Flame Heat Research Articles

The impact of equivalence ratio on the fire characteristics of Kerosene/air flame produced by NexGen burner for aeronautic application

This paper provides an experimental investigation of a Kerosene/air flame (produced by the NexGen burner designed by the FAA, and here calibrated for ISO 2685/AC20-135 standards fire tests conditions), which is used to generate flame/burnt gases impinging on material samples in the field of fire safety. The purpose of this study is to characterize this burner, and experimental means are implemented to better understand the effects of the equivalence ratio on the spatial distribution of the gas temperature (thermocouples) and the heat flux (heat flux gauge). A parametric study was carried out by varying the global equivalence ratio (ER); highlighting a flickering phenomenon, a relation between the ER and the heat flux, and determining empirical correlations. Hence, the measured flame temperature, heat flux, and heat release rate increase up to a critical value of equivalence ratio equal to 1.03 (flame temperature (1101.5 °C) and heat flux (140 kW/m2).

Large eddy simulations fire modeling of JIS A 1310 façade calibration test with respect to sidewall

AbstractThe full‐scale façade standard test is widely employed as a comprehensive method to assess the façade fire spread. Within this approach, the calibration test without combustible façade decouples the intricate interaction between gas‐phase combustion and material pyrolysis, which simplifies diagnostics and provides an ideal scenario for model validation. This paper presents large eddy simulations (LES) accompanied by comparisons of calibration tests in accordance with JIS A 1310. The calibration tests were conducted to obtain the flame morphologies, gas‐phase temperature, and heat flux of over‐ventilated façade fires, and the LES modeling is complemented by the modified eddy dissipation model for combustion, the one‐equation model for the sub‐grid scale turbulence, and the discrete ordinate method with the gray mean absorption‐emission approach for thermal radiation. The accuracy of LES data is discussed by comparing with measurements, and the mesh resolution is optimized as 2.5 cm for achieving mesh independency with good qualitative agreement. Furthermore, simulations are conducted to investigate the impact of sidewall distances on façade flame spread. The results highlight that the enhancement of sidewall in façade flame spread occurs under external heat release rate, and the 0.2 m sidewall distance for the designated JIS test is identified as a critical threshold increasing façade thermal load.

Flame acceleration, detonation limit and heat loss for hydrogen-oxygen mixture at cryogenic temperature of 77 K

Experimental investigations have been conducted in hydrogen-oxygen mixtures with equivalence ratio of 1.5 at cryogenic temperature (77 K) and initial pressure (P0) from 0.2 to 0.5 atm. The pressure sensors and optical fibers are used to measure the overpressure and flame velocity, respectively. The precursor shock wave is formed by the combination of the 1st and 2nd shock waves. When flame catches up with the precursor shock wave, detonation occurs. Strong flame acceleration was observed in all tested conditions, which can be well characterized and predicted by the Zel'dovich number and expansion ratio. The stuttering and galloping modes were observed at 0.5 and 0.3 atm, respectively. With the decrease of the initial pressure to 0.20–0.25 atm, detonation could not occur, only the deflagration mode was observed. The stability of the mixture can be indicated by the parameter χ, and the improvement of stability of mixture will shorten the initial pressure range where the galloping mode occurs. The heat loss effect on detonation limit has subsequently been examined for the present experiments as well as those with equivalence ratio of 2.6 reported in our previous study [Shen, X. et al., Proceedings of the Combustion Institute 2022]. For equivalence ratio of 2.6, the heat loss is found to have dominant effect on the appearance of detonation limit. Its influence can be quantitatively characterized by critical ratio of heat loss to heat release. However, for equivalence ratio of 1.5, the heat loss effect is found to have only relatively small effect on detonation limit.

