Flame Puffing Research Articles

Flame characteristics of backward-inclined pulsating combusting jet in crossflow

The flame appearances, thermal characteristics, and combustion product concentrations of a backward-inclined pulsating combusting jet in crossflow were experimentally studied. The time-averaged and streak instantaneous flame images were acquired using a still camera and a high-speed camera, respectively. The flame extinguishment processes were captured and analysed on the basis of streak instantaneous images. The flame temperatures and combustion product concentrations were measured using a homemade fine-wire thermocouple and a gas analyser, respectively. Three characteristic flame modes (crossflow dominated, transitional, and jet-dominated) were found when the backward-inclined jet flame in crossflow was subject to acoustic excitation. These characteristic flame modes were observed in different regimes of pulsation intensity and jet backward inclination angle. Discrete flame puffs with a frequency equal to the acoustic excitation frequency appeared in the jet-dominated flame. Periodic flame puffing became unstable when the pulsation intensity was increased to values near the instability limit. When the pulsation intensity was beyond the instability limit, a strong stretching effect caused the recirculation flame, which played the role of a flame ignitor, to almost vanish (misfire). The flame extinguishment then occurred from the area around the lee side of the burner tube tip. The temperature and gas concentration measurement results showed the significant improvement of combustion performance in low inclination angle even at small pulsation intensity in the regime of the crossflow dominated flame. To enhance combustion performance at a large jet backward inclination angle, the pulsation intensity must be large enough to push the flame in the regime of the jet-dominated flame.

Assessment of subfilter-scale turbulence-radiation interaction in non-luminous pool fires

The objective of this work is to investigate the effects of Turbulence-Radiation Interaction (TRI) in Large Eddy Simulation (LES) of ethanol pool fires ranging from medium to large sizes. LES of three pool fires are performed using a steady laminar flamelet (SLF)/presumed Filtered Density Function (FDF) combustion model and the Rank Correlated Full Spectrum k-distribution (RC FSK) for spectral gas radiation. Subfilter-scale (SGS) absorption TRI is neglected whereas the filtered emission term is modelled from the presumed FDF approach. The baseline case is the 0.5 m diameter pool fire investigated experimentally by Fischer et al. (Combust. Flame 70 (1987) 291–306). The two other synthetic pool fires were generated by scaling the pool diameter by factors of 2 and 4 while maintaining the fuel mass burning rate per unit area unchanged. Predictions in terms of temperature, mole fractions of radiating species, flame puffing frequency and radiant fraction are in good agreement with the available experimental data. Model results show that TRI accounts for at least 80% of the radiative loss. For LES that resolves more than 80% of the temperature variance, the SFS TRI contributes for a non-negligible part to radiative loss and its contribution increases significantly with the pool size. Neglecting it affects significantly the predictions of the fire plume structure and radiative heat flux even for medium-scale pool fires.

Effects of radiation models on steady and flickering laminar non-premixed flames

Four laminar non-premixed methane/air flames with increasing Reynolds numbers are investigated using a newly developed conjugate combustion-radiation laminar flame solver. The baseline model includes a forward Monte Carlo ray tracing solver coupled with a line-by-line spectral model, a 16-species skeletal chemical mechanism and no soot model. A backward Monte Carlo solver is developed for the nongray inhomogeneous combustion mixtures and verified using the forward Monte Carlo solver. A 25-species skeletal chemical mechanism is adopted for parametric studies. Effects of soot radiation on unsteady flames are investigated using a two-equation soot model. Good agreement with experiments in radiative flux and radiant fraction is observed for all flames using the baseline model. The radiative heat flux predicted by the backward Monte Carlo solver agrees well with that obtained from the forward Monte Carlo solver, with well-controlled standard deviations. Contours of radiation-related scalars indicate that the laminar flames are within the optically-thin limit. Results obtained from conjugate combustion-radiation simulations with various radiation solvers and spectral models show minor differences for the two smaller flames. For the two unsteady flames, radiative re-absorption has a more significant impact on the overall flame structure, especially when soot is considered. Parametric studies using the longest flickering flame suggest that the choice of chemical mechanism, the consideration of radiation, and the inclusion of soot radiation, affect the prediction of the instantaneous flame height. The prediction of flame puffing frequency is insensitive to the variations in chemistry, soot and radiation models. The radiative heat flux prediction is sensitive to the thermal boundary condition of the fuel nozzle, the inclusion of soot and its radiation with re-absorption. It is recommended that soot should be considered for future studies when assessing radiation models using this set of target flames.

