal conditions

However, even if researchers could reproduce fires under special environments through experiments and clarify the phenomenon, this would not solve all the problems. Major challenges still remain for actual firefighting.

Currently, fires inside spacecraft are extinguished by spraying an inert gas (extinguishing agent). At first glance, this may not seem to pose a problem, but the fire is occurring inside a completely sealed spacecraft. If you release a large amount of inert gas, the fire may go out, but the astronauts could suffocate and die. Also, if a small flame remains, not only will toxic gases continue to be released, but heat will also accumulate, which could damage the precise and expensive equipment and cause the spacecraft to lose its functionality.

To deal with this challenge, Prof. Nakamura proposed the "Vacuum Extinction Method." The idea is that if "blowing out" the fire is no good, why not "suck it up" to extinguish it? This is another example of a research topic that embodies Prof. Nakamura's motto of, "Let's do it because nobody else is doing it."

"Suppose you have a high vacuum box onboard the spacecraft. When you open the suction port valve on the box, rapid suction occurs due to the pressure difference. Using this suction force, everything, including the flames inside the spacecraft, the toxic gases caused by burning, and burning objects, are confined in the box. After that, you can close the valve and discard the box as trash outside the spacecraft. You could call it a fire extinguishing vacuum cleaner. You could also extinguish the fire with water, but, while this is an effective way to put out a fire, it would be difficult to return the spacecraft to normal conditions. With the vacuum fire extinguisher, you can quickly return the spacecraft to normal conditions after putting out the fire.

"Given how unusual my idea seemed to be, I was surprised to find that a heavy industry company was working on a patent for a similar concept. Their approach was to suck out all the gases inside the spacecraft and eject them into space to put out the fire. However, with that method you would lose much of the spacecraft's functionality, so it would be too damaging. Therefore, it's better to suck only the things you don't need into a box and throw them away," says Prof. Nakamura, explaining the advantage of his approach.

He made a conceptual diagram, presented it in a paper, and released it publicly last year. He was then asked if the system could be used in sealed spaces such as clean rooms or operating rooms. If commercialized, it may also be useful for preliminary fire extinguishing at museums and art galleries.

Finally, we asked Prof. Nakamura about his future plans. He answered that he wants to turn his model experiments into an educational program.

"With the model experiments, you don't have to deal with too many mathematical formulas, but you need to hone your physical senses to discover the acting forces. You can train your observational skills and insight, which are the key skills of engineers. So it can be a very meaningful educational tool for both university students and technical college students. There is a wide range of applications. Supporting Japanese engineering education through the theory of model experiments is another major goal for me as a researcher." concluded Prof. Nakamura.

Reporter's Note

Prof. Nakamura talked about his thoughts on education at the end of the interview. The conversation started from talking about his experience of studying abroad in the United States. Prof. Nakamura said that he had a tough time while studying abroad, due to both research and financial aspects as well as the language barrier.

"I went to the campus 365 days a year, studied hard, and I gained so much. There were several university faculty members who had also come from Japan, but most of them were just living abroad. When I saw that, I didn’t believe we should leave Japanese university education up to those people." says Prof. Nakamura.

And, of course, while we know that not all students want to become researchers, "Education is not about filling yourself with knowledge. Real learning is when you do what you want to do and actually understand something. This is what I want to convey. That's why I think students should do interesting research at university to investigate the mysterious things in the world that interest them as an individual, no matter how seemingly trivial. When they find it interesting, everyone works on their own initiative. My aim is to win an Ig Nobel Prize! (laughs)" he says. Given Prof. Nakamura's reputation for outstanding presentations, he is sure to liven up the awards ceremony if he wins!