Flame Spread Behavior Research Articles

Experimental validation of a correlation capturing the boundary layer effect on spread rate in the kinetic regime of opposed-flow flame spread

In opposed-flow flame spread over thermally thin solid fuels, an increase in flow velocity eventually leads to flame extinguishment. While the chemical time is independent of the flow velocity, the residence time of the oxidizer at the flame leading edge is inversely proportional to the flow velocity. A competition between the two leads to a situation where finite-rate kinetics dominates the flame spread behavior, leading to blow-off extinguishment. The ratio of the two competing, the Damkohler number, captures this finite-rate effect and has been used to correlate the non-dimensional spread rate with opposing flow velocity and ambient oxygen level. Although qualitatively successful, these correlations show considerable spread in the data despite the use of several variations of the definition of the Damkohler number. One possible reason could be the neglect of the developing boundary layer in front of the flame by the experimentalists. In this work we present new flame spread data over ashless filter paper acquired in an eight meter tall vertical steel chamber, in which a sample is moved at desired velocities to create a relative opposing flow. The developing boundary layer over the fuel sample is shown to have a significant effect on the measured spread rate and flame shape that does not correlate with opposing flow velocity if an Oseen flow approximation is used. Once the data is adjusted for the developing boundary layer using an effective flow velocity formula obtained from scaling arguments and refined through computational results, the correlation improves drastically, establishing the importance of the boundary layer in the kinetic regime. Much of the thermally thin fuel spread rate data available in literature does not mention the development length and must be cautiously used.

A Correlation for an Effective Flow Velocity for Capturing the Boundary Layer Effect in Opposed-Flow Flame Spread over Thin Fuels

In opposed-flow flame spread over solid fuels, an indefinite increase in flow velocity eventually leads to flame extinguishment. While the chemical time is independent of the flow velocity, the residence time of the oxidizer at the flame leading edge is inversely proportional to the flow velocity and, therefore, a completion between the two leads to a situation where finite-rate kinetics dominate the flame spread behavior, leading to flame extinguishment. Their ratio, the non-dimensional Damkohler number, is known to capture this finite-rate effect and has been used to correlate the non-dimensional spread rate. Although these correlations explain the behavior observed in the experiments, there is considerable spread in the data and several variations of the defs of Damkohler number exist in literature. With all the progress made in this area, it is still not possible to predict the blow-off extinction velocity for a given fuel at any given oxidizer condition. In this work, computational evidence to establish the importance of the developing boundary layer in the kinetic and microgravity regimes is presented. Based on scale modeling, a formula is proposed for the effective velocity experienced by a flame in terms of the flow Reynolds number and Prandtl number and the expression using computational spread rates is verified.

C125 走酸素性に基づく通路内燃え拡がり火炎挙動の予測

C125 走酸素性に基づく通路内燃え拡がり火炎挙動の予測

Experimental study on flame spread behavior along poly(methyl methacrylate) corner walls at different altitudes

The effects of altitude and intersection angle on the flame spread behavior and pyrolysis front characteristics along corner walls were experimentally studied. The experiments were conducted using mock corners made of poly(methyl methacrylate) slabs with intersection angles varying from 60° to 120° at two altitudes of 29.8 and 3658.0 m. Measurements were taken for the upward and lateral flame spread rates, the flame heights, the flame heat flux to the fuel surface, and the mass loss rates of the tested slabs. An “M”-shaped pyrolysis front was observed for all the intersection angles examined at both altitudes. The upward flame spread rate increases with decreasing intersection angles while no obvious trend was found for lateral spread rate when the intersection angle is varied. The high altitude leads to a spread rate about half of that at the lower altitude both for upward and lateral spread irrespective of the intersection angle. The measured mass loss rates are significantly higher than twice of that over a flat vertical slab.

