Radiative Fraction Research Articles

Experimental study on the burning rate and flame dynamics of oil reservoir pool fire under cross-wind: Effect of lip height

The study of combustion characteristics and heat feedback mechanisms of liquid fuels is important for fire safety engineering, particularly in the context of energy utilization. In thiswork, the effect of lip height on the evolution of n-heptane pool fire burning rates and flame heights, as well as the thermal feedback control mechanism, were investigated in a series of experiments in the presence of cross-wind. The results showed that the lip height of pool and cross-wind speed have a significant effect on the flame characteristics and heat feedback mechanism. For a given wind speed condition, the radiative feedback fraction and the convective feedback fraction showed non-monotonic changes with an increase in the lip height of pool. With the increase of lip height and cross-wind speed, the lift off phenomenon gradually appeared inside the oil pool, while the flame height outside the oil pool gradually decreased. A new correlation is proposed, which can well describle the relationship between the flame height and the ratio of the air entrainment caused by the cross airflow to that caused by the flame buoyancy, which can provide references for the utilization and management of energy storage strategies nowadays.

Controlling 229Th isomeric state population in a VUV transparent crystal

The radioisotope thorium-229 (229Th) is renowned for its extraordinarily low-energy, long-lived nuclear first-excited state. This isomeric state can be excited by vacuum ultraviolet (VUV) lasers and 229Th has been proposed as a reference transition for ultra-precise nuclear clocks. To assess the feasibility and performance of the nuclear clock concept, time-controlled excitation and depopulation of the 229Th isomer are imperative. Here we report the population of the 229Th isomeric state through resonant X-ray pumping and detection of the radiative decay in a VUV transparent 229Th-doped CaF2 crystal. The decay half-life is measured to 447(25) s, with a transition wavelength of 148.18(42) nm and a radiative decay fraction consistent with unity. Furthermore, we report a new “X-ray quenching” effect which allows to de-populate the isomer on demand and effectively reduce the half-life. Such controlled quenching can be used to significantly speed up the interrogation cycle in future nuclear clock schemes.

Distribution characteristics of radiant heat flux on tank surfaces exposed to wildland fires

AbstractThe expansion of wildland‐urban interface tends to expose fuel tanks to wildland fires, the frequency and intensity of which have increased in recent years due to climate change and global warming. This study aims to examine the distribution of radiant heat flux on a tank surface exposed to a wildland fire front, with a particular focus on extreme wildland fires. A theoretical framework is proposed, which consists of a flame length correlation, a thermal radiation model, and a correlation of time to failure, to calculate the radiant heat flux received by the tank surface and the potential for tank failure. The horizontal and circumferential distribution characteristics of the radiant heat flux are intensively discussed, in terms of the heat flux profile, heating area, total heat, time to failure versus the fireline intensity, fireline width, spacing, and tank volume. In particular, extreme wildland fires, characterized by large fireline intensities, could potentially destroy the tank within the safety spacing mandated by the current regulations. Therefore, a comparison between the time to failure and the radiant heating duration is suggested to ascertain the necessary safety spacing. The uncertainties in model calculations are also addressed, which arise from the use of different flame length correlations and flame radiative fractions.

Study on the flame radiative heat transfer and near-field radiation heat flux predictive model of vehicle fires in a tunnel

As a major form of heat transfer, flame thermal radiation has an important effect on the ignition of adjacent energy, fire spread and environmental damage. Forecasting the flame radiation heat flux can provide theoretical basis for fire hazard assessment, fire extinguishing and salvation. Meanwhile, the distinctive structure of the arc tunnel can result in significant variations in the local heat and mass transfer processes during tunnel fires compared to those in rectangular tunnels, leading to different fire characteristics parameters. In this study, experiments and theoretical analyses were conducted to investigate the flame length scale and near-field radiation heat flux on the lower surface of an arc-shaped ceiling induced by vehicle fires in a tunnel. The findings reveal that the flame length scale in the transverse direction beneath the arc-shaped ceiling decreases with an increasing burner aspect ratio and increases with the increasing heat release rate. A dimensionless model of the flame length scale under the arced ceiling with various burner aspect ratios was proposed based on the amount of remaining unburned fuel after flame impingement and the buoyancy component along the arc-shaped ceiling. Simultaneously, radiative heat fluxes on the lower surface also increase with rising heat release rate. The flame geometry beneath the arc-shaped ceiling is assumed to be a combination of a cuboid and an arc plate model based on the actual flame length scale. Subsequently, the view factor, radiative fraction, and flame surface area are analyzed to predict the radiative heat fluxes on the lower surface. The predictions of the cuboid and arc plate flame model show good agreement with the current experimental data. This work presents a dimensionless correlation of the flame length scale and calculates the view factor between the target and the arc plate to predict the near-field radiative heat flux more accurately. This approach also facilitates the understanding of flame spreading behavior and thermal hazards impacted by arced ceiling.

