Feng Zhu

Bio: Feng Zhu is an academic researcher from Chinese Academy of Sciences. The author has contributed to research in topics: Flame spread & Mechanics. The author has an hindex of 3, co-authored 5 publications receiving 33 citations.

Flame Spread and Extinction Over a Thick Solid Fuel in Low-Velocity Opposed and Concurrent Flows

Abstract: Flame spread and extinction phenomena over a thick PMMA in purely opposed and concurrent flows are investigated by conducting systematical experiments in a narrow channel apparatus. The present tests focus on low-velocity flow regime and hence complement experimental data previously reported for high and moderate velocity regimes. In the flow velocity range tested, the opposed flame is found to spread much faster than the concurrent flame at a given flow velocity. The measured spread rates for opposed and concurrent flames can be correlated by corresponding theoretical models of flame spread, indicating that existing models capture the main mechanisms controlling the flame spread. In low-velocity gas flows, however, the experimental results are observed to deviate from theoretical predictions. This may be attributed to the neglect of radiative heat loss in the theoretical models, whereas radiation becomes important for low-intensity flame spread. Flammability limits using oxygen concentration and flow velocity as coordinates are presented for both opposed and concurrent flame spread configurations. It is found that concurrent spread has a wider flammable range than opposed case. Beyond the flammability boundary of opposed spread, there is an additional flammable area for concurrent spread, where the spreading flame is sustainable in concurrent mode only. The lowest oxygen concentration allowing concurrent flame spread in forced flow is estimated to be approximately 14 % O-2, substantially below that for opposed spread (18.5 % O-2).

Microgravity diffusion flame spread over a thick solid in step-changed low-velocity opposed flows

Abstract: We report results from a microgravity combustion experiment conducted aboard the SJ-10 satellite of China, focusing on the structure and dynamics of diffusion flames spreading over a thick PMMA in low-velocity opposed flows. The width of the PMMA sample is chosen to be as large as possible in order to minimize the side diffusion effects of oxygen, and for each of the four oxygen concentration cases considered, four decrementally changing gas flow velocities are imposed such that a wide range of parameter values are spanned near the quenching limit. Two distinct flame spread modes are identified near the quenching limit, namely the continuous flame mode for gas flow velocities greater than an oxygen-concentration dependent critical value, and the flamelet mode for subcritical gas flow velocities. The transition process between these two spread modes due to a step change in the gas flow velocity is usually accompanied by flame oscillations, and diffusive-thermal instability of the leading flame front is identified as the mechanism controlling such transition. A correlation of the flame spread rate data among different oxygen concentrations indicates that, in the presently considered radiation-controlled regime the normalized flame spread rate deviates from the predictions of the thermal theory and decreases monotonically with the increase in the flame Damkohler number. Meanwhile, with the decrease in the flame spread rate, the standoff distance and the inclination angle at the flame leading edge show an increasing and decreasing trend, respectively. An energy balance analysis across the fuel surface beneath the flame leading edge indicates that the variation of the heat absorbed by the solid for vaporization is sub-linear with respect to the flame spread rate, thereby implying that the fuel regression depth has a tendency to increase with decreasing flame spread rate. Moreover, the energy balance analysis suggests that the quenching boundary and the marginal stability boundary identified on the flammability map are, respectively, intrinsically associated with a certain specific ratio of the overall heat losses to the total heat conducted from the flame, or equivalently, associated with a certain specific value of the flame spread rate.

Opposed-flow Flame Spread Over Solid Fuels in Microgravity: the Effect of Confined Spaces

Abstract: Effects of confined spaces on flame spread over thin solid fuels in a low-speed opposing flow is investigated by combined use of microgravity experiments and computations. The flame behaviors are observed to depend strongly on the height of the flow tunnel. In particular, a non-monotonic trend of flame spread rate versus tunnel height is found, with the fastest flame occurring in the 3 cm high tunnel. The flame length and the total heat release rate from the flame also change with tunnel height, and a faster flame has a larger length and a higher heat release rate. The computation analyses indicate that a confined space modifies the flow around the spreading flame. The confinement restricts the thermal expansion and accelerates the flow in the streamwise direction. Above the flame, the flow deflects back from the tunnel wall. This inward flow pushes the flame towards the fuel surface, and increases oxygen transport into the flame. Such a flow modification explains the variations of flame spread rate and flame length with tunnel height. The present results suggest that the confinement effects on flame behavior in microgravity should be accounted to assess accurately the spacecraft fire hazard.