Heat transfer mechanisms in horizontal flame spread over wood and extruded polystyrene surfaces

The heat transfer in horizontal flame spreading over the surface of construction materials was experimentally investigated at two different altitude conditions (the Lhasa plateau 3658m, and the Hefei Plain, 50m). The experiments were carried out with the charring material Whitewood and the thermoplastic material the extruded polystyrene (XPS) over a wide range of sample widths. The temperature profile in solid phase was measured. Different flame spread behaviors were found from the temperature profiles of the XPS and Whitewood. The XPS undergoes the melting stage in the preheated stage while the Whitewood undergoes the char burning stage after the pyrolysis stage. The heat transfer analysis suggests that the gas phase heat transfer is dominant for whitewood and XPS, even though the XPS is thermally thick. The flame spread rate and the surface heat flux with various sample widths were measured. They both dropped firstly and then rose with the increasing sample width. Furthermore, the spread rate was correlated with a pseudo-property Ф, which is the product of the surface heat flux and the preheated length. A good linear relationship was found between the spread rate and Ф, which agrees well with Quintiere’s model. Moreover, the sample width effects on flame spread rate were well explained by the effects on heat transfer. The horizontal flame spread behaviors with sample width were dominated by two different regimes: the spread rate drops for small sample width in convection regime but rises for large sample width in radiation regime.

A review on research of fire dynamics in high-rise buildings

Since serious fire occurred frequently in recent years, fire safety of high-rise building hasattracted extensive attention. A National Basic Research Program (973 program) of China has been setup by Ministry of Science and Technology (MOST) of China in 2012 to meet the research requirementsof fire safety in high-rise buildings. This paper reviews the current state of art of research onfire dynamics of high-rise buildings, including the up-to-date progress of this project. Thefollowing three subjects on fire dynamics of high-rise buildings are addressed in this review: theejected flame and fire plumebehavior over facade out ofthe compartment window, the flame spread behavior over facade thermal insulation materials, and the buoyancy-driven smoke transportationcharacteristicsalong long vertical channels in high-rise buildings. Prospective future works are discussed andsummarized.

Flame spread behavior of electric cables: Round-robin characterization of the IEC/EN 60332-1 tests

In the field of “reaction to fire,” the flame test is one of the easiest experiments available to assess the flammability of insulated wires or cables. The equipment and the method are completely described in the IEC/EN 60332-1 Parts 1 and 2 Standards. However, a round-robin study involving seven labs has highlighted a wide dispersion of the results. This may lead to a possible false performance conclusion on the product tested. To avoid this stumbling block, this article points out the necessity to know the test reliability in the product development frame, the quality survey, or homologation.

Behavior of Flame Spread on Thin PMMA near Extinction Limit at Low Oxygen Level

Behavior of Flame Spread on Thin PMMA near Extinction Limit at Low Oxygen Level

Classification of Flame Spread Behavior over a Solid Material by Scale Analysis

Classification of Flame Spread Behavior over a Solid Material by Scale Analysis

Numerical study of flame spread in an n-heptane droplet array in different ambient temperature and microgravity conditions

Flame spread in an n-heptane droplet array under conditions of microgravity, different ambient temperatures, and droplet spacing is studied numerically. The dimensionless distance, fuel type, and ambient temperature are characteristic parameters that could change the flame spread modes. Variations of the droplet spacing change the amount of the gas fuel on the centerline of droplets, and the flame can either appear between the droplets or pass above this space. A high-temperature environment causes an increase in the flame spread rate. Good qualitative agreement in the flame spread behavior between numerical results and microgravity experiments is obtained. Flame spread rates are obtained for different droplet spacings. This curve has a maximum, similar to experimental result.

Effects of sample width on flame spread over horizontal charring solid surfaces on a plateau

Experiments were conducted on the Tibetan Plateau to investigate the effects of sample width (W) on flame spread over horizontal charring solid surfaces. The results show that the rate of spread is influenced by the combined effects of convection heat transfer and flame radiation. Moreover, two different regimes depending on sample width were found in flame spread behavior. The rate of spread decreased with width in the convective regime (W⩽3 cm) but increased in the radiative regime (W>3 cm). Similar results were observed in comparative experiments conducted on a plain in Hefei, with the rate of spread reaching its minimum value at W=4 cm rather than 3 cm.

