Flame Radiation Research Articles

Numerical Simulation of Gas-solid Jet Fire in Natural Gas Pipeline Leakage

Aiming at the problem that gas-solid two-phase jet flame formed by jet fire entrained by natural gas pipeline leakage is less studied, the gas-solid two-phase jet and pure gas-phase jet flame models formed by natural gas pipeline leakage are established by FLUENT software. The differences between the gas-solid two-phase jet and the pure gas-phase jet flame were comparatively studied, and the effect of different particle size on flame height, temperature and radiation of gas-solid jet was analyzed. The results indicate that the solid particles entering the flame will lead to a lower temperature compared to a gas-jet-only flame. The larger the mass flow of solid particles is, the lower the flame temperature of the gas-solid jet will be, and the corresponding flame height and radiation hazards are also reduced. The present study findings can be used for the purpose of studying gas-solid two-phase jet flame in leakage of natural gas pipelines.

Experimental and analytical study on inclined turbulent fire in still air. Part 2. Round fire

This paper reports an experimental and analytical study on non-vertical round turbulent jet fire. A series of propane jet fire tests are performed under different initial flame angles (θ0) and initial fuel velocities. The two-dimensional velocity and temperature fields of the flame are measured, and the flame geometry is determined from video images. Experimental results show that the flame entrainment is enhanced as θ0 decreases. The flame deflection-induced buoyancy flow plays an essential role in the local flame structure and turbulence. A model of round fire combined with a kinetic energy equation is established, in which the flame entrainment coefficient is analytically correlated with the buoyancy and streamline curvature. Model calculations reproduce the flame entrainment and flame trajectory, consistent with the experimental indications. Under the same burner area and heat release rate, the entrainment coefficient of round fire is greater than that of line fire, closely associated with the difference in integral buoyancy triggered by different burner geometries. The results of this paper may help understand the flame motion and predict the flame radiation of turbulent jet fires.

Flame evolution and radiation hazards of rectangular fires between two parallel walls

The manuscript investigated experimentally the influence of two parallel walls on the flame geometric parameters and radiative heat flux distributions along the wall from fires on rectangular burners. Four sets of burners with equal area and different aspect ratios were used. The burner aspect ratios, separation distance between two parallel walls and the fire heat release rates were systematically varied in the tests. Measurements were conducted for the flame height evolution and radiation hazards. The results were analysed to build the change trends of the vertical flame height and radiant heat flux with the change of the separation distance between two parallel walls. A normalized flame height equation incorporating the separation distance was proposed. The results also revealed that the radiant heat fluxes along a vertical target do not change monotonously. Comparison between the measurements and the radiant heat fluxes calculated by some published empirical models revealed relatively large discrepancies. A view factor based formula was hence proposed by assuming the flame shape as a triangular prism based on the probability flame contours for relatively larger burner aspect ratios (n ≥ 3) and found to correlate well with the measurements.

Waste tires pyrolysis oil and its blend with diesel fuel spectral flame analysis, temperature contours, and emissions

The purpose of this research paper is to investigate the energy valorization from the pyrolysis of waste tires using coaxial continuous flame burners. For such an investigation, both the Tire Pyrolysis Oil (TPO) along with its blend (B1) with light diesel oil (LDO) were prepared, physically and chemically characterized, and then combusted in coaxial burners. To distinguish the spectral emission peaks, for the vaporization and combustion zone and the hot recirculation zone of the produced flame, flame spectroscopy techniques were utilized. The mass-specific emission indices and the axial inflame temperatures were used as indicators of the flame radiation intensity in the flame zones. The coaxial flame was inspected for excess air factors of 1.33, 1.04, and 0.9. In the study, a higher radiation intensity of TPO/LDO blend flame was recognized in the B1 flame length which was taller than that of LDO by 48% at λ=1.33. Furthermore, a decrease in CO emissions levels was also noticed in the combustion of waste tire pyrolysis oil and B1 by an average of 4%–12% compared to that from HDO, and the NOx emissions were also reduced by 8.5%–26%. The highest axial inflame temperature of 1,205 °C was recorded for TPO at condition λ=0.9, where the presence of oxygen molecules in fuel helps flame improvements.