Radiating biofuel-blended turbulent nonpremixed hydrogen flames on a coaxial spray burner

The low radiant intensity and luminosity of hydrogen flames can be enhanced by the addition of a small portion of sooting biofuels. To achieve higher effectiveness, the impact of blending turbulent nonpremixed hydrogen flames with liquid biofuels, by gas-assist atomisation, is investigated and compared with the introduction methods of prevapourisation and ultrasonic spray. The flame appearance, luminosity, radiant fraction, centreline temperature, and the near-field spray characteristics of four biofuel surrogates (eucalyptol, D-limonene, guaiacol, and anisole) blended into hydrogen flames are measured experimentally. Radiating biofuel/hydrogen flames are achieved on a coaxial needle spray burner by the addition of 0.1–0.3 mol% biofuel surrogates. Compared with the unblended hydrogen flame, the luminosity and radiant fraction are enhanced by 30%–500% and 2%–15%, respectively, with the addition of biofuel surrogates. The results show that adding the biofuel surrogates by gas-assist atomisation is more effective than prevapourisation and ultrasonic atomisation in luminosity and radiant fraction enhancement. It is found that the local fuel-rich conditions, which are beneficial for soot formation, are further facilitated by the larger droplets and spray objects generated by gas-assist atomisation. Of the additives tested, anisole is the most effective for luminosity and radiant fraction enhancement of a hydrogen flame while exhibiting the largest flame temperature drop due to the enthalpy of vapourisation and the radiative loss from the promoted soot formation. The viscosity and surface tension greatly influence the spray characteristics which in turn impacts the flame characteristics. Guaiacol, the representative of lignin, appears to have the lowest effectiveness in radiant fraction enhancement due to the presence of a hydroxy group, a higher bond dissociation enthalpy, and a coarser spray ascribed to higher viscosity and surface tension.

Fierce Feedback in an Obscured, Sub-Eddington State of the Seyfert 1.2 Markarian 817

Mrk 817 is a bright and variable Seyfert 1.2 active galactic nucleus (AGN). X-ray monitoring of Mrk 817 with the Neil Gehrels Swift Observatory in 2022 revealed that the source flux had declined to a lower level than recorded at any prior point in the then 19 yr mission. We present an analysis of deep XMM-Newton and NuSTAR observations obtained in this low flux state. The spectra reveal a complex X-ray wind consisting of neutral and ionized absorption zones. Three separate velocity components are detected as part of a structured ultrafast outflow (UFO), with v/c=0.043−0.003+0.007 , v/c=0.079−0.0008+0.003 , and v/c=0.074−0.005+0.004 . These projected velocities suggest that the wind likely arises at radii that are much smaller than the optical broad-line region. In order for each component of the outflow to contribute significant feedback, the volume filling factors must be greater than f ≃ 0.009, f ≃ 0.003, and f ≃ 0.3, respectively. For plausible, data-driven volume filling factors, these limits are passed, and the total outflow likely delivers the fierce feedback required to reshape its host environment, despite a modest radiative Eddington fraction of λ ≃ 0.008–0.016 (this range reflects plausible masses). UFOs are often detected at or above the Eddington limit; this result signals that black hole accretion has the potential to shape host galaxies even at modest Eddington fractions and over a larger fraction of a typical AGN lifetime. We discuss our findings in terms of models for disk winds and black hole feedback in this and other AGN.

Effects of low ambient pressure on the flame characteristics of ethanol−gasoline blended pool fire

The flame characteristics of Ethanol−Gasoline pool fire with different blend ratios were experimentally studied in a large−scale pressure−controlled compartment with ambient pressure of 40 kPa, 61 kPa, 80 kPa, and 101 kPa. The flame height, centerline temperature, and radiant heat flux were measured. The flame height increases notably with decreasing ambient pressure, but the flame temperature (intermittent and plume regions) slightly increases. With increasing ethanol ratio, the flame height and temperature decrease monotonously under large ambient pressure, but show a non−monotonic variation with a peak value at low ambient pressure and ethanol ratio of 10 %∼20 %. Heskestad’s flame height and temperature correlations were not suitable for blends pool fire under low ambient pressure. The new modified model for flame height correlation with ambient pressure and combustion heat of blends, and flame temperature correlation with flame height, fuel property parameter, and ambient pressure were developed with a better prediction. The addition of ethanol and decrease of ambient pressure led to the reduction of radiation loss from flame, and the non−monotonic variation with a peak value also occurs with increasing ethanol ratio at relatively large ambient pressure and ethanol ratio of 20 %. A new modified model based on the classical solid flame model was also developed to predict the radiative fraction, considering the ambient pressure, stoichiometric ratio, and the ratio of the combustion heat to the vaporization heat.

