Researchers from Toyohashi University of Technology, under the direction of Professor Yuji Nakamura of the Department of Mechanical Engineering, found that the flickering of flames may be flexibly controlled by moving two flames closer or farther away. It was previously understood that interference between flames that are spaced apart by a certain amount causes the flames to flicker whether they are in phase with one another or not. The state of “stopping the flickering of flames,” which should happen under stressful circumstances where the phase changes, could not be expressed in a stable manner, nevertheless.
By regularly altering the space between flames, the study team has successfully expressed the state of “stopping the flickering of flames” in a stable manner. This enables free control over the flames’ flickering as well as the clarification of the phenomenon’s fundamental characteristics. Physical Review Applied is the journal where the study is published.
Flame flickering is a common occurrence that is simple to watch. It is a complex phenomenon that is both intriguing and mysterious at the same time. For instance, only the stable flickering mode is selectively exhibited once flickering flames have interfered with one another. The “in-phase mode,” which fluctuates in the same phase as the flames, and the “anti-phase mode,” which fluctuates in the opposite phase, are selectively displayed depending on the distance between the flames. In certain modes, there are additional enigmatic occurrences like various fluctuation frequencies. These rules can be used to achieve different fluctuating states.
However, there isn’t a single instance that exemplifies “putting a stop to the flickering by interfering with the varying flames.” It has been demonstrated in the past that setting up three fires will produce this state. (known as “death mode” in reference to the complete absence of movement). Researchers still don’t know why using two flames prevents entering death mode, though.
The research team discovered after investigating this phenomenon that the death mode is indicated by varying the gap between the two flames in a cycle.
“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, head 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 earnestly research this theme.
“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.”
Professor Nakamura concludes, “Introducing this phenomenon at events such as academic conference is sure to capture the interest of the audience without exception. However, the audience will, without exception, raise questions regarding examples of practical application. For example, ‘How can these findings be used?’ I repeatedly answered such questions by posing my own question—’Actually, I only began researching this phenomenon out of personal curiosity, so I’d like to ask how 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.”
Future prospects
The research team intends to explore deeper into the issue not just through experiments but also through numerical and theoretical analyses, even if they are not yet considering the practical application of their findings. This will be accomplished by the explication of enigmatic phenomena, a type of basic research exclusive to universities.
The group intends to move further with Dr. Ju and numerous other international scholars who have shown interest in their work as part of an international joint research project. The team wants to demonstrate to the rest of the world that this kind of (now impractical) basic research can be vigorously pursued in Japan through the international dissemination of research seeds originating in Japan.
