Font Size

HOME > No.33, Jun. 2023 > "Magic" in the flickering of flames

"Magic" in the flickering of flames

Fluctuation can be controlled by adjusting the distance between two flames Yuji Nakamura
Professor Yuji Nakamura (Center) and his lab members
Professor Yuji Nakamura (Center) and his lab members

A research team, led by Professor Yuji Nakamura of the Department of Mechanical Engineering at Toyohashi University of Technology, discovered that the flickering of flames can be freely controlled by moving two flames closer together or further apart. Until now, it had been known that interference between flames separated by a certain distance causes the flames to flicker during in-phase or anti-phase. However, it was not possible to stably express the state of "stopping the flickering of flames" that should occur under critical conditions where the phase changes. The research team succeeded in stably expressing the state of "stopping the flickering of flames" by periodically adjusting the distance between flames closer and further apart. This makes it possible to freely control the flickering of the flames, and to elucidate the essence of flickering flames.

The flickering of flames is a very familiar phenomenon that is easy to observe. At the same time, it is also a mysterious and interesting phenomenon with vast complexity. For example, once the flickering flames have interfered with each other, only the stable flickering mode is selectively expressed. Depending on the distance between the flames, the "in-phase mode" that fluctuates in the same phase and the "anti-phase mode" that fluctuates in the opposite phase are selectively expressed. Also, the frequency of the fluctuation differs among these modes, which is a curious phenomenon. Although it is possible to achieve various fluctuating states in this way, there is no example that shows "stopping the flickering by interfering with the fluctuating flames." In the past, it was shown that this state can be achieved by arranging three flames (known as "death mode" in reference to the complete absence of movement). However, researchers have yet to understand the reason why death mode cannot be achieved with two flames.

When examining this theme, the research team found that the death mode is expressed by adjusting the distance between the two flames closer or further apart in a certain cycle.

Expression of death mode when flame flickering is stopped by adjusting the distance between flames Condition when flames are moved (left), Diagram of state (right)
Expression of death mode when flame flickering is stopped by adjusting the distance between flames Condition when flames are moved (left), Diagram of state (right)

"When conducting experiments involving flame-to-flame interference, flickering will temporarily stop if the flames are gradually brought closer or further apart," explains Dr. Ju Xiaoyu, lead author and researcher at the time of the project. "However, if the flames are kept in that position, they will eventually start flickering again. Since the flames eventually flicker, we know that flickering is a stable state. The fact that there is a delay period until the flames settle into a stable state means that if we can create a situation where flickering can be stopped within that time scale, the flickering should be stopped permanently. We were able to prove that this prediction is correct by periodically adjusting the distance between flames closer and further apart. We also demonstrated that the reason for this phenomenon can be explained by hydrodynamic properties. Moving forward, we will proceed with research aimed at constructing a theory."

"It has been known that the flame flickering mode is determined by interference between flames," says Professor Yuji Nakamura, leader of the research team. "Researchers in applied physics have attempted to explain this phenomenon as nonlinear physics instead of combustion engineering. Nevertheless, their explanation felt inadequate to me due to its failure to consider hydrodynamics. In response, I began to research this theme in earnest. I was amazed to witness a phenomenon in which flickering temporarily stopped in an intermediate state between in-phase and anti-phase flickering. I felt a strong desire to elucidate this mysterious transition state, a theme which has not been addressed by previous research. From the beginning, I had the idea of constantly adjusting the distance between flames to take advantage of the time delay until they settled into a stable state. Ultimately, I was able to organize this method with the help of Dr. Ju."

Expression of death mode when flame flickering is stopped by adjusting the distance between flames

Professor Nakamura concludes: "Introducing this phenomenon at events such as academic conference is guaranteed to capture the interest of the audience. However, it is just as certain that the audience will, raise questions about possible practical applications, such as ´What is it useful for?´ When facing such questions, I always respond by posing a question of my own—‘To be honest, I only began researching this phenomenon out of personal curiosity, so let me ask you, in what ways do you think my findings can be used?’ This experience has led me to start my research presentations by asking the audience to refrain from questions on practical application. I believe that one appeal of conducting basic research at a university is being able to purely immerse yourself in curiosity, without the need to consider practical application."

Although the research team is not considering practical applications of their research at the present time, they plan to delve deeper into the theme not only through experiments, but also through numerical and theoretical analysis. This will be done in the name of basic research that is unique to universities; that is, through the elucidation of mysterious phenomenon. The team plans to proceed as an international joint research in collaboration with Dr. Ju and many other international researchers who have expressed interest in their research. Through the international dissemination of research ideas originating in Japan, the team would like to showcase to the world that Japan is a place where this kind of (currently impractical) basic research can be pursued to the fullest extent.


Ju, X., Bunkwang, A., Yamazaki, T., Matsuoka, T., and Nakamura, Y., "Flame Flickering Can Cease under Normal Gravity and Atmospheric Pressure in a Horizontally Moving Dual Burner System", Physical Review Applied, Vol.19, No.1 (2023.1) eid 014060
DOI: 10.1103/PhysRevApplied.19.014060



中村 祐二



「炎と炎の干渉実験を行うにあたり、徐々に近づけるまたは遠ざけると、途中でゆらぎが一時的に止まります。ところが、その位置に留めておくと、いずれまたゆらぎ始めます。最終的にはゆらぐので、その状態が安定状態であることはわかるのですが、安定状態に落ち着くまでの「遅れ時間」があるならば、その時間スケール内でゆらぎを静止できる状況を作れば、ずっと静止させられる可能性があるのでは、と考えました。そこで、炎の距離を周期的に近づけたり遠ざけたりすることでこの推定が正しかったことを証明することができました。また、この理由は流体力学的な特性で説明できることを示しました。今後は理論構築に向けた研究を進めていきます。」と筆頭著者である研究員(当時)のJu Xiaoyu博士は説明します。




Share this story

Researcher Profile

Yuji Nakamura
Name Yuji Nakamura
Affiliation Department of Mechanical Engineering
Title Professor
Fields of Research Combustion / Fire Dynamics / Scale Modeling / Space Engineering / Safety Engineering