Computational fluid dynamics (CFD) study of heat radiation from large liquefied petroleum gas (LPG) pool fires

Liquefied petroleum gas (LPG) is flammable and has risks of pool fires during its transportation, storage, and applications. The heat radiation by LPG pool fires poses hazards to individuals nearby and can lead to potential failures of ambient facilities. Due to the high costs and invasive nature of experiments for investigating large-scale pool fires, computational fluid dynamics (CFD) is employed in this study as the cost-effective and noninvasive method to simulate the process and analyze the characteristics of large hydrocarbon pool fires. Specifically, an experimentally validated 3-D CFD model has been built to simulate surface emissive power (SEP) and incident radiation of large-scale LPG pool fires with three different diameters and wind speeds. Steady-state simulations with P1 radiation and probability density function (PDF) combustion models were employed to obtain reliable data after the optimizations based on the comparisons with experimental data and empirical models. The comparison with benchmark experimental data demonstrates that the CFD model employed in this study can accurately predict the incident radiation of large LPG pool fires. A new SEP correlation is also proposed, which is specifically for LPG pool fires with a diameter between 10 m and 20 m. Additionally, the safe separation distances between LPG facilities and surrounded objects have been estimated based on the CFD simulation results. The high-resolution CFD model for large LPG pool fires in this work provides noninvasive and direct quantitative evidence to enhance the fundamental understanding on the safety of large LPG pool fires and can assist regulatory agencies in refining the safety limits in the cost-effective and time-saving manners.     

Assessment of four turbulence models in simulation of large-scale pool fires in the presence of wind using computational fluid dynamics (CFD)

Pool fires are the most common of all process industry accidents. Pool fires often trigger explosions which may result in more fires, causing huge losses of life and property. Since both the risk and the frequency of occurrence of pool fires are high, it is necessary to model the risks associated with pool fires so as to correctly predict the behavior of such fires.Among the parameters which determine the overall structure of a pool fire, the most important is turbulence. It determines the extent of interaction of various parameters, including combustion, wind velocity, and entrainment of the ambient air. Of the various approaches capable of modeling the turbulence associated with pool fires, computational fluid dynamics (CFD) has emerged as the most preferred due to its ability to enable closer approximation of the underlying physical phenomena.A review of the state of the art reveals that although various turbulence models exist for the simulation of pool fire no single study has compared the performance of various turbulence models in modeling pool fires. To cover this knowledge-gap an attempt has been made to employ CFD in the assessment of pool fires and find the turbulence model which is able to simulate pool fires most faithfully. The performance of the standard k–? model, renormalization group (RNG) k–? model, realizable k–? model and standard k–ω model were studied for simulating the experiments conducted earlier by Chatris et al. (2001) and Casal (2013). The results reveal that the standard k–? model enabled the closest CFD simulation of the experimental results.     

水平风对不同液位深度酒精池火火焰长度的影响

通过改变风速、液位深度的大小,研究边界条件变化对酒精池火火焰长度的影响。研究结果表明：在无量纲液位深度小于一定值时,酒精池火的火焰长度会随着风速的增加而增大,当无量纲液位深度较大时,油盘外部酒精池火的火焰长度基本为0。风速相同而液位深度不同时,酒精池火的火焰长度会随着液位深度的增大首先增大,而后逐渐减小。这主要是因为,液位深度较大时,燃料表面的供氧量降低导致燃烧效率降低,最终导致火焰长度的减小。     

Extinguishment of hydrogen laminar diffusion flames by water vapor in a cup burner apparatus

