By Josh Perry, Editor
[email protected]

Scientists from Toyohashi University of Technology (TUT) in Japan have discovered that smoldering, non-flaming combustion, of a porous specimen can be sustained even when atmospheric pressure is under one percent, which will impact fire safety strategies for space exploration.

Effects on smoldering speed (propagation velocity) and extinction condition against the pressure at various adopted O₂ conditions. (Toyohashi University of Technology)

According to a release from the university, researchers were able to measure the thermal structure of a 2 mm burning specimen at near extinction using an embedded, ultra-fine thermocouple.

“Non-flaming combustion (i.e., smoldering) is an extremely slow burning process that emits toxic gas and white smoke during the burning event,” the article explained. “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 contained by a reduction of pressure, but non-flaming combustion can continue even at 1/100 of standard pressure if the ambient gas is fully oxygenated.

TUT researchers measured temperature distribution of a smoldering thin rod in a pressure-controlled chamber. “A tiny hole of 0.2-mm diameter was drilled through the fragile specimen,” the article continued. “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.”

This painstaking process revealed that heat is first transferred along the rod’s axis through radiation then heat is transferred to the ambient through convection when the total pressure was tens of kilo-pascals. The smoldering continues because the radiation heat transfer continues along the axis to avoid extinction.

Researchers admit there is still a lot to learn about this process, including how the specimen burns to generate heat locally. Further studies are planned.

The research was recently published in Proceedings of the Combustion Institute. The abstract stated:

“The smoldering behavior of porous combustibles near the extinction limit was examined under a highly-vacuumed environment at various oxygen concentrations. The considered ambient conditions of the present work is 1.0–90 kPa in absolute pressure filled with a nitrogen–oxygen mixture. A 2-mm-rod-shaped incense stick is considered as the burning specimen. The specimen was located vertically and forced ignition was made at the top of the specimen to complete the following downward propagation.

“Two well-known burning modes were identified; such as flame spread (flaming) and smoldering (non-flaming) depending on the imposed ambient conditions. The results showed that the smoldering rate near the extinction limit decreased dramatically as the ambient pressure decreased. Temperature profiles inside the smoldering specimen were successfully measured by fine thermocouple even at the near-extinction limit.

“The results imply that there is a peak located slightly below the top surface. Based on the simple radiative-conductive heat transfer model, the role of radiation heat transfer for a forward temperature profile is discussed within the range for which oxidative reaction is neglected. It is implied that the radiant heat transfer may play a role on the preheating ahead of the burning zone, especially when the peak temperature is relatively higher (>?1000  °C).

“To fully understand the whole temperature profile and extinction behavior, further continuous study is recommended.”