Turbulence-radiation interaction in large eddy simulations of turbulent reactive flows: a review

Turbulence-radiation interaction (TRI) arises from the highly non-linear coupling between fluctuations of radiative intensity and fluctuations of temperature and chemical composition of the medium. It is well recognized that neglecting turbulent fluctuations of the latter scalars when computing the mean radiation field, as is standard in Reynolds-Averaged Navier-Stokes (RANS) simulations, leads to mispredictions of the radiative heat fluxes and the radiant fraction in as much as one order of magnitude. Conversely, the importance of TRI in large eddy simulations (LES)—i.e., the effect of subfilter-scale (SFS) fluctuations on the filtered radiation field—has only started to be investigated in the last decade. The present text reviews the knowledge gathered so far on the subject. The instantaneous and the spatially filtered forms of the radiative transfer equation (RTE) are presented, and the unresolved correlations resulting from the filtering operation are discussed. Models for accounting for SFS-TRI are surveyed, with the most popular ones (namely, probability density function-based methods and empirical approximations) reviewed in detail. An appraisal of the literature on the importance and characteristics of SFS-TRI is performed, with special attention given to turbulent jet diffusion flames and pool fires, which are the two configurations for which SFS-TRI has been studied the most. The main findings of these studies are summarized, and recommendations are given on directions of future works on the subject.

Large eddy simulations of a buoyant turbulent line flame using conditional source-term estimation (CSE)

The objective of this paper is to assess the capabilities of the Conditional Source-term Estimation (CSE) approach applied to a laboratory scaled turbulent buoyant flame without extinction. CSE is coupled with the Large Eddy Simulation (LES) solver, FireFOAM. Tabulated detailed chemistry is included. Radiation is treated using the optically thin model using four different implementations and the effect of subgrid scale (sgs) Turbulence-Radiation Interactions (TRI) is considered. Predictions of time-averaged temperatures and corresponding root mean square (rms) are compared with the experimental measurements at several locations. Further, flame shape, unconditional and conditional species mass fractions, axial velocity and mixture fraction are also examined for qualitative analysis. The predicted temperatures are in good agreement with the experimental data, except very close to the fuel inlet. The predicted temperatures increase more slowly compared to the experimental data in the first 0.09 m above the burner. The temperature rms is in reasonable agreement with experimental data with the peak being overpredicted by approximately 17%. The predicted flame height closely matches the experimental value. The temperature predictions are consistent with previously published results. Best predictions are obtained when the effect of sgs TRI is included. For this flame, the optically thin assumption is found to be valid, in agreement with previous investigations. The calculated radiative fraction is found to be close to the experimental value, but the postprocessed heat release rate is larger than the experimental finding. This first LES-CSE study provides a good foundation for further developments, for example more detailed radiation models, for more complex fire cases.

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.

Experimental study of ullage height on the burning rate and heat transfer of medium-scale heptane pool fires

Ullage height (the distance between the fuel surface and the pool upper rim) is one of the most important and widely recognized factors that affect the burning rate and heat transfer of pool fires. This study investigated the effects of ullage height on the burning rate, heat feedback, and radiation fraction of medium-scale pool fires. Heptane pool fires were tested with a diameter (D) of 30 cm and the non-dimensional ullage height (h/D, h is the ullage height) being varied from 0 to 1.48. Results show that the non-dimensional burning rate tends to decrease exponentially with h/D. It has been proved that the current models used to predict the burning rate of pool fires are not applicable to those medium-scale pool fires that are affected by the ullage height. The radiative, conductive, and convective heat feedback rate overall decrease with the ullage height. The conductive heat feedback fraction weakly depends on h/D. The radiative heat feedback fraction firstly decreases then stabilizes with h/D, while the convective heat feedback fraction shows an opposite trend. The radiative fractions firstly increase then decrease with h/D. This paper presents a new correlation with considering the ullage height and the mean beam length within the pool ullage. The universality and accuracy of the proposed model were verified by comparing it with extensive experimental data, where D varied from 10 cm to 35 cm, and h/D varied from 0 to 2.0.

