Author: Sun, Peiyi
Title: Dripping ignition mechanism and fire risks of thermoplastic drips
Advisors: Huang, Xinyan (BEEE)
Usmani, Asif Sohail (BEEE)
Degree: Ph.D.
Year: 2022
Subject: Fires
Fire risk assessment
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Environment and Energy Engineering
Pages: xiv, 96 pages : color illustrations
Language: English
Abstract: Dripping is a common phenomenon in combustion and fire, e.g., in a burning candle. In fire accidents, the dripping phenomenon refers to the weight of melting fuel overcoming its surface tension to produce drips or dripping flows. The dripping of molten plastic fuels, such as cable insulations and building thermal insulation materials, can change fire behaviours, ignite nearby materials, and expand fire size, posing great fire risks. However, very little research has studied the fire phenomenon of dripping, especially its capability of igniting other fuels, mainly because of the larger complexity. This thesis is an attempt to provide important information to quantify the fire hazard of the dripping fire phenomenon and improve the understanding of ignition caused by dripping melts.
This thesis is presented in a manuscript style: the first chapter introduces the research background and motivations. The following chapters take the form of independent papers, which have been published or submitted to a journal publication. The final chapter summarizes the conclusions and points out the potential areas for future research.
Chapter 1 introduces the research background and motivation. The dripping of molten fuels is one of the most widely observed phenomena in combustion and fire. For example, the melted wax can flow down and generate drips in burning candles. Similarly, the molten drips also can be observed when the fire is involved with thermoplastics, but these thermoplastic drips are always falling carried with flame. In detail, dripping fire often occurs in wire fires and façade fires, where thermoplastic materials, such as polyethylene (PE), polyethylene chloride (PVC), polypropylene (PP), and expanded polystyrene (EPS), are widely used as the electric or thermal insulations. Burning drips transfer the fuel and the fire sources simultaneously and eventually start a new fire, posing a significant fire hazard. However, limited studies systematically explored the ignition mechanism by burning drips and quantified its fire risks before the research was conducted in this project.
Chapter 2 explores the burning behaviours and fire hazards of molten thermoplastics in the spacecraft. Pre-ignited droplets with a diameter of about 3 mm were continually generated and detached from burning PE tubes. Once the drop capsule started free-fall, droplets entered the microgravity environment with an initial velocity of 10-35 cm/s (Stage I). A comet-shape flame with an intense bubbling and ejecting process of the moving droplet was observed, and the burning-rate constant (K) was found around 2.6 ± 0.3 mm²/s. After the droplet landed on the floor, it could rebound with a near-zero velocity, showing as a spherical flame (Stage II). The combustion of PE droplets followed the classical d-square law with K = 1.3 ± 0.1 mm²/s. The measured large burning-rate constant (or the volume shrinkage rate) of the moving droplet was caused by the robust bubbling process, which reduced the bulk density of molten PE and ejected unburnt fuel (about 25% of total mass loss). However, the actual mass burning rate of the PE droplet should be smaller than most hydrocarbon liquids because of a smaller mass-transfer number (B ≈ 2). The flame burning rate of PE the droplet is 4 ± 1 g/m²-s per unit flame-sheet area that may be used to estimate the fuel mass-loss rate and fire heat release rate in microgravity. This novel microgravity combustion experiment on the thermoplastic droplet could expand the physical understanding of fire risk and the hazard of plastic material in the spacecraft environment.
Chapter 3 quantifies the ignition of thin papers (0.07 - 0.32 mm) by burning polyethylene drips with four sizes (2.6 - 6.2 mg) and dripping frequencies (0.8 - 1.8 Hz). The probability of dripping ignition as a function of key dripping parameters is quantified to determine the ignition limits. As the paper thickness increases, more drips and longer time are required for ignition, similar to the classical pilot ignition of thin fuels. The attached flame acts as the piloted source; heating effects from hot drips and dripping flame are comparable; and ignition occurs to the paper rather than landed drips. Moreover, the dripping-ignition capability is controlled by the dripping mass rate, which is the product of the drip mass and the dripping frequency. For the dripping mass rate of about 4.5 mg/s, the equivalent heat flux is 15 ± 3 kW/m². The dripping-ignition time is inversely proportional to the mass of the drip and the square of the dripping frequency, different from the piloted ignition under irradiation. This work provides important information to quantify the fire hazard of dripping and explores the ignition mechanism in the dripping fire.
Chapter 4 assesses the ignition capability of continual polyethylene drips with the size of 2.6-4.6 mg and the frequency of 0.3-1 Hz. These flaming drips land on four groups of materials, cardstock papers (>0.1 mm), thin papers (≤0.1 mm), cotton, and porous mineral materials. For igniting cardstock papers, the minimum drip number decreases with the drip size and frequency, and the ignition time follows the piloted-ignition theory. The thin permeable paper and cotton are soaked by drips, so ignition only requires a small and fixed number of drips. The soaking effect also helps anchor the flame on drips absorbed by other porous mineral materials, showing a notable fire risk. Theoretical analysis of the ignition limit and delay time is proposed to identify the boundary between the piloted dripping ignition and the flame anchored on drip-soaked material. This research reveals different ignition mechanisms of dripping fire and helps understand the fire hazard regarding the transport and soaking effect of molten fuels.
Chapter 5 investigates the burning behaviour and quantifies the fire hazards of the accumulated molten drips. The fully melted polyethylene (PE) vs polypropylene (PP) drips with controlled temperature (380-410 °C) and mass (2.0-2.6 g) were generated by a cylindrical heater. These large drips free-fall on a hot plate with a controlled area and initial temperature and then are ignited. Three burning patterns are observed and defined under different bottom boundary temperatures. When the boundary temperature is lower than the melting point of thermoplastic, burning Pattern I (near-limit flame) appears shortly before quenching. Above the melting point, the flame becomes stronger and lasts for a longer period at a higher boundary temperature (Pattern II: transitional flame). When the boundary temperature exceeds the fuel pyrolysis point, the flame becomes intensive and burns out all plastics (Pattern III: intensive flame). The burning processes of molten thermoplastics are further compared with the burning of ethanol and paraffin wax. This study promotes the understanding of the melting and burning of plastics in real fire scenarios and helps determine the hazards of dripping fires.
Chapter 6 summarizes the founding and contribution of this project and discusses the possible area for future research.
Rights: All rights reserved
Access: open access

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