Experimental investigation of soot and radiation characteristics in ethylene/propane buoyant diffusion flames

Experimental study was carried out on the soot and radiation characteristics of ethylene/propane buoyant diffusion flames. Characteristics reported include flame natural luminosity, average soot distribution, maximum soot volume fraction, probability density function, soot yield, radiation flux and flame radiation fraction. Results showed that for a constant burning rate of 1.0 kW, different soot and radiation levels of flames produced can be achieved by adjusting gas composition of fuel mixtures. Propane exhibited a synergistic effect on the soot formation, resulting in nonlinear relationship between the maximum soot volume fractions and blending ratios. Based on the analysis of characteristic lengths and characteristic mixture fractions, a correlation of characteristic soot volume fraction with blending ratio was proposed to predict the maximum soot volume fraction of buoyant diffusion flames. The skewed distribution of probability density function elucidates differences in the effect of blending ratio on soot intermittence in the flames. Flame radiation fraction was proportional related to maximum soot volume fraction and linear correlated with soot yield. The analysis provided new comprehensive understanding of relationship between soot formation and fuel mixture, and could further apply to establish flame radiation equivalence principles of real fire emulation in fire research, fire safety testing and fire-fighting training.

Determining spatially-resolved thermal radiation from non-intrusive measurements of soot properties

A procedure is presented that uses estimations of soot volume fraction and temperature fields to (i) calculate the spatially-resolved radiative intensity emitted from soot, and (ii) model the incident radiative power released from the flame by soot particles on a surface. The procedure is validated using both experimental and simulated data obtained from a canonical laminar diffusion flame about 5 cm high. First, estimations of soot volume fraction and temperature derived from non-intrusive experimental measurements are used to calculate the reference soot radiative intensity and to simulate the signals captured by radiometers located at different positions relative to the flame. Then, simulated radiometer signals are processed using different viewfactor methodologies to retrieve the values of integrated soot flame radiation, which are then compared to the reference intensity obtained from soot volume fraction and temperature by solving the radiative transfer equation. Results show that point-source methods accurately estimate the reference intensity (within 2% of error) when using simulated signals from radiometers positioned 7 and 10 cm from the flame axis and up to three times the flame height. Moreover, the double cylinder model accurately estimates flame radiation from measurements performed above three times the flame height. • Fields of radiative power determined from soot volume fraction and temperature. • Spatially-resolved radiative power used to simulate and compare radiometer signals. • Assessment of radiant fraction established from different viewfactor methodologies. • Simulated and experimental signals deviations increase radiative information.

Elucidating the challenges in extracting ultra-slow flame speeds in a closed vessel—A CH[formula omitted]F[formula omitted] microgravity case study using optical and pressure-rise data

Refrigerants with a low global warming potential (GWP) possess mild flammability. Hence, a fundamental understanding of their combustion characteristics is required to assess their fire-hazardous potential. The laminar burning velocity is one fundamental property to describe these under safety evaluation aspects. However, measuring laminar burning velocities for slow-burning components, such as low-GWP alternatives, is challenging. This is due to influencing effects such as buoyant flame deformation, stretch, radiation, and confinement, especially at the flammability limits. In the present study, we investigated near-limit flames of the representative refrigerant difluoromethane (CH2F2) with nitrogen-enriched oxidizer mixtures to elucidate the potentials and limitations at ultra-slow flame speeds of two widely used flame speed measurement methods in the closed vessel configuration: the optical flame speed measurement in the early quasi-isobaric regime and the flame speed determination from the pressure-rise during the later phase of the isochoric combustion process. Experiments were performed under terrestrial- and microgravity, conducted in a specially designed setup suitable for drop tower applications, using both methods in parallel. Recommendations for the extraction of flame speed data are derived from the non-buoyant reference investigations under microgravity and transferred to ultra-slow flames at terrestrial gravity. Near-limit flames under microgravity were found to be significantly affected by stretch effects for the optical method so that flame speed extrapolations to unstretched conditions are prone to errors. Stretch also affects the pressure method’s lower evaluation limit, and errors can be estimated by combining flame speed data from the pressure method and Markstein lengths from optical experiments.