Towards the prediction of the convective and the radiative heat fluxes in turbulent buoyant flames

The present paper aims at describing the flow dynamics of turbulent buoyant flames encountered in gaseous or liquid pool fires or horizontally oriented solid materials. Indeed, the predictive simulations of liquid pools or solid fuel fires requires the prediction of the thermal stress received by the evaporating liquid or the pyrolysing material, and therefore a fine description of the surrounding flame. Yet, reactive Large Eddy Simulation is a powerful tool in fluid mechanics to correctly simulate turbulent flows such as large-scale buoyant flames, so long as the discretization and the numerical schemes allow to resolve the large scale turbulent structures which determine the global flame shape.Thus, this study is divided into two steps. In the first step, Large Eddy Simulations of the SANDIA FLAME methane pool fire are performed using the CALIF3S-ISIS software, for different grid refinements and convective schemes. The ability of the different numerical choices in correctly describing this flame is compared in terms of velocity variance-to-average ratio, puffing frequency and turbulent structures resolution. In the second step, the flame above a 40×40 cm2 PMMA slab is simulated with the best numerical parameters found in the first step. The simulation of the flame is decoupled from pyrolysis aspects assuming by imposing the fuel mass flow rate observed at a particular stage of the experiment. The average temperature axial profile and the transverse profile of convective, radiative and total heat flux at the sample surface are compared to their experimental counterparts.It is shown that, at least for large scale pool fires such as the SANDIA methane flame, the increasing the mesh refinement permits to reach a good balance between the average velocity and the turbulent kinetic energy and to correctly recover the characteristic flame puffing frequency, whereas using non-dissipative centred schemes allows to catch a wider range of turbulent scales in the flowfield. Unfortunately, for smaller fuel sources such as the 40×40 cm2 PMMA slab, the present Large Eddy Simulations do not exhibit fully developed turbulence in most of the combustion zone, in spite of the use of centred schemes and of a fine grid. This uncertainty on the behaviour of the turbulence in such a configuration may be responsible for the observed anomalies, namely the overprediction on the average temperature and the radiative-to-convective heat flux ratio. Further experimental investigations would be useful to better understand the behaviour of turbulence on such mid-scale fires and prescribe relevant numerical model modifications.

Experimental study on thermal safety analysis of flexible polyurethane at various facade inclined structures under low ambient pressure condition

Experimental and theoretical investigations are conducted to analyze the influences of low atmospheric pressure on burning behavior of building facade insulation material flexible polyurethane (FPU). Comparison experiments for downward flame spread process over FPU under various facade inclined structures were performed at Hefei (99.8 kPa) and Lhasa (66.5 kPa), respectively. Characteristic parameters including burning rate (i.e. mass loss rate), flame spread velocity and flame height were studied in this paper. Firstly, morphological experimental results exhibited that the combustion was slower in reduced pressure condition and wider specimens showed more gradual flame spreading. Secondly, an inverted V-shaped spreading pyrolysis front was observed, and a “two-terminal” combustion behavior was found for narrow FPU board by partial molten flow combustion, which is critical to practical fire rescue techniques. Thirdly, power-law progressions of burning rate vs. pressure was proposed with an exponent ranging from 0.61 to 1.39, which can be illustrated by utilizing classical theory for sub-atmospheric pressure pool fire. Further, the increasing facade inclination angle significantly enhanced the flame spreading velocity, which suppressed the effect of pressure at high inclination angles. The flame temperature was measured to be increased at the lower pressure causing faster flame puffing. Finally, a linear relationship was determined between the pressure index values obtained by flame height vs. pressure and the inclination angle.

