Radiative Heat Feedback Research Articles

Experimental investigation of effects of ullage height on the burning rate and heat feedback in pool fires

Pool fire with varying ullage height is a common type of accident that occurs in industrial parks. This paper explores the ullage height effects on the burning rate and heat feedback in pool fires through the experimental and theoretical study. A series of experiments were conducted using heptane with four tray sizes and five ullage heights. The results show that, the burning rate initially decreases, then increases and finally decreases as the ullage height increases. These results are mainly dependent on the change of internal flame behavior and thus heat feedback mechanisms with increasing ullage height. In the small ullage height conditions, the convective heat feedback reduces substantially whereas the radiative one rises slightly. When the ullage height reaches to a certain value, the convective heat feedback suddenly increases because of the flame inside the tray. The radiative heat feedback decreases gradually as the ullage height increase continually, while the convective one becomes nearly constant. Based on these results, three regimes are identified in terms of the burning rate variation with heat feedback: (1) convection controlled, (2) convection-radiation controlled and (3) radiation controlled. The new controlling mechanism of the burning rate variation (with ullage height), as a further extension of the classic scaling model by Blinov and Khudyakov on the burning rate variation (with pool diameter), could lay a solid foundation for future studies of the other key parameters in tank fires.

Effect of flame radiation on aluminium particle combustion time

This work proposes a numerical model to estimate the radiant heat feedback from the flame of a burning aluminium particle. The objective is to evaluate its role on the burning time. It rests on a detailed combustion simulation of a single particle coupled with a simple two-flux approximation for the radiant flux emitted from the oxide smoke. Radiated flux from the flame is found to be around 1 MW/m 2 for atmospheric pressures, going up to more than 5 MW/m 2 for higher pressures (30 bar). Parametric simulations show that aluminium size has a small effect while oxygen-rich atmospheres can double the radiated flux. Overall, the role of flame radiation on aluminium burning rate is calculated to lie roughly between 1 and 25%, depending on conditions. This is the first study that clearly evaluates the role of flame radiation on aluminium combustion ; in most configurations, this effect, however, remains limited.

Numerical investigation and experimental validation of aluminized propellant combustion under high pressures: Critical effects of heat feedback

Aluminum is a common metal fuel in propellants, but its effects on the burning rate and the underlying mechanism are still unclear. Numerical calculations and temperature measurement experiments were carried out within the pressure range of 0.1–9 MPa, and the effects of the pressure and the ammonium perchlorate (AP) particle size on the combustion characteristics of aluminized propellants were studied. First, based on a two-dimensional micro-scale combustion model of an AP/hydroxyl‑terminated polybutadiene (HTPB) propellant, by considering the “heat sink” effect of aluminum on the burning surface and the chemical reaction of aluminum in the gas phase, a numerical calculation model of heat flux coupled with the steady-state combustion of an aluminized propellant was established. The flame structure of an aluminized propellant obtained by this model conformed to the multi-flame structure and could correctly reflect the influence of the AP particle size and pressure on the burning rate and other combustion characteristics. Subsequently, the influence of radiation heat feedback was further introduced into the model, which greatly improved its accuracy in predicting the burning rate at low pressures of 1–3 MPa. More importantly, the temperature distributions of the propellants were measured using embedded tungsten–rhenium thermocouples, and the flame temperature was obtained by further processing the temperature measurement data according to the thermocouple response characteristics. The temperature measurement results showed that with the increase in the pressure and the decrease in the size of the AP particles, the temperature gradient increased significantly, the flame temperature increased slightly, and the burning rate of the propellant increased. By analyzing how the temperature gradient and the burning rate changed with pressure, it was found that the conduction heat feedback was important to determine the burning rate.