Flame detection by heat from the infrared spectrum: Optimization and sensitivity analysis

Accurate detection of unwanted fires at their early stage is crucial for efficient mitigation and loss prevention. Moreover, the detection strategy must avoid false alarms and the associated disruptions in workplaces. Thermal radiation-based flame detection is the fastest detection method and is commonly used in critical industrial spaces, such as air hangars and petroleum manufacturing and storage. The main challenge is distinguishing the radiation of flames from other sources, e.g., hot objects or the Sun. The principles of radiation-based flame detection have been known for a long time, but open data and worked-out feasibility studies are rare. This work takes advantage of the recent advances in experimental and numerical methods of characterizing the infrared spectra. Combining high-resolution spectra from flames and blackbody emitters with virtual low-pass filters allows us to simulate the response of a hypothetical sensor. To maximize the difference between flame and blackbody responses, we use a pattern search algorithm to find optimal filtering wavelengths for two different detection strategies based on three or four optical low-pass filters. The optimal wavelengths are reported along with the sensitivity of the detection signal to the filter non-ideality. Our results give guidelines for design of efficient and highly selective flame-radiation-based fire detection sensors.

A new-proposed multi-inclined-cylinder radiation model of rectangular pool fire of heptane in longitudinal winds

A rectangular pool fire may be formed which is essentially affected by ambient wind in open space. In this work, the experimental investigations of flame radiation of heptane fire in longitudinal wind are conducted using five rectangular containers with a constant surface area, and variable length-width ratios of 1, 2, 4, 8, 16. The longitudinal air speeds are 0, 0.71, 0.96, 1.22, 1.47, 1.73, 1.98, 2.24 and 2.49 m/s respectively. Results show that as air speed increases, the incident heat flux increases initially and then decreases afterwards, and the threshold value of air flow is approximately at v = 0.96 m/s. The heat flux is primarily determined by burning rate at v ≤ 0.96 m/s whereas it is determined by the combination of flame morphology and burning rate at v > 0.96 m/s. The theoretical formula of radiation emissivity of rectangular pan is deduced using mass burning rate and aspect ratio. The multi-inclined-cylinder model incorporating aspect ratio, flame height, tilt angle and mass burning rate is proposed to evaluate the rectangular flame radiation in air flows. The theoretical values of heat fluxes are acceptably consistent with measurements in longitudinal wind-blown condition.

An experimental study on flame geometry and radiation flux of line-source fire over inclined surface

An experimental study was performed on line-source fire over an inclined surface (ground) to simulate downhill fire spread behavior. The flame geometry and the thermal radiation to both far-field surroundings and near-field inclined surface were investigated. As a basic configuration for wildland fire over a slope, the buoyancy induced natural convection flow along the inclined surface and the constraint of air entrainment by the inclined surface change the flame geometry as well as its radiation emission. Various surface (ground) inclination angles (from 0°-80°), fire source heat release rates and fuels were considered comprehensively with a total of 126 test conditions. Results showed that the flame perpendicular height decreased, while both the flame parallel length and base drag length along the inclined surface increased, with the increased inclination angle. A dimensional analysis was then performed based on the controlling mechanisms, with the dimensionless heat release rate, the density ratio of fuel vapor to air, along with sinα and cosα involved to represent the components in the parallel and perpendicular directions. The flame geometry parameters were well represented by the proposed dimensional analysis. Both the radiation fluxes to far-field surroundings and to near-field inclined surface decreased with the increased inclination angle. The far-field radiation was found to be well characterized by a model based on the soot volume fraction analysis according to single point source model. Concerning the near-field radiation to inclined surface, an inclined cuboid radiative modeling was developed. The predicting results by the proposed model and the experimental values showed good agreement. The present study has explained the controlling physics and proposed non-dimensional functions for flame geometry and modeling the downslope radiation of the line-source fire over the inclined surface, which facilitates the understanding of the wildland fire spread behavior over a slopping ground in the downhill direction.