Transient computations with full hydrogen chemistry were performed to reveal the flame structure and extinguishment process of co-flow, hydrogen diffusion flame suppressed by water vapor. As the concentration of water vapor was increased, the flame detached away from the burner brim and formed an edge flame at the flame base. Water vapor showed larger chemical inhibition effect than nitrogen when extinguishing hydrogen flame, which was attributed to its enhanced third body effect in the reaction H + O₂ + M = HO₂ + M. The minimum extinguishing concentration (MEC) of water vapor and nitrogen was predicted by Senecal formula and perfectly stirred reactor (PSR) model respectively. The MECs predicted by PSR model agree with the MECs calculated by Fluent, which shows that 1) the flame extinction is controlled by the flame base, and 2) radiation absorption is negligible. The measured MECs are in a reasonable agreement with the values calculated by Fluent, which demonstrates the accuracy of the CFD model. A simple model was used to investigate the relative importance of extinguishing mechanisms of water vapor. The results show that in a co-flow configuration the thermal cooling and chemical inhibition effect are the main extinguishing mechanisms in suppressing hydrogen diffusion cup burner flame.     

不同边沿高度油池火燃烧行为的实验和数值模拟研究

采用实验和FDS数值模拟相结合的方法,探讨了边沿高度对油池火燃烧特性的影响。在实验部分,研究了燃烧速率和表观火焰高度随边沿高度的变化趋势,并分别分析了各个阶段的热反馈机制。在实验获得不同尺度、边沿高度正庚烷油池火燃烧速率的前提下,建立相应尺度的不同边沿高度油池火的Fire Dynamics Simulator(FDS)计算模型以针对火焰高度进行了数值模拟研究,分析了实际火焰高度、火焰下探高度随边沿高度的变化趋势,并提出了相关的无量纲拟合式。     

Experimental study on the competing effect of ceramic pellets on premixed methane-air flame propagation in a duct

It is urgent to explore effective suppression methods for gas fires and explosions to ensure the safe utilizations of combustible gases in industrial processes. In this work, experiments are performed to study the effect of spherical ceramic pellets on premixed methane-air flame propagation in a closed duct. High-speed schlieren photography and pressure transducers are used to record the flame propagation and pressure transient, respectively. Behaviors of the flame propagating through a section of the duct filled with ceramic pellets in mixtures at different equivalence ratios are scrutinized. Three different diameters of pellets are considered in the experiments. The result shows that the flame can be quenched in the case with a smaller pellet diameter (3 mm) for a wide range of equivalence ratios from fuel-lean to fuel-rich mixture. For larger pellet diameter (5 or 10 mm), flame extinction occurs in fuel-rich mixtures (e.g. Φ = 1.1, 1.2). For the cases of flame surviving through the pellets bed, the pellets show a significant influence on the flame structure and behavior. The flame propagation depends on the porosity and the mean void diameter of the porous media in the pellets bed. Small void diameter is beneficial to flame quenching, while large porosity can accelerate the flame propagation. The pressure dynamics evolution is closely related to the interaction of flame with the pellets, and it depends on whether the flame quenches in the pellets bed. Overall, d = 3 mm ceramic pellets display the best suppression effect on flame propagation and pressure buildup in this study. The results of this study are of great significance to guide the safety design of spherical suppression materials in engineering applications for process safety researchers and engineers.     

CFD simulation of pool fires situated at differing elevation

When two or more pool fires happen to burn so close to each other that they interact, they are termed ‘multiple pool fires’ (MPF). Past accident analysis reveals that MPFs occur quite frequently in chemical process industries. Controlled experiments done so far to study MPFs have indicated that MPFs lead to increase in the fuel burning rate, flame height and heat release rate (HRR) but the nature and the extent of the impacts of different factors on these manifestations is as yet poorly understood. In this context computational fluid dynamics (CFD) appears to be a tool which can enable more detailed and realistic simulation of MPFs than other possible approaches, especially due to its ability to closely approximate the underlying physical phenomena. In tank farms there are situations where different storage tanks are placed at different elevations yet close to each other. If such tanks happen to catch fire, the resulting fires may influence each other in a manner that may be a function of the difference in the tanks’ elevation. However no CFD study has been carried out which addresses this type of situation. Hence an attempt has been made to employ CFD to study MPFs involving two pools with fuel surfaces are at different elevations. Results reveal that good correlation is possible between the experimental findings and the CFD simulations.     