Experimental study on the characteristics of horizontal flame spread over XPS surface on plateau

A series of comparative laboratory-scale experiments were carried out in the Lhasa plateau and the Hefei plain respectively to investigate the characteristics of flame spread over the extruded polystyrene (XPS) foam, a typical thermal insulation material. Flame shape and the temperature profile in solid phase were monitored, and the effects of altitude on the heat transfer process were analyzed. The results show that the temperature rise with time undergoes three stages: the preheating stage, the melting stage and the pyrolysis stage. The durations of the melting and pyrolysis stages on plateau are longer than that in plain, which sequentially results in a lower flame spread rate on plateau. Comparing of the temperature change rate curve on plateau with that in plain, it is found that the peak characteristics of the curves in the pyrolysis stage changed from single peak to multi-peaks, which suggests that the altitude difference might change the pyrolysis mechanisms of XPS material. Moreover, the sample scale effects on flame spread are also explored. Two different regimes are found in flame spread behavior with sample scale at the both altitudes. The spread rate drops with sample scale in convection regime and rises in radiation regime.

Microgravity experiments of flame spreading along a fuel droplet array in fuel vapor-air mixture

Combustion experiments of fuel droplet array in fuel vapor-air mixture were performed at microgravities to investigate growth mechanism of group combustion of fuel droplets. A 10-droplet array was inserted into the test section filled with a saturated fuel vapor-air mixture as a simple model of prevaporized sprays. Gas equivalence ratio of the fuel vapor-air mixture was regulated by the test section temperature. n-Decane droplets of 0.8 mm in the initial diameter were suspended at the crossing points of 10 sets of X-shaped suspenders. The first droplet was ignited by a hot wire to initiate flame spread along a fuel droplet array. Flame spread speed was obtained from the history of the leading edge position of a spreading flame. Effects of droplet spacing and gas equivalence ratio on the flame spreading behavior and the flame spread speed were examined. The droplet spacing and the gas equivalence ratio were varied from 1.6 to 10.2 mm and from 0.2 to 0.7, respectively. The gas equivalence ratio has little effect on the relationship between the flame spreading behavior and the droplet spacing. The flame spread speed increases as the increase in the gas equivalence ratio at all droplet spacings. The influence of the gas equivalence ratio on the flame spread speed becomes strong as the increase in the droplet spacings. The flame spread speed increases as the increase in the droplet spacing, and then decreases. The maximum flame spread speed appears in the range from 2.4 to 3 mm at all gas equivalence ratios.

Analysis of flame spread over aviation kerosene

Flame spread behaviors and temperature rise of fuel surface were experimentally investigated. A series of tests on aviation kerosene pool fires were conducted by using the schlieren image system, micro-thermocouple and infrared thermal imager. The velocity evolutions of flame spread and surface flow under different initial fuel temperatures were discussed. The characteristics of fuel surface temperature rise were revealed, and it was found that the preheating effect of surface flow on fuel decreased with increasing initial fuel temperature.

1010 Effects of Synthetic Material on Flame Spread Behavior over Combined Fabrics

1010 Effects of Synthetic Material on Flame Spread Behavior over Combined Fabrics

D113 電線被覆材ポリエチレンの燃焼量に対する周囲雰囲気圧力変化の影響(OS-11:火災安全(I))

Flame spread behavior over electric wire coated polyethylene (PE) in low pressure is studied experimentally. The objective of this study is to investigate the relationship between consumption rate of melting PE and ambient pressure. The ambient gas composition is fixed to the same as air and total pressure is set to sub-atmospheric range (30〜100 kPa). Dynamic behavior of molten PE under various conditions is precisely observed/recorded in order to analyze the growth rate of molten PE and increment of spread rate in time with various types of wires. It turns out that the growth rates in the molten PE of NiCr and Fe wires are larger than that of Cu wire. PE consumption rate of NiCr and Fe wires are weakly depended to pressure, on the other hand, that of Cu wire obviously decreases as the decrease in pressure. Since our results show that the time-depended behavior of molten phase is strongly related to the formation/consumption rate of molten matter and they are closely related to the flame behavior as well, precise modeling of molten layer is necessary to predict the time-depended character of wire combustion.