A Comparative Study of Near-Limit Flame Spread Over a Thick Solid in Space- and Ground-Based Experiments

Abstract: Microgravity experiments have been performed aboard the SJ-10 satellite of China to investigate flame spread behaviors over a thick PMMA in low-velocity opposed flow. Two variables are considered: opposed-flow velocity in a range of 0 to 9 cm/s, and ambient oxygen concentration in a range of 25% to 50%. It is found that, when the flow velocity is reduced, the initial extended flame may breaks into separate flamelets after a dynamic transition process. This is the first observation of the flamelets spreading over a thick solid fuel in microgravity. Flame and flamelet propagate with a steady spread rate, which increases with the increasing flow velocity and oxygen concentration. A flammability map using oxygen concentration and flow velocity as coordinates is established, which delineates the uniform regime, the flamelet regime, and extinguished regime. The flammability boundary was extended to lower oxygen concentrations and lower flow velocities by the flamelet regime. The microgravity results are compared with the counterparts in ground-based narrow channel apparatus (NCA) experiments. Results showed that although the NCA tests overestimate the flame spread rate and flammable area, also exhibit differences in detailed flamelet formation process, flame and flamelet behaviors agree well with that in microgravity in a qualitative manner.

A review of near-limit opposed fire spread

Abstract: Creeping fire spread under opposed airflow is a classic fundamental fire research problem involving heat transfer, fluid dynamics, chemical kinetics, and is strongly dependent on environmental factors. Persistent research over the last 50 years has established a solid framework for different fire-spread processes, but new fire phenomena and recent developments continue to challenge our current understanding and inspire future research areas. In this review, we revisit the problem of opposed fire spread under limited and excessive oxygen supply. Various near-limit fire phenomena, as recently observed in flaming, smoldering, and glowing spread under various environment and fuel configurations, are reviewed in detail. Particularly, aspects of apparent importance, such as transition phenomena and heterogenous chemistry, in near-limit fire spread are highlighted, and valuable problems for future research are suggested.

An experimental study on flame spread over electrical wire with high conductivity copper core and controlling heat transfer mechanism under sub-atmospheric pressures

Abstract: The present study revealed experimentally flame spread behavior over high-conductivity copper (Cu) core electrical wire in sub-atmospheric pressures, and compares those with relative low-conductivity nickel-chrome (NiCr) core electrical wire, to quantify the evolutions of “wire-driven” and “flame-driven” heat transfer mechanisms in supporting flame spread with pressure decreasing. Polyethylene (PE) insulated Cu electrical wires with inner core diameter ( d c ) of 0.30 mm, 0.50 mm 0.80 mm and insulation thickness ( δ p ) of 0.15 mm, 0.20 mm and NiCr core electrical wires with inner core diameter ( d c ) of 0.30 mm, 0.50 mm and insulation thickness ( δ p ) of 0.15 mm were studied with ambient pressure ranged from 100 kPa to 40 kPa. Results showed that the blue flame height (or the “flame standoff distance” from the bottom edge of the high temperature yellow flame region to wire surface), and the flame base width both increased as the pressure decreased. The flame spread rate (FSR) of NiCr-core electrical wire just increased; however, that of Cu-core electrical wire first decreased then increased as the ambient pressure decreased. The critical pressure (P*) for the turning of FSR variation of Cu-core wire with pressure was larger as the core diameter was smaller or the insulation was thicker. The “wire-driven” and “flame-driven” heat transfer mechanisms in controlling the flame spread behavior over electrical wires with metal core were well verified by the experimental results shown in the present study. The relative importance of “wire-driven” heat transfer mechanism decreased, while that of “flame-driven” increased, as the ambient pressure is reduced. The findings of the present study will be essential for simulative study of flame spread behavior over electrical wires concerning the core conductivity effect and the evolutions of controlling heat transfer mechanisms.