Laser interferometric detection of heat release rate perturbation field of acoustically perturbed laminar premixed flames

Laser interferometry, which was originally used to measure density perturbation, has been previously used to determine the heat release rate perturbation, which is a key factor in the analysis of combustion instability. However, its spatial accuracy is still unsatisfied. In the present work, an improved fringe Fourier analysis method is proposed to process the results obtained from the laser interferometry measurement, aiming to accurately determine the spatial distribution of flame heat release rate perturbation. The measurement of heat release rate perturbation was performed on a acoustic perturbed laminar premixed flame, with the burner outlet diameter of 8 mm. Interference patterns generated by two laser beams with a diameter of 70 mm were recorded at a frame rate of 6000 Hz. A spatial dependent fringe processing method, referred as the improved fringe Fourier analysis, was developed to demodulate the interference phase from the fringe patterns. The proper orthogonal decomposition analysis was performed on these results to analyze the component of interference signals. Validation of the present technique was conducted by comparing the heat release rate perturbation field resulting from the laser interferometric technique to those from the CH* chemiluminescence from the flame. Comparisons were performed both for the global and local values, and the low-frequency natural oscillation effects were analyzed. Finally, a refraction correction method based on background-oriented Schlieren is proposed to compensate the beam deflection caused by flame.

A simple synthesis of a guaiacol based reactive flame retardant and its application in epoxy resins

Under the premise of ensuring the flame retardancy and heat resistance of flame-retardant epoxy resin (FREP), the development of bio-based reactive flame retardants has a broad application prospect.

Modeling hydrogen–diesel dual direct injection combustion with FGM and transported PDF

This work extends the transported probability density function (PDF) method to model hydrogen–diesel dual direct injection (H2DDI) combustion, where a H2 jet is ignited by a small pilot diesel jet flame. The work is motivated by previous H2DDI engine tests where stable compression ignition engine operation was reported with up to 90 % H2 supply by energy share. To help explain this high performance and provide a tool to help further engine optimization, the Eulerian Monte Carlo Fields (EMCF) solution method is employed with which a transported PDF equation is solved in combination with Flamelet Generated Manifold (FGM) for high accuracy and computational efficiency. In all cases, the pilot fuel ignites before interacting with the H2 jet and thus reaction rate and chemical composition are computed as the weighted average of the corresponding values taken for two separated FGM tables generated for the two fuels. Pressure measurements and high-speed schlieren imaging performed in a preburn-type optical constant volume combustion chamber (CVCC) with n-heptane (nC7H16) pilot fuel were used to validate the EMCF+FGM model. The application focus is how H2 ignition and combustion is affected when the nC7H16 is injected prior to or after the main H2 jet. EMCF+FGM computed heat release rate and flame structure were compared with experimental data and results from conventional FGM model based on presumed PDF. When applied to H2DDI combustion, the EMCF+FGM model successfully reproduces flame structures and heat release rate under different injection strategies, including the transition towards partially-premixed combustion when the nC7H16 pilot follows the main H2 injection. Moreover, analysis of heat release rate from different stochastic fields can provide a useful indication about cyclic variability and combustion stability.

Effect of differential diffusion on head-on quenching of premixed NH[formula omitted]/H[formula omitted]/air flames within turbulent boundary layers

Direct numerical simulations (DNS) have been done for premixed laminar and turbulent NH3/H2/air flames propagating towards a cold wall. Two turbulent channel flows (with well-characterized turbulent boundary layers) at Reτ = 280 and 313 have been simulated for the turbulent flame cases. Near-wall low temperature chemistry has been considered. Effect of differential diffusion on the flame head-on quenching characteristics (quenching distance and maximum wall heat flux) has been checked in detail. Finally, two underlying mechanisms determining differential diffusion effects have been identified: (a) preferential diffusion of H radical increases the reaction rates of the near-wall low temperature (radical recombination) reactions, resulting in larger quenching distance and wall heat flux; (b) differential diffusion of fuels results in smaller flame heat release, thus, larger quenching distance and smaller wall heat flux. The effect of H radical preferential diffusion is suppressed in rich H2/air turbulent flames and strengthened in lean H2/air turbulent flames, due to the differential diffusion focusing/defocusing effects. The effect of differential diffusion of fuels is suppressed in turbulent flames, due to the enhanced mixing process, especially at low Damköhler numbers typical for NH3/H2/air flames interacting with highly turbulent flows.