An Experimental Study on Flame Puffing of a Swirl Partially Premixed Combustion under Varying Mass Flow Rate of Primary Air

It is of practical significance to understand the flame puffing behavior under varying mass flow rate of primary air ṁpri. An experiment was conducted to study the impact of ṁpri on flame puffing in a swirl partially premixed combustor, the puffing behavior of six significant flame properties was examined. The results showed that almost every spectrum had two fundamental frequencies, which is different from the single-peak spectrum of non-swirl flame. The flame heat-release rate, flame area, and flame equivalent width had identical dominant frequency and sub-dominant frequency, both decreased with the increasing of ṁpri. It was attributed to the decreased overall flame temperature caused by the improved mixing of fuel and primary air. All measured frequencies were in the range of 3–14 Hz, but the predicted frequencies from the theoretical models based on non-swirl flame were larger than the measured. This indicates the puffing frequency of swirl flame was much more sensitive to the variation in ṁpri than the frequency of non-swirl flame. Moreover, the amplitude of flame length was the smallest in all properties, with the most weakened oscillating intensity. While the amplitude of the flame area and flame equivalent width were the largest, with the strongest oscillation level. Consequently, the flame puffing is mainly attributed to the oscillation in width direction.

Evolution of pool fire plume characteristics during the depressurization process of an aircraft cargo compartment

This article presents an experimental investigation on the pool fire plume characteristics in a full-scale depressurized aircraft cargo compartment. The effects of decreasing pressure and vent flow rate on the fire characteristics such as flame shape, flame puffing, flame height, and centerline temperature were analyzed. The results show that during the depressurization process, the ventilation had an activation effect on the mass loss rate, and its increment had a linear relationship with the dimensionless ventilation factor. In addition, the larger depressurized rate caused the larger dimensionless ventilation factor and further resulted in the larger increment of mass loss rate. The flame puffing frequency was determined by the ratio of the gas density in the flame area of that in the ambient air, which increased with the drop of pressure. For flame centerline temperature, there was a counteraction area in the flame intermittent region, where the centerline temperature had almost no difference before and after the depressurization. The conclusions could provide the theoretical base and reference materials for the fire disaster in the cargo compartment of real aircrafts.

Effect of pressure on the heat transfer and flame characteristics of small-scale ethanol pool fires

Fire research with regard to high pressure is important for safety design and fire prevention under a high-pressure environment, such as nuclear containment shells during a pressurized period and deep-sea submersibles. To investigate the effect of high pressure on fire behavior, studies on ethanol pool fires using 1.5-cm- and 2.0-cm-diameter glass vessels were conducted under a wide pressure range of 1–5 atm. Results show that high pressure significantly impacts the heat transfer and flame characteristics of small-scale ethanol pool fires. As pressure increases, the fuel burning rate decreases slightly and then increases considerably, which is attributed to the effect of pressure on heat transfers. As pressure increases, the flame becomes unstable with the color changing from blue to yellow. Moreover, the flame height of the 1.5-cm-diameter pool fire increases over the entire pressure range whereas the flame height of the 2.0-cm-diameter pool fire increases until its peak value is reached at 2 atm and then decreases gradually. Then, a global correlation of flame height for small-scale ethanol pool fires is analyzed and verified using the experimental results. Flame puffing frequency increases with pressure for both pool sizes; hence, the Strouhal number correlates well with the Froude number.

Thermal and fire risk analysis of low pressure on building energy conservation material flexible polyurethane with various inclined facade constructions

This study investigated the factors affecting the fire safety analysis of the common construction material flexible polyurethane foam, based on combustion at various inclination angles and the ambient pressures at the altitudes of the cities of Hefei (99.8 kPa) and Lhasa (66.5 kPa). The effects of ambient air pressure, specimen width and inclination angle on the burning rate (as determined by mass loss), average flame spreading velocity, flame temperature and flame length were examined. The combustion rate was lower at 66.5 kPa and the wider specimens exhibited gradual spreading of the flame front in conjunction with melting and dripping. The burning rates at various inclinations were found to correlate with the pressure according to the expression ṁ∝pn,0.67<n<1.36, and this result can be explained based on pool fire theory. The average flame spreading velocity was found to correlate with pressure at various inclinations according to the relationship Va∝pn,2/3<n<1. The flame temperature was also observed to increase somewhat at the lower pressure, leading to an increase in the flame puffing frequency. Finally, a power law was used to describe the relationship between flame height and pressure, while a linear relationship was obtained between the index and the angle of inclination.

