Flame Spread Rate Research Articles

Upward Flame Spread over Two Parallel Paper Sheets under Reduced Ambient Pressure and Elevated Oxygen Concentration

ABSTRACT The effects of separation distance and environmental parameters (such as ambient pressure and oxygen concentration) on the upward flame spread over two parallel paper sheets were investigated in this work, which would be conducive to enriching the fire dynamic theories and providing a reference for the risk evaluation in building and space fires. It was found that, with the increase of separation distance, the flame spread regimes can be roughly divided into four classes: the flame quenching regime (without inner flame), where the interaction of two parallel papers is mainly dominated by the solid surface radiation; the complete fusion regime, limited by the oxygen diffusion; the partial fusion regime, dominated by the convection; the separated flames regime, dominated by the gas-phase radiation. The flames inside the narrow space are fused under the flame-induced horizontal flow, and extinguished for the oxygen starvation, which reflects the interaction of two parallel samples. Interestingly, although the flames inside and outside the parallel sheets often have different flame lengths and burning intensities, they spread at almost the same rate, which is ascribed to the overlapping of the pyrolysis region on both sides of the thermal thin paper. A critical separation distance was found, where the flame spread rate is largest due to the stoichiometric mixture fraction. The orange-blue layered flames were analyzed, which is related to the boundary layer of the flow field and the mixture of fuel and fresh air. The oxygen concentration has nearly equivalent impacts on the flame-induced buoyancy and horizontal flows, and thus a constant flame fusion angle is maintained. Nevertheless, the enhanced ambient pressure mainly promotes the buoyancy and reduces the flame fusion angle. The flames inside the narrow space are apt to be extinguished under lower ambient pressure with weak buoyancy similar to the reduced gravity and microgravity environments. The transformation of the dominating regimes were observed at a moderate separation distance: the flame quenching regime at pressures below 18 kPa, the complete fusion regime at pressures between 18 ~ 29 kPa, and the partial fusion regime at pressures over 29 kPa.

Exploring the brachistochrone (shortest-time) path in fire spread

The brachistochrone (shortest-time) curve is the path connecting two points that enables the shortest travel time. This work explores the “brachistochrone path” of fire spread connecting two points at the same altitude and with a fixed path length. The starting and ending points are connected by both thermally thin fuels (thin wires) and thermally thick fuels (PMMA bars). Flame-spread paths of triangular, rectangular, and circular shapes with different heights and inclinations are explored. Results show that having a local maximum flame-spread rate does not result in the shortest overall travel time. For thin-wire paths, the fastest overall-path fire spread occurs, when the upward spread path is vertical, and the path height reaches a maximum, as demonstrated by the theoretical analysis. Differently, for thick PMMA-bar paths, the brachistochrone condition occurs when the path length of the vertical upward spread reaches the maximum, because the upward spread is about ten times faster than the downward spread. This study extends the conventional problem of the fastest fire spread to the shortest-time problem of the whole fire path, and it may help optimize the fuel distribution inside the built environment and estimate available safe egress time in building and wildland fires.

Experimental Study of the Effect of Slope on the Spread and Burning Characteristics of a Continuous Oil Spill Fire

Elucidating the characteristics of continuous oil spill fires for different slope conditions can provide important theoretical support for the prevention of, and rescue strategies during, oil spill fire accidents. For this research, we conducted experiments to observe the spread and burning process of continuous oil spill fires under different slope conditions. The changes in physical attributes, such as flame spread rate, burning rate, heat convection at the bottom surface, and flame feedback radiation, were analyzed for the different slope conditions. The results showed that the shrinking phase becomes difficult to see, and the steady phase disappears when the slope increases in the spread and burning process. When the slope increases, the spread speed and spread area increase, and burning rate decreases. Compared with a non-burning process, the resistance to spread decreases in the burning process. We show that the slope directly affects the spreading process, and indirectly affects the burning process.

