Study on the flaming-transition behavior of a downwardly smoldering biomass stick utilizing low pressure

Abstract

The transition from downwardly (reverse) smoldering to flaming in a low-pressure environment during combustion of a thin-rod biomass stick with low porosity (high density and low permeability) was experimentally investigated to determine the dominant factor affecting the critical condition of this transition. After forced ignition at the top of the specimen, the downward smoldering progressed steadily, although the lengths of the luminous region slowly increased with time under certain conditions. The condition for a spontaneous transition to flaming was identified under various oxygen partial pressures. Such a spontaneous transition was not observed when the partial pressure of oxygen was sufficiently low (< 30 kPa), except in the case of a collapse of built-up ash, which happens naturally because built-up ash bends slightly during the smoldering process. Measurement using an image-processing method revealed that the luminous length at the point of transition is strongly related to the imposed partial pressure of oxygen. To examine this relationship, simple ignition models were introduced and discussed. The results show that the gas-phase model worked satisfactory to cover the range of experimental conditions, suggesting the boundary layer thickness plays an important role of the flaming. This would be the reason why the length scale of the smoldering region is a key factor for predicting the transition from smoldering to flaming.