Author: Lin, Shaorun
Title: Fundamental study of near-limit smouldering fire dynamics
Advisors: Huang, Xinyan (BEEE)
Degree: Ph.D.
Year: 2021
Subject: Fire
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Environment and Energy Engineering
Pages: xvii, 187 pages : color illustrations
Language: English
Abstract: Smouldering fire is slow, low-temperature and flameless, and is one of the largest and longest- lasting combustion phenomena on Earth, different from flaming fire regarding chemistry, transport processes and time scales. Smouldering fire is a heterogeneous process sustained when oxygen directly attacks the hot fuel surface. Smouldering fire is the dominant burning behaviours of porous and charring fuels, such as wood, char, peat, and polyurethane (PU). For example, smouldering wildfire is a significant disturbance to peatlands worldwide, and it contributes significantly to global carbon emissions and provides positive feedback to climate change. Despite its critical hazard to humans and the environment, our understanding of it remains limited. Compared to the flaming fire, smouldering fire can be initiated by a much weaker ignition source and provide a shortcut to flaming through smouldering-to-flaming transition. Traditionally, smouldering spreads in a creeping fashion, typically on the order of 1 cm/h, which is at least two orders of magnitude smaller than the spread rate of flaming fires. On the other hand, smouldering fire can be sustained in an extremely low oxygen concentration (~11%). However, very few works have been conducted to systematically study the near-limit smouldering fire dynamics before the research undertaken in this report. This thesis is divided into eleven chapters: except for the chapter of introduction (Chapter 1) and concluding remarks (Chapter 11), each chapter takes the form of an independent paper, which has been published or submitted to a journal or conferences.
Part A includes three chapters (2-4), with the focus on the ignition limits of smouldering combustion. Chapter 2 investigates the ignition limits of moist peat soil with moisture up to 100 wt.% under external radiation, and the critical ignition heat flux, ignition temperature, heat release rate and CO/CO2 ratio are compared thoroughly. Chapter 3 explores the effect of the diameter of irradiation spot on the ignition limit of smouldering combustion experimentally and numerically, which reveals that the lateral conductive cooling effect within the fuel becomes more dominant for a smaller spotting area. Chapter 4 describes the limits of transition from flaming to smouldering, where a unique wood combustion mode showing a near-limit blue flame was identified as an intermediate combustion mode between the buoyancy-controlled yellow flame and the smouldering combustion. PART B includes three chapters (5-7), focusing on the quenching limit and quenching distance of smouldering combustion. Chapter 5 explores the quenching limit and the applicability of quenching diameter in smouldering through laboratory-scale experiments, where the measured quenching diameter of smouldering was about 10 cm. Chapter 6 develops a 2-D numerical model based on open-source code Gpyro and a previously developed 5-step kinetics of peat to verify the quenching diameter of smouldering combustion and further explore the effects of lateral overall heat transfer coefficient, oxygen concentration and ambient temperature on the quenching limits. Chapter 7 explores the applications of quenching distance of smouldering combustion through constructing firebreak for extinguishing peat fire. PART C includes three chapters (8-10), with the focus on the environmental impacts on the smouldering limits. Chapter 8 quantifies the smouldering propagation rate on consolidated biomass and the blow-off limit under concurrent and opposed external airflows up to 50 m/s, where the effects of fuel diameter and density are thoroughly discussed. Chapter 9 assesses the underlying mechanism of rain in suppressing the smouldering peat fire in the shallow soil layer up to 15 cm deep through laboratory experiments, where the extinction limit of suppressing smouldering peat fire is found. Chapter 10 quantifies the minimum environmental temperature that allows the moist peat to smoulder, and then apply a typical vertical soil temperature profile to estimate the future depth of burn and carbon emissions in boreal peatland fires under the impact of global warming.
Rights: All rights reserved
Access: open access

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