On the dominant mode of heat transfer in downward flame spread

doi:10.1016/S0082-0784(79)80114-9

Symposium (International) on Combustion

Volume 17, Issue 1, 1979, Pages 1201-1209

https://doi.org/10.1016/S0082-0784(79)80114-9 Get rights and content

The dominant mechanism by which heat is transferred to the unburnt fuel ahead of the flame is analyzed by measuring the heat transfer parameters involved in the process of downward propagation of flames over two extreme thicknesses of a solid fuel. Measurements of the gas velocity fields and temperature fields for both fuel thicknesses are reported. Experimental techniques employed include Laser Doppler Velocimetry (LDV) and thermocouple probing. Qualitative information from the measurements as well as quantitative calculations of the pathways of heat transfer indicate that for thick fuels heat conduction through the solid phase is the dominant mechanism of heat transfer, and that as the thickness of the fuel decreases heat conduction through the gas is of increased importance. It is also shown that convective effects due to the entrainment of air by the flame plume play an important role in the ability of the heat to be conducted through the gas from the flame.

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The contribution of gas-phase and solid-phase heat transfer on opposed flow flame spread was quantified under a range of experimental conditions. The thermal gradient ahead of the flame front was determined using a temperature reconstruction method from time resolved temperature data. In all conditions tested, both gas phase and solid phase heat transfer displayed significant contributions to heating ahead of the flame front. PMMA samples were tested in three orientations - downward, horizontal, and lateral. It was found that the lateral configuration displayed the highest rate of flame spread (3.54 mm/min) compared to downward (2.67 mm/min) and horizontal (2.54 mm/min). Lateral spread also exhibited a lower magnitude of solid phase heat transfer compared to the other orientations. Trials that were externally heated showed lower contributions of solid phase heat transfer as the applied surface heat flux was increased. Both PMMA and POM samples were used to compare the effects of the presence of a sooting or non-sooting flame. The methodology provided in this work quantifies the relative contributions of gas-phase and solid-phase heat transfer in opposed flame spread scenarios.
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Determination of the dominant physical processes in downward-propagating flame spread over a solid fuel using machine learning
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Citation Excerpt :
Wu [8] investigated the structure of the thermal boundary layer of an opposed-flow FSS using experiments and analytical models, showing that flame spread is mainly controlled by the heat and mass transfer near the flame front. Kumar [9] and Fernandez-Pello [10] compared the dominant mechanisms in heat transfer, namely flame radiation, convection and conduction, for the opposed-flow FSS, providing a theoretical basis for model simplification. Additionally, some analytical models were introduced to simulate the thermal structure of downward-propagating FSS [11,12] and the corresponding flame geometries under varying flow velocities [13,14].
Flame spread over solid fuel (FSS) plays a key role in solid-fuel combustion and fire-related phenomenon. The mechanisms of flame spread over solid fuel are commonly described by means of dimensionless numbers or scaling analysis that describes the balanced relationship of several processes. However, these approaches rely on prior knowledge or explicit assumption of the relevant physical processes, and it is difficult to spatially distinguish among multiple physical processes. This work demonstrated a generalized way using an unsupervised machine learning method based on the Gaussian mixture models and sparse principal component analysis (GMM-SPCA) to automatically delineates the spatial domain of the FSS from a numerical simulation into several regions that are dominated by the balance between different physical processes. The idea of equation space is employed such that each coordinate in the equation space corresponds to a specific physical process as represented by the individual term in the corresponding governing equation. The dominant heat/mass transport processes for both gas and solid phases have been analyzed, and their spatial correspondence for the fields of temperature, flow, and species has been discussed. Some critical characteristics, such as the flame stand-off distance profile, the triple flame structure, and the pyrolysis zone of the solid fuel have been properly identified and quantified. It is demonstrated that the generalized GMM-SPCA method provides an intuitive insight into the heat and mass transfer processes of the FSS for further development of the flame spread model.
Opposed-flow flame spread over cylindrical fuels: Spread rate formulas and experimental verification
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Opposed-Flow flame spread over flat condensed fuel has been studied for many decades as it constitutes a fundamental problem of fire studies. Although cylindrical fuels offer the simplicity of axial symmetry, despite many previous attempts, there are no easy-to-use flame spread rate expressions that connect spread rate in this geometry to the exact solution for flame spread rate for flat fuels. Also no criterion exists to delineate a thermally thin cylindrical fuel from a thermally thick one. In this work, we use the scaling approach to develop closed-form expressions for flame spread rate for opposed-flow flame spread over thermally thin and thick cylindrical fuels. The approach is validated by reproducing the theoretical expressions of de Ris for flat fuels. By equating the heated layer thickness with the radius, a criterion is developed for delineating thermally thin cylinders from thermally thick ones. It is shown that the spread rate is always higher for a cylindrical fuel when compared to a flat fuel with a half-thickness equal to the radius of the cylindrical sample. The cylindrical spread rate depends on the same factors as that of flat fuels with an additional coefficient arising out of heat transfer enhanced by the curvature effect. For thermally thin cylinders, the spread rate is predicted to be at least two times faster than the corresponding flat fuel. For thermally thick fuels also the spread rate is significantly higher, but as the radius is increased the flat fuel limit is approached. Spread rates obtained from experiments performed with PMMA cylinders of different diameters in downward configuration are shown to agree well with the theoretical expressions. The thermally thick limit for downward spread, interestingly, is achieved for PMMA cylinders as narrow as 2.4 mm diameter.
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Observation of the deformation process of a thermoplastic rod is important because the burning behavior depends on the instantaneous shape. However, the observation is often difficult because the rod, particularly its top, is hidden by the flame. This study proposes a novel imaging technique that visualizes the deformation process of the burning rod by filtered backprojection of backlight images. An imaging system consisting of two sets of a backlight source and a camera was developed. Tests using cylinder and square rods of cast and extruded polymethyl methacrylate (PMMA) were conducted. The instantaneous three-dimensional shape of the burning rod was successfully visualized for all the samples, which proves the applicability of the proposed technique in the presence of molten material and corner edge. The mass loss was thereafter estimated from the images and compared with reference data obtained in another series of tests using an electric balance. For the cast PMMA rod, the mass loss agreed well with the reference data upon correction of the shrinkage of the cross-section. On the other hand, the result for the extruded PMMA rod showed a significant deviation from the reference data because the decrease in volume by the surface regression was canceled by the increase in the amount of molten material. It was also demonstrated that the vertical regression rate can be measured regardless of material types. The results showed that the vertical regression rate was less sensitive to initial shape for the cast PMMA rod during the steady-state, while it linearly increased with time and never reached the steady-state value within the test time for the extruded PMMA rod. The proposed technique can evaluate various characteristics of a burning thermoplastic that should vary in accordance with the deformation; therefore, it is particularly useful when the steady-state is not achieved.

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On the dominant mode of heat transfer in downward flame spread

Combust. Flame

J. Fire Flam.

J. Fire Flam.

Combust. Flame

Combust. Sci. Tech.

Flame Spread in Laminar Opposed Flow