Characterisation of turbulent non-premixed hydrogen-blended flames in a scaled industrial low-swirl burner

The performance of a scaled industrial, non-premixed, low-swirl burner design was experimentally investigated for hydrogen addition to natural gas. Two strategies for introducing hydrogen are considered, namely, conserving (i) heat input and (ii) velocity/volumetric flow of the original fuel. This work characterises the effects on key performance metrics of the burner as hydrogen fraction is increased. Compared with natural gas, the results with hydrogen showed a 33 % reduction in the radiant fraction and up to a 380 % increase in NOx emissions. The lift-off height was reduced by a maximum of 23 % and 51 % for addition of 10 and 30 vol% hydrogen addition, respectively, with 100 % cases becoming completely attached to the burner. The influence of hydrogen-addition strategy and air adjustment was shown to be significant with respect to NOx emissions but less significant than the resulting changes in fuel composition and heat input with respect to flame appearance, stability and radiant heat transfer.

Large eddy simulation of fire-induced flows using Lattice-Boltzmann methods

Large-eddy simulations (LES) of the near-field region of large-scale fire plumes are performed for the first time with a pressure-based Lattice Boltzmann method (LBM) with low-Mach number approximation. Two scenarios are considered: the large-scale non-reactive helium plume and the 1 m methane pool fire, both investigated experimentally at Sandia. In the second scenario, a simplified modeling of the combustion and radiation processes is introduced involving a one-step irreversible reaction eddy-dissipation concept-based combustion model and a radiant fraction model, respectively. In both scenarios, a quantitative agreement is observed with the experimental data and model predictions are consistent with previously-published numerical studies. Our simulations demonstrate the computational efficiency of the proposed LBM solver to tackle fire-induced flows, suggesting that LBMs are a good alternative candidate for the modeling of fire-related problems.

Numerical investigation on turbulence-radiation interaction in the UMD turbulent line fires

This work aims to study the turbulence-radiation interaction (TRI) in the UMD turbulent line fires with increasing levels of underventilation until strong local extinction occurs. Large eddy simulations (LES) are performed with a radiative flamelet/progress variable model to account for local extinction events. The effects of the resolved-scale and subgrid-scale (SGS) TRI on global radiant fractions and local radiative power are quantified. Results show that the total TRI makes a considerable contribution to radiation for all oxygen concentrations (XO2) tested in this work. Specifically, in the most under-ventilated case of XO2=13% featuring strong local extinction, more than 95% of the global radiant fraction is induced by the TRI. More interestingly, it is found that on a regular LES grid resolving more than 80% of the turbulent kinetic energy, the resolved-scale TRI plays a leading role in the XO2=21% case, while the SGS TRI becomes dominant in the XO2=13% case. Further investigations reveal that the temperature self-correlation and the absorption coefficient-temperature correlation are the key components for TRI, which is confirmed by the corresponding conditional statistics. This study highlights the importance of including an SGS TRI model to accurately predict radiation fields in LES of fires with local extinction.

Radiative decay rate and branching fractions of MgF

Radiative decay rate and branching fractions of MgF

A two-zone subgrid flame model for predicting radiant emission from fires

A simple two-zone subgrid temperature distribution is developed to model turbulence-radiation interaction in the Fire Dynamics Simulator. The new approach enforces consistency between the subgrid flame distribution and the heat release rate from the eddy dissipation model. To investigate the performance of the new model, we simulate the University of Maryland turbulent line burner and compare global radiative fraction as a function of coflow oxygen dilution. The results suggest that the model can improve grid resolution dependence in prediction of radiative emissions. Sensitivity of the model predictions to the assumed fuel carbon to CO fraction (a parameter of the two-step fast chemistry scheme) was found to be significant, which is not surprising since this parameter essentially controls the in-flame soot concentration and generally has a first-order effect on resolved emission. Four different model variations were tested, corresponding to varying degrees of complexity in modeling the subgrid correlations for the absorption coefficient. Differences in results between model implementations were on par with the variation in results obtained with different path lengths applied to the gray gas model for the absorption coefficient.