Experimental study on the influence of restricted distance on the discrete flame spread of different widths

In this paper, the effects of varying material widths on the propagation of discrete fires within diverse confined space conditions are investigated experimentally. The width-induced mechanisms influencing the flame spread of discrete solid surfaces in distinct confined spaces are comprehensively discussed, encompassing flame characteristics, pyrolysis front dynamics, flame spread rate, and mass loss rate. The findings reveal that the dimensionless flame height and width conform to a negative power relationship at varying restricted distances. Furthermore, a positive correlation between the width of the PMMA in confined spaces and both the flame spread rate and the mass loss rate is established. Notably, when the width is fixed, the confined space plays a dominant role in the propagation of flames. As the restricted distance decreases, the flames become more slender. Additionally, flame height, flame spread rate, and mass loss rate increase with the widening of the material. However, as the restricted distance becomes smaller, the effect of width becomes less pronounced. Under conditions of same restricted distance, an increased width results in a higher radiation factor, thereby enhancing the thermal feedback of the wall to the material, which in turn accelerates the spread rate of discrete flames. Moreover, due to the dominant role of radiant heat feedback provided by the confined walls to the discrete PMMA panels, the proportion of heat transferred by convection in the flames is minimal. Consequently, the influence of the convective coefficient's width variation on flame spread is relatively minor within confined spaces. Ultimately, an energy conservation model under confined spaces was established, which facilitated the analysis of the comprehensive mechanism of flame spread influenced by varying confined spaces and material widths.

Detailed modeling of the radiative feedback in the large eddy simulation of a lab-scale heptane pool fire

This article proposes a detailed modeling of gas/soot radiation for large eddy simulation (LES) of sooting pool fires with a focus on the radiative heat feedback through the fuel vapor dome. The numerical model combines a non-adiabatic steady laminar flamelet (SLF) model, a two-equation acetylene/benzene-based soot production model, the Rank-Correlated Full-Spectrum K (RCFSK) and a presumed filtered density function (FDF) approach to model the subgrid-scale (SGS) interactions between turbulence, chemistry, soot production and radiation. This LES model is applied to simulate 15 cm heptane pool fire. The reliability of the RCSFK and the acetylene/benzene soot model for heptane pool fire scenarios is investigated through a priori analysis. First, decoupled radiative heat transfer calculations along lines of sight, representative of heptane pool fires and covering a wide range of optical thicknesses, demonstrate that the RCFSK computes the absorption of the fuel vapor dome with a similar accuracy as the narrow-band correlated-k (NBCK) model. Second, simulations of laminar coflow diffusion flame are used to extend the acetylene/benzene-based soot production model, previously validated for C1-C3 hydrocarbons, to n-heptane flames. LES of the 15 cm heptane pool fire reproduces with fidelity the flame structure in terms of mean and fluctuating temperatures and mean soot volume fraction (SVF). Simulations with and without considering heptane vapor radiation evidences its effects on the radiative outputs and the heat feedback. The radiative contribution of heptane vapor noticeably enhances the radiative loss to the surroundings and the peak of axial radiative flux. On the hand, the heat feedback to the pool surface is predicted within the experimental uncertainty when this contribution is taken into account with the radiative component being reduced by more than 10 %.

Experimental research on radiation blockage of the fuel vapor and flame in pool fires

Pool fires are the most prevalent accidents in the process industry. Revealing the physical mechanism of pool fire has both fundamental and practical applications in process safety and risk analysis. This paper intends to study the radiation blockage phenomenon caused by fuel vapor and flame in pool fires. Combustion and evaporation (non-combustion) experiments under different external radiative heat fluxes have been conducted to differentiate the radiation blockage of the fuel vapor and the flame. Four different sooting fuels including methanol, ethanol, n-heptane and toluene were used in the experiments. The radiation blockage of fuel vapor was determined through evaporation experiments. The radiation blockage of flame and the total radiation blockage of pool fires were investigated by burning experiments. Based on the assumptions of radiation gray for flame radiation and external radiation, the effective radiation blockage was determined. It is found that the effective radiation blockage coefficient of the fuel vapor increases with fuel mass flux first, and then gradually approaches a constant value because the radiation absorption capacity of the fuel vapor tends to saturate with the increase of fuel mass flux. The correlations between the fuel vapor radiation blockage coefficient and fuel mass flux are established based on theoretical analysis and experimental data. Moreover, the flame blockage coefficient decreases with external radiation for methanol, ethanol and n-heptane because the fire expands, causing enhanced radiative heat feedback from the flame. For the heavily sooting fuel, toluene, the flame radiation blockage almost remains constant with external radiation due to high soot concentrations.