Enhanced stability and combustion performance of AlH3 in combination with commonly used oxidizers

Aluminum hydride (AlH3), as a promising fuel, has been utilized to improve the energy performance of propellants. Moreover, the inherent compatibility and mutual interaction between AlH3 and the commonly used oxidizers of solid propellants are the basics of propellant formulation design. Herein, the homogenous composites of AlH3/oxidizer have been prepared by the in-situ recrystallization method, and then their thermal stability, compatibility, and ignition performance have been investigated. The initial decomposition temperatures (Ti) of AlH3 in composites are increased by at least 16 °C, whereas the thermal decomposition peak temperatures (Tp) of involved oxidizers are lower than that of their pure state. The compatibility tests showed that AlH3 is compatible with the mentioned oxidizers. In particular, the induction time of the dehydrogenation of AlH3 in presence of these oxidizers is improved by almost 1.2 times that of raw AlH3, which means that the oxidizers have an unexpected strong stabilization effect on AlH3 due to strong hydrogen bonding. Moreover, the optimized contents of AlH3 in AlH3/HMX, AlH3/CL-20, and AlH3/AP composites have been determined to be 45 %, 40 %, and 33 %, which have the maximum flame temperatures of 1275.3, 1440.4, and 1616.8 °C, respectively. Besides, AlH3/CL-20 has the strongest flame radiation intensity (18.5 K) and shortest ignition delay time (32.2 ms) among the involved composites.

Critical temperature and reactant mass flux for radiative extinction of ethylene microgravity spherical diffusion flames at 1 bar

In microgravity combustion, where buoyancy is not present to accelerate the flow field and strain the flame, radiative extinction is of fundamental importance, and has implications for spacecraft fire safety. In this work, the critical point for radiative extinction is identified for normal and inverse ethylene spherical diffusion flames via atmospheric pressure experiments conducted aboard the International Space Station, as well as with a transient numerical model. The fuel is ethylene with nitrogen diluent, and the oxidizer is an oxygen/nitrogen mixture. The burner is a porous stainless-steel sphere. All experiments are conducted at constant reactant flow rate. For normal flames, the ambient oxygen mole fraction was varied from 0.2 to 0.38, burner supply fuel mole fraction from 0.13 to 1, total mass flow rate, ṁtotal, from 0.6 to 12.2 mg/s, and adiabatic flame temperature, Tad, from 2000 to 2800 K. For inverse flames, the ambient fuel mole fraction was varied from 0.08 to 0.12, burner supply oxygen mole fraction from 0.4 to 0.85, ṁtotal from 2.3 to 11.3 mg/s, and Tad from 2080 to 2590 K. Despite this broad range of conditions, all flames extinguish at a critical extinction temperature of 1130 K, and a fuel-based mass flux of 0.2 g/m2-s for normal flames, and an oxygen-based mass flux of 0.68 g/m2-s for inverse flames. With this information, a simple equation is developed to estimate the flame size (i.e., location of peak temperature) at extinction for any atmospheric-pressure ethylene spherical diffusion flame given only the reactant mass flow rate. Flame growth, which ultimately leads to radiative extinction if the critical extinction point is reached, is attributed to the natural development of the diffusion-limited system as it approaches steady state and the reduction in the transport properties as the flame temperature drops due to increasing flame radiation with time (radiation-induced growth.)