Experimental study of the thermal and burning characteristics of 15 cm pool fires fueled by heptane-toluene mixtures

This article presents an experimental investigation of the effects of the fuel sooting propensity on the thermal and burning characteristics of 15 cm diameter pool fires burning in air. Heptane is used as reference fuel and two other heptane/toluene blends with toluene volume concentration of 5% and 10% are considered to vary the fuel sooting propensity. An exhaustive characterization of the pool fires is performed with measurements of the flame geometry, puffing frequency, mass burning rate, combustion efficiency, local temperature statistics from a dual thermocouple technique, local mean soot volume fraction from laser light extinction, radiative loss to the surroundings and total heat flux to the center of the pool surface. This comprehensive dataset can be used to develop and validate models for soot formation and radiative heat transfer. The experimental results show that the increase in fuel sooting propensity, characterized by a smoke point height decreasing from 12.5 cm to 4.9 cm, results in a significant enhancement in soot production, with the peak of mean soot volume fraction increasing from 0.62 ppm to 1.11 ppm. Radiative loss to the surroundings is also affected, as radiant fraction and total heat flux to the center of the fuel surface increase from 0.37 to 0.45 and 18.5 to 22.2 kW/m2 respectively. This is accompanied by a significant reduction in combustion efficiency from 0.95 to 0.83. In an opposite way, the impact on the temperature statistics is minor.

Consequence analysis of heptane multiple pool fire in a dike

Multiple Pool Fire (MPF) reasonably enhances the flame height, the rate of fuel combustion and irradiation due to cascading effect and flame merging of multiple pools. The distribution of temperature and radiative heat flux from the source is the key parameter in predicting safety distance. The current study’s major objective is to develop a Computational Fluid Dynamics (CFD) model to calculate the safety distance of MPF and validate the predicted results (flame temperature, radiative heat flux, CO2 and O2 mass fraction) with the experimental data. The flame structure and its thermal properties have been evaluated using the Reynolds Averaged Navier Stokes (RANS) and the Large Eddy Simulation (LES) model. The CFD results are found to be in close agreement with the experimental findings. The LES turbulence model predicts more accurately the flame structures and the fluctuations with an error of less than 3%, while the Re-Normalization Group k-ε model predicts the average characteristics. This authenticates the accuracy of computational methodology and robustness of the LES turbulence model to predict MPF flame characteristics. Furthermore, this computational methodology can be utilised by the industries for quantitative risk assessment and the worst-case scenario of various pool fires to save the human and material of the workplace beforehand.

Geometrical features and global radiant heat of double turbulent jet flames

In energy industry, double jet fires are often reported to occur with a large release momentum at the leakage exit. The available literature considered the double buoyancy-controlled jet flames, and also neglected the restrictive effect of ground or tank surface. In this paper, an experimental setup is built to model the interaction of double turbulent jet fires with different spacings. Different nozzle diameters and exit velocities are chosen to achieve the transition from flame buoyancy to exit momentum for the dominance of turbulent jet flames. Comparative tests are also conducted between with and without a plate near the nozzle exit. It is found that the flame buoyancy, exit momentum and nozzle spacing significantly affect the flame merging behavior. A new dimensionless parameter coupling the Froude number (Fr) and the spacing is proposed to well fit the merging probability (Pm) of double jet flames dominated by the flame buoyancy (Fr < 105 for propane) and the exit momentum (Fr ≥ 105 for propane), respectively. The interaction of double jet flames leads to a less lift-off height than a single jet flame, and the lift-off height can also be reduced by a plate near the nozzle exit. A correlation coupling the spacing, the exit diameter and Froude number, is developed for the mean flame height of double jet fires. The distributions of radiant heat flux are intensively measured around the double jet flames, which can help to calculate the radiative fraction. The radiative fraction of double jet flames at the full merging state (Pm = 1) follows the law of a single jet flame, and it decreases at the intermittent (0 < Pm < 1) and no merging (Pm = 0) states, as compared to that of a single jet flame. In addition, the available line source radiation model of a single jet flame is revised to predict the thermal radiation of double jet flames in the three different merging states.

Enhanced heat transmission in conical slip flow of Walter’s-B nanofluid

This paper examines the thermal transfer attributes of ZnO-H2O Walter’s-B nanoliquid flow alongside an upright cone in the bearing of slip mechanism, radiative heat flux, and non-uniform heat source. We adopted suitable similarities to convert the nonlinear PDEs into ODEs and solved the transformed governing system by employing the bvp5c solver package in Matlab software. The prime novelty of this research is to compute the effective thermal diffusivity of the nanoliquid via the Maxwell and Xue nanomodels. Further, simultaneous results are deliberated for Maxwell and Xue cases via pictorial outcomes under various controlling limitations, such as velocity slip, radiative heat flux, buoyancy, viscoelastic, and volume fraction of nanomaterial. The thermal transmission rate of nanoliquid reported by the Xue nanomodel is higher than the Maxwell nanomodel. Therefore, the Xue nanomodel effectively works to enhance the thermal conductivity of the liquids with viscoelastic nature. Further, this study is helpful in industrial processing, where control over heat transport is required.