Propagation of ethylene–air flames in closed cylindrical vessels with asymmetrical ignition

A study of explosions in several elongated cylindrical vessels with length to diameter L/D = 2.4–20.7 and ignition at vessel's bottom is reported. Ethylene–air mixtures with variable concentration between 3.0 and 10.0 vol% and pressures between 0.30 and 1.80 bara were experimentally investigated at ambient initial temperature. For the whole range of ethylene concentration, several characteristic stages of flame propagation were observed. The height and rate of pressure rise in these stages were found to depend on ethylene concentration, on volume and asymmetry ratio L/D of each vessel. High rates of pressure rise were found in the early stage; in later stages lower rates of pressure rise were observed due to the increase of heat losses. The peak explosion pressures and the maximum rates of pressure rise differ strongly from those measured in centrally ignited explosions, in all examined vessels. In elongated vessels, smooth p(t) records have been obtained for the explosions of lean C₂H₄–air mixtures. In stoichiometric and rich mixtures, pressure oscillations appear even at initial pressures below ambient, resulting in significant overpressures as compared to compact vessels. In the stoichiometric mixture, the frequency of the oscillations was close to the fundamental characteristic frequency of the tube.     

Analytical modelling of hydrocarbon pool fires: Conservative evaluation of flame temperature and thermal power

As well known, risk is a combination of probability and consequences of an accident. In analyzing the consequence of accidental hydrocarbon fires and the potential for domino effects, the evaluation of the flame extent and temperature are of the utmost importance. Since the primary effects of pool fires are connected to thermal radiation and issues of interplant/tank spacing employees’ safety zones, firewall specifications are to be addressed on the basis of a proper consequence analysis. By means of real scale experimental tests it was verified that both the thermal power and the flame temperature, T_f, increase as the pool area increases, up to reach maximum values in connection with a “critical pool dimension”. Dealing with pool areas higher than the critical one, experimental results, performed by different researchers at different scales, show a decrease of T_f. An in-depth analysis of the different concurring phenomena connected to a pool fire development allowed identifying the limiting step controlling the flame temperature. In fact, the trend of T_f is mainly determined by the increasing difficulty of oxygen diffusion within the internal bulk of gaseous hydrocarbons. In this article, we propose a novel pool fire modelling approach based on the simplified physical phenomena occurring in a circular turbulent diffusion fire and suitable to provide a theoretical insight into the above-mentioned experimental trends and to obtain the maximum values of the flame temperature and of the thermal power.The geometry of the pool is dictated by the surroundings (i.e., diking) and the analytical models here presented were successfully applied to the common situation of circular pools.However, it must be remarked that the developed model, matching fairly well experimental data for different hydrocarbons, can be applied in modelling similar scenarios characterized by different geometric or environmental conditions (e.g. road and rail tunnel fires).     

LNG池火热辐射模型及安全距离影响因素研究

重点对LNG池火热辐射模型,模型应用方式,以及热辐射安全距离的影响因素做了详细研究,给出池火热辐射模型采用及安全距离计算的方法。对常用的热辐射计算模型(点源模型、LNGFire3和PoFM ISE模型)加以介绍,并对3种模型做了对比研究。PoFM ISE模型充分考虑大池火直径时不完全燃烧的因素以及风速对火焰高度的影响,建议当风速大于1.5 m/s,池火直径大于20 m时采用。同时,进一步研究影响LNG池火热辐射安全距离的各种因素,包括池火直径、风速、环境温度和湿度,从而得出不同条件下池火热辐射安全距离的要求。     

Improvement in the prediction of gasoline pool fire behaviour: CFD modelling and validation