The burning behaviors of pool fire flames under low pressure

SummaryTo investigate the influence of low pressure on pool fire behaviors, liquid n‐heptane pool fires with the diameters of 6, 8, and 10 cm were tested in a low‐pressure altitude chamber. The tests were conducted under different static chamber pressures, ranging from 40 to 60 to 80 to 101 kPa. Fuel mass loss and high‐speed flame video were recorded during the whole burning process of each test. The mass burning intensity is determined by the convective and radiative heat feedback to the fuel, and analysis shows that the convective part is the major contributor to fuel evaporation. Video recordings show that flame height increases with the reduction of pressure, as supported by the dimensionless analysis. The flame puffing amplitude increases under low pressure, resulting in a portion of the flame being quenched. A special phenomenon was observed from the flame videos in that flame rotation emerged at the ending burning stage, which was attributed to the Coriolis force effect. Copyright © 2014 John Wiley &amp; Sons, Ltd.

The effect of azeotropism on combustion characteristics of blended fuel pool fire

The effect of azeotropism on combustion characteristics of blended fuel pool fire was experimentally studied in an open fire test space of State Key Laboratory of Fire Science. A 30cm×30cm square pool filled with n-heptane and ethanol blended fuel was employed. Flame images, burning rate and temperature distribution were collected and recorded in the whole combustion process. Results show that azeotropism obviously dominates the combustion behavior of n-heptane/ethanol blended fuel pool fire. The combustion process after ignition exhibits four typical stages: initial development, azeotropic burning, single-component burning and decay stage. Azeotropism appears when temperature of fuel surface reaches azeotropic point and blended fuel burns at azeotropic ratio. Compared with individual pure fuel, the effect of azeotropism on main fire parameters, such as flame height, burning rate, flame puffing frequency and centerline temperature were analyzed. Burning rate and centerline temperature of blended fuel are higher than that of individual pure fuel respectively at azeotropic burning stage, and flame puffing frequency follows the empirical formula between Strouhal and Froude number for pure fuel.

Periodic puffing instabilities of buoyant large-scale pool fires in a confined compartment

This article presents an experimental study of the pulsating period of pool fires in a confined and ventilated compartment. Based on large-scale pool fire experiments, two modes of flame pulsation are identified. The first one is the common mode of flame puffing observed in an open atmosphere, and the second one is a perturbed pulsation mode induced by a hot vitiated environment. A description of these modes and their spectral analysis based on image processing are described. The second new mode differs from the common mode by a lower puffing frequency of the reacting gas phase. The effect of the pool diameter is analysed for the two modes.

Effects of low air pressure on radiation-controlled rectangular ethanol and n-heptane pool fires

Experiments on rectangular ethanol and n-heptane pool fires were conducted at different altitudes in Hefei (99.8kPa) and Lhasa (66.5kPa). The burners tested had the same fuel area of 900cm2, but with aspect ratio of long side to short side (n=l/w) varied from 1 and 8. The individual and combined influences of low pressure and aspect ratio on burning rate, temperature, puffing frequency, flame height and radiation for the two fuels were interpreted and formulated. First, burning rate was found to be proportional to ambient air pressure under radiation control, the main reason is that radiative heat flux decreased with pressure due to the pressure affecting the soot absorption coefficient. Flame temperature slightly increased, leading to higher flame puffing frequency at low pressure. Flame height was almost insensitive to pressure as H∝p0. Second, for aspect ratio n, flame temperature was constant and independent of fuel type and burner shape. With increasing n, burner wall temperature increased at the long side, and decreased drastically at the short side, especially n=8. This was attributed to the change of flame tilt and heating of the burner side, caused by variation of entrainment motion. Flame puffing frequency was found to increase with n as a function of f∼(ΔT/T∞)(n+1)/2n. The flame was observed to split into small clusters by enhanced asymmetric entrainment, and H decreased with increasing n as H∼(1/n)2/5Q˙4/15. Considering fuel differences, with increasing n, the burning rate of the ethanol pool fire decreased, and n-heptane showed the opposite trend.