Effect of sub-atmospheric pressure on the characteristics of concurrent/upward flame spread over a thin solid

The variation of ambient pressure is a potential tool for studying the driving parameters of fire dynamics and heat release in low-pressure environments such as high-altitude locations, aircraft, and spacecraft. The study of upward flame spread over a solid fuel has direct implications on material flammability and fire development, and low pressure environments have recently gained more attention for the possible comparison with the reduced gravity conditions encountered during space missions. In this work, we consider upward spreading flames over thin acrylic sheets in ambient pressures between 30 and 100 kPa. A forced flow velocity of 20 cm/s is added to the naturally-driven buoyant flow, creating a mixed flow field (natural and forced) that varies with pressure. Flame characteristics such as spread rate and standoff distance are measured from the video analysis of the experiments. It is observed that the former decreases with pressure while the latter increases. The larger flame stand-off distance at low pressures partially explains the decrease of the flame spread rate since the convective heat flux from the flame to the solid decreases. Additionally, volumetric concentrations of the combustion products are measured during the experiments. The results show lower O2 consumption and CO2 production rates at lower pressures. Based on these rates, we could calculate the heat release rate from upward spreading flames at low pressure, providing fundamental information for better understanding pressure-gravity correlations. According to the results, the volumetric heat release rate is proportional to pressure, which is consistent with previous studies on pressure modeling of fires. This suggests that chemical kinetics is not a constraint for the conditions tested in this study, which could help make future flammability tests comparable to low gravity ones.

Flexible Polyurethane Foam에서 화염확산에 대한 발화위치와 샘플 두께의 영향

Fire behavior of flexible polyurethane foam (FPUF) at different sample thicknesses and ignition positioning was investigated. Effects on flame height, mass loss rate and other parameters were tested, and the flame propagation mechanism was analyzed. A method for predicting equivalent combustion diameter (D) values of the dynamic change of liquid pool at different positions is proposed. Combined with data of sample mass loss rate, flame height can be predicted. Based on a transition state model, a method for predicting the fire risk of FPUF in late stage combustion by calculating the generation time of polyols is proposed. With edge ignition, FPUF burning produces an inclined surface during the combustion process which enhances the length of the preheating zone by means of heat conduction and heat radiation. Flame spread rate (FSV) in FPUF with edge ignition was greater than with center point ignition.

CFD Simulations of Fire Propagation in Horizontal Cable Trays Using a Pyrolysis Model with Stochastically Determined Geometry

In this paper, a pyrolysis model for a PVC cable is constructed using results from thermogravimetric analysis, microscale combustion calorimeter and cone calorimeter experiments. The pyrolysis model is used to simulate fire propagation in horizontal cable trays. The simulated arrangement corresponds to a cable tray fire experiment from OECD PRISME 2 project. As laying the cables loosely along the horizontal trays is a random process, a novel stochastic method is developed for making the simplified cable tray geometries for the computational fluid dynamics model. In addition, as the simplified cable tray geometry has significantly smaller surface area than a real tray full of cables, the surface area was parametrically adjusted. In contrast to most of the earlier published numerical approaches for simulating cable tray fires, the presented approach does not use empirical correlations for predicting fire propagation and does not require any results from full-scale experiments for calibrating the model. The simulation results are compared to experimental results in terms of heat release rate, mass loss, tray ignition times and lateral flame spread rates. The maximum heat release rate was overpredicted by 8% on average.

Experimental study on downward/opposed flame spread and extinction over electric wires in partial gravity environments

Downward/opposed flame spread over laboratory wire samples under varied gravity conditions were investigated in the range from 0 G to 1 G. Reduced gravity experiments are conducted by parabolic flights of an airplane. Limiting oxygen concentrations (LOCs) and flame spread rates (Vf) are obtained as a function of gravity level, with oxygen concentration, forced flow velocity, and wire characteristics such as insulation thickness and core material as experimental variables. The samples used in this study consist of low-density polyethylene (LDPE) insulation over metallic cores. Copper (Cu) and nickel-chrome (NiCr) were selected as core materials. It is found that the effect of gravity on the insulation flammability varies with the thermal conductivity of the wire core; the LOCs of the Cu sample are less affected by gravity, while those of the NiCr sample decrease with decreasing gravity level. On the other hand, Vf increase monotonically with increasing gravity level in the Cu sample, while Vf of the NiCr sample show a peak value under the low gravity conditions. It is suggested that these differences in the response of LOCs and Vf to the gravity level due to the difference in core materials are controlled by the fuel concentration in the reaction zone, which is a function of Vf. It is also found that the molten LDPE produced during the flame spread process shows unique behaviors depending on the gravity levels and wire characteristics. Some characteristics of the dynamic motion of the molten LDPE during the flame spread process, such as deformation and dripping, are also summarized in this paper. The experimental data obtained in this study provide useful information on the flammability of materials in a partial gravity environment and will serve as a database for fire safety design in future space exploration.