Combustion enhancement in a model scramjet by a simple pin-to-pin AC arc plasma

A simple pin-to-pin alternative-current (AC) arc plasma located upstream of the cavity was developed to achieve significant combustion enhancement in a Mach 2.52 C2H4-fueled and cavity-based model scramjet. The arc plasma-enhanced combustion process was recorded by high-speed CH⁎ chemiluminescence imaging, optical emission spectroscopy, static pressure measurements, and discharge waveform measurements. In the absence of the arc plasma, the flame heat release intensity was low, and the heat release region was located at the shear layer of the cavity; the flame stayed in a relatively weak combustion state and even appeared in a lower heat release intensity. Once the arc plasma was turned on, the static pressure of the model scramjet at the typical positions was increased by up to 9 %, and the flame area and the flame intensity were significantly enhanced. In the presence of the arc plasma, the position of the flame heat release region moved upstream and even caused the long-scale displacement for the flame with a higher heat release intensity several times. The arc plasma provided a large number of reactive species and Joule heat at the practical influence region of the arc plasma, which contributed to igniting an initial flame upstream of the cavity. The propagation and development of the continuous initial flame generated by the arc plasma can enable the flame to be more frequently transferred into the cavity-assisted jet-wake stabilized combustion mode, leading to significant combustion enhancement.

Calculations of CO2 emission and combustion efficiency for various fuels

The specific CO2 production of fuels has commonly been expressed empirically in terms of kgCO2 (kWh)−1. No specifications regarding combustion conditions, transformation technology used, and efficiency are usually presented in detail. In this paper, however, we propose replacing this approach by the use of rigorous chemical engineering calculations based on the composition and the combustion conditions of the fuels. The MS EXCEL program, which is provided as supplementary material to this paper, calculates the specific CO2 production considering energy losses, adiabatic flame temperature, calorific value and heat of fuel combustion, flue gas temperature, energy losses through flue gas, slag, and the overall energy efficiency of the whole energy conversion process. Results are presented for three fuel groups: coal, hydrocarbons, and renewable fuels. The calculated results are compared with the literature data. A benefit of our approach is that the procedure can be generalized for any fuel of known composition and for different types of combustion-based energy transformation.

Experimental and Numerical Study of the Trench Fire Spread Rule over a Sloped Uniform Fuel Bed: Rate of Fire Spread, Flame Morphology, and Heat Flux

Trench fires on sloped terrain are always complicated due to the corresponding flame dynamics and heat transfer mechanisms. Flame attachment may increase the rate of fire spread (ROS) by enlarging the heating area of unburned vegetation. In addition, variations in radiative and convective heat flux are of great importance to fire behavior characteristics. In this work, trench fire tests under different slopes (θ) and inclined sidewalls (A) were performed by numerical simulations based on the Lagrangian Particle Model (LPM) and Boundary Fuel Model (BFM) in the Fire Dynamics Simulator (FDS) and small-scale experiments, and the ROS, flame characteristics, and radiative/convective heat flux of the fire front are discussed in detail. The results indicate that the flame tends to adhere to the fuel bed with increasing slope angle and sidewall inclination. In particular, the flame becomes fully attached with a greater pressure difference than the buoyancy, which is caused by the unequal air entrainment between the front and behind the flame. When A = 90°, the critical slope angle of the flame adhesion (from slight tilt to full attachment) is identified as ~20°. The ROS (θ ≤ 15°) predicted by the BFM and LPM are closer to the small-scale experiments. The heat fluxes based on the experiments confirm the predominant mechanism of radiative heat transfer in trench fires at low slopes (θ ≤ 20°). Furthermore, convective heat transfer is more significant than radiative and becomes the main heating mechanism for θ ≥ 20°.

Effect of flame heating on thermal runaway propagation of lithium-ion batteries in confined space

As the thermal runaway (TR) of lithium-ion batteries (LIBs) may be induced in enclosed systems, thermal hazards from the ceiling fire contribute to the TR propagation in battery module. However, the characteristic of TR propagation in confined space, especially the heating effect of battery flame, is still unclear. To fill the knowledge gaps, a refined study has been conducted with LiFePO4 battery modules in a confined space. The TR propagation behavior, battery temperature, propagation time and the heat transfer are analyzed quantitatively with different plate heights and heating modes. The results show the TR propagation occurs at 2, 5 and 10 cm plate height while no propagation at higher height. The lower height leads to the increasing of flame heating effect and decreasing of TR propagation time. The flame heating reduces the demand of the heat transferred through the conduction for TR propagation. Further, the heat from ceiling flame has little change with different heights as it is mainly depended on the combustion time. Finally, the heating power of ceiling flame is analyzed and the maximum increases from 1400.3 to 2594.6 W when height decreases from 15 to 2 cm, while it is higher with continuous heating.