Transitional flame-spread and fuel-regression behaviors under the change of concurrent wind

Abstract: On the occurrence of an environmental wind, the steady-state flame spread will go through a transitional process to a new steady state of the concurrent flame spread. However, such a transition between two steady states was rarely studied, despite that it often happens to many fire events. This paper presents an experimental study on the transitional flame-spread behavior over the horizontal PMMA plate by applying different concurrent airflows from the still air. The flame spread rate at the pyrolysis leading edge and the regression rate in the fuel rear end were studied during this transition. Such a transition could be divided into three stages based on both the length of pyrolysis and the fuel regression rate, (1) increasing to maximum, (2) dropping from peak, and (3) reaching a new steady-state. The concurrent wind velocity showed different influences on the flame spread rate and the fuel-rear regression rate during the transition. This work provides novel experimental data on the transitional behavior of wind-assisted flame spread, which helps evaluate the fire hazard under the sudden change of environmental conditions.

Microgravity diffusion flame spread over a thick solid in step-changed low-velocity opposed flows

Abstract: We report results from a microgravity combustion experiment conducted aboard the SJ-10 satellite of China, focusing on the structure and dynamics of diffusion flames spreading over a thick PMMA in low-velocity opposed flows. The width of the PMMA sample is chosen to be as large as possible in order to minimize the side diffusion effects of oxygen, and for each of the four oxygen concentration cases considered, four decrementally changing gas flow velocities are imposed such that a wide range of parameter values are spanned near the quenching limit. Two distinct flame spread modes are identified near the quenching limit, namely the continuous flame mode for gas flow velocities greater than an oxygen-concentration dependent critical value, and the flamelet mode for subcritical gas flow velocities. The transition process between these two spread modes due to a step change in the gas flow velocity is usually accompanied by flame oscillations, and diffusive-thermal instability of the leading flame front is identified as the mechanism controlling such transition. A correlation of the flame spread rate data among different oxygen concentrations indicates that, in the presently considered radiation-controlled regime the normalized flame spread rate deviates from the predictions of the thermal theory and decreases monotonically with the increase in the flame Damkohler number. Meanwhile, with the decrease in the flame spread rate, the standoff distance and the inclination angle at the flame leading edge show an increasing and decreasing trend, respectively. An energy balance analysis across the fuel surface beneath the flame leading edge indicates that the variation of the heat absorbed by the solid for vaporization is sub-linear with respect to the flame spread rate, thereby implying that the fuel regression depth has a tendency to increase with decreasing flame spread rate. Moreover, the energy balance analysis suggests that the quenching boundary and the marginal stability boundary identified on the flammability map are, respectively, intrinsically associated with a certain specific ratio of the overall heat losses to the total heat conducted from the flame, or equivalently, associated with a certain specific value of the flame spread rate.

Opposed Flame Spread over Cylindrical PMMA Under Oxygen-Enriched Microgravity Environment

Abstract: The enriched oxygen ambient may be applied to China’s next generation space station. To understand the fire behaviors under oxygen-enriched microgravity environment, flame-spread experiments on extruded poly(methyl)methacrylate (PMMA) rods with 10-mm diameter were conducted in the SJ-10 Satellite. The opposed flame-spread behaviors were studied at the oxygen-enriched ambient (33.5% and 49.4%) under low flow velocities in the range of 0 to 12 cm/s. After the ignition in the middle of the sample, an opposed flame spread was achieved, rather than the forward flame spread. The flame-spread rate increases with the opposed flow velocity, due to the decreased flame width and the enhanced flame heat flux. Moreover, a blue flame sheet with a frequent burst of bubbles is found throughout the opposed-flow spread process, showing a near extinction behavior. For the oxygen concentration above 25%, normal-gravity experiments suggest that whether PMMA is cast or extruded should have a negligible effect on the opposed flame spread in microgravity. Compared to normal gravity, the microgravity flame spread rate in the oxygen-enriched atmosphere is slower which is the order of 0.1 mm/s, only one-tenth to one-fifth of that in normal gravity at the same nominal opposed flow velocity, and the acceleration of flame spread in microgravity by increasing oxygen concentration is also much smaller. This result suggests that (1) if the environmental gas flow is small, the fire hazard increased by raising oxygen level in microgravity space cabin can be much smaller than that on Earth; and (2) the fire risk of oxygen-enriched microgravity environment might be overestimated when a ground-based test method is employed to evaluate the burning characteristics of solid material.