The development and implementation of mathematical models through Computational Fluid Dynamics (CFD) techniques has been acknowledged as a promising tool for the prediction of hydrocarbon pool fires behaviours. In this sense, different approaches, with different assumptions and simplifications, and accounting for different phenomena, have been developed in the literature. However, the deviations in the predictions of the experimentally determined parameters, such as temperatures profiles, flame heights and radiative heat flux, by the implemented models are still high. Therefore, the implementation of these models to predict combustion phenomena and flame behaviours for various scenarios is limited. In this work, the software C3D is used to model gasoline pool fires of different diameters, and under different wind conditions, in order to improve the quality of the predictions of the flame behaviour. The modelled cases correspond to the experimental studies reported in literature. The results from the implemented model show an improved predictive quality when compared with other modelling works reported on literature for the same experimental cases. The deviations in the time averaged temperature, flame height, surface emissive power and radiative heat flux, has been calculated to be 5.0%, 0.05%, 6.32% and 3.82%, respectively.     

The study of burning behaviors and quantitative risk assessment for 0# diesel oil pool fires

The liquid fuel safety issues on fuel storage, transportation and processing have gained most attention because of the high fire risk. In this paper, some 0# diesel pool fire experiments with different diameters (0.2–1 m) were conducted with initial fuel thicknesses of 2 cm and 4 cm, respectively, to obtain liquid fuel combustion characteristics. Some key parameters including mass burning rate, flame height and the flame radiative heat flux, associated with fire risk, were investigated and determined. Subsequently, a detail quantitative risk assessment framework for 0# diesel pool fire is proposed based on the 0# diesel burning characteristics. In the framework, the probability of personal dead and the facility failure are calculated by the vulnerability models, respectively. In the end, 10 special tank fire scenarios were selected to show the whole risk calculation process. The tank diameter and the distance to pool fires were paid more attention in the cases. The safety distances in the cases are provided for the persons and nearby facilities, respectively. The paper enriches the basic experimental data and the provided framework is useful to the management of 0# diesel tank areas.     

Interaction of flame instabilities and pressure behavior of hydrogen-propane explosion

The flame destabilization mechanism of hydrogen-propane-air mixture is firstly revealed. The effects of unstable flame formation on pressure rise rate and burning rate are quantified. Finally, the theoretical prediction of explosion pressure behavior is performed by considering diffusive-thermal and hydrodynamic instability. The results demonstrated that before the explosion pressure starts to climbe, as the propane fraction increases, the effective Lewis number of lean and stoichiometric mixture undergoes the transition from Le_eff < 1.0 to Le_eff > 1.0, the stabilizing effect of diffusive-thermal instability continues to reduce for the rich mixture. After the explosion pressure starts to climbe, the hydrogen-propane flame becomes more unstable, which is mainly attributed to enhancing hydrodynamic instability. The maximum rate of pressure rise and burning rate should be augmented by unstable flame formation, the flame instabilities must be considered in the explosion pressure evaluation.     

Numerical and experimental study on flame spread over conveyor belts in a large-scale tunnel

Conveyor belt fires in an underground mine pose a serious life threat to the miners. This paper presents numerical and experimental results characterizing a conveyor belt fire in a large-scale tunnel. A computational fluid dynamics (CFD) model was developed to simulate the flame spread over the conveyor belt in a mine entry. Thermogravimetric analysis (TGA) tests were conducted for the conveyor belt and results were used to estimate the kinetic properties for modeling the pyrolysis process of the conveyor belt burning. The CFD model was calibrated using results from the large-scale conveyor belt fire experiments. The comparison between simulation and test results shows that the CFD model is able to capture the major features of the flame spread over the conveyor belt. The predicted maximum heat release rate, and maximum smoke temperature are in good agreement with the large-scale tunnel fire test results. The calibrated CFD model can be used to predict the flame spread over a conveyor belt in a mine entry under different physical conditions and ventilation parameters to aid in the design of improved fire detection and suppression systems, mine rescue, and mine emergency planning.     