Radiative characteristics of large-scale fire whirl: An experimental study

Flame radiative characteristics, depending on the flame type, flame scale, and fuel type, are key parameters that determine fire spread and thermal hazards. Accurate prediction of flame radiation in large-scale natural fires requires detailed information on radiative characteristics. This paper presented an experimental study on the radiative properties of large-scale fire whirls with RP-5 aviation fuel. The maximum flame height and mean width reached 21.40 m and 3.60 m, respectively. Unlike large-scale free pool fires, the radial stabilization effect of the fire whirl suppressed the unstable toroidal vortex structures and the smoke production significantly. Most of the flame was high-temperature and high-emissive power areas in the large-scale fire whirl, and the emissive power exceeded 100 kW/m2 on about 60 % flame area. The flame temperature and emissive power first increased with height, then stabilized and decreased in the upper part of the fire whirl. The peak flame temperature, mean flame temperature, and mean emissive power all increased with an increase in the flame width. The mean emissive power ranged from 93.06 to 157.47 kW/m2, 2.3–3.9 times the values in free pool fires of the same scale. The solid flame radiation model could effectively predict the instantaneous radiative heat fluxes from large-scale fire whirls to distant external targets with the corrections of atmospheric transmissivity. The semi-empirical correlation of the radiative fraction based on the turbulent dissipation showed that neither the flame size nor heat release rate affects the radiative fraction, consistent with the our and other researchers’ experimental data from medium to large-scale fire whirls.

Research on blending combustion characteristics of coaxial injector of oxygen/methane engine

Aiming at the demand of oxygen/methane rocket engine for future reusable spacecraft, the design of oxygen/methane coaxial injector engine and the study of flow mixing and combustion characteristics were carried out. The flow field characteristics, flame structure size and heat release state changes under different fuel-oxygen momentum ratio of coaxial injector were investigated. The results show that the increase of fuel-oxygen momentum ratio in the cold flow state increases the influence of the mainstream flow on the flow in the combustion chamber, and the length of the vorticity shear layer also increases. With the increase of fuel-oxygen momentum ratio, the flame's lifting distance gradually decreases, the rate of methane needed to stabilize the flame in the shear layer is over twice that of oxygen. At this time, as this ratio increases, the flame expansion angle increases while its length decreases. Rich combustion in the return zone is suppressed, but there is still some impact on component distribution ahead of main heat release. With the increase of fuel-oxygen momentum ratio, the starting position of the high temperature zone gradually advances, and the increase of fuel-oxygen momentum ratio makes the mainstream temperature change significantly intensified. Within a fuel-oxygen momentum ratio range of 0.5–2, the alteration of this ratio has a significant impact on the temperature profile of the gas close to the wall. However, as the ratio increases, the effect of this change on the gas temperature diminishes significantly.

P/N/Si‐containing synergistic flame retardants endowing lignin‐based epoxy resin with good flame retardancy, mechanical properties and heat resistance

AbstractBiobased epoxy resins are gaining attention as a renewable alternative to petroleum‐based resins. However, their inherent flammability requires flame‐retardant treatment. In this study, hexaphenoxycyclotriphosphazene (HPCP) and various synergists were utilized to improve the flame retardancy of lignin‐based epoxy resin (EP‐L). Results showed that (3‐aminopropyl)triethoxysilane (APTES) had the most effective synergistic effect for HPCP to form a P/N/Si‐containing system, resulting in a compact crosslinked char layer. Upon the incorporation of HPCP and APTES (6∶1, 12 wt% of EP mass) into EP, a V‐0 classification was achieved in the UL‐94 vertical burning assessment, with a limiting oxygen index of 33.6% and a 47.77% decrease in the peak of heat release rate. This system minimally influenced the tensile strength of the composite (a 16.51% reduction), and maximumly increased its initial degradation temperature by 83 °C, compared with EP‐L. © 2023 Society of Industrial Chemistry.