Effect of immersion time on the combustion characteristics of oil‐impregnated transformer insulating paperboard

AbstractTransformer insulating paperboard is an insulating material commonly used in the oil‐filled equipment in substations, which is usually immersed in the transformer oil. In this study, the effects of immersion time on the combustion characteristics of oil‐impregnated transformer insulating paperboard are studied using the thermogravimetric, UL94 vertical flammability, and cone calorimeter experiments. The experimental results show that the total oil content increases with increasing immersion time and reaches the saturation state in 240 h, with a saturation content of 14.7% ± 0.5%. However, the distribution of oil content inside the paperboard is uneven. At the saturation state, the surface oil content is 35.5%, being about 3 times of the core oil content. The results of UL94 vertical flammability experiments show that the flame spread rate increases with increasing immersion time and is controlled by the total oil content. In the cone calorimeter experiments, the time to ignition is significantly reduced compared to the clean paperboard. Unlike the flame spread rate, the ignition time is controlled by the surface oil content and barely varies with immersion time. The critical heat flux of oil‐impregnated transformer insulating paperboard is found as 3.9 kW/m2, being only 1/3 of clean paperboard. With increasing immersion time, the peak heat release rate increases, while the total heat release, fire growth index, and effective heat of combustion present linear correlations with the total oil content. The CO yield increases with increasing immersion time, and the released CO mainly comes from the combustion of transformer oil.

The fire hazard potential of thermally thin cellulose nonwovens with different overlap configurations

The fire hazard potential of nonwovens with different overlap configurations and various thicknesses between 0.5 and 5 mm were investigated to clarify the possible worsening situation induced by the difference in overlapping configurations. The fire hazard of thermally thin nonwovens is characterized by flame spread, flame height, temperature distribution, and mass loss. The flame spread rate and FDI (fire development index) increase and then decrease as the thickness of the nonwovens increases. The fire hazard potential of samples with OL configuration (overlapping layer by layer configuration) is higher than that with AO configuration (accordion-style overlap configuration), which is reflected in the high FDI, average fire spread rate, and flame height, and low ignition time. The difference between the fire risk of the two overlapping configurations was confirmed by analyzing the heat transfer mechanism. Due to the different packaging methods, the AO and OL configurations of nonwovens show different degrees of shrinkage and curling effects during combustion. The large air gap between the layers and the weight of the sample edge are the reasons for the weak contraction of the sample with an AO configuration. The sample shrinkage with OL configuration strengthens the flame spread to a certain extent by increasing the convective heat transfer in the preheat zone.

Impact of ventilation on the fire dynamics of an open‐plan compartment with exposed timber ceiling and columns: CodeRed #02

AbstractThe desire by developers and architects to build mass timber buildings using cross laminated timber (CLT) and glulam has significantly increased globally in the last decade due to its benefits with regards to sustainability as well as other architectural and commercial drivers. This paper presents novel experimental evidence from CodeRed #02, the second in a series of large scale fire experiments carried out inside a purpose‐built, open‐plan compartment to capture fire dynamics in large compartments with exposed timber. The experiment used a continuous wood crib (6 × 29 m) as a controlled movable fuel load. The aim of the CodeRed #02 experiment was to study the impact of reduced ventilation on fire dynamics by keeping all other parameters the same as CodeRed #01 (Kotsovinos et al., 2022), with the exception of ventilation area which was reduced by almost half. The reduction in ventilation was found to significantly impact the fire dynamics by slowing the fire spread and burning rate. The reduced ventilation led to an increased fire duration by 4 min 30 s, which corresponds to 20% longer duration compared to CodeRed #01. The reduced ventilation had a greater overall impact on the rate of flame spread across the CLT (−23%) than the crib (−8%) compared to CodeRed #01. While the maximum temperature and incident heat fluxes inside the compartment were approximately the same as in CodeRed #01, their evolution in time and space were significantly different. The external flames were higher than in CodeRed #01 (3–3.5 m compared to 2.5–3 m) and protruded further laterally (up to ~4–5 m compared to ~1 m) outside of the compartment from the large end openings. Unlike CodeRed #01, the external flaming from CodeRed #02 pulsated between visible flames and dark soot with significantly greater frequency. This was caused by the limited ventilation resulting in incomplete combustion in CodeRed #02. The peak heat release rate of CodeRed #02 was estimated to be 16.5% lower than CodeRed #01, despite having the same fuel load. The average char depth in the ceiling, as measured at the end of the experiment, was 28 mm. This was 11% greater than in CodeRed #01 and was likely due to the increased duration of the fire. After the extinction of the flaming, smouldering occurred for CodeRed #02 as it did for CodeRed #01. One of the glulam columns lost its restraint and fell to the ground because smouldering spread through the thickness of the member. The findings from this work can help engineers quantify the hazard and therefore identify appropriate solutions for the fire safety design of exposed mass timber buildings.