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.

Oscillatory burning regime in a gas-fueled compartment fire

The paper presents a series of 48 enclosure fire experiments carried out in a 1.40 m cubic compartment with a door-like opening of 0.80 m width and a variable height from 0.25 to 1.40 m. The fire source was a propane-fueled square burner of 0.17 m side length positioned in the center of the compartment and delivering a steady mass flux that has been varied in the tests from 0.5 to 5.0 g/s. In addition to the well-known well-ventilated and under-ventilated regimes, an oscillatory combustion regime has been uncovered for an opening height of 0.25 or 0.35 m. This regime is characterized by an alternation between the well-ventilated regime (combustion exclusively inside the compartment) and the under-ventilated regime (combustion inside and outside the enclosure). It is believed that the time for the onset of the oscillations is linked to the smoke filling time. Furthermore, depending on the specific conditions, the oscillations are either steady (until shut-off of the fuel supply) or transitory (leading to a sustained under-ventilated regime). The frequencies ranged from about 9 to 19 mHz. The oscillatory regime has previously been mainly reported for liquid fuels and thus associated to radiative heat feedback effects. The new data for a gaseous fuel calls upon a better understanding and a further analysis of the underlying physics.

Experimental evaluation and quantitative modeling of thermal behavior of surface-attached insulation polymers and steel structures under combustion

The surfaces of liquefied petroleum gas (LPG) storage vessels are usually coated with polymer insulation to maintain low temperatures. However, this insulation layer poses a potential fire risk. This study aimed to investigate the heat feedback mechanism and its impact on steel structures when bonded thermal insulation materials combust. Thermal insulation materials were bonded to Q345 steel surface, which is commonly used in LPG storage tanks, and experiments were conducted by designing a burning experimental rig of the combustible material attached to steel plate. Results revealed the existence of an equivalent temperature layer in the flame zone above the insulation material. Equations for predicting radiative and convective heat feedback were formulated. As combustion of the insulation material stabilized, the predicted heat feedback from the equations closely matched experimental values. A quantitative evaluation method, based on the inverse heat transfer problem, was proposed. This method determines the thermal decay function of heat feedback through the insulation material and steel plate. By establishing three key performance indicators, this work achieved independent quantification of potential thermal concerns and robustness evaluations of steel structures with surface-bonded insulation materials under combustion.

Modelling and effects of fuel radiation in methanol pool fires

This article proposes a computationally affordable radiative heat transfer model to predict accurately the feedback toward the fuel surface. It combines the multi-scale full-spectrum k (MSFSK) approach to model accurately the radiative interaction between CO2/H2O and the fuel and the rank correlated (RCFSK) scheme. The model achieves the narrow band correlated-k model accuracy with only five quadrature points for each of the two scales. The predictions are also weakly dependent on the Planck temperature which allows to store efficiently the MSRCFSK parameters in a flamelet library. The model is implemented in a well-validated numerical model to provide large eddy simulations of 30 cm and 1 m diameter methanol pool fires. The heat feedback predictions are improved when methanol radiation is considered. The overall radiative contribution of methanol results from two competitive mechanisms: an increase in emission in the hot part of the fuel dome and an increase in absorption close to the pool surface. For both pool fires, the enhancement in emission overall dominates, leading to higher radiative loss and heat feedback. The contribution of absorption increases with the pool size and, as a result, the effects of methanol radiation are more pronounced for the 30 cm pool than for the 1 m one.