Experimental study on the flame radiation fraction of hydrogen and propane gas mixture

Hydrogen added to hydrocarbon fuels can improve the combustion efficiency and reduce soot emissions. However, there are some safety problems with the storage and transportation of the hydrogen and hydrocarbon fuel gas mixture. In order to investigate the combustion characteristics of gas mixture, the propane and hydrogen are used as the fuel gas in this study. A series of experiments have been carried out to investigate the radiation fraction of the turbulent diffusion jet flame under ambient conditions (300 K, 1 bar). The experimental results showed that the dilution effect of hydrogen is limited for propane when the hydrogen flow rate is small. The radiation fraction changes slightly with the increase in the flow rate of hydrogen under the same heat release rate of propane. And then, it decreases sharply when the ratio of hydrogen volume flow rate to total flow rate is larger than 0.6. Based on the theoretical analysis of laminar combustion velocity, activationenergy, and the soot generation theory, a dimensionless model has been developed to describe the evolution of radiation fraction.

Effects of flame temperature and radiation properties on infrared light field imaging

In measuring the flame temperature and radiative properties, the light field camera is subject to the visible bands of the spectral snapshot measurement in a single shot. In this paper, the infrared light field camera is proposed to improve the spectral range of detection. The effects of the participating media on the infrared light field imaging are clarified. The temperature detection ranges of visible, near-infrared, and mid-infrared spectral bands are analyzed. The coupling effects of radiative properties of gas and soot on the mid-infrared light field imaging are compared. The light field imaging results show that the temperature detection range in the mid-infrared band is twice as much as that in the visible band. The gas and soot radiative properties have recognizable effects on the mid-infrared light field images. The absorption coefficient takes the primary role, while the scattering effects by 0.275. The mid-infrared light field camera is used to take snapshots of the flame. The temperature detection range is wider than the visible band, the gas and soot laden changes the flame structure. This work can be used as theoretical support for the design and application of infrared light field camera to diagnose the flame temperature and radiation properties.

Assessment of narrow-band and full spectrum gas radiation methods in a real industrial glass furnace configuration

This work is dedicated to a comparison of various methods of gaseous flame radiation in a three-dimensional configuration representative of a glass furnace studied at Saint Gobain Research Paris. For this purpose, several narrow band and full spectrum models of flame radiation are assessed against LBL calculations. Among narrow band models, the recently proposed l-distribution method is shown to be more accurate than the k-distribution method. Among global models, the RC-SLW technique provides a high accuracy and can be confidently used for radiative heat transfer calculations in three-dimensional high temperature configurations.

Investigation of the stability, radiation, and structure of laminar coflow diffusion flames of CH4/NH3 mixtures

The stability, radiation, and structure of laminar axisymmetric CH4/NH3/air diffusion flames have been studied using photographic images, spectrally resolved measurements of flame radiation, and the spatial distribution of temperature and major species mole fractions obtained by spontaneous Raman scattering. The fit procedure for the Raman spectra of NH3 includes a hitherto unquantified overtone feature, whose inclusion in the fit significantly improves the NH3 fraction obtained. Nitrogen is used to replace NH3 to separate chemical effects of NH3 addition from those due to dilution. The results show that NH3 addition drastically reduces radiation from carbon-containing species, with progressively increasing strong chemiluminescence from excited NO2 and NH2, indicating a substantial change in flame chemistry. While the Rayleigh/Mie scattering from soot particles is still observed in the Raman spectra at 28% NH3 addition, 46% NH3 in the fuel is seen to suppresses soot formation effectively. The measured axial and radial profiles of temperature and major species indicate a substantial contribution from radial transport from the reaction zone, seriously complicating the relation between composition, mixture fraction, and the corresponding equilibrium temperature and mole fractions.

