Quantifying the mechanisms of opposed flow flame spread
Creators: name: Morrisset, David
Publisher: name: The University of Edinburgh
Year: 2024
Subjects: Flame Spread
Fire

description: Experiments were conducted to investigate opposed flow flame spread over solid fuels, with a particular emphasis placed on defining and quantifying the relative contribution of key controlling mechanisms of flame spread. The subject of flame spread has been investigated in great detail over the past 50 years – with particularly significant research occurring in the 1970-1990’s. Much of the focus in fundamental flame spread studies has been dedicated to identifying what controlling mechanisms (e.g., specific heat transfer mechanisms or gas-phase processes) dominate the rate of flame spread. These controlling mechanisms largely break down into two categories: mechanisms of heat transfer ahead of the flame front (e.g., gas-phase heat transfer, solid-phase conduction) and relevant gas-phase phenomena (e.g., mixing, finite-rate chemical kinetics). For high Damkölher numbers (i.e., the “Thermal Regime”), the rate of flame spread is controlled by heat transfer ahead of the flame front. Defining the controlling mechanism in the thermal regime then relies on identifying which individual heat transfer mechanism most significantly drives the rate of spread (namely solid-phase vs. gas-phase heat transfer) – which has been studied for a range of conditions, and has led to much debate which continues today. For low Damköhler numbers (e.g., close to blow-off), the process of flame spread is driven by gas-phase phenomena such as finite-rate kinetics (as such this regime is referred to in the literature as the “Kinetic Regime”). The framework of establishing specific controlling mechanisms of flame spread is often a required simplification in developing simple analytical flame spread models. Ultimately it is demonstrated in this work that in many scenarios, no single mechanism can be considered in isolation. Most opposed flow flame spread scenarios must consider the combined effects of multiple heat transfer mechanisms, or a combination of heat transfer and relevant gas-phase phenomena, to fully describe the problem. As such this work sets out to explore opposed flow flame spread under a wide range of experimental conditions and to identify and, to the extent possible, quantify each relevant mechanism of flame spread. This study systematically investigates opposed flow flame spread starting with three canonical configurations (i.e., downward, lateral, and horizontal spread) at the bench-scale. In these cases, a method was developed to use relatively simple instrumentation to explicitly quantify the relative magnitude of solid-phase and gas-phase heat transfer. The balance of solid-phase and gas-phase heat transfer was then compared across each sample configuration, material, and external heating conditions at the surface. The complexity of the problem is then incrementally increased to explore the influence of forced flow conditions where both the thermal and kinetic regime can be explored. These experiments demonstrate the increased significance of solid-phase heat transfer with increasing flow in air, but also demonstrate that high oxygen environments (e.g., 40% O2 by volume) are indicative of increased gas-phase heat transfer at higher flow rates. The complexity of the flame spread problem is then increased again by investigating flame spread over spherical geometries. Opposed flow flame spread over a sphere demonstrated transient flame spread rates; this transience was then linked to key mechanisms of flame spread that changed over the experiment. These results demonstrate the transition from a problem dominated by gas-phase heat transfer to one being driven by heat transfer through the solid. In addition to increasing complexity in the experimental configuration, this work also demonstrates the application of advanced measurement techniques to provide new and fundamental insight into the drivers of opposed flow flame spread. Optical diagnostic techniques are widely used in the combustion science community, but are rarely applied directly to fire science problems. This study therefore explores the use of such diagnostic techniques including phosphor thermometry and CH* chemiluminescence to provide high-resolution measurements of surface temperature and the relative flame position in the context of opposed flow flame spread. These measurements allow for novel findings such as the superposition of flame location relative to the surface temperature distribution, providing definitive evidence to support the relative location of the onset of pyrolysis and the leading edge of the flame. This study therefore provides a fundamental investigation of laminar flame spread phenomena, using modern measurement techniques to expand the understanding of opposed flow flame spread. The findings in this work progress the development of flame spread theory and, where possible, provide an explicit characterization of the relative contribution of each controlling mechanism of flame spread.