Experimental study of the burning behavior and key parameters of gasoline pool fires with different ullage heights

Pool fires with different ullage heights are a common type of fire accident. A series of gasoline pool fire experiments with two sizes (D = 40 cm, 60 cm) and six ullage heights (h = 0, 0.2D, 0.4D, 0.6D, 0.8D, 1.0D) are conducted. The burning process, axial temperature profile, radiative heat feedback, and burning rate are measured and analyzed. The result shows that the fuel vapor layer and the down-reaching flame layer are distinguished based on the axial temperature profile for the steady burning stage. Meanwhile, the down-reaching flame length (Ldown) increases more profoundly for large tank diameters under the same ullage height. Subsequently, the dimensionless down-reaching flame length (Ldown* = Ldown/D) increases exponentially with the dimensionless ullage heights (h* = h/D). Finally, based on the classical burning rate model for the low ullage height and the heat transfer process from the flame to the fuel surface, a correlation with different ullage heights is established to calculate the burning rate, which is then validated against the experimental data in the paper and literature values. The results are of importance to understand the burning rate and the radiative heat feedback to the fuel surface for pool fires with different ullage heights.

Lip height effects on pool fire: An experimental investigation

This work studies lip height effects on the heat transfer and combustion characteristics of pool fire. Fire experiments of n-heptane and ethanol with pool diameters (D) of 0.1 m, 0.15 m and 0.2 m were carried out, in which the lip height (h) was increased from 0 until the burning could not be sustained. Results show that for both fuels, as the lip height increases, the flame enters the pool and the flame sheet becomes wrinkled and deformed. The n-heptane fire shows two distinct burning behaviors inside and outside the pool, and the burning rate decreases with the lip height due to heat feedback attenuation and air entrainment restriction. The conductive feedback rate remains almost unchanged with different lip heights for both fuels. For n-heptane, the radiative and convective heat feedback rates generally decrease with the lip height, and the convective heat feedback fraction increases as the flame enters into the pool, suggesting intensified turbulence motion. For ethanol, the radiative heat feedback rate decreases monotonically with the lip height, and the convective heat feedback rate increases slightly, followed by a decrease with h/D. A mass flux correlation based on the modified stagnant film theory is developed for the convection-dominated pool fires, which is in good agreement with the experimental data.

Flow and thermal radiation characteristics of a turbulent flame by large eddy simulation

Liquid fuel pool fire flame is a common combustion problem for liquid oil in many practical applications. Thermal radiation has considerable effect on its combustion process and fire spread. Based on the large eddy simulation method of the fireFOAM framework, the flame properties and thermal radiation characteristics of a 1 m diameter methanol pool fire are investigated. The flame structure features frequent buoyance-driven upward movement and air entrainment from surrounding environment due to turbulent mixing and combustion. The radiative heat feedback shows strong temporal and spatial variations, which is caused by the fluctuating behaviors of the temperature and species fields. The radiation transfer models have significant discrepancies in predicting the radiative heat feedback for all the surfaces with the weighted-sum-of-gray gases model producing better results. Ray effects of the solvers lead to unphysical non-uniform distributions of radiative heat flux on the bottom and side walls.

The burning process and temperature profile of double fires in a tunnel: An experimental study

In the event of double fires in tunnels, the combustion is strengthened and the smoke movement is restricted by the strong flame plumes. Heavy smoke accumulates rapidly between the two fires, which can result in serious casualties. For this paper, a series of experiments was carried out using a 1:10 scale tunnel, to primarily investigate the fire evolution and ceiling temperature distribution as a function of burner size and spacing. The burning process, the longitudinal ceiling temperature profile and the maximum temperature rise were measured and analyzed. The results show that the time from ignition to when the fire reaches a stable stage is obviously shorter than for a single fire. Furthermore, the steady burning rate is greater than that for a single fire with the same burner size. This higher steady-burning rate decreases as burner spacing increases, but increases with increasing burner size. This is due to the change in radiative heat feedback from the flame to the fuel surface. The longitudinal ceiling temperature profile shows a “double-hump”, with the hump values being dependent on the burner spacing. The ceiling temperature between the two fires in a double fire is obviously increased compared to that of a single fire, which can be attributed to the accumulation of smoke and heat between the two flames. The longitudinal ceiling temperature away from the double fires shows an exponentially attenuated trend and the corresponding model is revised. Finally, a maximum temperature rise model is established by introducing burner spacing. This model was validated with the experimental data. These new findings can be used for risk assessment of double fire accidents and to determine the best strategies for personnel evacuation from a tunnel.