Fabrication of talc reinforced transparent fire-retardant coating towards excellent fire protection, antibacterial, mechanical and anti-ageing properties

To enhance the fire safety of historic buildings, developing fire-retardant coatings with integrated decoration, fire protection, ageing resistance and antibacterial performances is imperative. Here, a novel multifunctional transparent fire-retardant coating with fire protection, smoke suppression, antibacterial and anti-ageing functions was successfully fabricated using talc as a multifunctional synergist. Mechanical tests show that the multifunctional coating containing talc shows an improvement in pencil hardness and adhesion classification while keeping a high transparency. Moreover, adding talc is conductive to fabricate a physical barrier with super compactness and intumescence, which endows the coatings with excellent fire protection and smoke suppression functions. Especially, when the mass ratio of PPB to talc is 95:5, the obtained MTPPB3 coating retains a high-level transparency of 85.8%, accompanying with a 72.6% reduction in flame spread rating and a 57.0% reduction in smoke density rating compared to MTPPB0 without talc. TG analysis shows that the multifunctional coatings containing talc exhibit sharp enhancements in thermal stability and char yield, and MTPPB3 retains 32.7% residue at 800 °C. Meanwhile, the addition of talc results in high antibacterial rate against E. coli bacteria and durable flame-retardant and smoke suppression effects, and MTPPB3 shows an antibacterial rate of 99.0% against E. coli bacteria. This work provides a promising way to fabricate multifunctional transparent fire-retarded coatings with outstanding functions of decoration and fire protection.

Influence of air gap ratio of the Chinese historical wooden window on the vertical flame spread performance

The fire protection of historical buildings with wooden structures is a topic that has attracted much attention over the last few years. As one of the critical structures of ancient buildings, wooden windows have high fire hazards. This study aims to clarify the influences of air gap ratio on the fire propagation mechanism of wooden windows by experimental investigating flame characteristics such as mass loss rate (MLR), flame spread rate (FSR), total burning duration (TBD), and maximum flame length (MFL), and propose corresponding fire protection strategies. In the experiment, the typical wooden window structure in ancient buildings was selected as the research object. The wooden windows with different porosity were installed on the vertical sample rack to carry out the combustion experiment, and the area air gap ratio ranged from 32.9% to 64.2%. The experimental results show a clear relationship between MLR and the area air gap ratio. TBD, FSR and MFL are related to the vertical linear air gap ratio of the wooden window structure. FSR first increases and then decreases with the linear air gap ratio, and the peak value of FSR appears on the sample with a vertical linear air gap ratio of 69.6%.