Numerical simulation of flame acceleration and deflagration-to-detonation transition of ethylene in channels

Ethylene (C₂H₄) is a hydrocarbon fuel and widely used in chemical industry, however, ethylene is highly flammable and therefore presents a serious fire and explosion hazard. This work is initiated by addressing the hazard assessment of ethylene mixtures in different scale channels (d = 5 mm, 10 mm and 20 mm) from the aspect of flame acceleration (FA) and deflagration-to-detonation transition (DDT) by using large eddy simulation (LES) method coupled with the artificially thickened flame (ATF) approach. The fifth order local characteristics based weighted essentially non-oscillatory (WENO) conservative finite difference scheme is employed to solve the governing equations. The numerical results confirm that flame velocity increase rapidly at the beginning stage in three channels, and the flame acceleration rate is slower in the subsequent stage, afterwards, the flame velocity has an abrupt increase, and the onset of detonation occurs. Due to the fact that wall effect is significant in the narrow channel (e.g.,5 mm), especially in the ignition stage of the flame, flames have different shapes in wider channels (10 mm and 20 mm) and narrow channel (5 mm). Both the pressure and temperature profiles confirm DDT run-up distances are 0.251 m, 0.203 m and 0.161 m in 20 mm, 10 mm and 5 mm channels, respectively, which indicates that a shorter run-up distance is required in narrower channel. The cellular detonation structures for the ethylene-air mixture in different channels indicate that multi-headed detonation structures can be found in 20 mm channel, as the channel width decreases to 10 mm, detonation has a single-headed spinning structure, as the width is further reduced to 5 mm, only large longitudinal oscillation of the pressure can be observed.     

Experimental study on vented deflagration of hydrocarbon fuel-air mixtures in a 20-L semi-confined cylindrical vessel with a slight static activation pressure

The overpressure peaks and flame propagation characteristics of hydrocarbon fuel-air mixtures vented deflagration in a 20-L cylindrical vessel with a slight static activation overpressure (P_ST = 2.5 kPa) and five vent opening ratio were studied by a series of experiments. The experiments focused on the effect of vent opening ratio on the overpressure peaks and flame propagation characteristics of hydrocarbon fuel-air mixture vented deflagration. The internal overpressure-time profiles and high-speed photographs of flame propagation processes were obtained. The results showed that three overpressure peaks were distinguished in the internal overpressure-time profiles, caused by the burst vent cover (p_burst), the acceleration of burnt gas (p_fv), and the fierce external deflagration of vented unburned fuel (p_ext), respectively. The changing of the vent opening ratio had almost no effect on the value of p_burst and (dp_burst/dt). With increasing vent opening ratio, the values of p_fv, p_ext, (dp_fv/dt) and (dp_ext/dt) showed a decreasing trend while the values of p_burst and (dp_burst/dt) were nearly constant. The flame presented a hemispherical shape before the vent cover ruptured then developed as a mushroom shape after accelerated to external field. There were three flame speed peaks during flame propagation process, resulted from venting flow acceleration, external deflagration, and axial heat flux formed by internal combustion. With the increase of vent opening ratio, all of the maximum flame speed, external average flame speed, maximum flame distance and external flame duration showed a downward trend, excepting for the internal average flame speed almost remained constant.     

Propane-air mixture deflagration hazard with different ignition positions and restraints in large-scale venting chamber

In order to better assess the hazards of explosion accidents, propane-air mixture deflagrations were conducted in a large-scale straight rectangular chamber (with a cross-section of 1.5 m × 1.5 m, length of 10 m, and total volume of 22.5 m³). The effect of initial volume, ignition position, and initial restraints on the explosion characteristics of the propane-air mixtures was investigated. The explosion overpressure, flame propagation, and flame speed were obtained and the computational fluid dynamics (CFD) software was used to simulate the flame-propagation process and field flow for auxiliary analysis. The hazards of large-scale propagation explosion under weak and strong constraints were evaluated and the different phases of flame propagation under weak and strong constraints were discriminated. Results indicate that the hazards caused by propane deflagration under weak constraint are mainly caused by flame spread. And the maximum overpressure under strong constraint appeared at the front part of the chamber under the large-scale condition, which is consistent with the previous small-scale test. Moreover, the simulations of flame structures under weak and strong constraint are in good agreement with experimental results, which furthers the understanding of large-scale propane deflagration under different initial conditions in large-scale spaces and provides basic data for three-dimensional CFD model improvement.     