Study on CO2* chemiluminescence of CH4/O2 co-flow diffusion flame based on the chemical kinetic mechanism

Chemiluminescence of CO2* is suitable for characterizing flame properties such as flame structure, equivalence ratio and heat release rate. However, experimental studies and numerical simulations of CO2* are not yet complete. In this work, the two-dimensional distributions of CO2* chemiluminescence were obtained by a high-resolution CCD camera. Numerical simulation results of CO2* were performed based on the detailed reaction kinetics mechanism of CO2* and were in good agreement with that of experiments. The differences of CO2* reactions at different axial positions were studied. The main CO2* formation reaction R2(CH + O2 = CO2*+H) at the burner outlet and R1(O + CO + M = CO2*+M) at the core reaction zone were determined. On the fuel side, the R1 reaction rate was mainly affected by temperature. On the oxidizer side, the R1 reaction rate was affected by temperature and reactant mole fraction. The main quenching reaction was R5(CO2*+H2O = CO2 + H2O). The oxygen-fuel equivalence ratio changed the reaction rate of formation and quenching reactions mainly by affecting temperature and reactant mole fraction.

An engineering model for creeping flame spread over idealized electrical wires in microgravity

Flame spread over an insulated electrical wire is a major source of fire scenario in a space vehicle. In this work, an engineering model that predicts the creeping flame spread over cylindrical wires in microgravity is developed. The model is applied to interpret experimental data obtained in parabolic flights for wires composed by a 0.25 mm radius nickel-chromium (NiCr) metallic core coated by low-density polyethylene (LDPE) of different thicknesses ranging from 0.15 mm to 0.4 mm. The model relies on the assumption that, in the pyrolysis region, the NiCr and the LDPE are in thermal equilibrium. This assumption is supported by more detailed numerical simulations and the model reduces then to solving the heat transfer equations for both NiCr and LDPE in the pyrolysis region and in the region ahead of the flame front along with a simple degradation model for LDPE, an Oseen approximation of opposed oxidizer flow and an infinitely fast gas-phase chemistry. The flame spread rate (FSR) is controlled by two model parameters, which are measurable from intrinsic material and ambient gas properties: the convective flame heat flux transferred to the solid ahead from the flame front and the gaseous thermal heat length near the flame front. These parameters are then calibrated from experimental data for a given wire geometry and the calibrated model is validated against experimental data for other wire geometries and ambient conditions. The heat transfer mechanisms ahead of the pyrolysis front are investigated with a special emphasis on the LDPE thickness and the conductivity of the metallic core. In addition to NiCr, metallic cores of lower and higher conductivities are considered. The polymer is shown to be thermally thick for all tested wire geometries and core conductivities. The flame heat flux is found to dominate the heat transfer in the preheat zone where it applies. The core has nevertheless a significant impact in the heating of the LDPE with its contribution increasing with the core conductivity and when decreasing the LDPE thickness.

Effects of Wind on Heat Transfer and Spread of Different Fire Lines Across a Pine Needle Fuel Bed

ABSTRACT In this work, a series of surface fire experiments are performed to investigate the effect of wind velocity on the surface fire behaviors over a pine needle fuel bed under nine wind velocities (0/0.25/0.5/0.75/1/1.5/2/2.5/3 m/s) and three fuel loads (0.2/0.5/0.8 kg/m2). The essential parameters are obtained and analyzed, including flame length, flame angle, flame spread rate, and heat flux. The main valuable conclusions have been drawn: (1) Under no or low wind velocity, the flame tilts in the direction of the burned fuel, and the fire line shows a linear shape; whereas as the wind velocity increases, the flame gradually shifts toward the unburned fuel and the fire line is observed to be curved. (2) Flame angle decreases with increasing wind velocity and is not sensitive to fuel load, while flame length, heat transfer, and fire spread rate increase with increasing wind velocity and fuel load. Moreover, the empirical flame length and fire spread rate models are proposed considering the wind velocity and fuel load. (3) The modified model of radiation heat flux for surface fire spread under wind conditions taking different fire line shapes into account, is developed, and the predicted values are in good agreement with the experimental data. This paper’s results can help better understand the fire spread and controlling mechanism of surface fire under wind velocity conditions.