Facile fabrication of multifunctional transparent fire-retardant coatings with excellent fire resistance, antibacterial and anti-aging properties

A series of organophosphate-grafted nano-TiO2 was synthesized and then applied in amino resin to obtain multifunctional transparent fire-retardant coatings with superior fire resistance, antibacterial and anti-aging properties. Transparency analysis reveals that the addition of nano-TiO2 slightly decreases the light transmittance of the resulting MTiPPBs coatings, which is still remained a high level of transparency. The results of fire tests show that the presence of nano-TiO2 exhibits well synergistic effect on strengthening the smoke suppression property and fire resistance of the MTiPPBs coatings, which is ascribed to the formation of more P-containing cross-linked and aromatic structures in the condensed phase that improve the barrier effect of intumescent char against fire, as supported by scanning electron microscope (SEM) and energy dispersion spectrum (EDS) analyses. When the weight ratio of PPB and nano-TiO2 is 99.7:0.3, the obtained MTiPPB3 coating shows a 69.5% decrement in smoke density rating and an 83.3% decrement in flame spread rating in comparison to MTiPPB0 coating without nano-TiO2. The accelerated aging test reveals that the presence of nano-TiO2 can minimize the blistering, powdering, and cracking phenomena of the multifunctional coatings, thus displaying durable fire-resistant and smoke suppression effects. Meanwhile, the addition of nano-TiO2 greatly enhances the antibacterial activity of the multifunctional coatings, and MTiPPB3 coating obtains a 45.6% improvement in antibacterial activity index against Escherichia coli compared to MTiPPB0 coating without nano-TiO2.

Experimental study of the effect of unheated starting segment upon flame spread over solid fuel under forced airflow

In the present work, to quantify the effects of an unheated starting length on the wind-driven flame spread rates, the flow field structure and flame characteristics over a horizontally placed thermally-thick poly (methyl methacrylate) slab (10 cm long, 10 cm wide, and 1.5 cm thick) are experimentally investigated for different lengths of an unheated segment (0–30 cm), where PMMA flames are stabilized under a concurrent forced airflow velocity of 0.75 m/s. The flow field, flame structure, flame spread rates, and flame standoff distances are recorded, measured, and calculated. The results show that the flow field in a reacting boundary layer is noticeably different for varying unheated lengths, which can be attributed to the reverse flow diffusion and vortex occurrence, which significantly affects the flame structure and flame spread rates. The laminar, reverse diffusion, and streaky flame regions can be easily demarcated with increasing unheated starting lengths. Eventually, the flame standoff distance, burning, and flame spread rates seem to have a non-monotonic dependence on unheated starting lengths. The work presented here reveals the functional mechanism of an unheated segment in reacting boundary layers and presents useful information that could enhance our understanding of the wind-driven flame spread rates.

Downward Flame Spread Rate Over PMMA Rods Under External Radiant Heating

Downward Flame Spread Rate Over PMMA Rods Under External Radiant Heating

Fabrication of Polypyrrole-Decorated Tungsten Tailing Particles for Reinforcing Flame Retardancy and Ageing Resistance of Intumescent Fire-Resistant Coatings.

Polypyrrole-decorated tungsten tailing particles (PPY-TTF) were prepared via the in situ polymerization of pyrrole in the presence of tungsten tailing particles (TTF), and then carefully characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and thermogravimetric analysis (TG) analyses. The effect of PPY-TTF on the flame retardancy, smoke suppression property and ageing resistance of intumescent fire-resistant coatings was investigated by a fire protection test, smoke density test and cone calorimeter test. The results show that PPY-TTF exerts excellent cooperative effect on enhancing the flame retardancy and smoke suppression properties of the intumescent fire-retardant coatings, which is ascribed to the formation of more cross-linking structures in the condense phase that enhance the compactness and thermal stability of intumescent char. The cooperative effect of PPY-TTF in the coatings depends on its content, and the coating containing 3 wt% PPY-TTF exhibits the best cooperative effect among the samples, showing a 10.7% reduction in mass loss and 35.4% reduction in flame-spread rating compared to that with 3% TTF. The accelerated ageing test shows that the presence of PPY-TTF greatly slows down the blistering and powdering phenomenon of the coatings, thus endowing the coating with the super durability of fire resistance and smoke suppression property. This work provides a new strategy for the resource utilization of tungsten tailing in the field of flame-retardant materials.