A numerical modelling of an influence of CH4, N2, CO2 and steam on a laminar burning velocity of hydrogen in air

The influence of additives of various chemical natures (CH₄, N₂, CO₂, and steam) at a laminar burning velocity S_u of hydrogen in air has been studied by numerical modelling of a flat flame propagation in a gaseous mixture. It was found that the additives of methane to hydrogen–air mixtures cause as a rule monotonic reduction in the S_u value with the exception of very lean mixtures (fuel equivalence ratio ? = 0.4), for which a dependence of the laminar burning velocity on the additive's concentration has a maximum. In the case of the chemically inert additives (N₂, CO₂, H₂O) the laminar burning velocity of rich near-limit hydrogen–air flames drops monotonically with an increase in the additive's content, but no more than 1.5 times, and the adiabatic flame temperature changes slowly in this case. In the case of methane as the additive, the laminar burning velocity is diminished approximately 5 times with an increase in the adiabatic flame temperature from 1200 to 2100 K. Deviations from the known empirical rule of the approximate constancy of the laminar burning velocity for near-limit flames are shown.     

Spill-over characteristics of peroxy-fuels: Two-phase CFD investigations

Two-phase CFD (Computational Fluid Dynamics) model for characterising the spill-over/dispersion of peroxy-fuels is presented. The model is independent of type and burning rate of the spilled/dispersed fuel and considers only overflow Reynolds number (Re) to characterise the spill/dispersion behaviour. Additional simulations are performed for LNG (Liquified Natural Gas) dispersion and it is found that the model can be used for different fuels within a defined range of Re. Different scenarios with Re = 100 to 3 × 10⁵ are investigated covering a wide range of mass flow rates, opening sizes and viscosities. Depending on Lower Flammability Limits (LFL) of the fuels spill/dispersion (vapour cloud) diameters (D_CFD) and heights (h_CFD) are predicted. A generalised correlation between D_CFD and Re is established to predict the dispersion occurring at varying scales. The model is validated by: (1) conducting an extensive grid independent study; (2) comparing the results with the existing analytical methods and (3) comparing against the standard field test data on LNG dispersions.     

Improved models of failure time for atmospheric tanks under the coupling effect of multiple pool fires

Fire accidents of chemical installations may cause domino effects in atmospheric tank farms, where a large amount of hazardous substances are stored or processed. Pool fire is a major form of fire accidents, and the thermal radiation from pool fire is the primary hazard of domino accidents. The coupling of multiple pool fires is a realistic and important accident phenomenon that enhances the propagation of domino accidents. However, previous research has mostly focused on the escalation of domino accidents induced by a single pool fire. To overcome the drawback, in this study, the failure of a storage tank under the coupling effect of multiple pool fires was studied in view of spatial and temporal synergistic process. The historical accident statistics indicated that the accident scenario of two-pool fires accounted for 30.6% in pool fires. The domino accident scenario involving three tanks is analyzed, and the typical layout of tanks is isosceles right triangle based on Chinese standard “GB50341-2014”. The thermal response and damage of a target tank heated by pool fires were numerically investigated. The volume of 500 m³, 3000 m³, 5000 m³ and 10000 m³ were selected. Flame temperature was obtained by FDS, and then was input onto the finite element model. The temperature field and stress field of target tanks were simulated by ABAQUS. The results showed that the temperature rise rate of the target tanks under multiple pool fires was higher than that under a single pool fire. The failure time of the tank under the coupling effect of multiple fires was lower than that under the superposition of multiple fires without the first stage. The stress and yield strength were compared to judge the failure of the target tank. The model of failure time for the tank under the coupling effect of pool fires was established. Through the verification, the deviation of this model is 4.02%, which is better than the deviation of 15.76% with Cozzani's model.