Wildfires are naturally occurring phenomena that result in significant and catastrophic
damage. Due to climate change, there has been a significant increase in
the frequency, severity, and extent of wildfires. Therefore, there is a growing need
to mitigate wildfire risk. In order to help mitigate the risk of wildfires, greater
understanding is required. One particular gap in knowledge is the impact of smouldering
combustion of potential fuel on wildfires. This thesis focuses on combustion
of fuel beds in wildfires. Specifically, the thesis targets smouldering combustion.
Smouldering combustion is a common type of combustion regime in wildfires and
hazard reduction burning (a wildfire mitigation measure). Smouldering is a slow
and low-temperature form of combustion, which shows no
flame. Smouldering is a
serious hazard because of its low ignition temperature, which makes it particularly
relevant to fire initiation and spread. Smouldering plays a vital role in wildfires,
as many forest biomass fuels such as grass, leaves and coarse woody debris are
prone to smoulder. Most previous studies of smouldering combustion have only
been carried out on polyurethane foam, due to its importance for residential fires.
However, smouldering has been scarcely investigated from the point of view of
wildfires. For example, smouldering combustion of forest fuel is scarcely studied.
Hence, the project aims to develop a greater understanding of the initiation of
smouldering combustion in biomass under different conditions with an emphasis
on wildfire.
Locating smouldering combustion in wildfires and hazard reduction burning
is difficult and time-consuming, as there is no effective method to identify the
initiation of smouldering combustion in biomass fuel beds. It is critical to know when and where smouldering combustion in a biomass fuel bed starts, as smouldering
combustion could transition to
flaming combustion under certain conditions.
Radiation is one of the important heat transfer mechanisms in wildfires; however,
there are few studies on smouldering combustion in biomass fuel beds started by
external radiant heat
flux. Although oxidiser
flow rate and oxygen concentration
have significant in
influences on the propagation of smouldering front, their effects on
the initiation of smouldering combustion in biomass fuels are not well understood.
Hence, the effects of oxidiser
flow rate and oxygen concentration on the initiation
of smouldering combustion are investigated. Fuels in a forest are diverse, and it
is essential to have a better understanding of what effects forest fuels have on
smouldering combustion. Thus, the effects of plant species and plant parts on the
initiation of smouldering in biomass fuel beds are also investigated. Within this
framework, the work presented in this thesis can be split into two main topics:
1. Conditions required to initiate smouldering combustion in bio-
mass fuel beds
The required radiant heat
flux and air
flow rate for the initiation of smouldering
and
flaming combustion in a biomass fuel bed are investigated in an experimental
testing rig. This investigation identifies and quantifies smouldering and
flaming
combustion in a biomass fuel bed based on the measurements of temperature,
product gas concentration and mass change, and the required radiant heat
flux
and air
flow rate for the initiation of smouldering and
flaming combustion are
determined. The effects of heating time and oxygen concentration on the initiation
of radiation-aided and self-sustained smouldering combustion are investigated in
the same testing rig. In this experimental study, the differences between radiation-aided
and self-sustained smouldering combustion are characterised based on the
measurements of temperature, product gas concentration and mass change, and
the required heating time and oxygen concentration for radiation-aided and self-sustained
smouldering combustion are determined.
2. Factors that influence smouldering combustion in biomass fuel
beds
The results from the first topic reveal that oxygen availability has significant effects
on the initiation of smouldering combustion in a biomass fuel bed. The air
permeability of a biomass fuel bed determines oxygen availability in that fuel bed.
Hence, the air permeability of natural forest fuel beds is investigated in an air
permeability testing rig. In this study, the air permeability of natural forest fuel beds is determined using experimental and theoretical methods. A comparison
between the experimental and theoretical methods is made. The effects of Euca-
lyptus species and plant parts on smouldering combustion are also investigated.
In this study, the different plant parts from different Eucalyptus species are characterised
based on the results of the thermogravimetric and ultimate analyses.
The results of this study show that the differences among the different plant parts
from different Eucalyptus can be characterised and quantified based on the results
of the thermogravimetric and ultimate analyses. It is also found that Eucalyptus
species and plant parts have significant effects on smouldering combustion.
Although this thesis covers a series of experimental studies of the initiation
of smouldering combustion in biomass fuel beds. There are still many important
factors to be considered. For examples, the thesis focuses on small-scale laboratory
experiments to better understand the fundamental studies of smouldering combustion
of biomass. However, the real-world conditions could be much more complex.
For example, forest fuel beds are composed with fuel particles with various sizes
and shapes. These factors also have effects on smouldering combustion.Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Mechanical Engineering, 201