| Author: | Qin, Yunzhu |
| Title: | Critical oxygen supply and combustion thresholds of smouldering fires |
| Advisors: | Huang, Xinyan (BEEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Subject: | Combustion Peat -- Combustion Fire Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building Environment and Energy Engineering |
| Pages: | xix, 95, vi pages : color illustrations |
| Language: | English |
| Abstract: | Smouldering is a slow, low-temperature, and flameless form of combustion, primarily governed by two key mechanisms: system heat loss and oxygen supply. The path and rate of oxygen supply are critical for sustaining the heterogeneous oxidation reactions that generate the heat required to balance endothermic processes such as drying, preheating, environmental cooling, and pyrolysis during smouldering propagation. This thesis addresses the fundamental issue of oxygen thresholds in near-limit smouldering combustion and examining the effects of fuel properties and environmental conditions. Both experimental and computational studies were conducted to investigate the characteristics of limiting oxygen supply under forced internal flow. In addition, large-scale laboratory experiments (1 metre in height) were performed to demonstrate persistent deep smouldering fires under natural diffusion. The insights gained from this work deepen our understanding of smouldering combustion, contribute to more effective prevention strategies for smouldering fires, and help optimise applied smouldering systems. The thesis is structured in a manuscript-style format, beginning with an introduction to the literature review and objectives in the first chapter. The following chapters each stand alone as an independent paper, which have been published in journals. In the final chapter, the overall outcomes are summarised and potential ideas for future research are outlined. Chapter 1 reviewed the current methodologies and insights of limiting oxygen supply characteristics of smouldering. The knowledge gaps were identified, and general research objectives and methodologies were determined to address these gaps. Chapter 2 experimentally determined the limiting oxygen supply to sustain smouldering propagation in peat soil. Oxidiser flow velocities (up to 14.7 mm/s) and oxygen concentrations ranging from 2% to 21% were controlled as forced internal flow to feed smouldering zone. The findings reveal a transition in smouldering behaviour: at higher velocities, smouldering propagates bidirectionally (forward and opposed), while reducing the velocity results in unidirectional propagation. At a velocity as low as 0.3 mm/s, smouldering ceases, with the minimum temperature recorded at around 300°C. As internal flow velocity increases, the limiting oxygen concentration (LOC) for sustaining smouldering decreases, reaching a minimum value below 2%. When the oxygen concentration exceeds 10%, the minimum oxygen supply rate stabilizes at 0.08 ± 0.01 g/m²·s. However, for oxygen concentrations below 10%, the required oxygen supply rate rises significantly due to enhanced convective cooling caused by the higher internal flow velocities. Chapter 3 investigated how bulk density and particle size control smouldering in porous pine needle beds (55–120 kg/m³) and wood samples of particle size (1–50 mm). The experiments demonstrate that the critical airflow velocity for self-sustained smouldering rises when bulk density decreases or particle size grows. At elevated flow rates, smouldering initially advances only against the oxidiser stream, then transitions to simultaneous forward and opposed spread. However, for larger pore geometries, either from coarser particles or lower packing density, oxygen readily penetrates the opposed reaction front, leading to persistent bidirectional propagation. A simplified thermochemical model further reveals that interparticle conductive heat transfer plays important roles on the oxygen flux needed to maintain smouldering. Chapter 4 presents a one-dimensional, physics-based simulation coupling multicomponent heat and mass transport with a five-step heterogeneous reaction scheme to quantify the oxygen thresholds for smouldering under forced internal flows. The model predicts that, for fixed flow velocity, the LOC for smouldering propagation is 3%, agreeing well with the experimental observations and theoretical analysis. Furthermore, the required flow rate increases with decreasing fuel density, lower ambient temperature, or higher moisture content, and the predicted maximum moisture content capable of supporting smouldering was about 110 %. At the smothering limit, the computed peak reaction-zone temperature and propagation rate are around 300 °C and 0.5 cm/h. Chapter 5 conducts large-scale laboratory experiments demonstrating that, under purely diffusive conditions, smouldering persists in deep peat layers for over ten days, independent of ignition depth. Four distinct propagation modes appear as the ignition point moves downward: (I) downward propagation, (II) upward-and-downward propagation, (III) in-depth propagation, and (IV) no propagation (local burning). Modes III and IV produce neither visible smoke nor surface collapse, underscoring the challenge of detecting subsurface peat fires. For ignition depths shallower than -40 cm, peak reaction-zone temperatures decline with depth. However, temperatures remained stable near 300 °C when initial ignition depth deeper than -40 cm, indicating that oxygen availability predominantly governs deep-layer dynamics. Despite persistent combustion, overall mass loss remains low owing to incomplete oxidation at these moderate temperatures. Near-surface CO levels span tens to hundreds of ppm, suggesting that real-time CO monitoring could serve as a pilot detection method for underground fire activity. Chapter 6 summarises the overall outcomes of oxygen thresholds and smouldering dynamics in oxygen-limited conditions. According to the present findings, the challenges the researcher need to overcome in the future are also discussed. |
| Rights: | All rights reserved |
| Access: | open access |
Copyright Undertaking
As a bona fide Library user, I declare that:
- I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
- I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
- I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.
By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.
Please use this identifier to cite or link to this item:
https://theses.lib.polyu.edu.hk/handle/200/13853