Experimental research on the thermal and mechanical response of flowing pipelines under the horizontal jet flame

The safe operation of parallel pipelines has become a focus of attention. The jet flame generated by the initial pipe accident may cause adjacent pipe section failure. Therefore, it is necessary to study the thermal impact of jet flame on adjacent pipelines. In this paper, we design and establish a thermal effect test platform to determine the change in pipe wall temperature and mechanical properties under the horizontal jet flame. Firstly, the flame combustion characteristics, such as flame form, temperature, and heat flux, are tested and analyzed. Then, the quantitative variation law of the pipe wall and oil temperature under different flow rates, wall thicknesses, and heating lengths is obtained. Combined with the mechanical test results of experimental pipelines, a pipe-bearing capacity calculation model is established via the GA-BP machine learning algorithm. Finally, the safety evaluation method and classification of the stress margin are established. The results indicate that the temperature and heat flux of the fire-facing surface is significantly higher than other clock orientations, where the values are 1287.38 ℃ and 12.37 kW/m2, respectively. As the flow rate decreases and the wall thickness and heating length increase, the pipe wall temperature rises linearly, and the tensile strength and impact toughness reduce logarithmically. The average relative error of the pipe strength prediction model is less than 2.44 %. The established safety evaluation method can be applied to determine the thermal risk level of steel pipelines under the jet flame.

Characterization of commercial thermal barrier materials to prevent thermal runaway propagation in large format lithium-ion cells

In recent years, electric vehicles (EVs) have been rapidly introduced as an environmental measure. It is said, however, one of the challenges to accelerate their spread rests in how to increase their driving range. Recent safety regulations for EVs have been stringent in the wake of the potential risk of thermal runaway of battery packs. The battery pack is expected to withstand intense ignition during the event of thermal runaway while measures to suppress cell-to-cell thermal runaway propagation are also being considered as a mandatory safety standard. Thermal barrier materials are viable system-level solutions to mitigate cell-to-cell thermal runaway propagation via electrical, thermal, and fire insulation to meet regulatory safety needs. Among the various researched thermal barriers, only a few materials are commercially viable. Hence, this paper explores different commercially available thermal insulating materials that can be accommodated in battery packs. The materials selected combine key characteristics such as flame resistance, heat insulation, and thickness. Subsequently, we experimentally investigate the thermal and fire-risk properties of the selected thermal barrier materials via flame torch tests and compare their performances. Among the tested materials, a potential material has been used to further validate its performance in mitigating thermal runaway propagation in battery module/pack level overheating tests. Additionally, the comprehensive results of single-configuration thermal barrier characterization tests aided in combining different thermal barrier materials, which displayed better performance than single-configuration materials.

Flame retardant, heat insulating and hydrophobic chitosan-derived aerogels for the clean-up of hazardous chemicals

Leakage of hazardous chemicals often causes significant casualties, enormous economic losses, and negative social benefits. Presently, fire rescue personnel lack efficient and eco-friendly disposal materials for hazardous chemical leakage accidents. In this study, chitosan (CS) aerogels with excellent flame-retardant performance were prepared via cross-linking by two phosphorus-containing vanillin-based compounds (DV and TV). The as-prepared chitosan aerogels were lightweight and porous. The introduction of DV and TV greatly enhanced the residual char yields of CS at 700 °C and the flame-retardant performance of chitosan aerogels. TCS-5.0 possessed the best flame-retardant performance, indicating that TV was more effective than DV in enhancing the flame-retardant performance of chitosan aerogels. The greatly improved flame-retardant properties could be attributed to TV effectively promoting the residual char formation of chitosan aerogels and reducing the formation of combustible gas phase products. To improve the hydrophobicity of chitosan aerogels, TCS-5.0 was treated with solution immersion to load siloxane molecules on its surface. The water contact angle of HTCS-5.0 was 136.1°. HTCS-5.0 had a high oil absorption multiplicity, absorbing up to 31 times its own weight of chloroform. HTCS-5.0 could continuously absorb organic solvents on the water surface with the assistance of a vacuum pump, indicating that HTCS-5.0 could be used for the clean-up of hazardous chemical leakage accidents.