Radiant heat feedback from a jet flame to the ruptured tank surface

The gas leakage of high-pressure tank can easily induce a jet fire which could heat the accidental tank by radiative heat feedback. In this paper, a new facility consisting of a jet fire apparatus and a hemispherical tank was used to model the jet flame ejected from the ruptured tank. The jet flame geometrical features were systematically measured under different nozzle diameters, exit velocities and jet directions. The radiant heat fluxes were also measured on the ruptured tank surface exposed to the jet fire. Two kinds of thermal radiation models were established to predict the radiant heat flux field on the tank surface. The first model only considers the effect of momentum on the flame geometry as the Froude number is large at the nozzle exit, while the other takes both buoyancy and momentum into account for the low Froude number. The model prediction agrees well with the experimental measurement. Finally, using the thermal radiation model and the heat balance equation, the distribution fields of radiant heat flux and temperature are calculated for the ruptured tank surface.

Experimental study on the characteristics of temperature distribution of two pool fires with different transverse locations in a naturally ventilated tunnel

The interaction between multiple fires is complex, in which the theory of single fire is not applicable. This study focuses on the combustion characteristics and temperature distribution of two pool fires with different transverse locations in a model tunnel under natural ventilation. In this study, 84 groups of combustion experiments of two ethanol pool fires were designed by varying the spacing between two pool fires (denoted by S), transverse distance between the geometric center of pool fires and tunnel sidewall (L), and side length of oil pan (D). The experimental results show that when L = 50 cm and L = D/2, mass loss rate decreases along with a bigger S/D, and reaches the minimum for L = 50 cm and the maximum for L = D/2. However, mass loss rate first increases and then decreases with S/D for L = 30 cm and L = 20 cm, which is controlled by the competition between air entrainment and radiant heat feedback. In order to describe the influences of fire source spacing and transverse distance, a prediction model of maximum temperature rise under the tunnel ceiling is developed, which matches the experimental results well. Under the case of no flame merging, longitudinal temperature distribution is insensitive to the transverse location rather than the spacing between the fire sources. Moreover, it is found that the vertical temperature distribution far from the fire sources matches the Gaussian distribution well. The results of this work can provide a significant reference for the risk assessment of tunnel fires with two fire sources.

Experimental study of concurrent-flow flame spread over thin solids in confined space in microgravity

Concurrent flow flame spread experiments are conducted over thermally thin solid fuels in microgravity aboard the International Space Station (ISS) under varying levels of confinement. Samples of cotton fiberglass blended textile fabric are burned in air flows in a small flow duct. Baffles are placed parallel to the sample sheet, one on each side symmetrically. The distance between the baffles is varied to change the confinement of the burning event. Three different materials of baffles are used to alter the radiative boundary conditions of the space that the flame resides: transparent polycarbonate, black anodized aluminum, and polished aluminum. In all tests, samples are ignited at the upstream leading edge and allowed to burn to completion. The results show that at low flow speeds (<17 cm/s), the flame reaches a steady state for all tested baffle types and baffle distances. The spread rates and flame lengths at the steady state increase first and then decrease when the baffle distance decreases, resulting in an optimal baffle distance for flame spread. Furthermore, there exists a limiting baffle distance below which the flame fails to spread. It is concluded that the confinement imposed by the baffles accelerates the flow during the combustion thermal expansion and the baffles reflect flame radiation back to the sample surface, both of which intensifying the burning. However, the confinement also limits the oxygen supply and introduces conductive heat loss away from the flame. At the same baffle distance and imposed flow speed, flame length and spread rate are largest for polished aluminum baffles, and lowest for transparent polycarbonate baffles. The differences are most prominent at intermediate tested baffle distances. While the radiative heat feedback from the baffles is expected to increase when the baffle distance decreases, flame length and flame spread rate are similar for all baffle types at small baffle distances as the combustion is limited by the reduced oxygen supply.

