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HOME > No.15, Nov 2018 > Can we have a fire in a highly-vacuumed environment?

Can we have a fire in a highly-vacuumed environment?

Towards better fire safety strategies for manned space missions By Yuji Nakamura
Yuji Nakamura

Yuji Nakamura and his research team at Toyohashi University of Technology have discovered that non-flaming combustion (smoldering) of a porous specimen can sustain itself, even at around less than 1 % of atmospheric pressure. The thermal structure of a 2-mm-diameter smoldering specimen, at a condition close to extinction, was successfully measured using an embedded ultra-fine thermocouple, clarifying the key issues that lead to fire extinction at low pressures. The outcome of this research will contribute to improved space exploration fire safety strategies.

Non-flaming combustion (i.e., smoldering) is an extremely slow burning process that emits toxic gas and white smoke during the burning event. This corresponds to the pre-flaming stage of burning a porous specimen, during which the blackened part grows, continuing the slow exothermic process. It eventually generates a flame that quickly accelerates the fire damage. Flaming combustion can be suppressed by reducing the pressure to nearly 1/3 of standard pressure (~30 kPa). Nevertheless, non-flaming combustion can sustain even at 1/100 of standard pressure (~1 kPa) if the ambient gas is fully-oxygenated. Extension of the critical pressure has been pointed out as an experimental fact; however, the actual reason is not known because it is extremely difficult to investigate the thermo-chemical status of near-critical conditions. Because the combustion intensity is very weak, sensor insertion may affect the status, resulting in failure to capture the actual physics.

Fig.1 Smoldering limit observed in low pressure. Effects on smoldering speed (propagation velocity) and extinction relative to the pressure at various adopted O2 conditions.

A research group led by Professor Yuji Nakamura, from the Department of Mechanical Engineering at Toyohashi University of Technology, took on the challenge of measuring the temperature distribution of a smoldering thin rod in a pressure-controlled chamber at near-critical conditions. To make this possible, special care was taken to adjust the sensor while avoiding the potential failure described above. A tiny hole of 0.2-mm diameter was drilled through the fragile specimen. Then, a 50-micron R-type thermocouple was embedded into the hole. By achieving steady-state burning, even near the critical condition under a well-controlled experimental environment, a repeatable 1-D temperature profile was obtained along the axis.

Fig.2 Temperature measurement device: fine thermocouple installed in 0.2-mm diameter hole in the specimen

The first author, Takuya Yamazaki, a PhD candidate, said, “The idea of drilling a miniscule hole into a small (2-mm) and fragile specimen such as the one we used, and then manually inserting a tiny thermocouple had never previously been considered. No doubt this was due to the high level of difficulty of such a procedure, which requires considerable patience and effort. Indeed I must admit that it was really exhausting to complete this task. Nevertheless, ultimately this process provided us sufficient insights into the near-critical condition thermal status to thoroughly understand the extinction mechanism. For instance, combustion heat is first transferred along the axis by radiation. After that, part of the transferred heat is lost to the ambient environment via natural convection when the total pressure is in the order of tens of kilo-pascals. Because the convective heat loss tends to be suppressed when the total pressure decreases, the heat transferred by radiation could remain in the specimen and so avoid extinction. Our success in being the first to demonstrate this fact by such a procedure is the fruit of our willingness to take on the ‘super’ challenge of measuring the precise temperature distribution of a smoldering specimen at near extinction.”

Fig.3 Heat balance analysis in the preheat zone. Thermal status during smoldering under low pressure: radiation heat transfer becomes significant, whereas convective cooling (i.e., heat loss) to ambient is negligible in the preheat zone.

“The present results will make a contribution to fire safety basically thanks to Takuya’s personal devotion. This outcome suggests that the vacuumed operation to extinguish fire in space may fail unless the proper condition is achieved. Otherwise, smoldering may survive, causing a fire which may result in secondary damage to the cabin. This work is just the first step in constructing a tentative fire safety (regulation) strategy for outer space habitats in this age of privatized space development”, explains Professor Yuji Nakamura.

We use the term “smoldering” frequently, but, in reality, no one knows how a specimen burns to generate heat locally. It has been considered that surface oxidation is the source of heat generation, and that gas-phase reaction is not required to be considered. However, based on recent numerical predictions by a Chinese research team (a member of an international collaboration team led by Prof. Nakamura), it was found that a gas-phase gentle heat generation can support or promote surface oxidation. To further the cause of understanding smoldering at low pressures, another international collaboration team in the United States, also led by Prof. Nakamura, will assume the challenge of experimentally identifying the reactivity in the gas-phase. This is a crucial step, because up to now scant attention has been paid to the reaction status of the micro-pores of a burning specimen.

Funding agency: JSPS program “Overseas Challenge Program for Young Researchers”


Yamazaki, T., Matsuoka, T., and Nakamura, Y., "Near-extinction Behavior of Smoldering Combustion under Highly-vacuumed Environment", Proc. Combust. Inst., Vol.37, in print.


By 中村 祐二


線香やたばこのように、空隙を多く持つ固体物質は炎を出さずに緩慢な燃焼を実現することができます。この現象を「くん焼(燻焼. 無炎燃焼。スモルダリング)」と呼びます。布団に火種が与えられると、暫く「くすぶり状態」が続き、その後、突然炎が立ち上がって火災が急速に拡がりますが、くん焼とはこの「くすぶり状態(の燃焼)」に相当します。通常、炎を出して燃える場合、ある程度の酸素を遮断すると炎が保てなくなって消えてしまいます。そのためせいぜい大気圧の1/3程度(~30 kPa)まで減圧すれば炎は消失します。ところがくん焼の場合、遥かに低い圧力まで消えることはなく、純酸素(酸素100%)条件では、大気圧の1/100程度(~1 kPa)でもくすぶりが続きます。くん焼は「しぶとい」ことは経験的に知られているものの、なぜそのような悪条件でも燃え続けられるのかは未解明のままでした。その主な理由は、消える直前での燃焼強度は著しく弱く、温度センサを挿入するとそれがきっかけで燃焼状態を変えてしまうからです。





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Researcher Profile

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