Flame-Spreading Behavior in a Fin-Slot Solid Propellant Rocket Motor Grain (Part II)

To accurately predict the overall ignition transient for the reusable solid rocket motors of the space shuttle booster with head-end fin slots, it is necessary to have the knowledge of the flame-spreading rates in the fin-slot region. This paper is the second of a two part study and deals with the development of a flame-spreading correlation in the fin-slot region. A subscale (1:10) pie-shaped fin-slot motor was designed to perform diagnostic measurements for studying the flame-spreading behavior on the exposed propellant surface. Dynamic similarity was considered in the igniter design so the impinging jet had a similar exit angle onto the propellant surface in the fin-slot section. Flame-spreading measurements were gathered using a high-speed digital camera and nonintrusive optical measurement methods through an array of 36 near-infrared fast-response photodetectors installed perpendicular to representative regions of the propellant surface. Results showed that the flame-spreading phenomena was highly nonuniform, starting in the downstream portion of the fin-slot region before traveling back toward the igniter. A correlation was developed for the dimensionless flame-spreading time interval showing that it was inversely proportional to the pressurization rate to a power of 0.62, which depends strongly upon the flow parameters of the igniter induced flow and local propellant grain geometry.

Behavior of Flame Spread Downward over Thick Wood Sheets and Heat Transfer Analysis

To investigate the flame spreading mechanism over charring materials under natural convection, wood sheets from 0.9 to 3 mm thickness have been chosen. The variation of mean flame spread velocity V with thickness δ is measured. Typical flame shapes over the wood surfaces are recorded using CCD. The temperature fields in the gas phase are examined by a set of fine thermocouples. On the basis of the results, the relative contributions of local heat transfer modes ahead of the pyrolysis front have been evaluated. It is found that heat conduction through solid phase increases notably with δ. A flame spreading model coupled with a pyrolysis structure is proposed.

Effects of carbon nanotubes on flame spread rate over 1-propanol

This paper discusses the potential for thermally conductive nanoparticles to reduce flame spread velocities over fuel dispersion mixtures. Multi-walled carbon nanotubes (MWNTs) were dispersed in 1-propanol at levels above the mixture's percolation limit, 0.5, 1.0, 2.0, 4, 8, and 12 wt%, and tested in a rectangular fuel tray and a shallow pan flame spread system. The MWNTs/1-propanol mixtures had much lower flame spread rates than the original fuel itself at temperatures below 1-propanol's flash point. A 2 wt% MWNT loading reduced the flame spread rate to 9–15% of the neat fuel rate (the corresponding increase in the average flame spreading time over a 16 cm horizontal test length is approximately 6–11 times) and significantly inhibited the pulsating flame spread behavior. Flame spread rate was calculated for both the pure fuel and MWNT mixtures with a surface-tension-driven spread rate equation. The calculations favorably compare with corresponding measured values despite several assumptions made. Thermally conductive high aspect ratio nanoparticles could be effective flame spread suppressants for liquid fuels.

Effects of Fuel Volatility on Flame Spread along Fuel Droplet Arrays in Microgravity

Microgravity combustion experiments of fuel droplet arrays were conducted with different fuel volatilities. N-octane, n-decane and n-hexadecane were used to examine effects of volatility. Experimental results showed that the flame spread rate took a maximum value at a specific inter-droplet distance for all the fuels and increased with fuel volatility. Effects of inter-droplet distance on flame spread behavior at room temperature were discussed based on positional relation between the leading flame and an unburned droplet. Effects of fuel volatility were discussed based on the droplet heating time. Simple formulas to estimate flame spread rates were proposed considering rate-controlling processes. Flame spread behavior were explained based on the droplet heating process for small inter-droplet distance and on the droplet heating process and the development of the flammable mixture layer around the next droplet for large inter-droplet distance.