Influence of low air pressure on combustion characteristics and flame pulsation frequency of pool fires

To investigate the effect of low air pressure on the combustion characteristics and puffing flame frequency of pool fires, ethanol and n-heptane pool fires were performed using 15 square burners of various size in both Lhasa (altitude: 3650m; air pressure: 65kPa) and Hefei (altitude: 24m, air pressure: 100.8kPa) fire laboratories. Comparison of the experimental results for pool fires of the same size in the two places shows that, firstly, the maximum rise in the centerline temperature in flames in Lhasa is generally larger than that in Hefei. Secondly, the dependence of the burning rate exponent n (m˙″∝pn) on the air pressure varies with the equivalent diameter D of the burner, with n<0 (D<7cm), (0–1) (7cm<D<10cm), (1–1.45) (10cm<D<19cm) and 1 (D>19cm). Thirdly, radiation fraction of the pool fire flames is smaller at low air pressure. Finally, the puffing frequency of the pool fire flames is higher at low air pressure. Compared with Hefei, in Lhasa, for the same size burner, a higher maximum temperature together with less radiation from the flame is observed. This shows that in low air pressure, the pool fire is more buoyant, which leads to stronger periodic oscillation and a higher flame puffing frequency.

A Review of Research and an Experimental Study on the Pulsation of Buoyant Diffusion Flames and Pool Fires

This paper reviews the past research, experimental techniques and scaling relationships used in the studies of oscillatory buoyant diffusion flames and reports an experimental investigation conducted to determine the pulsating characteristics of such flames. The experimental data were obtained by using three techniques, namely, pressure fluctuation measurements, thermal imaging and high-speed video photography. Present findings are compared with data sets reported in the literature and correlations for pulsation frequency suggested by previous studies are independently verified. Analysis of the experimental data on frequency of pulsations in different burners shows that for a fixed-diameter flame the pulsation frequency is almost independent of fuel flow rate. The equation f=1.68D-0.5 gives the best approximation for the relationship between pulsating frequency and diameter over a wide range of data. An alternative way of expressing the relationship between the key variables is St=0.52*(1/Fr)0.505. This proves to be a better way of expressing the relationship since it can include the effect of the fuel flow rate. Slight modifications to this expression allows prediction of flame oscillations under elevated/reduced gravity and isothermal buoyant plumes. This relationship and the observations of the present study confirm the hydrodynamic nature of flame puffing: interplay of buoyancy and fluid motion.

A Review of Research and an Experimental Study on the Pulsation of Buoyant Diffusion Flames and Pool Fires

This paper reviews the past research, experimental techniques and scaling relationships used in the studies of oscillatory buoyant diffusion flames and reports an experimental investigation conducted to determine the pulsating characteristics of such flames. The experimental data were obtained by using three techniques, namely, pressure fluctuation measurements, thermal imaging and high-speed video photography. Present findings are compared with data sets reported in the literature and correlations for pulsation frequency suggested by previous studies are independently verified. Analysis of the experimental data on frequency of pulsations in different burners shows that for a fixed-diameter flame the pulsation frequency is almost independent of fuel flow rate. The equation f=1.68D−0.5 gives the best approximation for the relationship between pulsating frequency and diameter over a wide range of data. An alternative way of expressing the relationship between the key variables is St=0.52*(1/Fr)0.505. This proves to be a better way of expressing the relationship since it can include the effect of the fuel flow rate. Slight modifications to this expression allows prediction of flame oscillations under elevated/reduced gravity and isothermal buoyant plumes. This relationship and the observations of the present study confirm the hydrodynamic nature of flame puffing: interplay of buoyancy and fluid motion. © 1996 by John Wiley & Sons, Ltd.