Experimental study on combustion and flame spread characteristics in horizontal arrays of discrete fuels

Abstract The combustion and flame spread behaviors of discrete fuels often depend on the distribution of the fuel arrays, including the fuel bed width and the spacing of the fuel arrays. In this study, scenarios with six different numbers of columns (n, 1–11 with an interval of 2) and nine different spacings (S, 1–9 mm) were designed. By varying n and S, 54 groups of experimental scenarios over thermally thick birch rod arrays were studied to determine the effects of fuel bed width and spacing. The experimental results showed that the critical spacing for multi-column arrays was larger than that for single-column arrays. Moreover, flame extinction was found when n > 1 and S = 1 mm . The value of S had a limited influence on the mass loss rate when S > 2 mm , but restrained the mass loss rate when S ≤ 2 mm . A piecewise correlation between a newly defined dimensionless mass loss rate and the exposed area ratio was proposed. Moreover, a prediction model of the mass loss rate was developed, which agreed reasonably well with the experimental data. With increasing spacing, the flame height showed three stages: increasing, relatively stable, and decreasing, which were dominated by the balance between air entrainment and radiant heat feedback. The three processes were further described by a piecewise correlation between the dimensionless flame height and fuel bed width. Based on the assumption of a constant incident heat flux, a prediction model of the global flame spread rate was built, which presented better predicted results than the previous models. In addition, it was found that the newly defined dimensionless global flame spread rate linearly increased with increasing porosity.

Hot-zone boilover suppression using floating objects in crude oil tank fires

Hot-zone boilover is dangerous in crude oil tank fires. Previous investigations have experimentally shown that floating objects can suppress hot-zone boilover with small scale pool fires, but not indicate a clear suppression mechanism. Thus, this study uses floating perlites (0, 1 and 2 layers) on large scale pool fires to examine the mechanism by performing three series of experiments. The first series applies perlites to 1m-diameter crude oil fires to evaluate the effect of perlites on the hot-zone formation; the second investigates the effect of perlites on heat release rate (HRR) and combustion efficiency, and the third studies the blockage effect of perlites on radiation heat feedback. Experimental results show that the perlites can significantly delay the hot-zone formation, and suppress boilover. Accordingly, this study presents a hypothesis to explain the mechanism of suppressing boilover by floating objects. The perlites form a heat blockage layer decreasing the radiation feedback, and act as a nucleation site, leading to local vigorous boiling. The enhanced local nucleation increases the fuel vapor generation, and further enhances the blockage effect. The perlites consequently raise the fuel regression rate, and decrease the heat transfer to the deep fuel and water, decreasing the hot-zone formation rate.

Experimental study on mass burning rate and heat feedback mechanism of pair of unequal circular pool fires of heptane

This paper aims to study the burning behaviors and heat feedback mechanisms of pair of unequal circular pool fires. A series of experiments on heptane pool fires with four diameters of 10, 15, 20 and 30 cm comprising 6 groups of unequal two pans was carried out. The pan edge spacing (S) was varied as a multiple of half of the diameter of the small pan in fire group and increased from 0 to the case of no flames merging. The fuel mass, flame shape and heat feedback components were measured. The results showed that the mass burning rate per unit area (m˙″) of the small pan (Ds) in fire group increases monotonously with reducing S. While m˙″ of the large pan (Dl) in fire group increases first and then decreases with reducing S, and the peak m˙″ presents at S/Dl = 0.167∼0.75. It is also found that for a given fire group, m˙″ from Ds and Dl are very close. From the determined heat feedback components and their fractions, it is indicated that for a given target pan, the three types of heat feedback fractions including conduction, convection and radiation are weakly affected by the adjacent pan scale. For the target pan Dt = 10 cm, the radiative heat feedback fraction (χrad) nearly maintains constant for the present limited S, the conductive heat feedback fraction (χcond) is comparable to χrad with small S and then it begins to decay as S increases. For Dt = 15–30 cm, the burning rate of each pan in fire group is radiation dominated. Based on the imposed heat feedback mechanism of MPF, a useful correlation is developed to estimate the mass burning rate of radiation-dominated MPF.