The flame mitigation effect of N2 and CO2 on the hydrogen jet fire

Hydrogen has attracted much attention from governments and enterprises around the world due to its zero pollution and recycling use. However, it may bring safety problems such as hydrogen jet fire due to its wide flammability range and low ignition energy. Therefore, the methods of preventing hydrogen jet fire need to be explored urgently. In order to investigate the effect of gaseous fire extinguishing agent on the hydrogen jet fire, a series of experiments were carried out to study the flame behaviors under the effects of nitrogen and carbon dioxide jets. The fire extinguishing agents were released through a jet nozzle with diameter of 3 mm at the pressure of 11 atm. The flame length, thermal radiant flux and total heat flux were analyzed. The results show that the reduction ratio of vertical flame length increases with the decrease of the installation height of the extinguishing agent nozzle. It indicates that when the fire extinguishing agent is injected at the root of hydrogen jet flame, the fire may be extinguished. Due to the reduction of flame size, the thermal radiant flux received by the radiometer decreases, which corresponds to the reduction of flame radiation fraction. The total heat flux received in the downstream region of deflected flame is determined by the joint action of thermal radiation and convection. The convective heat transfer of high-temperature gas may cause the value measured by the heat flux meter on the flow path of the deflected flame to increase. Besides, the extinguishing limit for hydrogen jet flame under the effects of nitrogen and carbon dioxide jets are analyzed.

Analysis of the danger of thermal radiation of a spherical flame

A new mathematical model of spherical flame propagation during emergency emission and ignition of mixtures of hydrocarbon gases in an open space has been developed. A comparative analysis of the results of the study made it possible to establish the main mechanisms of heating of a combustible gas mixture as a result of radiation from gorenje products. As a result of mathematical modeling, the resulting radiation model meets the real gorenje conditions of gas-air mixtures.

Experimental study and new-proposed mathematical correlation of flame height of rectangular pool fire with aspect ratio and mass burning rate

An experimental study is carried out on the combustion characteristics of rectangular heptane pool fire with a fixed area of 400 cm2 but different aspect ratios of (length-width ratio, n) n = 1, 2, 4, 6, 8, 10, 12 and 16. The results show that the mass burning rate decreases initially and increases afterwards with an increase in aspect ratio. Based on heat transfer theory, the mass burning rate of rectangular pool fires versus aspect ratio is divided into two regimes, namely heat convection and flame radiation coupled regime and heat conduction-dominated regime with a threshold value of aspect ratio n = 6. The flame height has a positive relationship with mass burning rate but a negative relationship with aspect ratio. Based on air entrainment theory, a new correlation of flame height against coupling of mass burning rate and aspect ratio is proposed and validated by experimental observations. The hydraulic diameter is introduced into the classical formula of pulsation frequency, and the experimental and modelling results show that the pulsation frequency of flame height increases linearly with the hydrodynamic diameter incorporating aspect ratio of rectangular pool in the correlation of f∝(n+1n)12.

Investigation of flame characteristics and cooling effectiveness of jet-cooled wall flameholders in vitiated flow

Experimental and numerical studies were performed to understand the flame characteristics and cooling effect of jet-cooled wall flameholders in a simplified augmented combustor. The time-averaged flame structure and wall temperature were investigated employing an instantaneous flame capturing system with image processing technology and a wall temperature measurement system, respectively. Flow and fuel distribution simulations were carried out to analyze the flame structure and cooling effect of the jet-cooled flameholders in high-temperature and high-velocity vitiated flows. It was found that the jet cooling type and the cooling jet angle on the oblique plate α and on the rear plate β greatly affect the flame features and cooling effect of the flameholder. The external-inhaled air cooling schemes can increase the flame luminosity and temperature and create a flame hollowed-out zone in the near-wall region to reduce flame radiation and heat convection, thereby achieving an excellent cooling effect. The pressure-driven gas jet cooling schemes yielded a weaker flame luminosity and its flame structure in the cavity more sensitive to the cooling jet angle than external-inhaled air cooling schemes. Moreover, the external-inhaled air cooling schemes have significantly better cooling effects than the pressure-driven gas jet cooling schemes under different mainstream conditions and fuel flow rates. Notably, the cooling schemes with α=30∘ and β=30∘ have more uniform wall temperature than that of α=90∘ and β=150∘ on the above two plates. Furthermore, the external-inhaled air cooling scheme with α=90∘ and β=150∘ has the best cooling performance under different mainstream and cooling air conditions and presents great cooling effectiveness in the case of enhancing cooling air temperature.