Surface inclination effects on heat transfer during flame spread acceleration based on FireFOAM

To study the influence of inclination on flame spread on solid surfaces and improve the accuracy of the theoretical model to predict the flame spread rate through the detailed heat flux distribution in the solid surface, a set of numerical simulations of fire spread on corrugated cardboard surface at different angles (0° ≤ θ ≤ 30°) is carried out. The simulations are performed using FireFOAM, a large eddy simulation solver with a “wall-resolved” approach. Great attention is paid to the explanation for the acceleration of fire spread and the influence of angle on flame heat transfer. Less attention is paid to the numerical models except for the treatment of turbulence. The simulation results are in good agreement with the experimental data qualitatively and quantitatively. The results show that the change of flow field structure, especially the vortex leads to the flame attachment. Moreover, the flame attachment length increases with angle; especially at θ ≥ 20°, it also increases with time. Based on the analysis of the influence of angle on the heat flux distribution in the preheating zone, a modified fire spread model considering the surface temperature is established, which effectively improves the accuracy of predicting the fire spread rate at large inclinations.

Experimental Study of the Fire Dynamics in a Semi-enclosure Formed by Photovoltaic (PV) Installations on Flat Roof Constructions

Flame spread experiments upon a BROOF(t4) compliant flat roof mock-up located below a vertical barrier were carried out for variations in gap height, inclination, subjacent insulation material, and the barrier type (stainless-steel board or photovoltaic (PV) module). A binary flame spread scenario was identified, where re-radiation from the flame facilitated self-sustained flame spread if the gap height to the horizontal panel was below 10 cm for the stainless-steel board and 11 cm for PV modules. These were defined as the critical gap heights. Inclination of the PV modules increased the critical gap height and caused a 25% faster flame spread rate (FSR) than the FSR below horizontal modules with the same gap height at the location of ignition. The faster FSR for inclined modules caused a 40% reduction of the maximum temperature measured at a depth of 70 mm in the insulation materials (242°C). Based on temperatures measured in the insulation materials, the 60 mm polyisocyanurate (PIR) insulation performed slightly better than the 50 mm mineral wool insulation. However, it is expected that the mineral wool would outperform the PIR insulation if tested with the same thickness, as it insulates significantly better at high temperatures. Finally, no sustained flame spread was observed on the back side polymer sheet of the PV modules, but one of the three PV module brands produced burning droplets. Based on the experiments, it can be concluded that the current standards are inadequate as the introduction of a PV system on a compliant roof construction enables flame spread.

The controlling mechanisms of horizontal flame spread over thick rods in upward cross flow

This work studies the flame spread over horizontal thick PMMA (polymethyl methacrylate) rods with three radii under different oxygen concentrations and upward cross flow velocities. The flame spread rate and the limit oxygen concentration are measured. Far away from the extinction limit, the flame spread rate increases with the flow velocity but decreases with radius. Near the extinction limit, the flame spread rate is insensitive to the flow velocity and radius. The flame spread rate can be correlated by the stretch rate, and it is found that the flame spread rates for different radii are close at the same stretch rate. A scaling analysis shows that the flame spread rates are approximately square-root dependent on the stretch rate. Prior to the extinction, the flame has entered the regressive burning regime where the flame leading edge will not spread forwardly but continuously retreat. The flame extinction is dependent on the local stretch rate and in-depth heat conduction. For a given radius, the limit oxygen concentration increases with the stretch rate. For a given stretch rate, the smaller cylinder can sustain at lower oxygen concentration due to the less solid-phase heat loss.

Data driven forecast of concurrent flame spread in micro-gravity

This paper presents a methodology that combines data assimilation and physical modelling of flame spread for fire growth forecast. The concurrent flame spreading over a flat solid sample in the absence of buoyant flows is considered as a simple canonical fire spread configuration. To predict flame spread rate and flame length evolution, the analytical solution to the two-dimensional boundary layer non-premixed combustion problem is combined with a CFD model which delivers the structure of the flow away from the fuel surface. In the process, two coefficients which intervene in the gas and solid heat transfer equations are assimilated using the pyrolysis length data as a flame spread rate surrogate input to absorb the shortcomings of the modelling. The robustness of the overall method is evaluated through convergence assessment of these two assimilated variables for different initial guesses, and for different values of the input invariants. Validation over large-scale microgravity data provides confidence in this approach, and demonstrates its potential to deliver flame spread predictions from initial measurements at a reduced computational cost. However, discrepancies between flame length measurements and predictions question the degree of correlation between pyrolysis and flame lengths.