別紙41 (課程博士(英文))Date of submission (month day, year)

Department
StudendD Nulnbel

Mechanical Engineering
PP11Ca11t'S I〕alne

D179104
Supervisors

January loth,2020

YujiNakamura
Akiyoshilida

TakuyaYamazaki

HidekiYanada

Abstract (Doctor)
Title oflhesis

Study on Limiting Behavior of smoldedng using Low pressure

Approx.80o words
Experimental study On Iimitin今 behavior of smolderin今 Combustion, thetransition to extinction or flamin今 from smoldering, was conducted with highreproducibility to clarify their precise mechanisms systematicaⅡy. The consideredambient cond此ions in this study were l.0 ‑10o kpa in absolute pressure varyin今
1.o in mass fraction of oxy今en. A
Oxygen concentrations within o.20

2'mm・rod・shaped biomass stick (incense stick) was considered as the test
Specimen as representative general charring fuels havin今 bulk density andPm'osity similar to general wooden nlaterials
The smoldering behavior of the thin・rod biomass stick at low pressures andVarious oxygen concentrations was exP田'imenta11y investi今ated for the verticaⅡy、and horizonta11y、oriented cases, aiming to examine the inauence of a low、pressure
enVⅡ'onment on smolderin牙 behaviors.1n a low"pressure environment, theSmoldering rates, the peak temperatures, and the thermal structures inside thebionlass sticks were found to be nearly identical for both vertical and horizontal
Cases. This fact implies that role of the natural convection on combustion process
Was effectively suppressed using low pressure, suggesting that the effect ofbuoyancy"induced aow on the smolderin宮 region becomes sufficiently sma11.1n
this way, the low・pressure method for examination for both extinction limit andU'ansition to aaming from smold田'in牙 Was verified.
The smoldering behavior near the extinction lim北 Was then studied using the10W・pressure method with the thinl'od biomass stick.1t was found that theextinction limits were clearly affected by the in〕posed oxygen concentration、Having lower oxygen concentration below o.4, the Hmit was achieved at certain
Smoldering rate. This limit was expected to be we11'contr0Ⅱed by the transport(SO"ca11ed transport'contr011ed re牙ime) since oxygen u'ansport to the smolderin曾
re牙ion by natul'al convection was adequate for smold田'ing combustion, while theheat losses are more e丘ective to lead to extinction. Having higher oxygen
Concentration above o.4, on the other hand, the lim北 appeared at sufficiently low
Pressure wheTe the transport process is severely suppressed. This limitin今
behavior is equivalent to one observed in the oxy今en'1imited re今ime, indicatingthat the oxy今en'deficient condition limited the oxidation reaction. The extinction
Iimit in the oxygen"1imited regime showed a strong dependency on the ilnposed
Pressure, which is first ever observed/confirmed by this work.

The transition from downwardly (reverse) smolderin今 to aamin今Ina10W"pressure environment was investigated to determine the dominant factor lnthe critical condition of the transition. The reverse smoldering progressed
Steadily, although the len牙ths of the luminous re今ion slowly increased with time
Under the conditions in which the partial pressure of oxygen is above 30 kpa
Introducing an image processing method,北 Was revealed thatthe luminous len牙that the point of transition showed S仇'ong dependency with the imposed partial
Pressure of oxy牙en. To examine their relationshゆ, a simple ignition modelrefa,red by semenov's theory w'as developed.1t was confirmed that the predicted
m'itica11engths when transition to aaming occurs were in 牙ood agreement with theexperimental data, implying that the occun'ence of the transition is mainly
Contr011ed by the heat balance between the heat release rate by oxidation reaction
Of the pyrolysis gases and the heat loss from the hot pyrolysiS 今ases to theSun'ounding area when the concentrations of pyrolysis gases are enoU今h to i今nite